Can Random Copolymers Serve as Effective Polymeric
Compatibilizers?
M. S. LEE,* ,1 T. P. LODGE, 2 C. W. MACOSKO 1
1
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave.,
Minneapolis, Minnesota 55455
2
Department of Chemistry, University of Minnesota, 207 Pleasant St., S.E. Minneapolis, Minnesota 55455
Received 17 April 1997; accepted 11 August 1997
ABSTRACT: We investigate the compatibilizing performance of a random copolymer in
the melt state, using transmission electron microscopy. Blends of polystyrene (PS) and
poly(methyl methacrylate) (PMMA) are chosen as a model system, and a random
copolymer of styrene and methyl methacrylate (SMMA) with 70 wt % styrene is used as
a compatibilizer. From TEM photographs it is clear that SMMA moves to the interface
between PS and PMMA domains during melt mixing, and forms encapsulating layers.
However, the characteristic size of the dispersed phase increases gradually with annealing time for all blend systems studied. This demonstrates that the encapsulating layer
of SMMA does not provide stability against static coalescence, which calls into question
the effectiveness of random copolymers as practical compatibilizers. We interpret the
encapsulation by random copolymers in terms of a simple model for ternary polymer
blends. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2835–2842, 1997
Keywords: polymer blends; random copolymer; compatibilizer; encapsulation; coalescence
INTRODUCTION
Blends of immiscible polymers often require the
addition of a compatibilizer to improve the dispersion and adhesion of phases and to stabilize the
morphology. Block or graft copolymers having the
same monomeric units as the blend components
have been successfully used for this purpose.1 – 4
Such compatibilizers can be introduced by adding
a premade copolymer, or by in situ reactive formation during melt processing.5 – 7 However, these
compatibilizers are typically expensive, and can
cause a substantial viscosity increase during processing.
Random copolymers may offer an alternative
* On leave from Chonnam National University, Kwangju,
Korea
Correspondence to: T. P. Lodge
Journal of Polymer Science: Part B: Polymer Physics, Vol. 35, 2835–2842 (1997)
q 1997 John Wiley & Sons, Inc.
CCC 0887-6266/97/172835-08
method for compatibilization of immiscible polymer blends.8 There are many experimental and
theoretical results that support this possibility.
First, random copolymers can enhance the interfacial adhesion between immiscible phases, 8–13
although the mechanism whereby a random copolymer layer strengthens the interface is still
controversial.10,14 Second, random copolymers located at the interface can reduce the interfacial
tension between immiscible phases. For example,
Balazs and co-workers recently examined the effect of molecular architecture of potential compatibilizers (random, alternating, and diblock copolymers) on their compatibilizing performance using
the self-consistent mean field approach.15 They
calculated the extent of the interfacial tension reduction as a measure of the efficiency of the compatibilizers, and predicted that long random copolymers are more effective than short diblocks
when they compare random copolymers with
diblocks of different molecular weight. However,
2835
8Q46
/ 8q46$$3012
10-20-97 05:06:51
polpa
W: Poly Physics
9713012
2836
LEE, LODGE, AND MACOSKO
at fixed molecular weight, diblock copolymers are
more effective in reducing interfacial tension.
Third, the spreading of random copolymers between immiscible phases is thermodynamically
favored. Winey and co-workers demonstrated that
encapsulation by random copolymers can occur
during drying of a solvated blend.16
It is not yet clear that random copolymers preferentially locate at the interfaces under melt mixing conditions, or whether they effectively stabilize the resultant morphology during annealing,
as is necessary for commercial application. Therefore, more data on the compatibilizing performance of random copolymers in the melt state
are needed, because polymer blends are generally
prepared by melt mixing. Furthermore, it is noteworthy that the efficiency of compatibilizers can
depend on the blend preparation method. For example, in melt compatibilization with premade
diblock copolymers, there appears to be an optimum molecular weight, 7 whereas when blends
are prepared by solution blending, compatibilizers with higher molecular weight are more effective in reducing the dispersed phase size.17
In this study we investigate the compatibilizing
performance of a random copolymer in the melt
state. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) are chosen as model blend components, and a random copolymer of styrene and
methyl methacrylate (SMMA) is used as the compatibilizer. Through this study we will determine
whether random copolymers move to the interface
between matrix and dispersed phase during melt
mixing and, if so, whether encapsulating layers
of random copolymers can stabilize blend morphology during melt annealing or not. Experimental answers to these two questions will be helpful
to determine whether random copolymers can be
commercially effective as compatibilizers.
