Morphologies of PDMS-containing Diblock Polymers
by
Charlotte Stewart-Sloan
Submitted to the Department of Materials Science and Engineering
in partial fulfillment of the requirements for the degree of
Master's of Science in Materials Science and Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2012
© Massachusetts Institute of Technology 2012. All rights reserved.
MASSACHUSETTS INSTl
OFTCHNLOGY
Author
...----------------RARIES
Engineering
and
Science
Department of Materials
February, 2012
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.................................................
Edwin L. Thomas
William and Stephanie Sick Dean of the George R. Brown School of
Engineering Professor, Rice University
Thesis Supervisor
C ertified by .....
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Accepted by ...................
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.
Christopher Schuh
Chairman, Department Committee on Graduate Students
Morphologies of PDMS-containing Diblock Polymers
by
Charlotte Stewart-Sloan
Submitted to the Department of Materials Science and Engineering
on February, 2012, in partial fulfillment of the
requirements for the degree of
Master's of Science in Materials Science and Engineering
Abstract
The morphologies of polydimethylsiloxane (PDMS)-containing diblock polymers are investigated as a function of volume fraction, segregation, processing procedure, and temperature. Strongly segregated polyisoprene-PDMS and polystyrene-PDMS diblocks are
prepared according to standard procedures in the literature by anionic synthesis in the
laboratory of Professor Apostolos Avgeropoulos at the University of Ioannina in Ioannina, Greece and their morphologies are investigated using small angle x-ray scattering
and transmission electron microscopy. Good agreement is found between this work and
other work on the structures of diblocks containing PDMS with a variety of complementary blocks and between this work and theoretical predictions for the morphologies of
diblock polymers. Different processing treatments including casting from solvents with a
range of preference for each block and a week-long anneal are tested to determine whether
processing has a strong effect on final morphology; it is found that in most cases the morphology displayed after processing is consistent independent of the processing treatment,
indicating that the morphologies are in equilibrium and fairly robust to preparation procedure. Finally, selected weakly segregated diblocks were studied at varying temperatures
using synchrotron small angle x-ray scattering. The diblock samples appeared to be affected by the prior x-ray dose that the materials had received. With limited prior dose,
the materials studied were ordered with little dramatic change in morphology throughout
the temperatures investigated; under continual irradiation by a 1.371A (9.1 keV) beam
for half an hour, the samples were damaged. The thesis concludes with a summary and
suggestions for future work, including a discussion of experimental and theoretical work
on the ways that equilibrium morphologies of block copolymers are perturbed when they
are spatially confined to dimensions on the order of several times their repeat period. The
small domain sizes achievable with and technological relevance of PDMS-containing diblocks make them ideal for use in microelectronics and information storage which provides
a motivation for exploring this topic further.
Thesis Supervisor: Edwin L. Thomas
Title: William and Stephanie Sick Dean of the George R. Brown School of Engineering
Professor, Rice University
2
Acknowledgments
I would like to acknowledge the help that I have received from many during this thesis
project. First, I'd like to thank my advisor, Professor Ned Thomas. His guidance has been
invaluable; I am so lucky to have had the experience to work in his lab, travel to national
facilities both with groupmates and on my own, write and submit proposals, and present
my work at conferences. His gift of the freedom to guide my own project and pursue my
own interests has been greatly appreciated. I would also like to acknowledge Professor
Apostolos Avgeropoulos from the University of Ioannina in Ioannina, Greece. Without
the careful synthesis work of his students, I would not have been able to study any of the
materials described in this thesis. His guidance during the summers that we have worked
together on TEM and SAXS have also been very helpful to my growth as a scientist. I
would also like to thank all the members of the Thomas group for friendship and helpful
discussions; I have enjoyed getting to know every one of you and look forward to more
collaboration in the coming years. Finally, I would like to thank the MIT Presidential
Fellowship and the NSF GRFP for financial support.
3
Contents
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and PI-PDMS Diblocks
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1 Introduction and literature review
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . .. . . .
1.2 Block Polymer Background . . . . . . . . . . . . . . . . . .
1.3 Characterization tools . . . . . . . . . . . . . . . . . . . .
1.4 Motivation for studying PDMS-containing block polymers
1.5 Prior work on PDMS-containing block polymers . . . . . .
1.6 Scope of Thesis Project . . . . . . . . . . . . . . . . . . . .
2
Strongly Segregated Morphologies of PS-PDMS
2.1 Introduction and motivation . . . . . . . . . . .
2.2 Materials synthesis . . . . . . . . . . . . . . . .
2.3 Experimental procedures . . . . . . . . . . . . .
2.4 R esults . . . . . . . . . . . . . . . . . . . . . . .
2.5 TEM results . . . . . . . . . . . . . . . . . . . .
2.6 SAXS results . . . . . . . . . . . . . . . . . . .
2.6.1 Morphology determination using SAXS .
2.6.2 D ata . . . . . . . . . . . . . . . . . . . .
2.7 D iscussion . . . . . . . . . . . . . . . . . . . . .
2.7.1 PS-containing diblocks . . . . . . . . . .
2.7.2 PI containing diblocks . . . . . . . . . .
3 Effect of Processing on Morphologies
blocks
3.1 Introduction . . . . . . . . . . . . . .
3.2 Sample preparation . . . . . . . . . .
3.3 Results . . . . . . . . . . . . . . . . .
3.4 D iscussion . . . . . . . . . . . . . . .
3.4.1 PS-containing samples . . . .
3.4.2 PI-containing samples . . . .
4
Morphologies of low molecular
as a function of temperature
4.1 Overview . . . . . . . . . . .
4.2 Experimental procedures . .
4.2.1 Sample descriptions .
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of PS-PDMS and PI-PDMS Di.
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weight PS-PDMS and PI-PDMS diblocks
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4.2.2 DSC procedures . . . . . . . . . . . . . . .
4.2.3 SAXS procedures . . . . . . . . . . . . . .
DSC results . . . . . . . . . . . . . . . . . . . . .
SAXS results . . . . . . . . . . . . . . . . . . . .
4.4.1 Minimal radiation exposure results . . . .
4.4.2 Initial protocol heating results . . . . . . .
4.4.3 Morphology change through the transition
4.4.4 Initial protocol cooling results . . . . . . .
4.4.5 Conclusions . . . . . . . . . . . . . . . . .
5 Conclusions and future work
5.1 Bulk morphologies . . . . . . . . . . . . .
5.2 Processing treatments . . . . . . . . . . .
5.3 Morphologies as a function of temperature
5.4 Outlook for the future . . . . . . . . . . .
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112
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List of Figures
The repeat units of the three polymers used in this thesis are displayed
ab ove. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-2 Schematic showing the structures of the various ordered diblock copolymer
morphologies. "f" indicates the volume fraction of component A. Taken
from [27]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-3 A theoretically computed phase diagram for block copolymers which shows
the expected morphology as a function of volume fraction of one of the
blocks and XN. Q229 corresponds to BCC packed spheres and Q23 0 indicates the double gyroid phase. CPS corresponds to a close packed sphere
structure (FCC or HCP) and is hard to distinguish experimentally from
BCC packed spheres. The figure is taken from [28]. . . . . . . . . . . . . .
1-4 Experimentally determined phase diagram for PS-PI diblock copolymers
taken from [16]. There are several differences between the data shown here
and the morphologies which are theoretically predicted to occur: ODTs occur at lower temperatures, the phase boundaries are not symmetric around
the 50:50 PS:PI composition, non-classical phases are present (e.g. HPL,
which is the hexagonally perforated lamellar phase), and unanticipated
Order-Order Transitions (OOT)s occur, among others. . . . . . . . . . . .
1-5 A schematic of a transmission electron microscope [30] (a) and a photograph of a transmission electron microscope [31] (b). On the right hand
side of (a), the path that electrons take as they travel through a TEM in
bright field imaging mode is compared to that of a beam of light travelling
through an optical microscope . . . . . . . . . . . . . . . . . . . . . . . . .
1-6 A schematic of a small angle x-ray scattering setup (a) and an image of
a SAXS instrument [32] (b). (a) shows the path of the x-rays from the
source, through the sample, and on to the detector after scattering. . . . .
1-1
6
19
20
21
21
25
26
1-7 Three composite plots showing previously observed morphologies in PDMScontaining diblocks as a function of XN and PDMS volume fraction. Eighty
six points are plotted in total; these include multiple plots of the same
sample on two sides of an observed phase transition. It is clear that the
locations of experimentally observed phases are not predicted perfectly by
the phase boundaries computed using SCFT. It appears that the range of
volume fractions at which the morphologies occur are broader than those
predicted and that those regions interpenetrate. This may be due to more
advanced factors such as conformational asymmetry, polydispersity, or be
the result of preparation procedures which produced metastable samples.
Graph (a) shows morphologies found at weak segregation; graph (b) shows
morphologies found at intermediate segregation; graph (c) shows morphologies found at strong segregation. The lines dividing the regions of expected
morphologies were created by plotting a smooth curve through the points
found in [15] and [27]; for values of xN outside the range computed in those
works, the boundaries between phases were assumed to be independent of
XN (vertical phase boundaries.) . . . . . . . . . . . . . . . . . . . . . . . . 30
1-7 Three composite plots showing previously observed morphologies in PDMScontaining diblocks as a function of xN and PDMS volume fraction. Eighty
six points are plotted in total; these include multiple plots of the same
sample on two sides of an observed phase transition. It is clear that the
locations of experimentally observed phases are not predicted perfectly by
the phase boundaries computed using SCFT. It appears that the range of
volume fractions at which the morphologies occur are broader than those
predicted and that those regions interpenetrate. This may be due to more
advanced factors such as conformational asymmetry, polydispersity, or be
the result of preparation procedures which produced metastable samples.
Graph (a) shows morphologies found at weak segregation; graph (b) shows
morphologies found at intermediate segregation; graph (c) shows morphologies found at strong segregation. The lines dividing the regions of expected
morphologies were created by plotting a smooth curve through the points
found in [15] and [27]; for values of XN outside the range computed in those
works, the boundaries between phases were assumed to be independent of
XN (vertical phase boundaries.) . . . . . . . . . . . . . . . . . . . . . . . . 31
7
1-8
1-8
1-8
Plots showing previously observed morphologies in PDMS-containing diblocks as a function of xN and volume fraction. Each graph shows the data
for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
DG (c), PDMS HPL (d), lamellae (e), other block HPL (f), other block
DG (g), other block cylinders (h), other block spheres (i), and disordered
(j). The lines dividing the regions of expected morphologies were created by
plotting a smooth curve through the points found in [15] and [27]; for values
of XN outside the range computed in those works, the boundaries between
phases were assumed to be independent of XN (vertical phase boundaries.)
Note that many phases are observed significantly outside of the regions of
volume fraction and values of XN that are predicted theoretically; different
phases also exist in close proximity to each other and in some cases the
phase boundaries appear to interpenetrate. . . . . . . . . . . . . . . . . . . 32
Plots showing previously observed morphologies in PDMS-containing diblocks as a function of XN and volume fraction. Each graph shows the data
for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
DG (c), PDMS HPL (d), lamellae (e), other block HPL (f), other block
DG (g), other block cylinders (h), other block spheres (i), and disordered
(j). The lines dividing the regions of expected morphologies were created by
plotting a smooth curve through the points found in [15] and [27]; for values
of xN outside the range computed in those works, the boundaries between
phases were assumed to be independent of xN (vertical phase boundaries.)
Note that many phases are observed significantly outside of the regions of
volume fraction and values of XN that are predicted theoretically; different
phases also exist in close proximity to each other and in some cases the
phase boundaries appear to interpenetrate. . . . . . . . . . . . . . . . . . . 33
Plots showing previously observed morphologies in PDMS-containing diblocks as a function of XN and volume fraction. Each graph shows the data
for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
DG (c), PDMS HPL (d), lamellae (e), other block HPL (f), other block
DG (g), other block cylinders (h), other block spheres (i), and disordered
(j). The lines dividing the regions of expected morphologies were created by
plotting a smooth curve through the points found in [15] and [27]; for values
of xN outside the range computed in those works, the boundaries between
phases were assumed to be independent of xN (vertical phase boundaries.)
Note that many phases are observed significantly outside of the regions of
volume fraction and values of XN that are predicted theoretically; different
phases also exist in close proximity to each other and in some cases the
phase boundaries appear to interpenetrate. . . . . . . . . . . . . . . . . . . 34
8
1-8
1-8
2-1
2-2
Plots showing previously observed morphologies in PDMS-containing diblocks as a function of XN and volume fraction. Each graph shows the data
for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
DG (c), PDMS HPL (d), lamellae (e), other block HPL (f), other block
DG (g), other block cylinders (h), other block spheres (i), and disordered
(j). The lines dividing the regions of expected morphologies were created by
plotting a smooth curve through the points found in [15] and [27]; for values
of XN outside the range computed in those works, the boundaries between
phases were assumed to be independent of xN (vertical phase boundaries.)
Note that many phases are observed significantly outside of the regions of
volume fraction and values of xN that are predicted theoretically; different
phases also exist in close proximity to each other and in some cases the
phase boundaries appear to interpenetrate. . . . . . . . . . . . . . . . . . . 35
Plots showing previously observed morphologies in PDMS-containing diblocks as a function of XN and volume fraction. Each graph shows the data
for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
DG (c), PDMS HPL (d), lamellae (e), other block HPL (f), other block
DG (g), other block cylinders (h), other block spheres (i), and disordered
(j). The lines dividing the regions of expected morphologies were created by
plotting a smooth curve through the points found in [15] and [27]; for values
of XN outside the range computed in those works, the boundaries between
phases were assumed to be independent of XN (vertical phase boundaries.)
Note that many phases are observed significantly outside of the regions of
volume fraction and values of XN that are predicted theoretically; different
phases also exist in close proximity to each other and in some cases the
phase boundaries appear to interpenetrate. . . . . . . . . . . . . . . . . . . 36
Updated phase diagrams (compare to figures 1-8 and 1-7) of PDMS-containing
diblocks reflecting the work done in this thesis. Extended data for the samples are available in Table 2.1. The SAXS and TEM data used for this table
was that taken after the sample underwent a one week cast from a 5 wt%
solution and then a one week anneal at 150 0C under vacuum. (a) shows
only the samples characterized in this thesis; the samples that have PS as
the complementary block are indicated by square markers while those that
have PI as the complementary block are indicated by circular markers. (b)
shows the samples characterized in this thesis plus those with values of XN
between 0 and 150 investigated in other works. The samples which were
studied here have larger markers. . . . . . . . . . . . . . . . . . . . . . . . 50
TEM images of the PS-PDMS samples which were characterized using this
0.37
technique. The PS cylinder morphology occurred when <pPDMS
low
molecular
weight
for the strongly segregated samples and for the only
sample. Lamellae occurred when <5PDMS > 0.43. See Table 2.1 for more
inform ation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
9
2-2
TEM images of the PS-PDMS samples which were characterized using this
technique. The PDMS sphere morphology occurred when
2-3
2-4
2-4
2-5
2-5
2-6
#PDMS
<
0.19.
PDMS cylinders were present when #PDMS= 0.23. Table 2.1 for more
inform ation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TEM images of the PI-PDMS samples which were characterized using this
technique. See Table 2.1 for more information. . . . . . . . . . . . . . . .
SAXS data from the PS-PDMS samples which are studied in this thesis.
See Table 2.1 for more information. . . . . . . . . . . . . . . . . . . . . .
SAXS data from the PS-PDMS samples which are studied in this thesis.
See Table 2.1 for more information. . . . . . . . . . . . . . . . . . . . . .
SAXS data from the PI-PDMS samples which are studied in this thesis.
See Table 2.1 for more information. . . . . . . . . . . . . . . . . . . . . .
SAXS data from the PI-PDMS samples which are studied in this thesis.
See Table 2.1 for more information. . . . . . . . . . . . . . . . . . . . . .
SAXS data from all of the low molecular weight samples investigated in
this thesis. See chapter 4 for more information. . . . . . . . . . . . . . .
Scattering from PS-containing samples studied in this thesis after different
processing treatments. The diblocks were cast from the relevant solvent for
one week at room temperature; half of each sample was then annealed for
one week at 150 0 C. See Tables 3.2 and 3.3 for more information. . . . . .
3-1 Scattering from PS-containing samples studied in this thesis after different
processing treatments. The diblocks were cast from the relevant solvent for
one week at room temperature; half of each sample was then annealed for
one week at 150 0 C. See Tables 3.2 and 3.3 for more information. . . . . .
3-1 Scattering from PS-containing samples studied in this thesis after different
processing treatments. The diblocks were cast from the relevant solvent for
one week at room temperature; half of each sample was then annealed for
one week at 150 0 C. See Tables 3.2 and 3.3 for more information. . . . . .
3-1 Scattering from PS-containing samples studied in this thesis after different
processing treatments. The diblocks were cast from the relevant solvent for
one week at room temperature; half of each sample was then annealed for
one week at 150 0 C. See Tables 3.2 and 3.3 for more information. . . . . .
3-1 Scattering from PS-containing samples studied in this thesis after different
processing treatments. The diblocks were cast from the relevant solvent for
one week at room temperature; half of each sample was then annealed for
one week at 150 0 C. See Tables 3.2 and 3.3 for more information. . . . . .
3-2 Scattering from PI-containing samples studied in this thesis after different
processing treatments. The diblocks were cast from the relevant solvent for
one week at room temperature; half of each sample was then annealed for
one week at 150 0 C. See Tables 3.2 and 3.3 for more information. . . . . .
. 52
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3-1
10
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3-2
Scattering from PI-containing samples studied in this thesis after different
processing treatments. The diblocks were cast from the relevant solvent for
one week at room temperature; half of each sample was then annealed for
one week at 150 0 C. See Tables 3.2 and 3.3 for more information. . . . . . . 74
4-1
Calculated values of XN(T) at different temperatures for the three samples
studied in this section. Values of X were computed based upon the work
in [74]. The annealing temperature and anticipated ODTs for each sample
are also shown; anticipated ODTs are based upon the figure in [15]. . . . .
The temperature programs for the S-64LMW samples described in Table
4.4. The temperature profiles used for S-64LMW-A and B were selected in
order to observe scattering at a wide and complementary range of temperatures and investigate whether or not the scattering was dependent upon
thermal history. The temperature profile for S-64LMW-C was selected in
order to observe scattering at many different and closely spaced temperatures and to detect where morphological transitions in this range occur
with fine precision. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The temperature programs for the S-45LMW samples described in Table
4.4. These temperature profiles were chosen in order to sample a wide
range of temperatures at which morphology transitions could occur and
to provide comparative data for morphology transitions upon heating and
cooling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The temperature programs for the I-39LMW samples described in Table
4.4. The two temperature profiles were selected to investigate structure in
I-39LMW at temperatures both above and below room temperature and to
investigate morphology transitions as a function of temperature, thermal
history (i.e. heating vs. cooling), and time. . . . . . . . . . . . . . . . . . .
Data from DSC scans of S-64LMW (a), S-45LMW (b), and I-39LMW (c).
The scans were performed at 5m nte and began at room temperature. The
samples were heated to 180 0 C, held for three minutes, cooled to -80 0 C,
held for 3 minutes, and then heated back to room temperature. The morphology transitions observed in SAXS can be seen to varying degrees in the
DSC data. For S-64LMW, the endothermic peak at approximately 50 0 C
to 70 0 C corresponds somewhat with the morphology transition observed in
SAXS between 80 0 C and 165 0 C. For S-45LMW, the endothermic peak at
approximately 30 0 C to 50 0 C is at the low end of the 40 0 C to 80 0 C range
where the morphology transition was observed to occur with SAXS. There
do not appear to be any thermal transitions in I-39LMW. The thermal
transitions are only observed during the heating cycles, consistent with
the faster kinetics of structural rearrangement observed during SAXS. The
large vertical lines which occur at 25 0 C are due to the rapid change in
4-2
4-3
4-4
4-5
80
84
85
86
temperature of the DSC pans to the initial experimental temperature. . . . 95
11
4-5
Data from DSC scans of S-64LMW (a), S-45LMW (b), and I-39LMW (c).
The scans were performed at 5m ate and began at room temperature. The
samples were heated to 180 0 C, held for three minutes, cooled to -80 0 C,
held for 3 minutes, and then heated back to room temperature. The morphology transitions observed in SAXS can be seen to varying degrees in the
DSC data. For S-64LMW, the endothermic peak at approximately 50 0 C
to 70 0 C corresponds somewhat with the morphology transition observed in
SAXS between 80 0 C and 165 0 C. For S-45LMW, the endothermic peak at
approximately 30 0 C to 500 C is at the low end of the 40 0 C to 80 0 C range
where the morphology transition was observed to occur with SAXS. There
do not appear to be any thermal transitions in I-39LMW. The thermal
transitions are only observed during the heating cycles, consistent with
the faster kinetics of structural rearrangement observed during SAXS. The
large vertical lines which occur at 250 C are due to the rapid change in
temperature of the DSC pans to the initial experimental temperature. . . . 96
Images recorded on the marCCD detector for a sample with the same characteristics as sample II-D where 40 second exposures were continuously
taken throughout the duration of the experiment; this sequence of images
obviates the need for the revised protocol and suggests that during x-ray
irradiation some event such as cross-linking which promotes the formation
of locked-in non-equilibrium morphologies or some event such as depolymerization of the PDMS occurs which irreversibly changes the equilibrium
morphology occurs. (a) shows the image taken at 25 0 C before any temperature treatment has been performed. (b) shows the scattering from this
sample at 90 0 C after raising the temperature. (c) shows data taken after
holding the sample at 90 0C for 40 minutes. (d) shows the image taken after
a quench to 60 0 C. (e) shows the data obtained after holding at 60 0 C for 18
minutes. (f) shows data taken after the sample was then cooled to 25 0 C
and held there for one hour and 15 minutes. (g) shows data taken after
this sample was removed, translated a small distance, and re-exposed to
the x-ray beam. This sequence shows that sample exposure to the x-ray
beam strongly affects its structure and the resulting data obtained. . . . . 97
4-7 Scattering data and caption taken from [79] showing scattering from a
PEO-PDMS and D3 blend. Compare this to the scattering shown in Figure 4-8. It is clear that the images obtained after x-ray irradiation look
qualitatively similar to those shown above, providing evidence for the hypothesis that the x-ray beam causes depolymerization of the PDMS to form
D 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8
4-6
12
4-8
4-9
4-9
4-10
4-10
Images of the scattering patterns recorded by the CCD camera for selected
temperatures for samples S-64LMW-A and S-64LMW-B. In these images,
the integrated intensity presented in Figure 4-10 does not fully capture the
x-ray scattering that occurred due to the development of orientation during
the experiment. S-64LMW-A at 80 0 C is shown in a; S-64LMW-B at 90 0 C
in b; S-64LMW-A at 105 0 C in c; S-64LMW-B at 110 0C in d; S-64LMWA at 120 0 C in e; S-64LMW-B at 125 0 C in f; S-64LMW-A at 150 0 C after
30 minutes in g and after approximately 70 minutes in h; S-64LMW-B at
165 0 C in i. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scattering data from samples S-45LMW-D and I-39LMW-C following the
revised protocol described. (a) shows scattering from sample S-45LMW-D
and (b) - (d) show scattering from I-39LMW-C. It appears that there is
very little change with temperature in the scattering data for S-45LMW-D
for the temperature range investigated (25-80 0C.) For I-39LMW-C, there
appear to be one prominent and one secondary peak from 10-80 0 C, which
becomes one peak which steadily loses intensity through 200 0 C. . . . . . .
Scattering data from samples S-45LMW-D and I-39LMW-C following the
revised protocol described. (a) shows scattering from sample S-45LMW-D
and (b) - (d) show scattering from I-39LMW-C. It appears that there is
very little change with temperature in the scattering data for S-45LMW-D
for the temperature range investigated (25-80 0 C.) For I-39LMW-C, there
appear to be one prominent and one secondary peak from 10-80 0 C, which
becomes one peak which steadily loses intensity through 200 0 C. . . . . . .
Scattering from S-64LMW-A, B, and C as a function of temperature. In
all cases, the data from the last exposure taken at a particular temperature
during the heating cycle is plotted. S-64LMW-A data is shown in red; S64LMW-B in green; S-64LMW-C in green. For all samples, it is clear that in
the region of 70 0 C-165 0 C the low temperature high q peak is suppressed and
a high temperature low q peak is formed. This transition occurs at slightly
different temperatures in each sample, indicating that there is both a time
and temperature effect in the morphological transition that is occurring;
the behavior, however, is the same in all cases. . . . . . . . . . . . . . . . .
Scattering from S-64LMW-A, B, and C as a function of temperature. In
all cases, the data from the last exposure taken at a particular temperature
during the heating cycle is plotted. S-64LMW-A data is shown in red; S64LMW-B in green; S-64LMW-C in green. For all samples, it is clear that in
the region of 70 0 C-165 0 C the low temperature high q peak is suppressed and
a high temperature low q peak is formed. This transition occurs at slightly
different temperatures in each sample, indicating that there is both a time
and temperature effect in the morphological transition that is occurring;
the behavior, however, is the same in all cases. . . . . . . . . . . . . . . . .
13
99
100
101
102
103
4-10 Scattering from S-64LMW-A, B, and C as a function of temperature. In
all cases, the data from the last exposure taken at a particular temperature
during the heating cycle is plotted. S-64LMW-A data is shown in red; S64LMW-B in green; S-64LMW-C in green. For all samples, it is clear that in
the region of 70 0 C-165 0 C the low temperature high q peak is suppressed and
a high temperature low q peak is formed. This transition occurs at slightly
different temperatures in each sample, indicating that there is both a time
and temperature effect in the morphological transition that is occurring;
the behavior, however, is the same in all cases. . . . . . . . . . . . . . . . . 104
4-11 Scattering from S-45LMW-A, B, and C as a function of temperature. In
all cases, the data from the last exposure taken at a particular temperature during the heating cycle is plotted. S-45LMW-A data is shown in
red; S-45LMW-B in green; S-45LMW-C in green. For all samples, it is
clear that in the region of 40 0 C-85 0 C the low temperature high q peak is
suppressed and a high temperature low q peak is formed. This transition
occurs at slightly different temperatures in each sample, possibly indicating that longer annealing times are necessary in this regime to achieve
equilibrium morphologies; the behavior, however, is the same in all cases. . 105
4-11 Scattering from S-45LMW-A, B, and C as a function of temperature. In
all cases, the data from the last exposure taken at a particular temperature during the heating cycle is plotted. S-45LMW-A data is shown in
red; S-45LMW-B in green; S-45LMW-C in green. For all samples, it is
clear that in the region of 40 0 C-85 0 C the low temperature high q peak is
suppressed and a high temperature low q peak is formed. This transition
occurs at slightly different temperatures in each sample, possibly indicating that longer annealing times are necessary in this regime to achieve
equilibrium morphologies; the behavior, however, is the same in all cases. . 106
4-12 Scattering from I-39LMW-A and B as a function of temperature. In all
cases, the data from the last exposure taken at a particular temperature
during the heating cycle is plotted. I-39LMW-A data is shown in red; I39LMW-B in green. For both samples, in the range of 30 0 C-50 0 C the low
temperature high q peak is suppressed and a high temperature low q peak
is form ed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4-13 SAXS data taken from diblocks undergoing structural evolution at a constant temperature. Ten sequential three minute exposures from samples
transitioning between ordered and disordered structures are shown and the
growth of the high temperature correlation hole peak at the expense of the
low temperature ordered structure peak is evident. (a) shows S-64LMW-A
at 80 0 C; (b)shows S-45LMW-A at 50 0 C; (c) shows I-39LMW-B at 40 0 C. . 108
14
4-13 SAXS data taken from diblocks undergoing structural evolution at a constant temperature. Ten sequential three minute exposures from samples
transitioning between ordered and disordered structures are shown and the
growth of the high temperature correlation hole peak at the expense of the
low temperature ordered structure peak is evident. (a) shows S-64LMW-A
at 80 0 C; (b)shows S-45LMW-A at 50 0 C; (c) shows I-39LMW-B at 40 0 C. . 109
4-14 SAXS data taken from all samples during their cooling runs. Note the lack
of structure development for any of the samples which received only a 30
minute anneal at the colder temperatures. I-39LMW-B shows morphology
development at longer temperatures due to the longer two hour anneal at
each temperature that it received. (a) shows data from S-64LMW-A and B;
(b) shows data from S-45LMW-A and B; (c) shows data from I-39LMW-A
and B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
4-14 SAXS data taken from all samples during their cooling runs. Note the lack
of structure development for any of the samples which received only a 30
minute anneal at the colder temperatures. I-39LMW-B shows morphology
development at longer temperatures due to the longer two hour anneal at
each temperature that it received. (a) shows data from S-64LMW-A and B;
(b) shows data from S-45LMW-A and B; (c) shows data from I-39LMW-A
and B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111
15
List of Tables
1.1
A chart detailing the morphologies found in the literature for PDMScontaining diblocks (it does not include triblocks or other multiblocks also
described in the text); it contains the data displayed in figures 1-7 and 1-8.
Some data found in the literature discussed was purposefully excluded if
there were significant reasons to believe that the data presented was either not sufficient to establish the morphology claimed, e.g due to large
amounts of obvious homopolymer contamination in the TEM images, etc.
Note that because much of the relevant information was not available in
the papers, in some cases calculations were made to arrive at the volume
fraction and XN values. If necessary, volume fractions were computed from
mole or weight fractions using commonly available densities. X values were
taken from [74] if available or calculated from solubility parameters found
in [75]; in the latter case, the reference volume was assumed to be 100".
The temperature used was that at which the last thermal treatment was
done (e.g. annealing, crosslinking, etc.) or room temperature if the sample
was not thermally treated after casting. For works in which several samples were examined at a large variety of temperatures, the chart displays
multiple entries for each transition temperature ±50 C: the lower temperature for the phase present below the transition temperature and the upper
temperature for the phase present above the transition temperature. . . . . 43
16
2.1
The list of samples analyzed in this thesis and their relevant characterization data. These materials were synthesized in the laboratory of Professor Apostolos Avgeropoulos at the University of Ioannina in Ionannina,
Greece. The molecular weights and volume fractions of the samples were
also determined there; gel permeation chromatography (GPC) was used
to establish the former and nuclear magnetic resonance (NMR) the latter
(except where noted that the characterization was performed using GPC.)
The x-ray measurements and TEM measurements were performed at the
Institute for Soldier Nanotechnologies at the Massachusetts Institute of
Technology in Cambridge, Massachusetts. - means that the relevant measurement was not performed. N/P means that there were not sufficient
peaks in the SAXS data to assign the value with certainty. * Indicates
that toluene was the casting solvent and no annealing treatment was performed. ** Indicates that cyclohexane was used as the casting solvent and
the annealing treatment was that described in the text. . . . . . . . . . . . 49
3.1
Solubility parameters for the polymers and solvents studied in this thesis.
64
. . ... .. ..
. . .. ..
All values are taken from [75]. . .. . . . .. .
Summary of sample morphologies obtained by SAXS after different casting
and annealing treatments for the samples whose morphology was discussed
in Chapter 2. The diblocks were cast from the relevant solvent for one week
at room temperature; half of each sample was then annealed for one week
at 150 0 C. Morphologies were determined by the relative q values of the
positions of the peaks in the scattering spectrum, which are characteristic
of different morphologies. S indicates a spherical morphology, C indicates a
cylindrical morphology, and L indicates a lamellar morphology. - indicates
that the experiment was not performed and N/P indicates that there were
insufficient peaks in the SAXS data to assign a morphology. . . . . . . . . 66
Summary of d-spacing values obtained by SAXS after different casting and
annealing treatments for the samples whose morphology was discussed in
Chapter 2. The diblocks were cast from the relevant solvent for one week
at room temperature; half of each sample was then annealed for one week
3.2
3.3
at 150 0 C. D-spacings are obtained from the relationship d = g.
In some
cases, the ratios of q at which the main peaks appear indicate a morphology
change, so the changes in peak spacings are attributable to changes in
both the sizes of the domains and the distances between the first planes
from which scattering appears. See Table 3.2 for more information about
the latter. - indicates that the experiment was not performed and N/P
indicates that there were insufficient peaks in the SAXS data to assign a
m orphology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
17
4.1
4.2
4.3
4.4
The composition and anticipated ODT's of the low molecular weight diblocks studied in this section. The molecular weights were determined by
GPC and the volume fractions were determined by NMR. Estimated values
of XN at the ODT were estimated based on the figures in [15] and the value
of x was determined from the equations presented in [74]. . . . . . . . . . .
Physical properties of the chains that form the diblocks investigated in
this thesis. The entanglement molecular weights and radii of gyration are
taken from [77]; note that these assume infinitely long chains and so may
overestimate the radii of gyration. The first number given for radius of
gyration is the ratio of the radius of gyration to the square root of sample molecular weight; the latter are the calculated values for the relevant
samples. The glass transition data is taken from [75]; the measured values
were performed on longer chains and so may overestimate the values of Tg.
The second procedure used in SAXS studies at BNL. . . . . . . . . . . . .
Summary of different sample preparation procedures used during this experiment. The initial scattering pattern displayed at 25 0 C was consistent
across prior preparation methods. The thermal programs for samples receiving continuous x-ray exposure can be found in Figures 4-2, 4-3 and 4-4
while those for the samples not receiving continuous x-ray exposure can be
found in Table 4.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
81
82
87
88
Chapter 1
Introduction and literature review
1.1
Overview
A-B diblock polymers are materials which are composed of two homopolymers which
are chemically linked at a junction point. The combination of chemical incompatibility
between the homopolymers and their topological linkage forces spatial segregation between
the blocks into predictable, ordered structures with typical periodicities in the tens of
nanometers. These structures have been of interest to researchers who work on developing
materials where such features are important, including as templates for deposition of
magnetic material [1] and as photonic crystals [2-8], as well as to scientists studying
fundamental questions about the effects of confinement on material properties [9-13].
Because of block copolymers' relevance to many different research efforts, many studies
have focused on the factors which determine block copolymer structure [14-20] and ways in
which the equilibrium morphologies of these materials can be tuned [21-26]. The focus of
this thesis is an investigation into the structures formed by two types of diblock copolymers
which contain polydimethylsiloxane (PDMS): polystyrene (PS)-PDMS and polyisoprene
(PI)-PDMS (see Figure 1-1 for the repeat units of these materials). PDMS-containing
diblocks have received much research interest due to the combination of their ability to
form structures with small length scales due to strong spatial segregation between the
component blocks at comparatively low molecular weights and the difference in chemical
reactivity between PDMS and other polymers with a variety of materials which allows
for removal of one of the blocks while the other retains its initial morphology; these
PDMS
PS
P1
Si
Figure 1-1: The repeat units of the three polymers used in this thesis are displayed above.
19
Figure 1-2: Schematic showing the structures of the various ordered diblock copolymer
morphologies. "f" indicates the volume fraction of component A. Taken from [27].
two features make PDMS-containing systems ideal block copolymers for pattern transfer
applications.
1.2
Block Polymer Background
Because block copolymers have received substantial interest from engineers interested in
applying their unique physical properties to technical problems, much is known about
their structure and the way that they behave. The simplest class of block polymers
(and the class that this thesis focuses on exclusively) are diblock copolymers, which are
composed of two homopolymers which are chemically linked at one end to each other.
At thermodynamic equilibrium, diblocks form five known structures depending on the
volume fraction of the minority block: a disordered and phase-mixed structure, a body
centered cubic array spheres composed of the minority block chains in a matrix composed
of the majority block chains, an array of hexagonally packed cylinders composed of the
minority block chains in a matrix composed of the majority block chains, a structure with
two interpenetrating gyroid networks composed of the minority block chains in a matrix
composed of the majority block chains, and alternating lamellae composed of chains from
each block (see Figure 1-2). This has been verified theoretically for idealized diblocks [15]
(see Figure 1-3) which are Gaussian chains (not valid for shorter chain polymers), have a
X which is independent of composition, and have known polydispersity (often assumed to
be one) and ratio between segment size (often assumed to be one.) The morphology phase
diagram has also been studied experimentally for the widely used diblock consisting of
polystyrene (PS) and polyisoprene (PI) [16] (see Figure 1-4.) These microdomain phases
are also present in other diblock systems, although the phase diagram boundaries have
received less interest, as it has been assumed that they are similar to the PS-PI case.
Theoretically, the thermodynamics of block copolymers is well understood; the structure at equilibrium is that with the lowest free energy and is typically calculated using
self-consistent mean field theory (SCFT). This procedure involves numerically minimizing
the free energy of the system while assuming that each chain is subject to a mean field
instead of computing individual chain-chain interactions. While this requires extensive
computational power and the results are not readily apparent before the calculations are
performed, a basic understanding of the factors influencing equilibrium structure can be
easily explained and is also quite illuminating.
20
100
W 4
80
ti
L
H
60
XN
40
20
2Y
CPS
CPS
DIS
0
0.2
0.4
f
0.6
0.8
1
Figure 1-3: A theoretically computed phase diagram for block copolymers which shows
the expected morphology as a function of volume fraction of one of the blocks and xN.
Q229 corresponds to BCC packed spheres and Q230 indicates the double gyroid phase. CPS
corresponds to a close packed sphere structure (FCC or HCP) and is hard to distinguish
experimentally from BCC packed spheres. The figure is taken from [28].
IfxIA
LAM
Hx
40
1011
Disordered
0
0 0.1
-
-
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Figure 1-4: Experimentally determined phase diagram for PS-PI diblock copolymers taken
from [16]. There are several differences between the data shown here and the morphologies which are theoretically predicted to occur: ODTs occur at lower temperatures, the
phase boundaries are not symmetric around the 50:50 PS:PI composition, non-classical
phases are present (e.g. HPL, which is the hexagonally perforated lamellar phase), and
unanticipated Order-Order Transitions (OOT)s occur, among others.
21
For two dissimilar polymers which are unattached, i.e. an A-B homopolymer blend,
there are two factors which influence their tendency to demix: the loss of entropy upon
confining each polymer to specified regions of space and the gain in enthalpy achieved by
minimizing unlike monomer-monomer contacts. Both of these can be determined by the
Flory-Huggins theory, which states that the configurational entropy of mixing per lattice
site is given by
ASmix = -k
(1
N1
In 01+
n4
N2
(1.1)
2
and that the sum of the resultant changes in entropy and enthalpy due to increased spatial
proximity between unlike monomers per lattice site is
(1.2)
AHmix = k - T - X-#1-42
In equations 1.1 and 1.2, N is the degree of polymerization for each polymer, # is the
volume fraction, k is Boltzmann's constant, T is temperature, x is the interaction energy
resulting from having unlike monomers in close proximity scaled by Boltzmann's constant
multiplied by temperature. X can be computed by
x
= C - [eAB -
2
(eAA
+
(1.3)
EBB)]
where C is a constant accounting for the number of monomer-monomer contacts associated
with the lattice and Exy is the pairwise interaction energy between monomers X and Y.
As in all materials, the equilibrium morphology is that which minimizes the Gibbs free
energy of the system. This occurs when the chemical potential of each component is equal
in all phases; for this to be true, 9 (where nA is the number of moles of A in phase 1)
does not vary with phase. The chemical potential in any mixed phase is given by
p 1 =jtp
1 o+k-T,-n#+
(1-41-
1-
+ (1 - #1)
(1.4)
where 1 is the chemical potential of species 1 in a mixed phase and piLo is the chemical
potential of species 1 in its pure phase. This equation can be plotted to yield the binodal
curve which defines at which volume fractions and values of x, NA and NB phase separation occurs. For block copolymers, the Flory-Huggins polymer blend theory is modified
by the presence of a junction linking the homopolymers together. Because of this, the size
of phase-separated domains is limited, increasing the contacts between unlike monomers
in the microphase separated state. The attached homopolymers are also stretched away
from the junction point, which decreases the configurational entropy. These two effects
both raise the free energy of the phase separated state. For example, in a 50:50 blend of
chains of equal length, phase separation occurs at x -(NA + NB) of 2, if the two chains are
attached the x -(NA + NB) at which phase separation occurs is raised to ~ 10.5. For other
compositions, the required x - (NA + NB) is increased further. Boundaries between morphologies in diblocks occur when the sum of chain stretching energy and surface energy
22
becomes minimized for a different microdomain structure.
Experimentally, it became possible to perform precise investigations into block copolymer structure after anionic polymerization was discovered by Szwarc in 1956 [29]. Before
development of this technique, polymers were synthesized through either free-radical polymerization or through step growth polymerization, both which resulted in highly polydisperse products. In the former, initiators decompose to form two free radicals which
react with monomers to form long chains; in this case, polymerization is terminated when
two propagating radicals meet and there are chains of all lengths present during the polymerization process. In step growth polymerization, chemical reactions occur between
monomers which result in the formation of ester, amide, or other bonds; the reaction
proceeds until equilibrium is reached. Neither of these two methods is well suited for
creating model polymers: the choice of monomers is limited and there is no ability to
precisely control their sequence on the chain and form ideal diblock chains.
In anionic polymerization, a living anion is formed which is coordinated to a cation.
When this anion reacts with monomers in the reaction vessel, the chain grows while
maintaining a negatively charged end group. If initiation occurs at the same time, all
of the chains in the solution grow at approximately the same rate and the composition
of the chain can be tuned by changing the composition of monomers present during the
reaction. Because of this, precisely defined chains with uniform lengths and compositions
can be achieved, allowing for the creation of model materials whose structural properties
can be studied as a function of volume fraction/composition.
This thesis investigates diblocks composed of pairs of the following homopolymers:
PS, PI and PDMS. PS is a polymer with a backbone composed of carbons connected by
a single bond; to half of these backbone carbons, a phenyl ring is attached. PS has a
glass transition of approximately 100 0 C. PI has a backbone made up of an alternating
pattern of two single bonds and one double bond; a methyl group is attached to one of the
double-bonded carbons. Both it and PDMS are rubbery at room temperature. PDMS has
a backbone with alternating silicon and oxygen atoms; two methyl groups are attached to
each silicon atom. See Figure 1-1 for more detail about the molecular structures of these
monomers.
1.3
Characterization tools
This thesis primarily relies upon two characterization tools to understand block copolymer structure: transmission electron microscopy (TEM) and small angle x-ray scattering
(SAXS). These two techniques provide complementary information about sample structure: TEM provides two-dimensional projections of real-space of selected small areas of
the structure while SAXS experiments yield information about the characteristic feature
sizes and spacings over volumes of cubic millimeters. The use of both of these techniques together allows for determination of the size and shape of microdomain features
(and hence microdomain structure) in the material of interest and their orientation with
respect to each other.
TEM studies involve electrons which have either been transmitted (forward scattered)
through a sample (bright field) or scattered at a defined angle (dark field). Because the
23
polymers studied in this thesis were amorphous at room temperature and thus do not
have a crystalline diffraction pattern, only bright field imaging was used. In this case,
regions of the sample through which many electrons have been transmitted remain bright
while areas through which many electrons were scattered appear darker. Because the
scattering power of an atom is dependent upon its atomic number, there is significant
inherent contrast between the carbon-based blocks and the PDMS blocks, removing the
need for any heavy metal stain. While the TEM can readily produce images which appear
to show true structure, understanding the actual morphologies of the block copolymers
being examined has more subtlety: the data that is being recorded is a two-dimensional
projection of a selected location of the sample and so the image can be affected by the
angle of the grain with respect to the electron beam, the thickness of the sample, the
number of unit cells present in the projection, etc. Moreover, as samples are typically
prepared by cutting very thin slices (less than 100nm) from a piece of bulk material, it
is possible that additional artifacts may be introduced into the image due to deformation
during cutting or that the area under examination may not be representative of the
larger overall sample. For this reason, it is necessary to examine multiple locations in
the slice, corroborate TEM-based structure assignment with SAXS data, and be aware
of how commonly found structures typically look in projections at different angles. The
microscope must also be appropriately focused in order to accurately measure distances
and generate the most representative image. Despite these limitations, it is the best way
to perform real space imaging of structures whose characteristic length scales match those
of block copolymers. See Figure 1-5 for a schematic of how images are produced in a TEM
using bright field imaging and a picture of a TEM instrument.
SAXS data is obtained by irradiating a piece of bulk material (typically 1 mm thick)
with an x-ray beam and recording the intensity of scattered radiation at a set distance
(typically 1-2m) from the sample. If there are ordered, periodic variations in the electron
density of a material, x-rays will be scattered at a characteristic angle given by
nA = 2dsin6
(1.5)
where n is any integer, A is the wavelength of the incident radiation, d is the spacing
between of the periodic electron density variation in the sample, and 6 is half of the
scattering angle. This is due to the constructive interference of waves when the path
difference between each plane is an integer multiple of the x-ray wavelength. In small
angle scattering, 6 is on the order of several degrees and q is given by
q =
A- sin0 = 2-r
d
(1.6)
For a d-spacing of 35 nm and a wavelength of 1.54 A (which is the typical setup in
laboratory x-ray diffractometers which use Cu-Ka radiation), 6 is approximately .002
radians and q is approximately 0.18 nm- 1 .
Therefore, knowledge of the first location in q of the maximum scattered intensity
allows for a direct calculation of the spacing between the planes in the material with
identical electron density distributions which have the largest spacing. Peaks in the scat24
i0
W,
Jn
Figure 1-5: A schematic of a transmission electron microscope [30] (a) and a photograph
of a transmission electron microscope [31] (b). On the right hand side of (a), the path that
electrons take as they travel through a TEM in bright field imaging mode is compared to
that of a beam of light travelling through an optical microscope.
tering intensity at higher q values are present when there are additional scattering angles
at which constructive interference occurs. This happens when n > 2 in equation 1.3 and
when planes other than those responsible for the lowest q scattering scatter x-rays. The
ratio of the q's of these higher-order peaks is determined by the morphology of the sample:
both the geometry of the features themselves and the lattice type that they occupy affect
the positions and intensities of scattering angle maxima. The influence of feature shape
on the scattering pattern is called the form factor; at different angles, different geometries
will have different intensities of radiation scattered due to interference of the waves that
are scattered from each part of the object. The influence of the lattice that the features are
ordered on is called the structure factor; it describes how the minima and maxima in the
intensity of the scattered radiation depend on the interference of the radiation scattered
by each object. The product of these two terms describes the total intensity. Also, as
the perfection of the lattice increases, additional higher order peaks appear and the peaks
become sharper; thus, SAXS data can corroborate the morphologies established by TEM
images and provide insight into structural uniformity across macroscopic length scales.
See Figure 1-6 for a schematic of how diffraction patterns are obtained using SAXS and
an image of a laboratory-scale instrument.
25
LiW
source
beam stop
sample
and
scattered
beams
direct
pinholes
a
1 meter
detector
Figure 1-6: A schematic of a small angle x-ray scattering setup (a) and an image of a
SAXS instrument [32] (b). (a) shows the path of the x-rays from the source, through the
sample, and on to the detector after scattering.