EXPERIMENTAL
The source, molecular weight, and melt viscosity
of the polymers used in this study are listed in
Table I. Note that at the blending temperature
the ranking of viscosities is hSMMA ú hPS ú hPMMA .
All polymers have broad molecular weight distributions, and were provided in pellet form; they
were used as received. Blend samples were prepared using a Haake Rheomix 600 batch mixer
operating at 2107C for 20 min. The rotor speed
was 50 rpm, which corresponds to a maximum
shear rate of 65 s 01 . After mixing, the samples
8Q46
/ 8q46$$3012
10-20-97 05:06:51
were quenched in ice water. The sample code 70/
20/10 denotes 70% PS, 20% PMMA, and 10%
SMMA by weight. To investigate the melt stability, as-blended samples were annealed in an oven
at 2007C for a specified period of time and then
quenched in ice water. Specimens were wrapped
in aluminum foil to prevent flow during annealing.
The blend morphology of as-blended and annealed samples was characterized using a JEOL
1210 transmission electron microscope (TEM),
operated at 120 kV. Sections of 70 nm thickness
were microtomed at room temperature with a
Reichert Ultracut S ultramicrotome. The sections
were stained for 15 min with the vapor of 0.5%
RuO4 in water solution. The number ( Dn ) and volume-to-surface (DVS ) average Waddel diameter
(the diameter for an equivalent sphere) of the dispersed phase were calculated from TEM photographs, averaging over 200 to 400 particles. For
the case that SMMA forms encapsulating layers
between PS and PMMA domains, the layers were
included to calculate Dn or DVS .
RESULTS AND DISCUSSION
As styrene is preferentially stained under the conditions used in this study, 18 PS appears dark,
SMMA gray, and PMMA white in bright-field
TEM. This contrast makes it possible to differentiate the SMMA phase in the blends. Figure 1
shows TEM photographs of three as-blended samples, where PS comprises the majority component
at a constant concentration of 70 wt %. As seen in
Figure 1(b) and (c), SMMA forms encapsulating
layers between the PS matrix and the PMMA particles during melt mixing. The encapsulating
SMMA layer for the 70/20/10 blend is clearly seen
in the magnified TEM photograph inserted in Figure 1(b). The ratio of PMMA to SMMA has no
discernible effect on the observed encapsulation
by SMMA, although for the 70/10/20 blend [Fig.
1(c)] some pure SMMA domains exist. Although
the blends were prepared by melt mixing rather
than solution casting, the morphologies seen in
Figure 1 are equivalent to those observed by
Winey and co-workers.16 This suggests that encapsulation by the random copolymer is independent of the blend preparation method. Moreover,
because SMMA has the highest melt viscosity under the mixing conditions employed, we can conclude that the driving force for the encapsulation,
that is, the interfacial tension reduction by random copolymers, is strong enough for random co-
polpa
W: Poly Physics
9713012
RANDOM COPOLYMERS AS EFFECTIVE POLYMERIC COMPATIBILIZERS
2837
Table I. Polymers Used for This Study
Polymers
Source
Mw (g/mol)
PIa
hb (Pa.s)
PS
PMMA
SMMAc
Dow
Polysciences, Inc.
Polysciences, Inc.
200,000
100,000
270,000
1.8
1.9
1.9
894
660
1383
a
Polydispersity index, measured using gel permeation chromatography.