26
1.4
Motivation for studying PDMS-containing block
polymers
Due to their unique properties, PDMS-containing block polymers have been used for
many different applications. These include both the development of new technologies and
the creation of materials which can be used to measure scientific properties not readily
accessible by other means. In the former category, works which take advantage most
directly of the different chemistries of PDMS and carbon-backbone polymers typically
involve removal one of the component blocks after structure formation. PDMS can be
removed from a material by an etch in hydrofluoric acid or tetrabutylammonium fluoride;
this has been used to create many different advanced materials. Materials with PS serving
as the matrix have been made which contain either spherical or gyroid shaped voids [33].
Cross-linked PI has also been of interest for use as a matrix material because its shape
after removal of the PDMS can be tuned by crosslink density; PI with cylindrical voids
can retain its initial structure when highly crosslinked [34] but will have voids which are
collapsed until swollen with a solvent when lightly crosslinked [35]. Polybutadiene (PB),
another elastomer, has also been used to make a matrix containing voids which are gyroid
shaped or performated lamellar shaped [36]. In all of these cases, the etching rate of the
PDMS by the fluorinated chemical is sufficient for preparation of bulk porous samples
which could be used to create large-area applications.
Other work has focused on exploiting the morphologies found in thin film PDMScontaining block copolymers. Investigators have typically used oxygen plasma to simultaneously remove the carbon-containing block and convert PDMS into a silicon oxy carbide
ceramic for this application due to its higher safety compared to fluorinated etchants and
because the smaller distances that the etchant must penetrate allows the use of slower and
less aggressive methods. When PDMS is the majority phase of the block copolymer being
treated, oxygen plasma treatment yields a thin film nanoporous structure similar to those
described above for bulk materials; when another olefin is placed under it and etched, a
topographic pattern is formed where material is removed from the areas the pores covered [37]. When PDMS is the minority phase, the ceramic formed is left free-standing on
the substrate where the PDMS was located initially; dots [38] as well as other structures
have been formed this way. In some cases, the locations where the carbon-backbone polymer was located can be in-filled with a technologically relevant, multifunctional material
such as Chromium [1].
Perturbing the block copolymer structure from its equilibrium morphology can expand the range and utility of the morphologies of the resulting ceramic. This can be
accomplished by creating templates using interference lithography into which the block
copolymer can be deposited. The presence of surfaces in close proximity to each other
raises the energy of defects and so forces the creation of highly aligned, nearly defect-free,
and uniformly spaced structures. For example, cylinders [39] can be formed when long
and narrow trenches are used to confine the material and concentric tori when shallow
cylinders are [1]. When low molecular weight block copolymers are placed in these templates, free standing cylinders with very small periods (17 nm) and line widths (8 nm)
are formed. Another way to tune the morphology formed prior to etching is to perform a
27
two solvent treatment, where the first step swells the PDMS as the film is being deposited
and then the second promotes contraction so that rings are the final structure that is
formed [40].
The high immiscibility of PDMS with other polymers has been another area which has
driven application development. The low cohesive energy density of PDMS compared to
other materials means that the chemicals that it is soluble in and can solvate overlap very
little with those for the blocks it is commonly linked to. When the block copolymer is the
solute in a preferential solvent, spherical and cylindrical micelles with a core comprised of
the block which is insoluble are formed; these solutions can be dried and the previously
solvated structures collapsed into tablets and ribbons [41]. When the block copolymer
is the solvent, materials such as C60 [42] and gold nanoparticles [43] are localized in the
non-PDMS block. In the latter two examples, the concentration of solute in the diblock
is low enough that well-ordered and uniform morphologies are formed by the solutions,
increasing the technological relevance of this patterning method.
The differential solvation properties of block copolymers can also be used in instances
where a uniform and ordered morphology is not required but the presence of two materials
with different physical properties in close spatial proximity is desired. For these cases,
small amounts of diblock copolymer can be added to a blend composed of the constituent
homopolymers (i.e. PDMS and the block that it is linked to) to reduce the free energy of
the interface between the homopolymers. This stabilizes homopolymer domains against
coarsening and thus preserves the morphological integrity of the original material over its
lifetime. PDMS has been combined in a diblock with PS [44-46], PI [47], and polyethylene
oxide (PEO) [48] for this purpose. PDMS-containing diblocks can also lower the energy of
free surfaces by forming an interfacial layer where the PDMS block is in contact with the
air interface and the other block is in contact with the substrate. This property has been
exploited by experimentalists to form stamps for nanoimprint lithography where a high
surface energy polymethyl methacrylate (PMMA) tethers the diblock to the substrate
while the low surface energy PDMS allows for facile mold-resist separation [49] as well as
lubricating [50] and low energy [51] surfaces.
Other uses of PDMS-containing block polymers where the primary effect of interest
occurs due to the close spatial presence of two materials with differing properties are
numerous. In the field of selective membranes, PDMS has been used as both the more
permeant conductive block and the less conductive block; when paired with sulfonated
polystyrene in a membrane [52], it increases the ratio of water conducted to ethanol conducted while when combined with PMMA it decreases the ratio of water conducted to
methanol conducted [53]. The former occurs because PDMS retards both methanol and
water permeation through the membrane by restricting the size of the more highly conductive pathways present; methanol is retarded more than water so the overall membrane
is more selective. The latter occurs because both PMMA and PDMS are less conductive
for these polar materials than sulfonated PS but the rubbery PDMS allows many more
molecules to permeate than the glassy PMMA due to its larger free volume; as ethanol
has a higher conductivity than water in PDMS, increasing the PDMS volume fraction
in the membrane increases its preference for conducting ethanol over water. PDMS has
also been used to increase the solubility of ionically conducting polypyrrole in order to
28
aid processability [54]. Other practical uses of PDMS-containing block polymers include
flame retardant transparent materials when PDMS is attached to polycarbonate [55] and
materials for external coatings for spacecraft when combined with polyimides due to their
resistance to etching by the oxygen present in outer space and ability to couple mechanically with the commonly used underlying polymer layers [56].
In addition to being widely known for their use in practical and technical applications,
PDMS diblocks have also received attention for their ability to be formed into materials on
which scientific experiments about fundamental polymer physics can be performed. These
are primarily investigations into the mechanical and physical-chemical properties of one
of the blocks when under confinement. Examples include investigations into the rubbery
dynamics of PDMS under soft confinement for PI-PDMS diblocks [57] and PDMS under
hard confinement either tethered at one end in PS-PDMS diblocks [58] or at both ends
in PS-PDMS PS triblocks [59]. Other work that PDMS has been used for in this area
includes a research program dedicated to studying polymer dynamics under solvation
and tethering; PDMS is an ideal block to attach to polymers under study because its
low surface energy pins it to the surface of fluids which allows for the creation of model
systems in which many different experimental factors such as surface grafting density,
chain length, etc. can be independently varied [60].
1.5
Prior work on PDMS-containing block polymers
While many theoretical advances have been made in understanding block copolymer structure and how it depends upon composition, segregation strength between the component
blocks, and other factors, experimental knowledge of practical systems is necessary for
engineers who wish to make use of specific materials. Because of this, there have been
several studies examining the bulk structures PDMS-containing block polymers. For these
works, PDMS has been combined with many other polymers, including glasses, rubbers,
or polymers commonly used in biological or electronic applications. In many cases, X is
quite large for the resultant diblocks; the PDMS backbone is covered by methyl groups
which have a lower cohesive energy density than many other arrangements of atoms and
so the two components are often immiscible even for short chain lengths and high temperatures. Prediction of x for PDMS-containing diblocks has been treated theoretically
in [61] and [62].
Another interesting feature of block polymers containing PDMS is the experimental
observation of structures present at volume fractions which are substantially different than
expected. Theoretical and experimental descriptions of the classical phase diagrams of
diblock copolymers predict a symmetric phase diagram in which the four ordered phases
occur at defined volume fractions independent of the chemical identity of the component
blocks. However, this is not true for many actual systems. In Table 1.1 and figures 1-7
and 1-8, all of the bulk morphologies which have been observed in the literature and
accompanied by reasonably supporting data are displayed as a function of the volume
fraction of PDMS xN; these include morphologies discussed in the previous section as well
as those treated below. Many authors have noted the presence of morphologies occurring
in unexpected regions of the theoretical phase diagram; figures 1-7 and 1-8 show that
29
50
100
45
95
40
90
35
85
*
30
80
z25
75
20
70
15
65
10
60
5
55
0
so
a
0
0.2
0.4
1
0.8
0.6
b
0
0.4
0.2
PDM S
sphees
a PDMScyhnders
4 PDMS DG
I PDMIS perforated
Lamena
1
PDMS volume fraction
PDMS volume fraction
a
0.8
0.6
U .ameeiJ
6
Oth
b1
k peforatc
d
me.
Oa
Ot!
blot k D,
a Othrl biock
vhldLts
UOther
block
phtee
4 Diordered
igure 1-7: Three composite plots showing previously observed morphologies in PDMS-containing diblocks as a function of
nd PDMS volume fraction. Eighty six points are plotted in total; these include multiple plots of the same sample on two s
f an observed phase transition. It is clear that the locations of experimentally observed phases are not predicted perfectly
he phase boundaries computed using SCFT. It appears that the range of volume fractions at which the morphologies oc
re broader than those predicted and that those regions interpenetrate. This may be due to more advanced factors such
onformational asymmetry, polydispersity, or be the result of preparation procedures which produced metastable samp
Graph (a) shows morphologies found at weak segregation; graph (b) shows morphologies found at intermediate segregat
raph (c) shows morphologies found at strong segregation. The lines dividing the regions of expected morphologies w
reated by plotting a smooth curve through the points found in [15] and [27]; for values of xN outside the range compute
hose works, the boundaries between phases were assumed to be independent of xN (vertical phase boundaries.)
1500
1300
1100
z
900
700
500
300
100
0
0.2
0.4
0.6
0.8
1
PDMS volume fraction
U PDMS phce
U PDMScyhnde
S PDNIS DG
PDNIS
eforIted
lamdlae
dl
a
Lamelie
Other bicxk perfo ted aimele
Othe biotk D6
a Othe block
cihader
a Othe blod
Spheles
a Doot r ed
ontinued Figure 1-7: Three composite plots showing previously observed morphologies in PDMS-containing diblocks a
unction of xN and PDMS volume fraction. Eighty six points are plotted in total; these include multiple plots of the sa
ample on two sides of an observed phase transition. It is clear that the locations of experimentally observed phases
ot predicted perfectly by the phase boundaries computed using SCFT. It appears that the range of volume fractions
hich the morphologies occur are broader than those predicted and that those regions interpenetrate. This may be due
ore advanced factors such as conformational asymmetry, polydispersity, or be the result of preparation procedures wh
oduced metastable samples. Graph (a) shows morphologies found at weak segregation; graph (b) shows morphologies fo
intermediate segregation; graph (c) shows morphologies found at strong segregation. The lines dividing the regions
xpected morphologies were created by plotting a smooth curve through the points found in [15] and [27]; for values of
utside the range computed in those works, the boundaries between phases were assumed to be independent of xN (vert
hase boundaries.)
PDMS Cylinders
PDMS Spheres
1600
500
S
450
1400
400
1200
350
1000
300
800
z 250
Z
600
400
200
0
ao
200
KL
0.2
150
100
50
0
0.4
0.6
0.8
1
b0
8 PDMS
suheie
PDMS cyhndcr
PM)IS DG
PDMS pd.0oatd OamIah
0.8
0.6
0.4
0.2
1
PDMS volume fraction
PDMS volume fraction
a t amellae
Other boc k perorated
lanmlle
Othe' blo k
DG
a Ot her Wok
vlide
* Othe block
spheree
a
Disorder'd
igure 1-8: Plots showing previously observed morphologies in PDMS-containing diblocks as a function of xN and vol
action. Each graph shows the data for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS DG
DMS HPL (d), lamellae (e), other block HPL (f), other block DG (g), other block cylinders (h), other block spheres (i),
isordered (j). The lines dividing the regions of expected morphologies were created by plotting a smooth curve through
oints found in [15] and [27]; for values of XN outside the range computed in those works, the boundaries between phases w
ssumed to be independent of XN (vertical phase boundaries.) Note that many phases are observed significantly outsid
he regions of volume fraction and values of XN that are predicted theoretically; different phases also exist in close proxim
o each other and in some cases the phase boundaries appear to interpenetrate.
DG
300
PDMS HPL
PDMS DG
100
90
250
80
70
200
60
z150
S50
40
100
30
20
50
10
0
0
0.2
C 0
0.4
0.8
0.6
1
d 0
0.2
PDMS volume fraction
8
PDMS
B PDMS
8pre
ybnlmder
9 PDMIS DG
- PDMIS perforated
Iamelej
0.4
0.6
0.8
1
PDMS volume fraction
a
Lamllae
* Othef block perforte
f
d
amellae
Other
Lock DG
0 Other block t yhnde
3 0ther
bock
3 D 1Odered
Wpe
ontinued Figure 1-8: Plots showing previously observed morphologies in PDMS-containing diblocks as a function of XN
olume fraction. Each graph shows the data for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
), PDMS HPL (d), lamellae (e), other block HPL (f), other block DG (g), other block cylinders (h), other block spheres
nd disordered (j). The lines dividing the regions of expected morphologies were created by plotting a smooth curve throu
e points found in [15] and [27]; for values of xN outside the range computed in those works, the boundaries between pha
ere assumed to be independent of XN (vertical phase boundaries.) Note that many phases are observed significantly outs
the regions of volume fraction and values of XN that are predicted theoretically; different phases also exist in close proxim
each other and in some cases the phase boundaries appear to interpenetrate.
Other Block HPL
Lamellae
300
50
45
250
40
35
200
30
z
150
z 25
20
100
15
10
50
5
0
e
0
0
0.4
0.2
0.6
0.8
1
f
0.2
0
PDMS volume fraction
U PDMS
sphece
a PDMS ybdes
8
PDMIS DG
PDMSperoiated
lamelfaf
0.4
0.6
0.8
1
PDMS volume fraction
a talnwlae
0 Other bock perfoted
lamellae
Other -o k D
*ther
block cybooers
a Othelr
bkxk
phere.
a Dordered
ontinued Figure 1-8: Plots showing previously observed morphologies in PDMS-containing diblocks as a function of xN
olume fraction. Each graph shows the data for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
), PDMS HPL (d), lamellae (e), other block HPL (f), other block DG (g), other block cylinders (h), other block spheres
nd disordered (j). The lines dividing the regions of expected morphologies were created by plotting a smooth curve thro
e points found in [15] and [27]; for values of XN outside the range computed in those works, the boundaries between pha
ere assumed to be independent of xN (vertical phase boundaries.) Note that many phases are observed significantly outs
the regions of volume fraction and values of XN that are predicted theoretically; different phases also exist in close proxim
each other and in some cases the phase boundaries appear to interpenetrate.
Other Block DG
Other Block Cylinders
50
250
45
40
200
35
30
150
z
z25
20
100
15
10
so
5
0
g0
0
0.2
0.4
0.6
h0
0.8
0.2
PDMS volume fraction
a PDMS
sphere
U PDMSt vbndef
0 PDMS DG
PDMS plror aed
lelma
0.4
0.6
0.8
1
PDMS volume fraction
a
.LIela
a Othe
blok k pirtorted
lanea
i
OtIr
Luo k DO
a Other bOck ylmdels
I
a Other blot k 'pheres
a Dsordored
ontinued Figure 1-8: Plots showing previously observed morphologies in PDMS-containing diblocks as a function of xN
olume fraction. Each graph shows the data for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
), PDMS HPL (d), lamellae (e), other block HPL (f), other block DG (g), other block cylinders (h), other block spheres
nd disordered (j). The lines dividing the regions of expected morphologies were created by plotting a smooth curve throu
e points found in [15] and [27]; for values of xN outside the range computed in those works, the boundaries between pha
ere assumed to be independent of xN (vertical phase boundaries.) Note that many phases are observed significantly outs
the regions of volume fraction and values of XN that are predicted theoretically; different phases also exist in close proxim
each other and in some cases the phase boundaries appear to interpenetrate.
Other Block Spheres
200
50
180
45
160
40
140
35
120
30
z100
l25
80
20
60
15
40
10
20
5
0
0
i Q
Disordered
0.2
0.4
0.6
0.8
1
D
0
0.2
0.4
PDMS volume fraction
U PDMS pherv
a PDMScyhtnder-
U PDMS DG
P)MS perfoated
lamela
0.6
0.8
1
PDMS volume fraction
a
Lamellae
* OtIot )k
peffoahd
aia1m
Ot 1hr
I DG
U Other block
vhlder
s
Other
lotik
pheices
a DI, pdred
ontinued Figure 1-8: Plots showing previously observed morphologies in PDMS-containing diblocks as a function of XN a
olume fraction. Each graph shows the data for a distinct morphology: PDMS spheres (a), PDMS cylinders (b), PDMS
), PDMS HPL (d), lamellae (e), other block HPL (f), other block DG (g), other block cylinders (h), other block spheres
d disordered (j). The lines dividing the regions of expected morphologies were created by plotting a smooth curve throu
e points found in [15] and [27]; for values of xN outside the range computed in those works, the boundaries between pha
ere assumed to be independent of xN (vertical phase boundaries.) Note that many phases are observed significantly outs
the regions of volume fraction and values of XN that are predicted theoretically; different phases also exist in close proxim
each other and in some cases the phase boundaries appear to interpenetrate.
in many cases structures appear at volume fractions of PDMS closer to 0.5 than would
be predicted theoretically. Reference [63] examined possible causes for the presence of
PDMS-minority phases occurring at higher PDMS volume fractions than expected by
considering the effect of the conformational asymmetry of PDMS and PS, which is the
difference in the ratios between flexibility (measured by the radius of gyration of the
homopolymer chains) and specific volumes of each chain and can sometimes account
for differences in morphologies obtained; stiffer chains prefer to form structures which
have less domain surface curvature. However, the calculated conformational asymmetry
parameter was much too small to explain the pronounced shift that they observed. The
presence of morphologies occurring in unexpected regimes is important both technically
and scientifically: it is necessary for synthetic chemists to understand what compositions
they should target in order to achieve their desired morphologies and it is interesting to
polymer scientists to understand why unexpected behavior occurs.
The first comprehensive study of PS-PDMS morphology was undertaken by the Register group in 1995 [64]; in this paper, the bulk morphologies of six diblocks of PS-PDMS
were studied using TEM, SAXS, and Differential Scanning Calorimetry (DSC.) The microdomain structures observed were different than those which were expected based upon
accepted PS-PI behavior: while samples with a PDMS majority behaved as anticipated,
two samples with a slight PS majority which were predicted to fall inside of the lamellar
region instead formed morphologies which consisted of PDMS cylinders in a PS matrix. The authors discussed the possibility that the casting solvent (toluene) might have
preferentially swollen the polystyrene matrix during casting and that a non-equilibrium
structure may have been locked in as the polystyrene went through its glass transition
during solvent evaporation. They concluded, however, that this explanation may not be
accurate because the Mark-Houwink exponents for PS and PDMS in toluene are very
similar. Because this exponent describes scaling of the viscosity of a polymer with chain
molecular weight and intrinsic viscosity is directly proportional to the volume of a chain
in solution, using its value to as a way to compare swelling and solvent quality is quite
reasonable.
Moreover, similar results were found in a study that analyzed the SAXS patterns of
five different diblocks [63]: samples with volume fractions and values of XN predicted to
have morphologies which were disordered, cylindrical, and lamellar range showed SAXS
patterns consistent with theoretical predictions but one sample close to the PDMS double
gyroid-lamellar boundary had a scattering pattern typical of cylinders. This study also
examined a diblock which was expected on the basis of volume fraction to form BCC
spheres of PDMS in a PS matrix, but the scattering pattern did not have sufficient peaks
to be interpretable.
The discovery of the gyroid phase in PS-PDMS further supports the idea that the
volume fraction boundaries between phases are shifted towards larger volume fractions of
PDMS in samples containing a PDMS minority. In [65], the double gyroid was found to
exist at strong segregations in the region which lamellae occur in theoretical predictions.
The protocol used in this work was similar to that in the study performed by the Register
group: the polymers were cast from toluene and annealed at 130 0 C for one week before
examination using TEM and SAXS. Four samples, all in the expected lamellar region,
37
were discussed: one which had a morphology consisting of PDMS cylinders, two samples
contained a PDMS double gyroid, and one sample was composed of lamellae.
One other study on block polymers containing PS and PDMS showed even stranger
phase behavior [66]; a PDMS-PS-PDMS triblock with a 50:50 PS:PDMS volume composition displayed a morphology consisting of large PDMS spheres in a PS matrix. While
this sample was obtained in an unknown way from a film of unknown thickness (not from
a bulk sample) and cast from toluene and annealed at 100 0C for 1.5 hours, making it
probable that its bulk equilibrium structure was not obtained, the formation of a morphology consisting of spheres where lamellae are predicted is highly unexpected! If the
predicted phase boundaries are accepted, this would require the PS phase to be swollen
at least seven times as much as the PDMS phase at the glass transition of PS (which
itself will not happen at high solvent concentrations) and then exist in a deep potential
well so that it is not erased by annealing. While this is possible, it is more probable that
there is some equilibrium phase that is not lamellae which is present at this composition.
Further support for the conclusion that this morphology is not an artifact of preparation
methods is its formation when cast from both toluene and bromobenzene [66].
Work has also been done on the hydrogenated analogs of PS-PDMS-PS triblocks
(polycyclohexane-PDMS-polycyclohexane) [62]. Here, a selected set of triblocks were
studied using SAXS, TEM, and DMA in order to locate the ODTs and morphologies
of these samples. All were within the lamellar volume fraction region and all displayed
lamellae.
Prior work on the equilibrium bulk morphology of PI-PDMS diblocks consists of an
examination of the ODTs of two PI-PDMS diblocks using SAXS and dynamic mechanical
analysis (DMA) [67]. In this study, the presence of a double gyroid morphology where
lamellar morphology was expected was also found. While both samples investigated had
morphologies which were expected to be within the lamellar region (with volume fractions
of 0.46 and 0.63), the DMA data was consistent with lamellae for the sample closer to
50:50 PI:PDMS and the SAXS data was consistent with the double gyroid morphology
for a sample close to the lamellae-double gyroid phase boundary.
Similarly to PS-PDMS and PI-PDMS, Poly(ethylenepropylene) (PEP)-PDMS has received attention due to the high x parameter between the component blocks and associated ability to form ordered structures on small length scales. These morphologies
have been studied using SAXS, small angle neutron scattering (SANS), and DMA. The
typical experiment that has been performed is the location of the ODT using SAXS,
SANS, and/or DMA, determination of the scattering form factor, and the assignment
of a morphology based upon this data. The morphologies found agreed with theoretical
predictions in some cases: in [68] BCC cylinders of PDMS were found in the expected volume fraction range and in [67] when two samples which were expected to display lamellae
at room temperature were examined by SAXS, the resulting data were consistent with
this hypothesis. One of the samples which had a volume fraction closer to those in the
double gyroid range, however, went through two OOTs upon heating; first into a layered structure and then into a double gyroid structure before ultimately disordering at
the ODT. Another study [69] also found an anomalous pressure dependence of the ODT
for a dilock with almost symmetric composition; instead of having a monotonic change in
38
ODT as pressure increased, the ODT had a minimum at one of the intermediate pressures
examined. The combined evidence from these three studies shows that the well-known
PS-PI phase diagram cannot fully describe the behavior of PEP-PDMS diblocks.
The morphologies of PEO-PDMS block copolymers have also received much interest
due to the small scale highly segregated structures that can be found and the formation
of ordered liquid structures. This system has been extensively characterized above the
PEO melting point using optical microscopy and SAXS in [70]. While many of the volume
fractions studied in this work were formed by creating blends of diblocks (which changes
the thermodynamics of mixing by altering the chain stretching that occurs; however, the
volume fraction boundaries between morphologies should be close to those found in pure
diblock systems), an extensive range of blend compositions at varying temperatures was
covered; the authors were able to present a phase diagram as a function of composition and
the ratio between molecular weight and temperature, which scales the same way as xN.
In this case, the phase behavior was quite different from that found in most other diblock
systems: only lamellae, cylinders, and a phase with cubic symmetry were found. In this
case, the phase diagram is again remarkably asymmetric with respect to volume fraction;
while on the PDMS-rich side of the phase diagram the boundaries between morphologies
are in the expected locations, only lamellae are present in the PDMS-poor region and the
region in which they are found extends into much higher volume fractions than for PS-PI.
Most other studies have focused on the PDMS-poor side of the phase diagram, so this
thesis work provides a nice complement.
Polybutadiene (PB) is another polymer that has been combined with PDMS to form
a diblock in order to take advantage of the phase separation at small length scales offered by PDMS-containing diblocks; in this case, it is the crystallization of short and
spatially-confined PDMS chains that is of interest. One study [71] examined nine different PB-PDMS diblocks to answer this question. Unfortunately, in this case little can be
said except that the presence of PDMS spheres and some combination of PDMS spheres,
PDMS cylinders, and lamellae were present at the varying volume fractions; the preparation procedure was not fully described, only one descriptive micrograph was provided
for each image, and no other technique was employed to confirm the assigned phases.
The morphologies were assigned based upon examination of a sample cast from a THF
solution (temperature, time of casting, and whether or not thin film or bulk samples were
investigated was not discussed); crystallization behavior was investigated using a DSC on
bulk powders. However, if the morphology claims in the paper are accepted at face value,
it appears that PDMS spheres were present well into the region where PDMS cylinders
are expected to be found. One other paper [72] has also examined PB-PDMS morphology.
Here, one diblock with a volume fraction where cylinders are expected was synthesized
and analyzed using SAXS and DMA. As it was heated, it underwent a transition from a
cylindrical morphology through BCC packed spheres into a disordered structure, as would
be expected from theory.
PDMS block copolymers containing polyamides have been synthesized to probe the
effect of crystallization of one of the component blocks on overall morphology and to understand how structure varies with volume composition and processing [73]. In this work,
it was found that the observed morphologies for two sphere-forming diblocks (one which
39
has a volume fraction just on the disorder side of the disordered-spherical boundary and
the other which has a volume fraction within the cylinder region near the cylinder/double
gyroid boundary) depend strongly upon both the volume fraction and processing treatment. It was found that the diblock with a very low percentage of PDMS would develop
different structures when solvent cast than when cooled from a melt; in the former case,
the PDMS separated from the solvent before crystallization of the nylon block and so
spheres of PDMS are dispersed in spherulites while in the latter, a morphology with
a much more regular arrangement of PDMS spheres within a nylon 6 matrix with less
crystallinity was obtained by quenching in the structure present above the crystallization
temperature of nylon 6. For the diblock with a larger amount of PDMS, more regularly
sized and placed spheres of PDMS in a matrix of nylon 6 were obtained by both solvent
casting and melt casting; there was also no significant difference in the nylon crystallinities
obtained.
40
Sample morphology
#PDMS
0.04
0.05
0.08
0.10
0.12
PDMS Spheres
0.12
0.17
0.18
0.20
0.20
0.25
0.29
0.18
0.23
0.23
0.26
0.27
0.29
PDMS Cylinders
0.32
0.33
0.33
0.34
0.37
0.39
0.39
0.44
41
XN
15000
93
93
35
35
20
78
24
43
33
97
960
27
50
80
28
380
28
24
380
39
95
45
420
460
110
Reference
Argon, [73], 1991
Berg, [33], 2003
Rueda, [63], 2010
Lodge, [68], 2002
Lodge, [68], 2002
Lodge, [68], 2002
Berg, [33], 2003
Valles, [72], 2008
Kunieda, [70], 2003
Kunieda, [70], 2003
Huang, [71], 1992
Argon, [73], 1991
Valles, [72], 2008
Rueda, [63], 2010
Berg, [33], 2003
Ndoni, [35], 2009
Uragami, [53], 1999
Ndoni, [36], 2011
Ndoni, [34], 2004
Uragami, [53], 1999
Huang, [71], 1992
Ndoni, [34], 2004
Rueda, [63], 2010
Uragami, [53], 1999
Hill, [65], 2009
Register, [64], 1995
Sample morphology
#PDMS
PDMS DG
0.37
0.39
0.39
0.41
0.42
PDMS HPL
Lamellae
0.39
0.39
0.36
0.36
0.37
0.39
0.39
0.43
0.44
0.45
0.47
0.48
0.48
0.50
0.50
0.53
0.53
0.56
0.57
0.58
0.60
0.65
0.65
0.65
0.71
Other block HPL
0.65
0.65
0.64
Other block DG
0.64
0.65
0.65
0.60
0.60
0.67
0.67
0.82
0.84
Other block Cylinders
42
XN
22
23
21
290
170
20
11
50
38
41
26
25
120
62
210
46
10
9
44
44
67
52
92
260
57
260
120
16
14
230
13
12
19
14
12
10
68
52
78
60
47
200
Reference
Berg, [33], 2003
Ndoni, [36], 2011
Ndoni, [36], 2011
Hill, [65], 2009
Hill, [65], 2009
Ndoni, [36], 2011
Ndoni, [36], 2011
Kunieda, [70], 2003
Kunieda, [70], 2003
Huang, [71], 1992
Ndoni, [36], 2011
Ndoni, [36], 2011
Huang, [71], 1992
Huang, [71], 1992
Hill, [65], 2009
Huang, [71], 1992
Bates, [67], 1996
Bates, [67], 1996
Kunieda, [70], 2003
Kunieda, [70], 2003
Deloche, [59], 2003
Huang, [71], 1992
Rueda, [63], 2010
Uragami, [53], 1999
Deloche, [59], 2003
Uragami, [53], 1999
Register, [64], 1995
Bates, [67], 1996
Bates, [67], 1996
Uragami, [53], 1999
Bates, [67], 1996
Bates, [67], 1996
Bates, [67], 1996
Bates, [67], 1996
Bates, [67], 1996
Bates, [67], 1996
Kunieda, [70], 2003
Kunieda, [70], 2003
Kunieda, [70], 2003
Kunieda, [70], 2003
Richter, [57], 2010
Register, [64], 1995
Sample morphology
Other block Spheres
Disordered
qPDMS
0.89
0.76
0.76
0.80
0.80
0.92
0.02
0.12
0.18
0.39
0.46
0.48
0.64
0.65
0.76
0.80
0.86
0.94
XN
51
33
28
31
29
190
39
20
22
11
11
9
28
10
27
28
29
25
Reference
Richter, [57], 2010
Kunieda, [70], 2003
Kunieda, [70], 2003
Kunieda, [70], 2003
Kunieda, [70], 2003
Register, [64], 1995
Rueda, [63], 2010
Lodge, [68], 2002
Valles, [72], 2008
Ndoni, [36], 2011
Bates, [67], 1996
Bates, [67], 1996
Bates, [67], 1996
Bates, [67], 1996
Kunieda, [70], 2003
Kunieda, [70], 2003
Kunieda, [70], 2003
Kunieda, [70], 2003
Table 1.1: A chart detailing the morphologies found in the literature for PDMS-containing
diblocks (it does not include triblocks or other multiblocks also described in the text); it
contains the data displayed in figures 1-7 and 1-8. Some data found in the literature discussed
was purposefully excluded if there were significant reasons to believe that the data presented
was either not sufficient to establish the morphology claimed, e.g due to large amounts of
obvious homopolymer contamination in the TEM images, etc. Note that because much of
the relevant information was not available in the papers, in some cases calculations were
made to arrive at the volume fraction and XN values. If necessary, volume fractions were
computed from mole or weight fractions using commonly available densities. x values were
taken from [74] if available or calculated from solubility parameters found in [75]; in the
latter case, the reference volume was assumed to be 100O.
The temperature used was
m01
that at which the last thermal treatment was done (e.g. annealing, crosslinking, etc.) or
room temperature if the sample was not thermally treated after casting. For works in which
several samples were examined at a large variety of temperatures, the chart displays multiple
entries for each transition temperature ±50 C: the lower temperature for the phase present
below the transition temperature and the upper temperature for the phase present above
the transition temperature.
1.6
Scope of Thesis Project
This thesis seeks to expand upon previous work by experimentally elucidating the determinants of PDMS-containing block polymer structure by examining the effect of volume
43
fraction, temperature, and processing history on the morphologies of selected PS-PDMS
and PI-PDMS diblocks of varying volume fractions and molecular weights. The majority
of the PS-PDMS diblocks investigated have a XN of ~ 100 at 150 0 C (the temperature
at which they were annealed and thus where final structure formation occurred) and
the majority of the PI-PDMS diblocks examined have a xN of ~ 100 at 25 0 C (due to
the rubbery nature of both blocks, structural changes can occur at room temperature).
The structures of three low molecular weight diblocks (two of PS-PDMS and one of PIPDMS) were examined with several minute resolution while undergoing differing thermal
programs by using SAXS at Brookhaven National Laboratory in order to determine the
effect of processing on final morphology and to explore the temperature dependence of
the phase diagram at varying values of <pPDMS-
44
Chapter 2
Strongly Segregated Morphologies of
PS-PDMS and PI-PDMS Diblocks
2.1
Introduction and motivation
This chapter describes the determination of the bulk morphologies of selected PS-PDMS
and PI-PDMS diblocks using SAXS and TEM. The diblocks were designed to provide
insight into the strongly segregated morphologies of these materials (all samples had
values of XN close to 100 at the final processing temperature); diblocks in this regime are
of interest for application development because they are relatively stable with temperature
(the boundaries between different morphologies in 4>PDMs become nearly independent of
composition as XN increases) and because the stronger segregation of material means
there is less molecular-scale mixing so the different properties associated with each block
(e.g. etch resistance) are more pronounced. The samples span a range of volume fractions
(12%-65% PDMS; see Table 2.1) and three different morphologies (PDMS spheres, PDMS
cylinders, and lamellae) are experimentally observed.
2.2
Materials synthesis
The polymers used in this thesis were synthesized in the laboratory of Professor Apostolos Avgeropoulos at the University of Ioannina in Ioannina, Greece by his graduate
students. The information described below was kindly provided by Professor Avgeropoulos. The total process consisted of three steps: purification of the reagents and solvents,
polymerization, and molecular characterization.
Because of the high reactivity of species present during the polymerization process,
ultrapure reagents, solvents, and polymerization environments are necessary for the elimination of premature products and side reactions while allowing for production of monodisperse products. Therefore, extensive procedures were undertaken to prevent contaminants
from being present during the reaction.
As the polymerization procedures were performed in both benzene and tetrahydrofuran
(THF) in different phases of the reaction, both of these solvents needed to be purified.
45
This was accomplished by using ground, degassed calcium hydride as a drying agent and
performing a distillation. In the case of THF, two additional distillations were performed:
the first using a sodium mirror to indicate purity and the second containing a potassium:
sodium alloy in a 3:1 ratio.
The three monomers used in synthesis: styrene, isoprene, and hexamethylcyclotrisiloxane (D3 ) were also purified. These reagents were similarly dried over ground calcium
hydride before further preparation. At the conclusion of the preparation procedures, the
monomers were distilled into and stored in an apparatus containing ampoules and break
seals. Commercially synthesized styrene was also reacted with dibutylmagnesium in an ice
bath to remove impurities. After drying, commercially synthesized isoprene was distilled
into a flask containing a small amount of n-butyllithium and stirred for thirty minutes
at 00C twice. As D3 is a solid at room temperature, it was dissolved in benzene during
its purification procedure and before introduction to the reaction. After the solution was
formed, it was distilled and degassed. Then, living PSLi anions were added to the flask
and benzene and D3 was distilled again. The resulting product was stored at -20 0 C.
The final purifications that must occur before synthesis occurs are those for the initiator which begins the reaction and the termination agent which terminates it. sec-BuLi,
the initiator, was received in a 1.4M hexane solution which was first introduced into a
vacuum line and the hexane was removed. Then, benzene was added and the solution
was stored in an ampoule before usage during polymerization. Methanol, which was used
to terminate the reaction, was first dried using calcium hydride and then degassed and
distilled into ampoules.
After the necessary materials have been prepared, polymerization was performed. The
polymerization contains of four steps: preparing the polymerization vessels, performing
the PS or PI polymerization, performing the PDMS polymerization, and termination. The
reaction vessel consists of a round-bottomed flask connected to a vacuum line and purge
section and to which all of the necessary reagents are connected by break seals. During
polymerization, pure benzene was used as a solvent. Before performing the reaction, this
benzene was first mixed in the purge section with n-BuLi (which reacts with any impurities
that may be present), frozen with liquid nitrogen and degassed, and then distilled into
the main flask. After this step, the purge section was sealed off from the main flask and
the polymerization can begin.
Polymerization of both the PS-PDMS and the PI-PDMS diblocks was accomplished by
first polymerizing the styrene or isoprene monomers in benzene at room temperature using
sec-BuLi as an initiator. In both cases, the monomer and initiator were introduced into
the flask and polymerization was allowed to occur (for 18 hours in the case of PS and 24 in
the case of PI). After polymerization of the first block was complete, the polymerization
of the PDMS block was performed. The procedure in both cases was identical, except
that in the case of the PI-PDMS diblocks THF was first added to the reaction and mixed
for 20 minutes before performing the second polymerization. D3 was polymerized to form
PDMS in one of two multi-step procedures. The first step in both cases was to add D3
to the reaction vessel and allowed to react for 18 hours. Then, enough THF was added
to the mixture to form a solution with equal amounts of benzene and THF. Following
these two steps, one of two approaches were taken. The first was to allow a four hour
46
polymerization of D3 so that half of the monomers could react. The second was to lower
the reaction temperature to -20 0 C for seven days to allow complete polymerization of
the D3 . At the conclusion of the polymerization, approximately one milliliter of methanol
was added to flask to terminate the reaction.
Upon the conclusion of the polymerization, each polymer was analyzed using membrane osmometry (MO), gel permeation chromatography (GPC) and nuclear magnetic
resonance (NMR) to determine the molecular weight and composition of the product. For
GPC, a PL-GPC-120 (Polymer Laboratories) equipped with three PLgel 5m MIXED-C
columns and an RI detector light source upgrade (PL-GPC 120/220) was used. The
experiment was performed in THF at 35 0 C with a flow rate of 1 milliliter per minute.
NMR studies were performed using a Bruker AC-250 spectrometer at 250 megahertz. The
polymers were dissolved in CDCl3 and examined at 25 0 C. The following chemical shifts in
the 1H-NMR spectrawere used for calculation of the volume fraction: 6.7ppm-7.7ppm for
the five aromatic protons of polystyrene, 4.9ppm-5.1ppm for the one proton of the double
bond in PI(1,4), 4.6ppm-4.7ppm for the two protons of the double bond in PI(3,4) and
0.3ppm-0.6ppm for the six protons of PDMS.
2.3
Experimental procedures
The diblocks studied in this chapter were all prepared in a uniform way in order to ensure
consistency in final morphology assignment (see Chapter 3 for more information about
how processing conditions can affect observed morphology). The procedure used here
is similar to those described in [64] and [65], making comparison of results with these
works the most clear-cut. In each case, approximately 0.1 gram of each diblock was
dissolved in toluene to form a 5 wt% solution. These solutions were poured into ceramic
crucibles and between one and ten crucibles were placed symmetrically around a central
crucible containing toluene inside a hood. This setup was covered with a glass dish and
the solvent was allowed to slowly evaporate; total drying time was approximately seven
days. Then, these samples were placed inside a vacuum oven and annealed for seven
days at 150 0 C under vacuum. The samples were then slowly cooled to room temperature
under vacuum by turning down the oven temperature to 230C. This final cooling step
required approximately 12 hours (in some cases noted on the chart structure formation
was not observable using SAXS but was if another solvent was used or annealing was not
performed and data from those trials was used to make the phase diagram.)
Samples were prepared for TEM examination by using a cryo-microtome to cut very
thin slices which were then picked up and placed on copper grids using a sucrose solution.
First, the samples were trimmed using a glass knife and then they were cut on the diamond
knife (final slices taken were ~ 50 nm thick.) The sample, air, and knife temperatures were
all set at -140 0 C during cutting (well below the ~ -120 0 C glass transition of PDMS, the
~ -70 0 C glass transition of PI, and the ~ 100 0 C glass transition of PS) to ensure limited
sample deformation during cutting. The sample speeds were ~ 1" during trimming and
during cutting. The grids onto which the slices were deposited were examined
~ 0.2"
using an optical microscope to look for regions where thin pieces of sample had deposited.
These grids were then imaged using a JEOL 2000FX TEM operating at 120 kV. No
47
staining was necessary due to the inherent scattering contrast between the silicon and
oxygen in the PDMS backbone and the carbon in the PS and PI backbones (scattering
cross-section increases with increasing Z of atomic nuclei.)
Samples were prepared for SAXS examination by placing the annealed materials
between two pieces of Kapton tape (which is nearly x-ray transparent at the Cu-Ka
wavelength). The materials were then studied using a Rigaku SAXS instrument using a rotating anode and producing Cu-Ka x-rays with a wavelength of 1.54 A . The
sample chamber was evacuated before each one hour data collection session began. Image plates were used to record the raw scattering data and Polar software (available at
http://precisionworksny.homestead.com/EN/Technical-software-polar.html) was used for
analysis. Silver behenate was used to calibrate the distance between the samples and the
image plate each time the sample holder was removed and replaced (silver behenate has
a characteristic peak at q = 1.076nm- 1 .)
2.4
Results
The morphology assignments and characteristics of samples investigated are displayed in
Table 2.1. These results are also shown graphically on their own and in comparison to
the diagrams shown in Chapter 1 in Figure 2-1. It is clear from both of these figures that
the samples investigated here have morphologies which occur at volume fractions within
the ranges of those found in the literature and for the most part in the ranges predicted
by SCFT. The glaring exception to this is S-64LMW. This may be explained by the fact
that at its lower molecular weight (and associated lower value of XN at both room and the
annealing temperatures) means that it exists in the region of the phase diagram where
the phase boundaries all curve together. In this region, the phases are closer together
in volume fraction and the domains are less strongly segregated; therefore, it takes less
perturbation in volume fraction to cause the preferred phase to shift and other factors
such as chain packing length, etc. may have more of an influence.
2.5
TEM results
The TEM images for the samples investigated using this technique are shown in Figures
2-2 (PS-containing samples) and 2-3 (PI containing samples.) It is clear that the PScontaining samples studied here contain PDMS cylinders and lamellae and that the PIcontaining samples display PDMS spheres and cylinders. Each image other than that
from S-64LMW contains multiple grains at different orientations to the incoming electron
beam; therefore different projections of the individual morphologies can be observed. For
instance, samples S-17, S-22, and S-37 contain both end-on and in-plane cylinders and S43 and S-53 both show lamellar-lamellar grain boundaries. These multiple views confirm
that the morphological assignment is valid and not an artifact of projection along a special
axis.
48
Identity of
non-PDMS
block
PI
Total molecular
weight
(kg/mol)
44.9
41.2
43.8
53.5
43.2
36.1
66.1
31.1
45.9
46.1
7.0
4.3
50.9
73.4
60.7
43.8
42.9
10.7
0.16
0.17(GPC)
0.21
0.22
0.30
0.34
0.37
0.43 (GPC)
0.53
0.56
0.64
0.45
0.12
0.19
xN
at
150 0 C
91
84
95
110
95
76
130
70
110
110
14
10
70
100
S-16
S-17
S-21
S-22
S-30
S-34
S-37
S-43
S-53
S-56
S-64LMW
S-45LMW
1-12
1-19
0.23
0.25
0.44
0.39
82
59
57
14
1-23
1-25
1-44
I-39LMW
#PDMS
Sample
name
SAXS
d-spacing
(nm)
23*
34
40
N/P
39**
37
66
39
48
47
13
10
36
N/P
38
53*
65
16
SAXS
morphology
TEM
morphology
S*
C
C
N/P
C**
C
N/P
L
L
L
N/P
N/P
N/P
N/P
C
C*
L
N/P
N/A
PDMS C
PDMS C
PDMS C
PDMS C
L
L
PDMS C
PDMS S
PDMS S
PDMS C
ble 2.1: The list of samples analyzed in this thesis and their relevant characterization data. These materials were synthesiz
the laboratory of Professor Apostolos Avgeropoulos at the University of Ioannina in Ionannina, Greece. The molecu
eights and volume fractions of the samples were also determined there; gel permeation chromatography (GPC) was u
establish the former and nuclear magnetic resonance (NMR) the latter (except where noted that the characterizati
s performed using GPC.) The x-ray measurements and TEM measurements were performed at the Institute for Sold
anotechnologies at the Massachusetts Institute of Technology in Cambridge, Massachusetts. - means that the releva
easurement was not performed. N/P means that there were not sufficient peaks in the SAXS data to assign the value w
rtainty. * Indicates that toluene was the casting solvent and no annealing treatment was performed. ** Indicates th
clohexane was used as the casting solvent and the annealing treatment was that described in the text.
140
120
100
Z
80
U
U
60
40
8 PDMS spheres
20
a PDMS cylinders
a
0
0
a
0.2
0.8
0.6
0.4
1
PDMS DG
PDMS perforated lamellae
8 Lamellae
PDMS volume fraction
* Other block perforated larrellae
140
Other block DG
Other block cylinders
-
120
=
100
U Other block spheres
-
- Disordered
60
40
20
0
0
0.2
0.4
0.8
0.6
1
PDMS volume fraction
Figure 2-1: Updated phase diagrams (compare to figures 1-8 and 1-7) of PDMS-containing
diblocks reflecting the work done in this thesis. Extended data for the samples are available in Table 2.1. The SAXS and TEM data used for this table was that taken after the
sample underwent a one week cast from a 5 wt% solution and then a one week anneal at
1500C under vacuum. (a) shows only the samples characterized in this thesis; the samples
that have PS as the complementary block are indicated by square markers while those
that have PI as the complementary block are indicated by circular markers. (b) shows
the samples characterized in this thesis plus those with values of XN between 0 and 150
investigated in other works. The samples which were studied here have larger markers.
50
igure 2-2: TEM images of the PS-PDMS samples which were characterized using this technique. The PS cylinder morphol
ccurred when #PDMS < 0.37 for the strongly segregated samples and for the only low molecular weight sample. Lame
ccurred when 4PDMS> 0.43. See Table 2.1 for more information.
ontinued Figure 2-2: TEM images of the PS-PDMS samples which were characterized using this technique. The PD
phere morphology occurred when #PDMS < 0.19. PDMS cylinders were present when 'PDMS = 0.23. Table 2.1 for m
formation.
. .
........................
..
gure 2-3: TEM images of the PI-PDMS samples which were characterized using this technique. See Table 2.1 for m
formation.
2.6
2.6.1
SAXS results
Morphology determination using SAXS
As discussed in Chapter 1, SAXS is a powerful tool for determining sample morphology
because it can be used to determine the characteristic spacings at which there are correlations in electron density and thus can be used to determine the space group of materials.