Measured using a Rheometrics DSR rheometer at a shear rate of 50 s01 and 2107C.
c
70 wt % styrene.
b
polymers to move to the interfaces during melt
mixing. A second observation from Figure 1 is the
change in the size of dispersed phase with the
addition of SMMA. The dispersed phase size of
the 70/20/10 blend is significantly reduced compared to the 70/30/0 blend. As predicted by Balazs and co-workers, 15 this results from the interfacial tension reduction by SMMA layers located at the interfaces. However, as seen in the
70/10/20 blend, SMMA used in excess does not
contribute to further size reduction; rather, the
thickness of the SMMA shell increases.
In accord with other experimental results referred to in the Introduction, Figure 1 clearly
demonstrates that random copolymers are polymeric compatibilizers: SMMA moves to the interface during melt mixing and contributes to reduce
the dispersed phase size. However, as noted in
the Introduction, to be actually effective as a compatibilizer the layer located at the interface must
provide melt stability during other processing
conditions, such as annealing. Figure 2, TEM photographs of annealed blend samples, may provide
an answer. In all cases, samples show that after
60 min at 2007C, the domain size has increased
significantly compared to that of the as-blended
samples. This is similar to the coarsening process
observed for uncompatibilized polymer blends.19,20
Although the encapsulating layer of SMMA is well
developed for 70/20/10 and 70/10/20 blends after
annealing, it is clear that the layer does not prevent static coalescence. Considering that the
physical properties of polymer blends depend critically on the size of dispersed phase as well as on
the adhesion between the matrix and the dispersed phase, the coarsening of encapsulated particles may be a crucial disadvantage in commercial applications.
The change in the domain size with annealing
time is summarized in Figure 3(a). Upon annealing, the particle size increases significantly, irrespective of the blend composition. As shown in
Figure 3(b), the coarsening rate calculated from
8Q46
/ 8q46$$3012
10-20-97 05:06:51
the plot of (Dn ) 3 vs. annealing time is almost the
same within the experimental error.
Figure 4 shows the morphology change before
and after melt annealing for a 20/70/10 blend
with a PMMA matrix. As with blends with a PS
matrix, SMMA encapsulates the minor phase and
dispersed phase coarsening is observed after annealing. Thus, the matrix type has no significant
effect on the encapsulation by random copolymers
and coarsening of encapsulated particles.
Recently Macosko et al. attributed the role of
block copolymers in preventing static coalescence
to the steric stabilization mechanism of colloidal
particles, 7 i.e., the copolymer forms a brush at
the interface. They proposed an elastic repulsion
between approaching particles, caused by the reduction in the compatibilizer chain conformation.
From the balance between van der Waals attraction between approaching particles and the
entropic repulsion, they postulated that Ç 20%
surface coverage by block copolymers is necessary
to impart static stability. In contrast, the conformation of SMMA at the interface is very different
from that of a block copolymer brush; in effect,
the SMMA chains may be considered as a separate phase. Consequently, there is no repulsion
between encapsulated particles.
Figure 5 illustrates how the particles with encapsulating layers grow during annealing. The
photographs were obtained from a 70/10/20 blend
annealed for 60 min, and reorganized according to
the coalescence mechanism of immiscible polymer
blends proposed by Chesters and others.21 – 23 As
seen in Figure 5, coalescence of the encapsulated
particles is two-step process: coalescence of encapsulating particles in the PMMA matrix, followed
by coalescence of dispersed domains within the
encapsulating SMMA layer. The latter process
might be slower than the first because the melt
viscosity of SMMA is higher than that of PS. It is
clear that the encapsulating layer acts as a discrete phase and does not provide stability against
static coalescence. The force associated with the
polpa
W: Poly Physics
9713012
2838
LEE, LODGE, AND MACOSKO
particle movement is sufficient to squeeze out the
encapsulating SMMA layer when the encapsulated PMMA particles collide. After a bridge between two inner particles is formed by rupture of
Figure 2. TEM photographs of samples annealed at
2007C for 60 min: (a) 70/30/0 (Dn Å 1.15; DVS Å 1.75);
(b) 70/20/10 (Dn Å 0.85; DVS Å 1.19); (c) 70/10/20 (Dn
Å 0.87; DVS Å 1.18). The unit of Dn and DVS is mm.