In diblock polymers, the four ordered morphologies belong to different space groups and
so SAXS can be used to distinguish between them. As stated before, scattering occurs
when x-ray beams which are scattered constructively interfere (and thus is intimately
dependent upon material structure) and the spacing between the planes in the sample
containing the scattering centers is given by d = sr. =
The ratios of d-spacings
at which scattering occurs for a particular morphology can be analytically determined
based upon the location of scattering centers in that morphology; the d-spacing between
planes is affected by the lengths of the axes of the unit cell and the distance between
planes for which constructive interference occurs depends upon the locations of scattering
centers which are not at the corners of the unit cells. Therefore, the ratios between the q
values at which peaks are present in the SAXS data can be used to assign morphologies
to materials.
The lamellar structure is the simplest ordered morphology which occurs in diblock
polymers and it has periodicity along one axis. The constructive interference that results
in appreciable intensity of scattered x-rays must therefore occur for q values which are
perpendicular to this direction. The distance between scattering planes for any given
scattering peak is thus to a first approximation the wavelength of the scattered light
divided by the number of wavelengths present between lamellae. This means that the
ratio of d-spacings where scattering is present should be 1:j:j:1 etc and thus the ratio
of q-values should be 1:2:3:4 etc. In practice, due to the influence of the form factor,
which modulates scattering intensity based upon the interference of waves emanating
from different parts of one scattering center, certain reflections can be absent or strongly
suppressed.
Determination of the d-spacings and scattering vectors for which constructive interference occurs for more complex morphologies is aided by the use of the concept of the
reciprocal lattice (much of the following discussion is adapted from course notes from
Bruce Clemens' MSE 205 class at Stanford University.) The reciprocal lattice is made up
of the points defined by the translation vectors a*, a*, and a* which are related to the
vectors a1 , a 2 , and a3 of the real space lattice for a material by:
a,
a 2 xa 3
ai - (a 2 x a3 )
(2.1)
a
a 3 xa
a 2 - (a 3 x a1)
(2.2)
a* =
, x(2.3)
a 3 - (a1
54
x a2)
Each vector of the reciprocal lattice is perpendicular to the two vectors of the real
lattice which do not have the same index (as can be easily seen by observing that the
numerator is the cross product of these two vectors) and has a length that is equal to
the reciprocal of that of the real lattice vector which shares its index (as can be seen
from the cancellation of the magnitude of the cross product terms in the numerator and
denominator and the presence of the real space vector in the denominator). The spacing
where Ghkl is a vector which
G
between planes which have intercepts -k1
al a2 a3 is given by IJhk1I
is perpendicular to these planes and is defined by ha*+ka*+la*. The former can be verified
by observing that the diffracting planes can be defined by any two vectors connecting any
of the three intercepts and noting that the dot product of these vectors with Ghkl is always
zero (e.g. 1 - 2 - Ghk1 =1 - ha* - 1 - ka* = 0 (any other terms drop out due to the
perpendicularity of reciprocal lattice vectors to two of the real lattice vectors.) The latter
can be seen by noting that the distance between the scattering planes is given by the dot
1
ha*1 -- [GlkiI
G1 I ( h
product of gh and the unit normal to the planes or h -aIIGhkLI = IGhkL
With this knowledge, determination of the expected ratios of d-spacings of planes
from which scattering occurs for cylindrical and spherical morphologies can be performed
easily. Diblock polymers with cylindrical morphology have periodicity along two axes
and their morphologies can be conceived of as two-dimensional plane lattices with plane
group of p6mm. The primitive unit cell for this lattice consists of a parallelogram with
two 600 angles and two 1200 angles. The planes from which scattering occurs can be
thought of as lines within this plane lattice. In the reciprocal lattice for this system,
where i and j are the unit vectors in the x and
-+ _3) and a* = 2 (ii
a* =
y directions for a Cartesian plane lattice. Thus, IGhk I is given by
d-spacings of these planes are
planes are therefore 1: ~: : :
.
2___2
2
2VrhT+k -+hk
2,h/
k2 +hk
and so the
The ratios between the d-spacings of scattering
etc. and the ratios of the q values of these spacings are
1:x/5:2:F7 etc.
Diblocks which have a morphology consisting of spheres occupying a BCC lattice have
a reciprocal lattice given by a* =t2, ai = j, and a* = f. Therefore, |GhktI is given
+1 In this case, scattering does
by rh2 + k 2 + 12 and d-spacings are given by
not occur from any arbitrary plane because unlike in the previously discussed cases the
real lattice for this morphology is non-primitive (it contains more than one lattice point)
and scattering from an object (PDMS sphere) on this additional lattice point can cause
destructive interference of x-rays. Determination of the planes from which constructively
interfered waves are scattered requires a more advanced treatment, the results of which
indicate that allowed planes are those for which h + k + 1 is even. Therefore, the ratio of
d-spacings at which scattering occurs in this lattice is given by 1: : : etc at q values
of 1:V2:V25:2 etc.
Expected scattering from samples with a double gyroid morphology can be determined
in a similar way but is not discussed here due to the lack of such samples in this study.
With this knowledge, the morphology of a sample containing more than one peak
in its SAXS pattern can be easily determined by comparing its scattering with that of
the known diblock morphologies. It most likely has the morphology whose predicted
scattering intensity it most closely matches.
55
It is important to note here that disordered morphologies can also give rise to correlation hole scattering due to the relatively higher frequency of intra-chain contacts as
opposed to interchain contacts which results in variations in electron density at a characteristic distance.
2.6.2
Data
Figures 2-4 and 2-5 show small angle scattering from the PDMS-containing diblocks studied here. It is encouraging to see that SAXS patterns with more than one peak predict
the same morphology as that displayed in their TEM images.
2.7
2.7.1
Discussion
PS-containing diblocks
Using a combination of TEM and SAXS, it was possible to assign morphologies to eleven
samples, the results of which are summarized in Table 2.1. There was one sample which
was found to display the PDMS sphere morphology (S-16), which was assigned on the
basis of its volume fraction (16% PDMS) and SAXS pattern. The SAXS pattern for this
sample shows three distinct peaks with a ratio in values of q of approximately i:/2:v'/5,
which is consistent with a spherical morphology and not a cylindrical morphology. The
d-spacing was determined to be 23 nm using the SAXS data.
Six high molecular weight samples samples show the PDMS cylinder morphology (S17, S-21,S-22, S-30, S-34, and S-37). with volume fractions ranging between 17% and 37%
PDMS. TEM images were used to establish the morphologies of S-17, S-22, S-34, and S37; for all of these samples except for S-22, SAXS is able to provide corroboration for the
assignment. The TEM images show either end on cylinders (which appear as circles) or
cylinders in the plane of the section (which appear as lines.) The morphologies S-21 and
S-30 were determined solely using SAXS; both sample, however, have three peaks which
rules out the possibility of a lamellar structure and makes the presence other structures
highly unlikely. For most samples, strong {110} and {210} peaks are observed in addition
to the primary peak while the {200} peak is strongly suppressed. S-17 is an exception
where the {200} peak is displayed but not the {210} peak. For sample S-37, only the
{210} peak is observed (explained below.) D-spacings for these samples with the exception
of S-37 were determined by assuming that the first peak in the diffraction pattern was
due to the {100} peak and using the relationship d = '.q Since S-37 has a large molecular
weight (66.1 ±9)) it is not unreasonable to assume that the first peak would be inside
the beam stop. For this to be the case, the peak that is displayed could be no lower
index than {210}. The d-spacings for all of the other samples cluster together, which
makes sense because their molecular weights and segregation strengths at the annealing
temperature are similar. If we assume that the domain length scales with the molecular
weight of the sample to the two thirds power, it is not unreasonable to estimate that the
peak present in S-37 is from the {210} planes: we can look at the ratio of (')3
= 1.5
and estimate that the d-spacing for S-37 should be approximately 55 nm. This is not too
56
PS-containing samples with cylindrical morphology
PS-containing sample with spherical morphology
I I
II
II
210
100
S-37
100
y.v
110
210
S-34
100
110
111
110
210
S-16
Sj
S-30
i
ii.
,
1 ; 4P",Y( ,,iN,14V1
*1
i
I.
S-22
100
I
c
110
210
S-21
100
110
200
/1
A
S-17
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
Figure 2-4: SAXS data from the PS-PDMS samples which are studied in this thesis. See Table 2.1 for more information
PS-containing samples with lamellar morphology
f
I
I
I
I
I
100
200
300
S-56
100
300
i
I.
S-53
100
V
i
300
S-43
', Y
0
0.1
0.2
7
V
0.3
0.4
q (1/nm)
0.5
0.6
0.7
ontinued Figure 2-4: SAXS data from the PS-PDMS samples which are studied in this thesis. See Table 2.1 for m
formation.
Pi-containing samples with spherical morphology
Pl-containing samples with cylindrical morphology
100
110
210
1-19
1-25
100
100
i
110
I.
E
C
1-12
210
300
.2
1-23
1
h~
v
V
A.1
4
'If
0
[1
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
0
I
i
I
0.1
0.2
0.3
I
0.4
q (1/nm)
I
I
0.5
0.6
I
0.7
Figure 2-5: SAXS data from the PI-PDMS samples which are studied in this thesis. See Table 2.1 for more information
PI-containing sample with lamellar morphology
I
0
0.1
i
i
0.2
0.3
i
0.4
q (1/nm)
|
|
0.5
0.6
0.7
ontinued Figure 2-5: SAXS data from the PI-PDMS samples which are studied in this thesis. See Table 2.1 for m
formation.
Room temperature SAXS for processed LMW samples
1-39LMW
S-45LMi
I
S-64LMV\
'~
IV
A
4t
0
0.2
0.4
0.6
A
A
0.8
1
q (1/nm)
gure 2-6: SAXS data from all of the low molecular weight samples investigated in this thesis. See chapter 4 for m
formation.
far from the value of 66 which is found when the diffraction peak present arises from the
{210} planes.
One low molecular weight sample, S-64LMW, also displays the PDMS cylinder morphology, as can be seen by an inspection of its TEM images. The results from SAXS,
which displayed only one peak, were inconclusive. Note that the cylinders for this sample
are much smaller than those present in the larger molecular weight samples, as would be
expected. In order to observe the cylinders at all, a higher magnification is needed than
was typically used for the other samples; to see them well, a large amount of additional
power is needed. An inspection of the image with higher magnification shows that the
cylinders have a large volume fraction compared to the matrix, making the morphology
not unreasonable despite its odd placement on the phase diagram.
S-43, S-53, and S-56 all display the lamellar morphology. TEM images were obtained
of S-43 and S-45, which both display dark stripes (PDMS) on a lighter background. All
three samples have SAXS patterns which are consistent with cylinders: all samples display
both the {100} and {300} peaks while S-56 also shows a {200} peak. As the molecular
weight decreases from 46
-kmot
of the
for S-56 to 31.1 ISmot for S-43, the q vector
{100}
peak
moves outwards, as expected. This trend is also consistent with the expected scaling as
a function of molecular weight: the weight ratio (}6j)2 = 1.3 while the d-spacing ratio
-=
1.2.
39
2.7.2
PI containing diblocks
The morphologies for six PI-containing diblocks: 1-12, 1-19, 1-23, 1-25, and 1-44 were also
established using the procedures described above. These samples spanned a range of
volume fraction from 12% to 44% PDMS and displayed morphologies of PDMS spheres
and cylinders, and lamellae.
1-12 and 1-19 are identified as displaying a morphology consisting of PDMS spheres in
a PI matrix; their TEM images show dark spheres on a lighter background. 1-12 displayed
one peak in the SAXS pattern, while 1-19 displayed zero. This is probably due to a lack
of long range order in the samples.
1-23 and 1-25 both show a PDMS cylinder morphology. For 1-23, this is established
both by TEM image which shows both end-on and in plane dark cylinders in a lighter
matrix and the SAXS data which consists of four well-resolved peaks corresponding to
the {100}, {110}. {210}, and {300} diffracting planes. 1-25 can also be assigned a
cylindrical morphology because of its SAXS data: the three {100}, {110}, and {210}
peaks are characteristic of hexagonally packed cylinders. It is surprising that 1-25 has the
larger d-spacing despite it's lower molecular weight (53 nm vs 58 nm); however, this may
be because the sample was unannealed and thus a solvent-swollen state may have been
preserved. No order was displayed in the annealed sample, but perhaps it would have had
a smaller d-spacing if extended annealing were performed.
1-44 is the only PI-containing sample examined which displays a lamellar microstructure. This can be seen by examination of the SAXS pattern, which shows the {100},
{200}, and {300} peaks. The d-spacing for this sample is 65 nm.
62
Chapter 3
Effect of Processing on Morphologies
of PS-PDMS and PI-PDMS Diblocks
3.1
Introduction
Chapter 2 discussed the structures of samples which had undergone a very specific processing treatment. However, knowledge of the true equilibrium morphologies of these
materials is impossible to obtain because it is impossible to determine when samples have
achieved equilibrium. To provide some insight into whether or not the morphologies observed above occur after many different treatments (providing strong evidence for their
equilibrium nature or at least strong metastability) and to investigate the possibility of
using different processing treatments to tune ultimate structure, different casting procedures were studied with and without annealing treatments. In this chapter, only the
higher molecular weight diblocks were examined; the effect of temperature treatments and
chain mobility of the lower molecular weight diblocks is discussed in 4. SAXS was used
to investigate the morphologies observed in each case.
Examination of Table 3.1 suggests that the solvent used during casting has the potential to have a profound effect on the final morphology. It is clear that PDMS has a lower
solubility parameter than either PS or PI and so solvents which are good for PDMS are
less for PS and PI and vice versa. When diblock polymers are dissolved in a solvent, their
equilibrium structure depends on the solvent concentration: solvation dilutes contacts
between unlike species and so lowers the effective x parameter while also increasing the
size of the chains as they pervade more volume. Of course, at low block copolymer concentration the system is a homogeneous solution. With decreasing solvent concentration,
a disorder to order transition occurs where the block copolymer forms a swollen periodic
microdomain structure. If a particular solvent preferentially segregates to the domains
of one of the blocks, then that block pervades a larger volume than it does in the neat
block copolymer solid. Additionally, the presence of a solvent introduces mobility into the
structure and thus lowers the glass transition of the blocks. Combining these two effects, it
is expected that as the solvent evaporates the structure observed after drying is complete
would be that which was present when one of the blocks experienced its glass transition
(i.e. the glass transition temperature reaches room temperature) such that chain mobility
63
Material
PDMS
PS
PI
Toluene
Cyclohexane
Methyl cyclohexane
Octane
Solubility parameter (
15.7
18.3
16.7
18.2
16.8
16.0
15.6
)
Block preference
PS/PI
neutral/PI
PDMS
PDMS
Table 3.1: Solubility parameters for the polymers and solvents studied in this thesis. All
values are taken from [75].
becomes appreciably slowed. If the solvent is preferentially swelling one of the blocks during this "freezing in" of the structure, it is possible that its effective volume fraction may
be larger; for materials with volume fractions near morphology boundaries, there exists
the possibility of locking in a non-equilibrium phase during casting. Annealing raises the
temperature to above the glass transition of all polymers studied here and increases chain
mobility, making structural rearrangement possible and thus increasing the probability of
achieving the morphology which is at equilibrium for the conditions present during this
treatment. However, for strongly segregated and well entangled polymers, the kinetics of
rearrangement may be slow enough to require very long annealing times.
3.2
Sample preparation
To examine the influence of casting solvent (and thus relative swelling of the PDMS
block during casting) and subsequent annealing treatment on PDMS-containing diblocks,
each sample was cast from three (for PS-containing materials) or four (for PI-containing
materials) solvents. These samples were then cut in half and half was examined directly
using SAXS and the other half was annealed. Sample casting, annealing, and SAXS
preparation procedures were identical to those described in Chapter 2 except for the
solvent used during casting. Similarly, SAXS measurements were conducted in an identical
way for consistency.
3.3
Results
Tables 3.2 and 3.3 describe the morphologies observed after casting from different solvents
before and after annealing. Figures 3-2 and 3-1 show the SAXS data. Data is only shown
for samples which were cast from multiple solvents. In many cases there is no effect on
morphology (which is encouraging when assigning an appropriate equilibrium structure);
the absence of higher order peaks indicate that the morphology is less ordered after some
treatments compared to others but does not indicate structural change. Larger effects
64
are found in the d-spacings of the materials studied; in most cases it appears that the as
cast d-spacing can very greatly but that the final d-spacing after annealing is relatively
consistent independent of initial casting results. The latter shows that the annealing
treatment employed is appropriate for allowing structural relaxation and rearrangement to
a morphology which is more stable than any initial one. The former is possibly due to nonequilibrium swelling which results in domains or matrices under tension or compression
after the solvent has been removed but which can rearrange when mobility but not solvent
is reintroduced. Thus, it is fair to say that the protocol used in Chapter 2 is appropriate
for establishing the equilibrium bulk morphology of the PDMS-containing diblocks studied
and that using casting solvent to tune morphology may require different samples (possibly
those having longer chains, which would order at higher solvent concentrations magnifying
differential swelling effects and have reduced mobility both above and below the glass
transition temperature.)
3.4
3.4.1
Discussion
PS-containing samples
Nine PS-containing samples are investigated in this chapter, three of which show morphologies which can be tuned by changing the processing treatment. These three samples
do not display many similarities; they all have molecular weights which are in the middle
of the range studied in this thesis (so they may be large enough to have sufficient entanglements to promote stable morphology formation after solvent evaporation but short
enough to rearrange in kinetically accessible time frames during processing.). All of the
samples investigated here do have two things in common: they only show long range order
under certain conditions (which are unique to each sample), and that each sample mostly
displays consistent d-spacings after annealing regardless of the initial solvent treatment.
S-16 has two scattering patterns which contain enough information to assign for a
morphology to be assigned to the relevant processing treatment: casting without annealing from toluene and casting without annealing from cyclohexane. In the first case, three
peaks which have ratios of the scattering vector of 1:v12:/ are shown and in the second
two peaks which have ratios of the scattering vector of 1:/5 are displayed. The scattering
that occurs from the toluene cast sample fairly unambiguously indicates a spherical morphology; this is the only morphology that exists in this volume fraction range which has
a ratio of the first two peaks of 1:V/ and the next peak q value is V3 times larger than
the initial value, which is also consistent with a spherical morphology. The S-16 sample
which was cast from cyclohexane only displays two peaks which have a ratio of 1:v5.
In this case, it is possible that the v5 peak is suppressed or that the sample displays a
cylindrical morphology; either case indicates a change in morphology, as the intensity of
the higher order scattering peaks depends on the volume fraction of the minority block.
Therefore, the PS matrix becomes less swollen as a more neutral solvent is used and either
smaller spheres or cylinders are formed. There is a large change (~ 25%) in the d-spacing
when cyclohexane is used during casting instead of toluene, indicating that a significant
morphology shift occurs.
65
Sample name
S-16
S-17
S-22
S-30
S-34
S-37
S-43
S-53
S-56
1-12
1-19
1-23
1-25
Molecular weight
(kg/mol)
44.9
41.2
53.5
43.2
36.1
66.1
31.1
45.9
46.1
50.9
73.4
60.7
43.8
4PDMS
Toluene
as cast (annealed)
0.16
0.17
0.22
0.3
0.34
0.37
0.43
0.53
0.56
0.12
0.19
0.23
0.25
S (N/P)
C (C)
N/P (N/P)
N/P (N/P)
C (C)
C (N/P)
N/P (L)
C (L)
L (L)
N/P (N/P)
N/P (N/P)
N/P (C)
C (N/P)
Morphologies as cast from:
Methyl cyclohexane
Cyclohexane
as cast (annealed)
as cast (annealed)
S/C (N/P)
C (C)
N/P (N/P)
N/P (C)
L (C)
C (C)
N/P (L)
N/P (L)
N/P (L)
N/P (N/P)
N/P (N/P)
N/P (N/P)
N/P (C)
N/P (N/P)
N/P(C)
C (C)
N/P (N/P)
N/P (C)
N/P (N/P)
N/P (L)
N/P (L)
N/P (L)
N/P (N/P)
N/P (N/P)
N/P (N/P)
C (C)
Octane
as cast (annea
-
(-)
(-)
(-)
(-)
(-)
- (-)
- (-)
- (-)
- (-)
N/P (N/P
S (S)
C (C)
C (-)
able 3.2: Summary of sample morphologies obtained by SAXS after different casting and annealing treatments for
mples whose morphology was discussed in Chapter 2. The diblocks were cast from the relevant solvent for one week at ro
mperature; half of each sample was then annealed for one week at 150 0 C. Morphologies were determined by the relativ
lues of the positions of the peaks in the scattering spectrum, which are characteristic of different morphologies. S indica
spherical morphology, C indicates a cylindrical morphology, and L indicates a lamellar morphology. - indicates that
periment was not performed and N/P indicates that there were insufficient peaks in the SAXS data to assign a morpholo
#PDMS
S-16
S-17
S-22
S-30
S-34
S-37
Molecular weight
(kg/mol)
44.9
41.2
53.5
43.2
36.1
66.1
0.16
0.17
0.22
0.3
0.34
0.37
Toluene
as cast (annealed)
23 (N/P)
31 (34)
28 (N/P)
N/P (N/P)
35 (37)
43 (66)
S-43
31.1
0.43
N/P (39)
S-53
S-56
1-12
1-19
1-23
1-25
45.9
46.1
50.9
73.4
60.7
43.8
0.53
0.56
0.12
0.19
0.23
0.25
41 (48)
44 (47)
31 (36)
38 (N/P)
N/P (38)
53 (56)
ample name
d spacings (nm) as cast from:
Methyl cyclohexane
Cyclohexane
as cast (annealed) as cast (annealed)
29 (N/P)
28 (34)
N/P (30)
N/P (39)
30 (38)
31 (47)
N/P (N/P)
N/P(35)
36 (36)
31 (39)
N/P (38)
N/P (50)
31 (38)
30 (39)
42
38
37
N/P
N/P
51
(46)
(44)
(34)
(N/P)
(N/P)
(55)
34
36
31
N/P
N/P
52
(45)
(49)
(36)
(N/P)
(N/P)
(55)
Octane
as cast (annea
-
(-)
(-)
(-)
(-)
(-)
(-)
- (-)
- (-)
- (-)
35 (35)
N/P (N/P
47 (47)
52 (-)
ble 3.3: Summary of d-spacing values obtained by SAXS after different casting and annealing treatments for the samp
ose morphology was discussed in Chapter 2. The diblocks were cast from the relevant solvent for one week at ro
mperature; half of each sample was then annealed for one week at 150 0 C. D-spacings are obtained from the relationsh
= 2. In some cases, the ratios of q at which the main peaks appear indicate a morphology change, so the changes in pe
acings are attributable to changes in both the sizes of the domains and the distances between the first planes from wh
attering appears. See Table 3.2 for more information about the latter. - indicates that the experiment was not perform
d N/P indicates that there were insufficient peaks in the SAXS data to assign a morphology.
S-17 scattering as a function of processing conditions
S-16 scattering as a function of processing conditions
100
Methyl cyclohexane; annealed
110
Methyl cyclohexane; annealed
Methyl cyclohexane; as cast
Methyl cyclohexane; as cast
100
Cyclohexane; annealed
110
Cyclohexane; annealed
c
100
I.
Cyclohexane; as cast
110
100
110
Cyclohexane; as cast
c
100
110
200
Toluene; annealed
Toluene; annealed
100
100
110
110
111
Toluene; as cast
Toluene; as cast
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
igure 3-1: Scattering from PS-containing samples studied in this thesis after different processing treatments. The diblo
ere cast from the relevant solvent for one week at room temperature; half of each sample was then annealed for one wee
50 0 C. See Tables 3.2 and 3.3 for more information.
....
S-22 scattering as a function of processing conditions
I
I
I
S-30 scattering as a function of processing conditions
I
I
100
Methyl cyclohexane; annealed
110
200
Methyl cyclohexane; annealed
Methyl cyclohexane; as cast
100
100
110
110
Methyl cyclohexane; as cast
210
Cyclohexane; annealed
i
$
I
Cyclohexene; annealed
Cyclohexane; as cast
C
Cyclohexane; as cast
Toluene; annealed
Toluene; annealed
Toluene; as cast
Toluene; as cast
-
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
A
V'K
0.6
0.7
ontinued Figure 3-1: Scattering from PS-containing samples studied in this thesis after different processing treatments.
blocks were cast from the relevant solvent for one week at room temperature; half of each sample was then annealed for
eek at 150 0 C. See Tables 3.2 and 3.3 for more information.
S-37 scattering as a function of processing conditions
S-34 scattering as a function of processing conditions
100
110
210
Methyl cyclohexane; annealed
Methyl cyclohexane; annealed
Methyl cyclohexane; as cast
Methyl cyclohexane; as cast
100
100
110
110
ii
210
210
*1
1
Cyclohexane; annealed
Cyclohexane; annealed
I
100
200
100
110
Cyclohexane; as cast
Cyclohexane; as cast
100
210
110
210
Toluene; annealed
Toluene; annealed
100
100
110
110
\1
Toluene; as cast
Toluene; as cast
..Ab
III
0
N1 -
I
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
ontinued Figure 3-1: Scattering from PS-containing samples studied in this thesis after different processing treatments.
iblocks were cast from the relevant solvent for one week at room temperature; half of each sample was then annealed for
eek at 150 0 C. See Tables 3.2 and 3.3 for more information.
S-53 scattering as a function of processing conditions
S-43 scattering as a function of processing conditions
100
100
300
300
Methyl cyclohexane; annealed
Methyl cyclohexane; annealed
100
Methyl cyclohexane; as cast
Methyl cyclohexane; as cast
100
300
i
300
Cyclohexane; annealed
Cyclohexane; annealed
I
I
I
I
Cyclohexane; as cast
Cyclohexane; as cast
100
100
300
300
Toluene; annealed
Toluene; annealed
100
110
200
210
Toluene; as cast
Toluene; as cast
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
ontinued Figure 3-1: Scattering from PS-containing samples studied in this thesis after different processing treatments. T
blocks were cast from the relevant solvent for one week at room temperature; half of each sample was then annealed for
eek at 150 0 C. See Tables 3.2 and 3.3 for more information.
S-56 scattering as a function of processing conditions
100
400
200
300
Methyl cyclohexane; annealed
Methyl cyclohexane; as cast
100
200
i
300
I.
I
Cyclohexane; annealed
Cyclohexans; as cast
100
300
200
Toluene; annealed
100
200
Toluene; as cast
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
ontinued Figure 3-1: Scattering from PS-containing samples studied in this thesis after different processing treatments.
blocks were cast from the relevant solvent for one week at room temperature; half of each sample was then annealed for
eek at 150 0 C. See Tables 3.2 and 3.3 for more information.
1-19scattering as a function of processing conditions
1-12 scattering as a function of processing conditions
I
II
Octane; annealed
Octane; annealed
Octane; as cast
Octane; as cast
Methyl cyclohexane; annealed
Methyl cyclohexane; annealed
Methyl cyclohexane; as cast
I
Cyclohexane; annealed
IL -
0
I
aI
II
0.1
0.2
0.3
ii.
Methyl cyclohexane; as cast
I.
I
Cyclohexane; annealed
Cyclohexane; as cast
Cyclohexane; as cast
Toluene; annealed
Toluene; annealed
Toluene; as cast
Toluene; as cast
I
0.4
q (1/nm)
I
I
I
0.5
-
-
ai
0.6
0.7
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
igure 3-2: Scattering from PI-containing samples studied in this thesis after different processing treatments. The diblo
ere cast from the relevant solvent for one week at room temperature; half of each sample was then annealed for one wee
50 0 C. See Tables 3.2 and 3.3 for more information.
1-25 scattering as a function of processing conditions
1-23 scattering as a function of processing conditions
100
100
110
210
Octane; annealed
100
Octane; as cast
100
110
210
Octane; as cast
Methyl cyclohexane; annealed
100
Methyl cyclohexane; annealed
210
Methyl cyclohexane; as cast
100
Methyl cyclohexane; as cast
*1
Cyclohexane; annealed
I.
I
210
Cyclohexane; annealed
Cyclohexane; as cast
Cyclohexane; as cast
100
110
Toluene; annealed
210
Toluene; annealed
100
110
210
Toluene; as cast
Toluene; as cast
t
I
0
0.1
0.2
0.3
0.4
q (1/nm)
0.5
0.6
0.7
0
I
0.1
I
0.2
j
0.3
I
0.4
I
0.5
I
I
0.6
0.7
q (1/nm)
ontinued Figure 3-2: Scattering from PI-containing samples studied in this thesis after different processing treatments.
blocks were cast from the relevant solvent for one week at room temperature; half of each sample was then annealed for
eek at 150 0 C. See Tables 3.2 and 3.3 for more information.
S-17 displayed sufficient SAXS data to assign a morphology in five different processing
cases: casting from toluene with and without annealing, casting from cyclohexane with
and without annealing, and casting from methyl cyclohexane with annealing. In all of
these cases, there is remarkable similarity between the SAXS data obtained: all samples
have two peaks with a scattering vector ratio of 1:vr5, and the sample cast from toluene
and then annealed displays an additional peak at a q value of twice the principal peak.
This is strong evidence for the microdomain morphology of PDMS cylinders inside of a
PS matrix. This is undoubtedly the case for the sample which was cast from toluene
and then followed by an anneal because of the additional peak. The other scattering
patterns could arise from a cylindrical morphology displaying only two peaks, but this
is unlikely because the q values at which the peaks occur are similar across samples
and because a spherical microdomain morphology would be most favored after a toluene
casting (where the scattering data unambiguously points to a cylindrical structure.) The
d-spacings present after annealing are almost identical (34 nm in two cases and 35 nm
in the third) and fairly similar before annealing (31 nm and 28 nm); however, there is
a significant increase in the d-spacing after annealing. This indicates that one of the
domains (probably PDMS) is swollen after the casting treatment but that this artifact
can be eliminated by appropriate annealing.
S-22 only exhibits ordering after casting from methyl cyclohexane; the morphology
that forms in this case is stable with respect to annealing. In both cases, the first two
peaks show a ratio of 1:v/5, providing strong evidence for a cylindrical morphology at
this volume fraction and segregation strength. The sample which had also been annealed
shows a third peak which is twice as large as the principal scattering vector, providing even
stronger evidence for this morphology assignment. With the exception of the third peak,
both scattering patterns look qualitatively similar which is confirmed by their identical
d spacings (36 nm.) The other two samples which display a prominent peak are those
which have been cast from cyclohexane and annealed and cast from toluene with no
further treatment. Both of these samples have SAXS patterns which look qualitatively
similar except that the large broad peak is shifted to a lower q value for the toluene cast
sample. It is difficult to conclusively assign d-spacings to these patterns, as assuming that
these peaks arise from the {100} peak gives relatively small d-spacings (30 nm and 28
nm) while assuming that they are from the {110} peak of a cylindrical sample gives the
large d-spacings of 52 nm and 48 nm (it is unlikely that this would display a different
morphology based upon its volume fraction and the morphologies of the nearby samples
on the phase diagram.) The former seems more reasonable because the values are closer
to those from the methyl cyclohexane casting conditions, but in this case it cannot be
determined unambiguously.
S-30 has one sample with a SAXS pattern which contains enough peaks to definitely
assign it a morphology. This is the sample which was cast from cyclohexane and then
annealed. It contains three peaks with q values which have a ratio of 1:V35:v/7, indicating a
cylindrical morphology with a d-spacing of 39 nm. After casting from methyl cyclohexane
and then performing an annealing treatment, only one prominent peak is displayed, but
it does have the same d-spacing as the prior sample, indicating that the morphologies are
probably similar (although the latter has much less long range order.) The sample which
75
was cast from methyl cyclohexane but left unnanealed also contained only one prominent
peak, the d-spacing in this case is the substantially smaller 31 nm. Again, it appears that
methyl cyclohexane swells one of the domains (probably PDMS) during casting but that
this effect is erased upon annealing.
S-34 is one of the three samples that does exhibit a change in morphology with processing treatment. While in four cases the morphology displayed is unambiguously cylinders,
S-34 has a lamellar morphology when cast from cyclohexane and left unannealed. This
can be seen by the ratio in the q values of the two scattering peaks present; it is 1:2.
While it is possible that this is indicative of a cylindrical morphology with the v5 peak
suppressed, the shape and locations of the peaks that are present are dramatically different from the other cases displaying cylinders (including those from this same sample after
annealing.) The other four cases (annealed and not after casting from toluene, annealed
after casting from cyclohexane, annealed after casting from methyl cyclohexane) all have
the ratio of 1:v/5 for the first two scattering peaks, indicative of a cylindrical morphology.
All of these except for the unannealed sample cast from toluene also have a v? peak,
strengthening the validity of this assignment. Once again the d-spacings are all quite similar after annealing (37-38 nm) and the unannealed toluene cast sample has a d-spacing
which is only slightly smaller (35 nm). The sample which was cast from cyclohexane and
not annealed has a much smaller d-spacing of 30 nm, indicating that the morphology
present here is substantially different from that observed in the other cases. It is unexpected that the neutral solvents induces a different morphology formation from the two
solvents which are preferential for opposite blocks; to induce a transition from PDMS
cylinders to lamellae, the PDMS would have to be swollen which would be more likely
to occur in methyl cyclohexane than in cyclohexane. This may be due to differences in
volatility between the solvents: if the methyl cyclohexane evaporated more slowly, there
would be have been more time for the chains to rearrange while there was still solvent in
the system and thus a metastable structure would be more challenging to lock in.
S-37 is another sample whose SAXS patterns change greatly when the processing
treatment is varied. Although all of the indexable patterns are consistent with a cylindrical
morphology, even a quick glance at the data shows that the peak positions and shapes are
inconsistent across differing preparation procedures. Four morphologies can be assigned
without ambiguity: three from the SAXS data and one from the TEM data (see chapter
2 for more information about indexing the SAXS data from S-37 which has been cast
from toluene and subsequently annealed). The unannealed toluene cast sample and the
unannealed cyclohexane cast sample both have two peaks in their scattering patterns
with a q value ratio of 1:v/5. When the cyclohexane cast sample is annealed, the V7_ peak
develops, providing confirmation of the cylindrical morphology assignment. The two
samples which were cast from methyl cyclohexane have insufficient peaks for morphology
determination. The d-spacings for this sample vary widely (more than 100%!), showing
that the morphology is not very robust to processing treatment. However, across all cases
the d-spacing does increase with annealing.
In contrast to S-37, S-43 displays remarkably similar SAXS data across the processing
conditions examined here. In all cases, the unannealed samples are insufficiently ordered
to provide useful SAXS data for morphology determination but the annealed samples
76
are indicative of a lamellar morphology with two peaks with a q value ratio of 1:3 and
a d-spacing of 38-39 nm. The samples which were cast from cyclohexane and methyl
cyclohexane but were not annealed also are remarkably similar: they have a single peak
in their diffraction pattern and d-spacings of 31 and 30 nm, respectively. Here, again,
annealing promotes the formation of larger periodicities.
S-53, however, is another sample which displays differing morphologies depending
upon the processing treatment. Lamellae are present after annealing for every sample,
but cylinders are present if the sample is cast from toluene but not annealed. The lamellae
display only the 1 and the 3 peak and have d-spacings ranging between 45 and 48 nm.
The sample with the cylindrical morphology has four peaks whose scattering vectors have
the ratio 1:v/5:2:,F and a d-spacing of 41 nm. The sample cast from cyclohexane but
not annealed has a similar d-spacing of 42 nm (making it possible that it also shows a
cylindrical morphology), while that cast from methyl cyclohexane and left unannealed
has the substantially smaller d-spacing of 34 nm. It is likely that the cylinders which
are present are made up of PDMS because the cylinders are present when the solvent is
neutral or PS-preferential, indicating that PS is being swollen. As S-53 is heavier than
S-43, it is possible that it is easier to trap S-53 in a metastable state and explaining the
presence of a trapped morphology for S-53 but not S-43.
S-56 unambiguously shows a lamellar morphology for all processing treatments except
casting from cyclohexane without subsequent annealing and casting from methyl cyclohexane without subsequent annealing. This is given by the ratios of the scattering peaks,
which are 1:2 in the toluene cast but annealed sample, 1:2:3 in the toluene cast and annealed sample and the cyclohexane cast and annealed sample, and 1:2:3:4 in the methyl
cyclohexane cast and annealed sample. In all cases, annealing increases the long range
order in S-56, which is reflected in the increased presence of higher order peaks. The
d-spacings after annealing are also similar, ranging from 44 nm to 49 nm and increasing
from the unannealed value in all cases. The unannealed samples have d-spacings of 44 nm,
38 nm, and 36 nm for the samples cast from toluene, cyclohexane, and methyl cyclohexane, respectively. The decreasing d-spacing with increasing preference of the solvent for
the PDMS block means that the smaller d-spacings arise when the PS undergoes a glass
transition with more solvent in the PDMS block. The PDMS block can then contract
farther as the solvent evaporates, as it is rubbery throughout the casting and annealing
processes.
3.4.2
PI-containing samples
Four PI-containing samples are discussed here. It is surprising that any of these samples
should display sensitivity to casting solvent and annealing: both PI and PDMS have glass
transitions substantially below room temperature so it is expected that any rearrangement
from a metastable to a stable morphology should occur at room temperature. However,
while no morphology transitions are observed, it is clear that certain processing conditions
do a much better job at promoting long range order than others. These are discussed
below.
For no processing case studied here does 1-12 exhibit sufficient long range order to have
77
a SAXS pattern from which a morphology can be inferred. In all cases, the d-spacings
observed from the one scattering peak are relatively similar (ranging between 31 and 37
nm), with an even closer correspondence between the peaks coming from samples which
have been annealed (ranging between 34 and 36 nm.)
In the case of 1-19, processing using octane promotes the formation of spheres. The
ratio between the peak spacings (1.75) is not consistent with spherical spacing, but is
consistent with scattering due to poorly ordered monodisperse spherical domains [76].
In this case, scattering is expected when qR=5.76 and qR=9.10, which have a ratio of
1.58. This information can also be used to compute the sphere radius, which is 28.8 nm.
Measuring the spheres in chapter 2, a value of - 40 nm is obtained for the diameter. This
means that either substantial deformation occurred during microtoming while the toluene
cast and annealed sample was prepared for TEM examination or that the casting solvent
has a profound effect on ultimate d spacing.
1-23 is another sample which only displays long range order under certain processing
conditions. When cast from toluene and subsequently annealed, it displays good long
range order with four peaks in its SAXS pattern with a q ratio of 1:V'5:V7:3, indicating
that the sample has a cylindrical microdomain structure. It also displays appreciable long
range order after casting from octane (with and without subsequent annealing), showing
peaks with a q ratio of 1:V35. Since the cylindrical morphology appears both when the
solvent is preferential to PI and when it is preferential to PDMS, it is clear that cylindrical
microdomains comprise the equilibrium morphology for this material. The d-spacings are
quite different depending on the solvent used (38 nm for toluene vs 47 for octane); octane
should swell the cylinders but since both blocks should be mobile it is unclear why there
is this large discrepancy.
1-25 also displays a cylindrical morphology for several different processing conditions.
It has sufficient peaks in its SAXS pattern to be interpretable for all cases except for
that cast from toluene and subsequently annealed and that cast from cyclohexane and
not subsequently annealed. The other six cases contain scattering patterns with both
the {100} and {210} peaks, in addition to the {110} peak for the toluene cast but not
annealed sample. These patterns all indicate a cylindrical morphology, meaning that
is the equilibrium microdomain structure. The d-spacings here follow the same trends
observed above: the d-spacings after annealing are relatively constant (55-56 nm) and
slightly larger than the unannealed d-spacings (51-53 nm.) In this case, the unannealed
s-spacings are also relatively close together, indicating that casting solvent has little effect
on ultimate morphology in this case.
78
Chapter 4
Morphologies of low molecular
weight PS-PDMS and PI-PDMS
diblocks as a function of temperature
4.1
Overview
The study of the morphologies of weakly segregated PDMS-containing diblocks provides
a complement to investigations into the strongly segregated PDMS-containing diblock
morphologies studied in chapters 2 and 3 because it allows for both insight into the morphologies that are formed in a different segregation regime and an increased understanding
of factors that affect morphology development from either the disordered or metastable
equilibrium structures present during thermal processing. Therefore, the structures of
weakly segregated PDMS-containing diblocks at varying temperatures were investigated
using SAXS at Brookhaven National Laboratory's X27C beamline. Due to the high flux
of synchrotron x-ray sources, it was possible to record sufficient scattering intensity for
forming an image of the sample's diffraction in less than one minute. This capability was
coupled with the depression of the glass transition of the PS blocks and enhanced mobility
of the PI and PDMS blocks due to the short chain lengths allowed for structural rearrangements on readily observable laboratory timescales. The following sections describe
this work and the results that were obtained.
4.2
4.2.1
Experimental procedures
Sample descriptions
Two PS-PDMS diblocks and one PI-PDMS diblock were investigated. These materials
were synthesized at the laboratory of Professor Apostolos Avgeropoulos at the University
of Ioannina in Ioannina, Greece. Their chain structural characteristics are described in
4.1 and the physical properties of the component polymers are described in 4.2. The
morphological characterization data for S-64LMW is given in chapters 2 and 3. The
anticipated values of XN as a function of temperature are shown in 4-1. All of these
79
XN as a function of temperature
30
Annealing temperature
25
20
z
Anticipated ODT
15
S-64LMW
10
1-39LMW
Anticipated ODT
Anticipated ODT
S-45LMW
5
0
0
50
100
150
Temperature (Celsius)
200
250
300
Figure 4-1: Calculated values of XN(T) at different temperatures for the three samples
studied in this section. Values of x were computed based upon the work in [74]. The
annealing temperature and anticipated ODTs for each sample are also shown; anticipated
ODTs are based upon the figure in [15].
materials have a composition where the volume fraction of PDMS is relatively close to
0.5 and the value of xN is close to the value at the ODT (~-10-15) at room temperature,
indicating that these samples feature some of the smallest molecular weights that can be
present in phase-separated PDMS-containing materials.
4.2.2
DSC procedures
DSC experiments were performed on diblocks with the molecular weight and volume fractions as listed in Table 4.1. These samples were cast from toluene and annealed at 150 0 C
using the protocol for sample preparation that was described in Chapter 2. During the
experiments, 4-6 mg of sample were heated from room temperature to 180 0 C; held there
for three minutes; cooled to -80 0 C; held there for three minutes; and then heated to room
temperature. In all cases, the rate of temperature change was 50 C per minute. A reference pan also underwent this same procedure and the heat flow to it was subtracted from
the heat flow to the pans containing the samples. These parameters were chosen in order
to both mimic the experimental conditions present during the first set of SAXS studies
80
Sample
name
S-64LMW
S-45LMW
I-39LMW
Molecular weight of
non-PDMS block ( )
4.3
2.3
6.6
Molecular weight of
PDMS block (-)k
2.1
1.9
4.2
PDMS
0.64
0.45
0.39
Estimated xN
at ODT
13
11
12
Estimated ODT
(EC)
180
85
220
able 4.1: The composition and anticipated ODT's of the low molecular weight diblocks studied in this section. The molecu
eights were determined by GPC and the volume fractions were determined by NMR. Estimated values of XN at the OD
ere estimated based on the figures in [15] and the value of X was determined from the equations presented in [74].
Polymer
Entanglement molecular
PS
PI
PDMS
weight (kg/mol)
13.3
5.4
12.3
tran
Radius of gyration normalized by the 0Glass
square root of sample molecular weight A)
0.659 (PS-PDMS1 = 43 A) (PS-PDMS2 = 32 A)
0.971 (PI-PDMS1 = 64 A)
0.676 (PS-PDMS1 = 31 A) (PS-PDMS2 = 30 A) (PI-PDMS1 = 44
A)
temperatur
100
-70
-123
able 4.2: Physical properties of the chains that form the diblocks investigated in this thesis. The entanglement molecu
eights and radii of gyration are taken from [77]; note that these assume infinitely long chains and so may overestimate
dii of gyration. The first number given for radius of gyration is the ratio of the radius of gyration to the square root of sam
olecular weight; the latter are the calculated values for the relevant samples. The glass transition data is taken from [
e measured values were performed on longer chains and so may overestimate the values of Tg.
described below (heating up to 180 0 C), to provide information about transitions during
both heating and cooling, and to provide data on possible low temperature transitions.
4.2.3
SAXS procedures
Three different trips were made to Brookhaven to study the scattering of these diblocks
at elevated temperatures and different experimental procedures were used based upon an
evolving understanding of the morphology changes the samples underwent when subject
to x-ray irradiation and temperature changes. In total, ten different experiments were
performed. The different procedures are summarized below and by sample in table form
in Table 4.4.
The first procedure was used for S-64LMW-A, S-64LMW-B, and S-64LMW-C; S45LMW-A, S-45LMW-B, and S-45-LMW-C; and I-39LMW-A and I-39LMW-B. The second was used for S-45LMW-D and I-39LMW-C. Both procedures involved initially preparing the samples, sandwiching them between Kapton tape, and placing them inside an
Instec HCS600V hotstage. The experiments were performed in air, x-ray wavelength was
set at 1.371 A and silver behenate powder was used to calibrate the sample to detector
distance.
The first procedure involved continuously performing three minute exposures for each
sample as they completed the thermal programs described in Figures 4-2, 4-3 and 4-4.
Several different strategies were employed to prepare the samples before performing the
dynamic temperature experiments. S-64LMW-A and S-45LMW-A were first cast from
toluene, annealed at 150 0 C for one week, and then annealed at temperatures slightly above
the temperature where they began to macroscopically soften but did not flow (at approximately 80 0 C and 50 0 C, respectively.) S-64LMW-B and S-45LMW-B were also cast from
toluene and then annealed for one week at 150 0 C but did not receive the last pretreatment. S-64LMW-C and S-45LMW-C were taken from uncast material. I-LMW39-A was
cast from toluene and annealed for one week at 150 0 C; I-LMW39-B consisted of uncast
material. In all cases, the cooling from the annealing temperature to room temperature
occurred in air. As can be seen in a later section, the substantially mobile short chains
at room temperature allow for structural rearrangements on the time scale of hours; the
highest molecular weight diblock with a glassy block (S-64LMW samples) attained identical equilibrium morphologies after different cooling programs. Possible differences in
peak sharpness at the initially observed temperature may be due to beam alignment with
particularly organized or disorganized regions of sample; enhanced ordering after cooling
may be due to preferential alignment on the Kapton tape surface. I-39LMW-A, which has
two blocks with glass transition temperature below room temperature, showed structural
rearrangement on the time scale of minutes during cooling.
The second procedure, described in Table 4.3, consisted of a multistep process described below for temperatures anticipated to be below the ODT.