Figure 1. TEM photographs of as-blended samples:
(a) 70/30/0 ( Dn Å 0.80; DVS Å 1.03); (b) 70/20/10
(Dn Å 0.28; DVS Å 0.36); (c) 70/10/20 ( Dn Å 0.47; DVS
Å 0.56). The unit of Dn and DVS is mm.
8Q46
/ 8q46$$3012
10-20-97 05:06:51
the SMMA layer, a coalesced particle, which has
a well-developed encapsulating layer, is formed.
The encapsulation by random copolymers can
be interpreted with a model proposed by Hobbs
and co-workers for ternary polymer blends.24 They
polpa
W: Poly Physics
9713012
RANDOM COPOLYMERS AS EFFECTIVE POLYMERIC COMPATIBILIZERS
2839
When both l31 and l13 are negative, we can expect
two separate dispersed phases of 1 and 3. In other
words, 3 moves to the interface and encapsulates
minor phase 1 or 2, when the sum of the interfacial tensions associated with 3 is smaller than
the interfacial tension between the original blend
components. In mean-field theory gij is proportional to the square root of the Flory–Huggins
interaction parameter xij 25 as
kT
gij Å 2
b
r
xij
6
(2)
where b is the effective length per monomeric
unit, k is the Boltzmann constant, and T is temperature. Since for homopolymer/copolymer blends
xij can be estimated from a binary interaction
model, 26 we can estimate lij for the PS/PMMA/
Figure 3. (a) variation of the number-average particle diameter with annealing time; (b) plots of (Dn ) 3 vs.
annealing time. Slopes are proportional to coalescence
rates.
considered the spreading coefficient lij , which determines whether the third component should encapsulate the minor phase or not. For the case
that a polymer, 3, is added to an immiscible polymer blend of components 1 and 2, l31 is defined
as
l31 Å g12 0 [ g31 / g32 ]
(1)
where gij is the interfacial tension between i and
j. Encapsulation of 3 onto 1 will occur if l31 ú 0.
8Q46
/ 8q46$$3012
10-20-97 05:06:51
Figure 4. TEM photographs of PS/PMMA/SMMA
20/70/10 blends: (a) as-blended; (b) annealed at 2007C
for 60 min.
polpa
W: Poly Physics
9713012
2840
LEE, LODGE, AND MACOSKO
Figure 5. Coalescence of encapsulated particles: (a) approach and overlap of encapsulating layer; (b) deformation of dispersed particles and squeezing out the encapsulating
layer; (c) rupture of the encapsulating layer; (d) mass flow and coalescence. Note that
these pictures do not represent a time series of two particular drops.
SMMA system. We designate PS, PMMA, and
SMMA as 1, 2, and 3.
For blends of PS, PMMA, and SMMA, x13 and
x23 are obtained from the binary interaction
model as follows:
x13 Å x12 (1 0 f ) 2
x23 Å x12 f
2
(3)
(4)
where f is the volume fraction of styrene in
SMMA. Assuming that the b values of PS, PMMA,
and SMMA are the same, we can approximate eq.
(1) as
q
q
q
l31 É x12 0 [ x13 / x23 ]
8Q46
/ 8q46$$3012
10-20-97 05:06:51
(5)
The values of bPS and bPMMA obtained from neutron scattering measurements are 0.68 and 0.69
nm, respectively.27 From eqs. (3), (4), and (5),
we can calculate lij for a given matrix. The lij
values calculated, and the corresponding morphologies, are given in Figure 6. The calculation
predicts that, except for the case of a random copolymer matrix, encapsulation by random copolymers is thermodynamically favored, regardless of
the content and type of comonomer and matrix.