In this case, the high temperature was selected to be above the anticipated ODT based
upon prior work (800C for Sample II-D and 50 0 C for Sample III-C). For temperatures
above this anticipated ODT, only steps 6-9 were performed at each temperature. Each
exposure was only performed for 40 seconds for these samples in order to minimize sample
83
Temperature programs for S-64LMW diblocks
200
180
S-64LMW-B
S-64LMW-
S-64LMW-C-
160
140
0
a)
----
120 100
80
60
40
I
20
0
100
200
300
400
Time (minutes)
500
600
700
Figure 4-2: The temperature programs for the S-64LMW samples described in Table 4.4.
The temperature profiles used for S-64LMW-A and B were selected in order to observe
scattering at a wide and complementary range of temperatures and investigate whether
or not the scattering was dependent upon thermal history. The temperature profile for
S-64LMW-C was selected in order to observe scattering at many different and closely
spaced temperatures and to detect where morphological transitions in this range occur
with fine precision.
84
Temperature program for S-45LMW diblocks
120
S-45LMW-B
110
100
90
80
F
70
S-45LMW-A
(D
a0
E
6
6
S-45LMW-C
O
50
40
30
20
0
100
200
300
400
500
Time (minutes)
600
700
800
900
Figure 4-3: The temperature programs for the S-45LMW samples described in Table
4.4. These temperature profiles were chosen in order to sample a wide range of temperatures at which morphology transitions could occur and to provide comparative data for
morphology transitions upon heating and cooling.
85
Temperature program for
I-39LMW diblocks
70
1-39LMW-A
60
50
-39LMW-B
I40
0
-
E
0
-
--
20
10
0
0
100
200
300
400
500
600
Time (minutes)
700
800
900
1000
Figure 4-4: The temperature programs for the I-39LMW samples described in Table
4.4. The two temperature profiles were selected to investigate structure in I-39LMW at
temperatures both above and below room temperature and to investigate morphology
transitions as a function of temperature, thermal history (i.e. heating vs. cooling), and
time.
86
Step
Step
Step
Step
Step
Step
Step
Step
Step
Step
1
2
3
4
5
6
7
8
9
10
Perform initial exposure at 25 0 C
Heat to highest temperature(TH)
Perform exposure
Anneal at TH for 40 minutes
Perform exposure
Cool to temperature of interest
Perform exposure
Anneal at temperature of interest for 40 minutes
Perform exposure
Repeat steps 2-9 for all temperatures of interest.
Table 4.3: The second procedure used in SAXS studies at BNL.
exposure to the x-ray beam.
4.3
DSC results
The raw DSC data is shown in Figure 4-5. By convention, endotherms are shown as
negative peaks. It is apparent that both S-64LMW and S-45LMW display endothermic
peaks (from approximately 500 C - 70 0 C and 30 0 C - 50 0 C, respectively) during the heating
runs, indicating a phase transition. Because PS and PI should all be amorphous both
above and below these temperatures, and PDMS should be amorphous in this range (it
has a melting temperature of appromixately -50 0 C) it is likely that this is related to the
ordering of phase separated domains or due to depressed glass transitions in the PS block
due to short chain lengths which allows for increased mixing and diffusion. According
to [78], the ODT should be a first-order phase transition, so it is not unexpected that it
may be associated with a release of heat. The lack of a measured transition for I-39LMW
may be due to the decreased interfacial energy between PI and PDMS chains compared
to PS and PDMS chains so that less heat is required to break the ordered structure; the
amount required may be too small to be observable. Or, it may be due to the presence
of an ODT at a higher temperature than those investigated here or the rubbery PI and
PDMS may be able to mix and diffuse sufficiently at room temperature so that there is
no one temperature (i.e. the glass transition) above which mixing can occur but below
which the structure may be relatively immobile.
While endothermic peaks signifying a first-order phase transition are present, two
expected features of the DSC curves are absent. These include the crystallization of
PDMS at approximately -50 0 C and the glass transition of polystyrene in between room
temperature and the ODT. The glass transition of polystyrene may be suppressed due to
the combination of its short chain length and the ODT. Its short chain length allows for
rearrangements below its nominal glass transition at any given chain length (see Section
4.4.3) and thus does not prevent the occurrance of an ODT due to limited mobility while
mixing with PDMS above the ODT plasticizes the PS so that it is no longer glassy.
The endothermic peak may also arise due to the glass transition as described above and
87
Prior preparation procedure
S-64LMW-A
Continuous x-ray exposure
during x-ray data collection?
Yes
S-64LMW-B
Yes
1) Cast from toluene for one week at 25 0 C
2) Anneal at 150 0 C for one week
S-64LMW-C
S-45LMW-A
Yes
Yes
Precipitated from reactor
1) Cast from toluene for one week at 250 C
2) Anneal at 150 0 C for one week
3) Anneal at 50 0 C for one day
S-45LMW-B
Yes
1) Cast from toluene for one week at 25 0 C
2) Anneal at 150 0 C for one week
S-45LMW-C
Yes
Precipitated from reactor
S-45LMW-D
I-39LMW-A
No
Yes
Precipitated from reactor
1) Cast from toluene for one week at 25 0 C
2) Anneal at 150 0 C for one week
I-39LMW-B
Yes
Precipitated from reactor
I-39LMW-C
No
Precipitated from reactor
Sample Name
1) Cast from toluene for one week at 25 0C
2) Anneal at 150 0 C for one week
3) Anneal at 80 0 C for one day
Table 4.4: Summary of different sample preparation procedures used during this experiment. The initial scattering pattern displayed at 25 0 C was consistent across prior
preparation methods. The thermal programs for samples receiving continuous x-ray exposure can be found in Figures 4-2, 4-3 and 4-4 while those for the samples not receiving
continuous x-ray exposure can be found in Table 4.3.
88
thus mask the smaller change that would be present due to a glass transition. The
crystallization of PDMS may be suppressed due to the small domain sizes (amorphous
PDMS may be able to pack better into smaller areas or may have a lower surface tension
with PS) or possible intermixing of PS into the domains due to weak segregation which
disrupts packing.
4.4
SAXS results
This section contains results obtained by following the two different protocols outlined
above. The results from the second protocol will be discussed first, as they are cleaner
and a better representation of the equilibrium morphologies of the diblocks investigated
as a function of temperature. First, a little-known fact is demonstrated in Figure 4-6:
the PDMS-containing samples studied here are quite beam-sensitive and have structures
which are strongly affected by x-ray dose. In the case shown below, a sample was heated
from room temperature to 90 0 C, quenched to 60 0 C, and then quenched to 250 C while being
continuously exposed to x-rays (except when the shutter was closed briefly in between
images.) The total time exposed to the x-ray beam was approximately one hour. Then,
the sample was translated slightly such that the x-ray beam illuminated a new area of
the sample and re-exposed; a completely different (and representative of the sample's
expected morphology based upon initial data) scattering image was revealed. This means
that the scattering measured using the first protocol was dependent on the temperature
of the material when the images were taken, the thermal history of the sample, and the
cumulative x-ray dose the sample had received before that point in time. Because the
latter was not controlled and is less technologically relevant (because samples are almost
never exposed to high intensity x-rays while undergoing processing), the data obtained
using this procedure is very interesting but less useful to other scientists. It is also more
difficult to compare to other experiments in this work.
Clues to determining the factors responsible for the producing the differences in scattering between regions which have and have not been exposed to the x-ray beam for an
extended period of time can be found in [79]; here, the structures of blends of PEOPDMS diblocks and D4 (the monomer from which PDMS is polymerized) were studied
using SAXS. A figure from this paper which shows scattering data from blends of D4
and PEO-PDMS is reproduced below. A comparison with scattering data from samples
S-64LMW-A and S-64LMW-B in this work (see Figure 4-8) shows a very interesting correspondence. It appears that the x-ray beam may depolymerize some of the PDMS to form
D 4 ; this may be accompanied by some cross-linking which makes the old structure unrecoverable upon cooling, or the new structure formed by a D4 and diblock blend may be
stable across a range of elevated and room temperature. The presence of what appears to
be two superimposed scattering patterns for these samples examined using the initial protocol (see later sections for more details) might be due to the coexistence of two mixtures
in thermodynamic equilibrium or partial conversion of the prior equilibrium morphology
(which was locked in due to x-ray crosslinking) to the new equilibrium morphology.
89
4.4.1
Minimal radiation exposure results
The SAXS results obtained for samples S-45LMW-D and I-39LMW-C are shown in Figure
4-9. It is clear that for the temperatures examined, there is little change in the structure
as observed by scattering for S-45LMW-D and loss of a higher-order peak between 90 0 C
and 130 0 C for I-39LMW-C. There is no indication that either sample goes through the
ODT; at the ODT, the scattering peak should broaden appreciably and the intensity
should dramatically decay. The decreasing intensity of the secondary peak in I-39LMW-C
could indicate a transition between a strongly ordered morphology and one that is weakly
ordered, meaning that the ODT is not too much higher than the temperatures investigated
here. The subtle changes in peak location for S-45LMW-D and changing intensity of the
secondary peak in I-39LMW-C indicate that structural rearrangements are not limited
by kinetic factors in this case and that the scattering patterns are representative of the
actual structures.
The representative character of the scattering from samples after relatively short annealing times at the temperatures of interest is important from a technological perspective. This is because samples are often processed through several steps and allowed to
rest for undetermined amounts of time before their morphology is examined. Rapid kinetic change means that treatment steps may not be responsible for the final effect that
is measured and slow kinetic change means that structures that are determined cannot
be reliably predicted to exist for arbitrary lengths of time. Therefore, technological applications which rely on block copolymer structures require knowledge of the predictability
of morphological change at room temperature.
Any structural rearrangement at laboratory temperatures, means that the sample has
substantial mobility despite the fact that polystyrene at this molecular weight should be
a glass at room temperature. The increased mobility could be due to the plasticization of
PS by PDMS while the morphology is disordered and/or the fact that the PS chains are
much shorter than their entanglement molecular weight of 13.3 1 and the PDMS chains
are shorter than their entanglement molecular weight of 12.3 6, both determined in [77].
The lack of entanglements means that structural rearrangements occur much more quickly
because the chains can relax due to Rouse dynamics instead of diffusing through a network
of entanglements. This means that any processing steps which purposefully change the
displayed morphologies of these materials must be preserved through some means other
than the PS glass transition if it is desired for these non-equilibrium structures to persist
over laboratory or product time scales.
4.4.2
Initial protocol heating results
The last scattering patterns measured during the heating cycles of the first protocol are
discussed in this section . For these experiments, all seven samples studied displayed a
predictable change in peak character as the temperature increased: an initially sharp peak
at room temperature broadened asymmetrically (only towards lower q with a sharp dropoff
in intensity at higher q) as the temperature increased resulting in a final high-temperature
broad peak at a significantly lower q than the original peak. The asymmetric broadening
is characterized by peaks which are negatively skewed (the maximum in intensity occurs
90
at a higher q than the mean q of the peak); in some cases, it appears as if two peaks can
be resolved in one scattering pattern (one at lower q which is growing and one at higher
q which is shrinking). The low q peak may be due to a blend between depolymerized D 3
and the diblock; the D3 may swell the PDMS, increasing its effective volume fraction and
the domain spacing and causing scattering at a lower value of q. The low temperature,
high q peak occurs due to scattering from ordered structures in the phase separated state
as described in section 1.3.
Figure 4-10 shows the integrated intensity obtained from the last scattering patterns
taken at selected temperatures during heating runs of samples S-64LMW-A, S-64LMW-B,
and S-64LMW-C. From the data shown in this figure, it is clear that while the basic trend
described above is consistent across all three samples, the actual scattering patterns vary
greatly depending upon thermal history. Overall, it is possible to generalize that from
approximately 80 0 C to 165 0 C, the high q scattering peak is replaced with a low q. Figure
4-8 (described initially above) shows several scattering profiles as recorded on the CCD
camera for data whose character is not fully described by integrated intensity curves. All
of the patterns shown display increasing texture. For S-64LMW-A, preferred orientation
begins to develop at 80 0 C, increases at 105 0 C and 120 0 C, and transitions into many large
0
grains (indicated by the speckled diffraction peaks in the image) at 150 C. In all of these
images, the sharp high q boundary of the high q peak and the gradual development of a low
q peak as temperature is raised is evident. For sample S-64LMW-B, four-fold symmetry
0
0
begins to develop at 90 0 C which transitions to six-fold symmetry at 110 C and 125 C.
At 120 0 C and 165 0 C, speckles begin to develop in sample S-64LMW-B, again indicating
the presence of large and highly-ordered grains with random alignment around the xray beam. The development of a low q peak and decrease in intensity of the high q peak
with increasing temperature can also be seen in these samples. S-64LMW-C, all S-45LMW
samples, and all I-39LMW samples do not display the textures shown in figure 4-8 (instead
displaying cylindrically symmetric rings around the beam), and so this data is only shown
for S-64LMW-A and S-64LMW-B. The development of orientation and speckles may be
due to the broad nature of the transition due to a gradual cumulative effect of x-ray beam
dose: ordered grains transform piecewise from an ordered morphology to a disordered
one; scattering vectors disappear at one orientation from the higher q peak to reappear
at the lower q peak (see (d) and (f) in 4-8). The combination in S-64LMW of longer
chains than those in S-45LMW and more hindered motion than I-39LMW causes slower
morphology rearrangements and thus slower transitions between morphologies, allowing
for observation of this phenomenon. The lack of thermal pretreatment for S-64LMW-C
means that fewer large grains were formed to begin with, making this effect essentially
impossible to detect.
The scattering patterns of S-45LMW-A, B, and C show a similar trend with temperature as those of S-64LMW-A, B, and C. The scattering intensities displayed as a function
of q and temperature are shown in figure 4-11 In this case, the lower total molecular
weight reduces XN and thus the transition temperature for the decay in ordered structure
scattering to modified scattering due to the combination of heating and x-ray dose to
between 45 0 C and 80 0 C.
The scattering data from I-39LMW-A and B are shown in figure 4-12. These samples
91
display an increase in low q scattering occurring at approximately 40 0 C to 50 0 C followed
by almost total suppression of scattering above 500 C.
One interesting thing to note about the data obtained from the samples as they were
heated is that the transition temperatures are similar to those detected by DSC in the
case of S-64LMW (transition temperatures are just above that observed using DSC) and
S-45LMW (transition temperature range overlaps with that observed by DSC at the low
end.) If the DSC is measuring the macroscopic softening of the sample, this makes sense
(i.e. below this temperature, any physical interactions between the x-ray and the sample
may not yield a change in morphology due to the glassiness of the PS block.) This indicates
that there appears to be both a temperature and a dose effect during the heating run.
Perhaps when the x-ray dose is small, the effect of the sample and x-ray interaction is
not seen until the sample begins to rearrange on its own due to metastability of the lower
temperature ordered structure; at this point, its rearrangement occurs piecewise until the
entire sample takes on the new equilibrium diblock-D 3 blend morphology. Then, when
the sample is cooled (described below), the initial morphology is not recovered due to the
high dose of x-ray changing the equilibrium morphology and limiting the diffusivity and
mobility of the chains which fixes the morphology.
4.4.3
Morphology change through the transition
The transition from the initial morphology to the final morphology can be seen most
clearly by examining the scattering from a transitioning sample as a function of time.
Continuous three minute exposures over a total time period of 30 minutes were taken at
each temperature for which scattering data is measured; selected series of these at the
lowest temperatures during which morphology transitions occurred are shown in Figure
4-13.
The scattering from S-64LMW-A at 80 0 C is shown in (a); here, the relatively sharp
initial peak moves inwards, broadens, and flattens as time increases. At this temperature
and dose, two peaks cannot be discerned; instead, the scattering data displays a fairly flat
top. The movement to low q of the entirety of the peak is due to the replacement of the
initial morphology with a new morphology containing a larger d-spacing. This could be
due to the depolymerization of parts of the PDMS domains to form D3 which then swells
the remaining PDMS chains and increases the PDMS domain size and spacing. This
trend is present across all three samples. The piecewise nature of the transition could
be due to the grains which are less strongly segregated forming the new structure first,
leaving behind the sections of sample which scatter at lower q values. The broadening can
easily be attributed to decreasing perfection of the ordered structure as it transitions to a
disordered morphology and the flattening could be due to the superposition of a high dose
peak and a lower dose peak. Similar trends are apparent in S-45LMW-A at 50 0 C (shown
in (b) of this figure): an initially relatively narrow and intense peak broadens, shifts to
lower q, and decreases in intensity as time increases for similar reasons. In contrast,
in I-39LMW-B at 40 0 C, it is apparent that there is the development of a lower q peak
while the higher q peak decreases in intensity, broadens, and shifts inwards. In this case,
it is clear that the x-ray dose dependent peak grows at the expense of the initial peak
92
and that the initial phase peak shifts slightly to lower q due to some limited swelling of
the structure. The temperatures investigated here were all slightly above the transition
temperatures detected by DSC: thus their equilibrium phases should either be different
from their low temperature phases or more kinetically achievable; it is curious that this
is where the effects of x-ray dose begin to be seen. It could be due to a weakening of the
driving force for ordering of the neat sample combined with an increasing concentration
of dose-affected material.
4.4.4
Initial protocol cooling results
In Figure 4-14, the last scattering patterns obtained at each temperature during the
cooling runs performed are presented; S-64LMW-A and B are shown in (a), S-45LMW-A
and B in (b), and I-39LMW-A and B in (c). In this case, the effect of x-ray dose is readily
apparent (compare to Figures 4-10, 4-11, and 4-12 and 4-9. For the most part, during
cooling the dose-dependent peak decreases in intensity without any compensatory growth
in the initial peak: the initial structure is never recovered due to irreversible changes
in the molecules and hence the morphology upon prolonged x-ray exposure. The curves
become fairly featureless as increasing exposure destroys more and more of the PDMS
block and the sample becomes composed of PS or PI hooked to ever shorter PDMS in
a blend with D 3 and show almost no evidence of any consistent periodic variations in
electron density.
I-39LMW-B does display some ordering at lower temperatures, but the annealing
time at these temperatures is much longer than that used in the other samples studied
(two hours instead of thirty minutes) and both blocks are in the rubbery regime at all
temperatures investigated and so have higher mobilities. Note also that the peaks are
present at a lower q value than expected based upon the trend in peak movement during
heating: the maxima are present at q- 3.8 at moderate temperatures during heating runs
and at q- 3.2 lower temperatures during cooling runs. These peaks are actually closer in
q to the high temperature peaks that occur at - 3.3 - indicative of irreversible structural
change. Perhaps if these samples were exposed to x-rays for longer periods of time they
would experience similar changes in scattering data as those shown for the PS-containing
samples. Or, the difference in the chemistries of PS and PI may be responsible for the
differing behavior in the x-ray beam of PS-PDMS and PI-PDMS diblocks.
4.4.5
Conclusions
While DSC experiments show a range of temperatures at which some thermodynamic
transition is occuring for the two PS-containing samples, only inconclusive data is available from SAXS to confirm this or to establish an ODT for the PI-containing diblock.
Because of this, it is impossible to extract a x parameter for PS-PDMS or PI-PDMS
based upon the work done here or to conclusively establish the ODT for these samples. It
is possible, however, to say that even low molecular weight diblocks which can rearrange
in dramatically brief timescales (on the order of minutes) are strongly segregated enough
to produce scattering indicative of ordered structures at a variety of temperatures. This
93
means that the X parameter is very high in both of these systems, with promising applications for pattering due to the small length scale and stability of the structures obtained
independent of processing treatment. There has also been evidence presented that these
samples are highly sensitive to x-rays when exposed at elevated temperatures, providing
important information for experimentalists who wish to do further work on the structures
of these materials and the kinetics of molecular rearrangements. It also provides the possibility for new processing treatments; novel dose-dependent structures are formed which
might be of interest for applications.
94
DSC plot for S-64LMW
DSC plot for S-45LMW
0.5
0.8
0.6 0.4
0
0.2
0
-0.5
-0.2
-0.4
-0.6
-0.8
-1.5
-1.2
-2
-100
0
100
Temperature(Celsius)
(a)
150
200
-1.4
-100
-50
0
50
Temperature(Celsius)
100
1
200
(b)
igure 4-5: Data from DSC scans of S-64LMW (a), S-45LMW (b), and I-39LMW (c). The scans were performed at 5 mi
nd began at room temperature. The samples were heated to 180 0 C, held for three minutes, cooled to -80 0 C, held fo
inutes, and then heated back to room temperature. The morphology transitions observed in SAXS can be seen to vary
egrees in the DSC data. For S-64LMW, the endothermic peak at approximately 50 0 C to 70 0 C corresponds somewhat w
e morphology transition observed in SAXS between 80 0C and 165 0 C. For S-45LMW, the endothermic peak at approxima
00 C to 500 C is at the low end of the 40 0 C to 80 0 C range where the morphology transition was observed to occur with SA
here do not appear to be any thermal transitions in I-39LMW. The thermal transitions are only observed during the hea
ycles, consistent with the faster kinetics of structural rearrangement observed during SAXS. The large vertical lines wh
ccur at 25 0 C are due to the rapid change in temperature of the DSC pans to the initial experimental temperature.
DSC plotfor 1-39LMW
0.5
-0.5
-1.5
-2
-100
-50
0
50
Temperature(Celsius)
100
150
200
(c)
ontinued Figure 4-5: Data from DSC scans of S-64LMW (a), S-45LMW (b), and I-39LMW (c). The scans were performed
0
0
mOt and began at room temperature. The samples were heated to 180 C, held for three minutes, cooled to -80 C, held
minutes, and then heated back to room temperature. The morphology transitions observed in SAXS can be seen to vary
egrees in the DSC data. For S-64LMW, the endothermic peak at approximately 50 0 C to 70 0 C corresponds somewhat w
e morphology transition observed in SAXS between 80 0 C and 165 0 C. For S-45LMW, the endothermic peak at approximat
0 C to 500 C is at the low end of the 40 0 C to 80 0 C range where the morphology transition was observed to occur with SA
here do not appear to be any thermal transitions in I-39LMW. The thermal transitions are only observed during the hea
ycles, consistent with the faster kinetics of structural rearrangement observed during SAXS. The large vertical lines wh
ccur at 250 C are due to the rapid change in temperature of the DSC pans to the initial experimental temperature.
..........
.......
..
......
......
......
.....
0
a
0
b
c
o
d
0
e
o
0
f
Figure 4-6: Images recorded on the marCCD detector for a sample with the same characteristics as sample II-D where 40 second exposures were continuously taken throughout
the duration of the experiment; this sequence of images obviates the need for the revised
protocol and suggests that during x-ray irradiation some event such as cross-linking which
promotes the formation of locked-in non-equilibrium morphologies or some event such as
depolymerization of the PDMS occurs which irreversibly changes the equilibrium morphology occurs. (a) shows the image taken at 25 0 C before any temperature treatment has
been performed. (b) shows the scattering from this sample at 90 0 C after raising the temperature. (c) shows data taken after holding the sample at 90 0 C for 40 minutes. (d) shows
the image taken after a quench to 60 0 C. (e) shows the data obtained after holding at 60 0 C
for 18 minutes. (f) shows data taken after the sample was then cooled to 25 0 C and held
there for one hour and 15 minutes. (g) shows data taken after this sample was removed,
translated a small distance, and re-exposed to the x-ray beam. This sequence shows that
sample exposure to the x-ray beam strongly affects its structure and the resulting data
obtained.
97
(a)
(a)
(b)
I Oi# I.,
(~)
(C)
t
(d)
Figure 8.2
2D-patterns in the q,, q, range
0.24. Row (b), m
25: 5 samples around the
L,- H transition with f
0, 0,05, 0 10,
-
0,71 nm ' (20 range - 1 ) of oriented
morphologies in the system Si,,C,Ek m/D,
58"C, with the central structure featuring
0.15, and 0.20. Row (c), m = 25: 5 samples
at
around the H-
parasitic scattering (hmited by the 3' sIt),
I,k O, transition with
beam-stop and attenuated dirert beam
image. Row (a), m - 14: 5 typical L,,
4 = 0.25, 0.35, 0,40, 0 45, and 0,50 Row
(d). m = 51.6: 5 samples around the
H-1,-0, transition with dek
0, 0.075,
samples with A, - 0, 0.04, 0.12, 0.20, and
0.17. 0.20, and 0.50.
-
Figure 4-7: Scattering data and caption taken from [79] showing scattering from a PEOPDMS and D3 blend. Compare this to the scattering shown in Figure 4-8. It is clear
that the images obtained after x-ray irradiation look qualitatively similar to those shown
above, providing evidence for the hypothesis that the x-ray beam causes depolymerization
of the PDMS to form D3 .
98
A
B
A
B
A
00
b
B
f
c
d
A
A
e
B
h
igure 4-8: Images of the scattering patterns recorded by the CCD camera for selected temperatures for samples S-64LMW
nd S-64LMW-B. In these images, the integrated intensity presented in Figure 4-10 does not fully capture the x-ray scatter
at occurred due to the development of orientation during the experiment. S-64LMW-A at 80 0 C is shown in a; S-64LMW
90 0 C in b; S-64LMW-A at 105 0 C in c; S-64LMW-B at 110 0C in d; S-64LMW-A at 120 0C in e; S-64LMW-B at 125 0 C i
64LMW-A at 150 0 C after 30 minutes in g and after approximately 70 minutes in h; S-64LMW-B at 165 0 C in i.
....
...
................
...
of temperature
for1-39LMW-C
Scattering
asa function
Scatteding as a function of temperature for S-45LMW-D
70C
I
800
60C
I
50C
65C
I
I.
I
S
55C
40C
I'
30C
I
45C
2SC
I
35C
20C
I
10C
25C
0
0.2
0.6
0.4
0.8
1
0
0.2
0.4
0.6
q (1/nm)
q (1/nm)
(a)
(b)
0.8
1
igure 4-9: Scattering data from samples S-45LMW-D and I-39LMW-C following the revised protocol described. (a) sh
cattering from sample S-45LMW-D and (b) - (d) show scattering from I-39LMW-C. It appears that there is very little cha
ith temperature in the scattering data for S-45LMW-D for the temperature range investigated (25-80 0 C.) For I-39LMW
ere appear to be one prominent and one secondary peak from 10-80 0 C, which becomes one peak which steadily loses inten
rough 200 0 C.
............
.........
-
..
....
.....
Scattering
as a function
oftemperature
forI-39LMW-C
Scattering as a function of temperature for
1-39tMW-C
150C
200C
140C
130C
19C
c
cc
120C
I
II
80C
110C
100C
170C
90C
160C
80C
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
q (1/nm)
q (1/nm)
(c)
(d)
0.8
1
ontinued Figure 4-9: Scattering data from samples S-45LMW-D and I-39LMW-C following the revised protocol describ
) shows scattering from sample S-45LMW-D and (b) - (d) show scattering from I-39LMW-C. It appears that there is v
tle change with temperature in the scattering data for S-45LMW-D for the temperature range investigated (25-80 0 C.)
39LMW-C, there appear to be one prominent and one secondary peak from 10-80 0 C, which becomes one peak which stead
ses intensity through 200 0 C.
.......
..
.............
Scattering as a function
of temperature
forS-64LMW
Scattering as a function of temnperature for S-64LMW
95C-C
65C-C
I
I
9C-C
65C-8
90C-B
6C-C
soc-A
800-C
8CC5CC-CC
0
0.2
0.4
0.6
q (1/nm)
0.8
5CC-A
S0C-A
25c-B
75C-C
25-A
70C-C
1
0
0.2
0.4
0.6
0.8
q (1/nm)
igure 4-10: Scattering from S-64LMW-A, B, and C as a function of temperature. In all cases, the data from the last expos
ken at a particular temperature during the heating cycle is plotted. S-64LMW-A data is shown in red; S-64LMW-B
een; S-64LMW-C in green. For all samples, it is clear that in the region of 70 0 C-165 0 C the low temperature high q p
suppressed and a high temperature low q peak is formed. This transition occurs at slightly different temperatures in e
mple, indicating that there is both a time and temperature effect in the morphological transition that is occurring;
ehavior, however, is the same in all cases.
Scattering
as a function
oftemperature
forS-64LMW
120C-C
-
-
0
0.2
-
0.4
0.6
q (1/nm)
Scattering as a function of temperature for S-64LMW
0.8
150C-C
-
120C-A
150C-A
115C-C
145C-C
1100-C
140C-C
110C-8
135C-C
105C-C
130C-C
105C-A
130C-B
1O0C-C
125C-C
1
0
0.2
0.4
0.6
0.8
q (1/nm)
ontinued Figure 4-10: Scattering from S-64LMW-A, B, and C as a function of temperature. In all cases, the data fr
e last exposure taken at a particular temperature during the heating cycle is plotted. S-64LMW-A data is shown in
-64LMW-B in green; S-64LMW-C in green. For all samples, it is clear that in the region of 70 0 C-1650 C the low temperat
gh q peak is suppressed and a high temperature low q peak is formed. This transition occurs at slightly different temperatu
each sample, indicating that there is both a time and temperature effect in the morphological transition that is occurri
e behavior, however, is the same in all cases.
Scattering
as a function
oftemperature
forS-64LMW
180C-8
180C-A
165C-B
0
0.2
0.6
0.4
0.8
1
q (1/nm)
ontinued Figure 4-10: Scattering from S-64LMW-A, B, and C as a function of temperature. In all cases, the data f
e last exposure taken at a particular temperature during the heating cycle is plotted. S-64LMW-A data is shown in
-64LMW-B in green; S-64LMW-C in green. For all samples, it is clear that in the region of 70 0 C-165 0 C the low temperat
gh q peak is suppressed and a high temperature low q peak is formed. This transition occurs at slightly different temperatu
each sample, indicating that there is both a time and temperature effect in the morphological transition that is occurri
e behavior, however, is the same in all cases.
Scattering
asa function
of temperature
forS-45LMW
Scattering as a fuinction of temperature for S-45LMW
150C-C
I
52C-
36C-C
-
-
- -50C-C
34C
50CA
32C-C
48C-C
30C-
44C-C
30CAC
44C-C
0
0.2
0.4
0.6
q (1/nm)
0.8
25C-B
42C-C
25C-A
40C-C
1
0
0.2
0.4
0.6
0.8
1
q (1/nm)
igure 4-11: Scattering from S-45LMW-A, B, and C as a function of temperature. In all cases, the data from the last expos
ken at a particular temperature during the heating cycle is plotted. S-45LMW-A data is shown in red; S-45LMW-B
een; S-45LMW-C in green. For all samples, it is clear that in the region of 40 0 C-85 0 C the low temperature high q p
suppressed and a high temperature low q peak is formed. This transition occurs at slightly different temperatures in e
mple, possibly indicating that longer annealing times are necessary in this regime to achieve equilibrium morphologies;
ehavior, however, is the same in all cases.
.............
forS-45LMW
of temperature
asa function
Scattering
Scattering as a function of temnperature for S-45LMW
95C-B
I110C-85C-B
8OC-B
75C-B
75C-A
100C-B
65C-A
0
0.2
0.6
0.4
q (1/nm)
0.8
1
0
0.2
0.6
0.4
0.8
1
q (1/nm)
ontinued Figure 4-11: Scattering from S-45LMW-A, B, and C as a function of temperature. In all cases, the data from
st exposure taken at a particular temperature during the heating cycle is plotted. S-45LMW-A data is shown in red
5LMW-B in green; S-45LMW-C in green. For all samples, it is clear that in the region of 40 0 C-85 0 C the low temperature h
peak is suppressed and a high temperature low q peak is formed. This transition occurs at slightly different temperature
ach sample, possibly indicating that longer annealing times are necessary in this regime to achieve equilibrium morpholog
he behavior, however, is the same in all cases.
.....
.....-
for1-39LMW
Scattering
as a function
oftemperature
500-A
I
48C-B
46C-8
44C-8
42C-B
1~
II
40-B
II
4*C-A
30C-A
250-A
0
0.2
0.4
0.6
0.8
1
q (1/nm)
igure 4-12: Scattering from I-39LMW-A and B as a function of temperature. In all cases, the data from the last expos
ken at a particular temperature during the heating cycle is plotted. I-39LMW-A data is shown in red; I-39LMW-B in gre
or both samples, in the range of 30 0C-50 0C the low temperature high q peak is suppressed and a high temperature lo
eak is formed.
.........
0.3
Scattering at constant temperature for S-45LMW-A at 50C
Scattering at constant temperature for S-64LMW-A at 80C
Increasing time
Increasing time
.
0.35
0.4
0.45
q (1/nm)
0.5
0.55
0.6
0.45
0.5
(a)
igure 4-13: SAXS data taken from diblocks undergoing structural evolution at
inute exposures from samples transitioning between ordered and disordered
gh temperature correlation hole peak at the expense of the low temperature
-64LMW-A at 80 0 C; (b)shows S-45LMW-A at 500 C; (c) shows I-39LMW-B at
0.55
0.6
q (1/nm)
0.65
0.7
(b)
a constant temperature. Ten sequential th
structures are shown and the growth of
ordered structure peak is evident. (a) sh
40 0 C.
...........
Scattering at constant temperature for 1-39LMW-B at 40C
Increasing time
0.2?
0.3
0.35
q (1/nm)
0.4
0.45
(c)
ontinued Figure 4-13: SAXS data taken from diblocks undergoing structural evolution at a constant temperature.
equential three minute exposures from samples transitioning between ordered and disordered structures are shown and
rowth of the high temperature correlation hole peak at the expense of the low temperature ordered structure peak is evid
a) shows S-64LMW-A at 80 0 C; (b)shows S-45LMW-A at 50 0 C; (c) shows I-39LMW-B at 40 0 C.
cooling
forS-45LMW
during
oftemperature
as a function
Scattering
Scattering as a function of temperature durin cooling for S-64LMW
105C-B
1650-
I
I
1150-4
95C-B
-
130C-I
9OC4B
120C-A
65C-B
80C-B
75C-B
105C4
I I
J
7C-A
90C-B
BOC-A
50C-A
30C-A
0
0.2
0.6
0.4
0.8
1
0
0.2
0.5
0.4
q (1/nm)
q (1/nm)
(a)
(b)
0.8
i
igure 4-14: SAXS data taken from all samples during their cooling runs. Note the lack of structure development for any of
amples which received only a 30 minute anneal at the colder temperatures. I-39LMW-B shows morphology developmen
onger temperatures due to the longer two hour anneal at each temperature that it received. (a) shows data from S-64LMW
nd B; (b) shows data from S-45LMW-A and B; (c) shows data from I-39LMW-A and B.
during
coolngforI-39LMW
Scattering
asa frctIlon oftemperature
4CC-A
35C-8
3CC-B
250-8
20C-8
IS-1
15C-B
1CC-A
C
0.2
0.4
0.6
0.8
q (1/nm)
(c)
ontinued Figure 4-14: SAXS data taken from all samples during their cooling runs. Note the lack of structure developm
r any of the samples which received only a 30 minute anneal at the colder temperatures. I-39LMW-B shows morpholo
evelopment at longer temperatures due to the longer two hour anneal at each temperature that it received. (a) shows d
om S-64LMW-A and B; (b) shows data from S-45LMW-A and B; (c) shows data from I-39LMW-A and B.
.. ....
.............
...
...........
.............
.....
....
...
Chapter 5
Conclusions and future work
In this thesis, several different topics related to determining the morphologies of PDMScontaining diblocks were investigated. These included work on the structures of highly
segregated materials both at equilibrium and as a function of processing treatment (casting solvent and annealing) as well as attempted determination of the ODT for several low
molecular weight diblocks. Overall, the results were mixed: while the volume fractions
at which bulk morphologies of the larger molecular weight diblocks were observed were
reasonable given prior work and theory and they were relatively stable independent of the
sample preparation procedure, determination of an ODT or interpretation of the scattering data obtained at elevated temperatures was inconclusive. The most promising future
directions for this work involve further study of the elevated temperature SAXS data and
determination of what is causing the dose-dependent scattering effects and the application of bulk morphology knowledge to the creation of devices for which it is suited and to
fundamental studies investigating the perturbation of known and well-characterized morphologies under confinement. The former is most probably due to the depolymerization
of PDMS to form D4 and cross-links; the former causes new structures to become the
equilibrium morphologies and the latter arrests structural change by impeding the chain
mobility required for morphological transitions.
5.1
Bulk morphologies
The bulk morphologies for strongly segregated diblocks found in this thesis match fairly
well with predictions based upon prior experimental and theoretical work. Figure 2-1
shows this quite nicely. In part a, it appears as if the lamellar volume fraction range
is smaller than would be expected: there are samples with a cylindrical morphology in
PI- and PS-containing samples with volume fractions at the low end of the expected
lamellar region. Similarly, there is a PI-containing sample with a cylindrical morphology
at a volume fraction at the high end of the expected lamellar region. The combination
of these two effects means that the lamellar range may be narrower than theoretically
expected; the phase diagram may be compressed inwards from both ends. Other work,
most notably [64] suggests that the phase diagram may be asymmetrical; however, it
appears that the samples investigated with a PDMS majority composition fell well within
112
the anticipated ranges for the morphologies (i.e. not close to morphology boundaries) and
in two of the three cases were in fact closer to the boundary with larger PDMS content
than that with lower so that the narrowing of the central region would not be observed.
Examination of part b of Figure 2-1 further shows that the morphologies obtained in this
work fall in similar regions of volume fraction and segregation strength as others previously
observed in other PDMS-containing systems. In many of these cases, it appears that the
boundaries between morphologies are closer to 50% PDMS on both the PDMS-rich and
PDMS-poor sides of the phase diagram. These results are encouraging because they verify
prior work and indicate that PDMS-containing diblocks display consistent morphologies
which match theoretical predictions in many different experiments. The one exception to
this is the one low molecular weight sample on the phase diagram, which although having
a majority of PDMS by volume displays (64 % ) a PDMS cylinders morphology. This may
be explained by the weakly segregating nature of the system; at the bottom of the phase
diagram, all the theoretical boundaries curve together and the predicted morphologies are
in less of a energy well than at stronger segregations. Here, other effects such as block
flexibility, local chain chemistry, etc. may affect the morphology that is displayed.
Another encouraging result from the work on bulk morphologies is the correspondance
between SAXS and TEM results. In cases where both tools were used to investigate
morphology and there were enough peaks in the SAXS data to assign a morphology, the
morphology assignment was consistent across TEM and SAXS for samples which were
prepared in the same way. SAXS morphology assignment can be less than clear-cut due
to possible ambiguity in peak location (especially for broad or low intensity peaks.) TEM
morphology assignment can be misleading if anomalous grains are investigated or they
do not occur at sufficient angles with respect to the electron beam for enough different
projections to be observed. However, when measurements made by the two different
techniques indicate the same morphology it means that there is a high probability that
the assigned morphology is correct.
Future work in this area could expand upon the results presented here in several ways.
These could include pairing a larger diversity of other chemical blocks (see Chapter 1
for other blocks which have previously been investigated; the diversity is endless) with
PDMS and determining whether the boundaries between morphologies as a function of
chiN and volume fraction continue to be consistent. In a similar vein, PDMS-containing
triblocks, starblocks, and other architectures could be studied by these same methods
to determine the influence of archeology on morphology. Other opportunities include
synthesizing more samples with volume fractions inside the observed cylinder-lamellar
transition region (around 40% PDMS by volume) to better pin down how the morphology
varies here. Finally, advanced theoretical modelling which explicitly accounts for the
chemistries and chain characteristics of PDMS, PS, and PI could be performed to see if
it can account for the data presented here.
5.2
Processing treatments
Additional support for the assignment of accurate equilibrium morphologies to the diblocks studied based upon the TEM and SAXS work done on samples cast from toluene
113
and then annealed is provided by the robustness of these morphologies to processing
treatments. It was shown here that the morphologies obtained did not vary significantly
with the preference of the solvent for PDMS or the attached block and with whether or
not an annealing treatment took place; this means that even if one of the blocks was
swollen and an alternate morphology was present at some point during casting, this effect
was erased by the time that the final structure was formed. This could be due to the
strong segregation between the blocks and thus strong driving force for the formation
of an ordered morphology and/or of substantial mobility being present in the material
throughout the casting process. In any case, since the morphologies found were for the
most part independent of initial processing, the final morphology can be said to be robust
and easily attainable. The d-spacings for some samples did vary somewhat with casting
solvent prior to annealing, but since they all obtained the same value after annealing it
is fairly clear that the annealing treatment was sufficient to relax the morphology to its
equilibrium structure.
Further work in this area could be performed to investigate the affects of different
annealing times and temperatures, the use of solvent annealing or combined solvent and
thermal annealing. Faster casting procedures could also be used to attempt to lock in
swollen structures by evaporation of solvent before the polymers have time to rearrange
into their preferred morphologies. Since PDMS-containing diblocks are increasingly used
in academia and industry for patterning and other technologies where the ultimate morphology is extremely relevant to the ultimate application, especially in thin films, the
importance of processing steps in determining final structure is of great importance.
5.3
Morphologies as a function of temperature
Overall, the results from this study are somewhat less clear-cut than those discussed
previously. It is fair to say that both low molecular weight PS-PDMS samples had a
transition which was detectable by DSC which matched temperatures in which there were
changes in the scattering patterns of continuously x-ray exposed diblocks. The PI-PDMS
sample did not display a transition in either the DSC experiment or the SAXS experiment
with minimal dosing across a larger range than that measured by DSC; however, some
decrease in order is apparent upon increasing temperature because a secondary peak
in the scattering pattern is lost as the temperature is raised. In all cases, it appeared
that exposure to the x-ray beam had substantial effects on the SAXS data: experiments
where the samples were continuously exposed to x-rays exhibited peaks which broadened
asymmetrically towards low q (in some cases showing increased orientation) and appeared
similar to SAXS data obtained from blends of PEO-PDMS and D4 while diblocks with
very limited exposure to x-rays displayed peaks which were fairly stationary and only
changed in intensity (if at all) in the same temperature ranges. It is also of note that the
kinetics for structural arrangement were very fast in the latter case: while the samples
were held at each temperature for approximately 40 minutes before taking the final data,
this data differed imperceptibly from the data taken after initially quenching to the target
temperature. This is probably due to the short chain lengths of all the blocks which both
depress the glass transitions of all of the blocks thus providing good chain mobility and
114
result in a lack of entanglements above the glass transitions. In the former case, however,
the transition occurred relatively slowly (on the order of at least 30 minutes during heating
cycles and sometimes with no change at all in 30 minutes during cooling cycles), indicating
some sort of crosslinking and/or change of equilibrium state and/or other effect occurring
due to the x-ray flux which made the structure more resistant to change.
There is much more work that could be done to provide further insight into all the
phenomena that are present here. It could be interesting to use other methods (such
as DMA) to locate the ODT to provide confirmation of the ranges found. Additionally,
more experiments could be done using the revised protocol to both pin down ranges over
which the transition occurs and then precisely target the ODT. Experiments could be performed significantly above the ODT in order to determine the mean-field and correlation
hole scattering, from which X and its possible composition dependence can be extracted.
Other work could build upon the studies discussed above and investigate the effect of
solvent concentration on structure by obtaining scattering from solvated diblocks with
different concentrations of solute. This could determine whether or not there are other
morphologies present during casting which could be locked in by fast solvent evaporation,
cross-linking, or other means. Other studies could focus on better determining what is
happening during x-ray exposure; these could take the form of exposing large regions of
sample to similarly intense x-rays (or precisely determining which region of sample is in
the beam path) and then examining the properties of the resulting material using GPC,
FTIR, NMR, DSC, and TEM to determine any chemical or structural changes in the
material. Deep UV illumination could also be investigated as a route to promoting the
formation of these morphologies. The x-ray induced morphologies might be well-suited for
certain technological applications and in any case would expand the repertoire of diblock
morphologies which are currently available to engineers; thus, determining precisely how
they are induced and what the resultant structures are would be quite interesting.
5.4
Outlook for the future
The primary reason that PDMS-containing diblock polymers are interesting and relevant
to study is that their chemical properties are ideal for nanoscale patterning applications.
The combination of a large x paramter which causes ordered morphologies to exist even
in very low molecular weight materials with a large differential etch resistance of PDMS
and other polymers to oxygen plasma, HF, and other etchants which allows for removal
of one block allows for the creation of highly ordered and topological patterns. For the
most part, PDMS-containing diblocks have been used extensively for thin film patterning. However, there is a large variety of geometries into which these materials could be
patterned. A better understanding of the factors that determine PDMS-containing diblock morphology in the bulk allows for better designed and interpretable studies about
how these morphologies are perturbed if confinement is introduced in any of the three
dimensions.
While the majority of experimental and theoretical work on block copolymer structure
has focused on determining bulk structures, understanding structures that are present
inside of confined geometries allows for the further development of block polymers as
115
technologically relevant materials. Technologies that make use of bulk block polymers
are limited by the types and qualities of morphologies that are thermodynamically and
kinetically accessible as well as by the range of available chemistries that can be used in
synthesis. The presence of non-equilibrium defects such as grain boundaries and dislocations are also problematic and can be difficult to eliminate.
Recently, research has been pursued to address these issues by using a variety of techniques to change the thermodynamic boundary conditions that are present when the block
polymer microstructure is formed. Elimination of randomly located defects and control of
the orientation and spatial registration of the domains have been pursued using "directed
self assembly" [80]. Approaches have included affecting the energetics of candidate morphologies by chemical means, designing casting conditions in which the block polymer
undergoes a disorder to order transition while geometrically confined to dimensions at
which the energies of the confining surfaces have a significant effect on the overall free
energy of the material, and the combination of these two strategies. For all of these cases,
block polymers have mainly been confined between surfaces of constant curvature (flat,
cylindrical or spherical surfaces), and so are subject to the same geometrical constraint
at all locations within the material. In general, the symmetries of the confining geometry
and of the block polymer's equilibrium morphology interact to produce the final morphology; the symmetries of the confining surfaces should be satisfied if possible in the
equilibrium morphologies, so the domain spacing and IMDS shapes are usually perturbed
from their bulk values and forms. Novel morphologies found under confinement include
the perturbation of cylinders to helices or network structures, the formation of structures
which are concentrically nested inside of each other, and many others including the necessary introduction of "equilibrium defects." In most cases, this is due to the surface
energetics, maintenance of the repeat period, or maintenance of the minority domain's
relevant length while conserving the volume fraction of material throughout the confined
morphology. The understanding of morphologies that form as a function of imposed
boundary symmetries and boundary surface preferences is thus a powerful tool for the
prediction of new structures; it also allows the exclusion of morphologies which strongly
violate the required symmetries, and surface boundary conditions, and provides insight
into how the bulk morphologies may be manipulated. PDMS-containing diblocks are ideal
for investigating the formation of such structures for several reasons: their morphologies
are well-characterized, they have ideal properties for adapting the new structures into relevant technologies, the mass thickness contrast between PDMS and many other polymers
makes imaging easy, and the strong segregation at small length scales means that a variety
of chain lengths can be experimentally accessed so that large ranges of confinement ratios
can be studied. For more information about prior work in this area and its technological
potential, see [81].
116
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