This prediction is consistent with the TEM results
of Figures 1 and 4. For the SMMA matrix, Winey
et al. reported that two separate PS and PMMA
domains are formed in the SMMA matrix and no
encapsulation between PS and SMMA occurs.16
Their observation is also in agreement with the
polpa
W: Poly Physics
9713012
RANDOM COPOLYMERS AS EFFECTIVE POLYMERIC COMPATIBILIZERS
Figure 6. Spreading coefficients, lij , for PS/PMMA/
SMMA and possible morphologies for a given matrix.
prediction of Figure 6. Further evidence for the
encapsulation by random copolymers may be
found for polypropylene (PP)/polyethylene (PE)/
ethylene–propylene rubber (EPR) blends, where
EPR encapsulate the minor PE phase.28
The encapsulation by random copolymers may
be extended to other blend systems. For miscible
polymers, the interfacial tension is effectively
zero. Therefore, it is possible to prepare polymer
blends with encapsulation by random copolymer
when we replace one or both of blend components
with corresponding miscible components. Because
polycarbonate (PC) is partially miscible with
PMMA, and poly(phenylene oxide) (PPO) is completely miscible with PS, 3,29 PS/PC/SMMA, PPO/
PMMA/SMMA, and PPO/PC/SMMA blends are
candidates for encapsulation. From TEM photographs we have found that in all cases, SMMA
moves to the interface during melt mixing and
forms an encapsulating layer.
SUMMARY
We investigate whether random copolymers can
act as effective compatibilizers for blends prepared by melt mixing. Similar to previous experimental and theoretical studies, the possibility of
random copolymers acting as polymeric compatibilizers can be seen from TEM photographs of asblended samples. SMMA moves to the interfaces
between PS and PMMA domains during melt mixing and forms encapsulating layers. As a result,
the size of the dispersed phase is significantly reduced. When SMMA is used in excess, separate
domains of pure SMMA are formed. This is analogous to micelle formation of block copolymers in
the case of compatibilization by premade block
copolymers. However, when the blends are an-
8Q46
/ 8q46$$3012
10-20-97 05:06:51
2841
nealed at high temperature, the size of the encapsulated particles increases with annealing time.
This means that the encapsulating layer of random copolymers does not provide stability against
static coalescence. The amount of SMMA and the
matrix type, PMMA or PS, have no effect on the
encapsulation by SMMA and the coarsening process of encapsulating particles. It is not yet clear
whether the coarsening behavior observed in this
study is fatal for the physical properties of polymer blends compatibilized by random copolymers
or not. To answer this question further experiments will be necessary.
The morphologies observed after annealing are
equivalent to those seen after long drying of a
solvated blend.16 Moreover, the same morphology
was also seen in as-blended samples. It is clear
that encapsulation by SMMA is thermodynamically favored and a driving force effectively acts to
locate SMMA at a preferred position, the interface
between PS and PMMA phases. The driving force
for encapsulation is apparently the reduction of
interfacial tension caused by the addition of random copolymers. From spreading coefficients calculated from Helfand and Tagami theory and a
binary interaction model we can predict that the
encapsulation by random copolymers is thermodynamically favored. Also, the encapsulation by
random copolymers is independent of copolymer
type and comonomer composition. However, to establish whether this result is a general rule will
require further work.
This work was supported in part by the Center for Interfacial Engineering, an NSF-sponsored Engineering
Research Center at the University of Minnesota. M.S.L.
thanks the Korea Science and Engineering Foundation
(KOSEF) and Chonnam National University (CNU)
for fellowship support.
REFERENCES AND NOTES
1. D. R. Paul, in Polymer Blends, Vol. 2, D. R. Paul
and S. Newman, Eds., Academic Press, New York,
1978.
2. L. A. Utracki, Polymer Alloys and Blends, Hanser
Publishers, Munich, 1989.
3. L. A. Utracki, Ed., Encyclopaedic Dictionary of
Commercial Polymer Blends, ChemTec Publishing,
Toronto, 1994.
4. S. Datta and D. Lohse, Polymeric Compatibilizers,
Hanser Publishers, Munich, 1996.
5. C. D. Park, W. H. Jo, and M. S. Lee, Polymer, 37,
3055 (1996).
polpa
W: Poly Physics
9713012
2842
LEE, LODGE, AND MACOSKO
6. P. Guegan, C. W. Macosko, T. Ishizona, T. Hirao,
S. Nakayama, Macromolecules, 27, 4993 (1994).
7. C. W. Macosko, P. Guegan, A. K. Khandpur, A. Nakayama, P. Marechal, and T. Inoue, Macromolecules, 29, 5590 (1996).
8. See, for example, R.-J. Roe and D. Rigby, Adv.
Polym. Sci., 82, 105 (1987).
9. H. R. Brown, K. Char, V. R. Deline, and P. F.
Green, Macromolecules, 26, 4155 (1993).
10. C. A. Dai, B. J. Dair, K. H. Dai, C. K. Ober, E. J.
Kramer, C. Y. Hui, and L. W. Jelinski, Phys. Rev.
Lett., 73, 2472 (1994).
11. K. Cho, T. O. Ahn, H. S. Ryu, and K. H. Seo, Polymer, 37, 4849 (1996).
12. R. Kulasekere, H. Kaiser, J. F. Ankner, T. P. Russell, H. R. Brown, C. J. Hawker, and A. M. Mayes,
Macromolecules, 29, 5493 (1996).
13. M. Sikka, N. N. Pellagrini, E. A. Schmitt, and K. I.
Winey, Macromolecules, 30, 445 (1997).
14. S. T. Milner and G. H. Fredrickson, Macromolecules, 28, 7953 (1995).
15. Y. Lyatskaya, D. Gersappe, N. A. Gross, and A. C.
Balazs, J. Phys. Chem., 100, 1449 (1996).
16. K. I. Winey, M. L. Berba, and M. E. Galvin, Macromolecules, 29, 2868 (1996).
8Q46
/ 8q46$$3012
10-20-97 05:06:51
17. S. Thomas and R. E. Prud’homme, Polymer, 33,
4260 (1992).
18. J. S. Trent, J. I. Scheinbeim, and R. R. Couchman,
Macromolecules, 16, 589 (1983).
19. U. Sundararaj and C. W. Macosko, Macromolecules, 28, 2647 (1995).
20. B. Christ and A. K. Nesarikar, Macromolecules, 28,
890 (1995).
21. A. K. Chesters, Trans. Inst. Chem. Eng., 69, 259
(1991).
22. A. Saboni, C. Gourdon, and A. K. Chesters, J. Colloid Interface Sci., 175, 27 (1995).
23. I. Fortelny and A. Zivny, Polymer, 36, 4113 (1995).
24. S. Y. Hobbs, M. E. J. Dekkers, and V. H. Watkins,
Polymer, 29, 1598 (1988).
25. E. Helfand and Y. Tagami, J. Polym. Sci., Lett., 9,
741 (1971).
26. G. ten Brinke, F. E. Karasz, and W. J. MacKnight,
Macromolecules, 16, 1827 (1983).
27. D. G. H. Ballard, G. D. Wignall, and J. Schelten,
Eur. Polym. J., 9, 965 (1973).
28. F. C. Stehling, T. Huff, C. S. Speed, and G. Wissler,
J. Appl. Polym. Sci., 26, 2693 (1981).
29. T. Kyu, J. M. Saldanha, and M. J. Kiesel, in TwoPhase Polymer Systems, L. A. Utracki, Ed., Hanser
Publishers, Munich, 1991.
polpa
W: Poly Physics
9713012