814
Macromol. Rapid Commun. 21, 814–819 (2000)
Communication: A novel technique to contact (ultra-)
thin polymer layers is presented which enables to compare
the molecular dynamics in grafted films of poly(c-benzylL-glutamate) (PBLG) to that of the bulk polymer by
means of dielectric spectroscopy. Two relaxation processes are observed which are assigned to restricted fluctuations of the helical main chains and to the dynamic
glass transition of the side chains. Furthermore, the swelling behavior of PBLG is studied.
Sketch of contacting the sample by use of a solid mica-electrode. An aluminium stripe (b) is evaporated on a sheet of
freshly cleaved mica (a), thus forming the upper electrode.
For the base plate, aluminium stripes (d) are evaporated on
cleaned glass plates (e). PBLG-films (c) are grafted subsequently on the aluminium stripes and are contacted by applying the evaporated side of the mica sheet, thus composing
one sample capacitor. Wires are electrically connected to contact pads by use of silver paint (f)
Molecular dynamics of grafted PBLG in the swollen and
in the dried state
Lutz Hartmann1, Thomas Kratzmüller2, Hans-Georg Braun2, Friedrich Kremer* 1
1
University of Leipzig, Department of Physics, Linnéstr. 5, 04103 Leipzig, Germany
kremer@physik.uni-leipzig.de
2
Institute of Polymer Research, Hohe Straße 6, 01069 Dresden, Germany
(Received: January 17, 2000; revised: April 6, 2000)
Introduction
While the structure of grafted polymers is well explored
by a variety of experimental techniques (ellipsometry1),
neutron-reflectivity2) and X-ray-reflectivity measurements3)), little is known about the molecular dynamics in
these systems. Dielectric spectroscopy is a powerful tool
to probe these dynamics since its sensitivity increases
with decreasing thickness of the sample capacitor
arrangement4). On the other hand, it is a technical challenge to prepare capacitors with thicknesses in the order
of 10 nm containing grafted polymers. In this communication we describe a novel experimental approach to
solve this problem. For the first time, it enables to study
the molecular dynamics in (ultra-) thin grafted layers of
e. g. poly(c-benzyl-L-glutamate) (PBLG) in a broad frequency and temperature range. However, this technique
is applicable in general for thin organic films and offers
therefore the chance to investigate possible deviations of
Macromol. Rapid Commun. 21, No. 12
the dynamics in (ultra-) thin polymer layers with respect
to bulk behavior. The results are compared to those
obtained by evaporating the upper electrode directly on
the polymer film and to measurements on bulk material.
Furthermore, the molecular dynamics after swelling the
grafted polymer layer is studied.
The rigid rod-like polymer PBLG (Fig. 1 A) carries a
large dipole moment parallel to its helical axis. By grafting one end of the helix to a surface, one has to expect a
considerable net dipole moment perpendicular to the surface. Thus, the possibility is given to control structure,
orientation and dynamic properties by an external electrical field. This could lead to possible applications in the
areas of non-linear optics, liquid crystal displays, separation membranes or biosensors. Grafted layers of PBLG
have already been studied by several groups with regard
to molecular orientation5), electromechanical properties6)
and formation of microstructured layers7). In particular,
i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000
1022-1336/2000/1208–0814$17.50+.50/0
815
Molecular dynamics of grafted PBLG in the swollen and in the dried state
means of a nitrogen gas jet, thus obtaining a stability better than l 0.05 K10).
For the analysis of dielectric spectra, the imaginary
part e99 of the dielectric function is fitted using a superposition of a conductivity contribution and a generalized
relaxation function according to Havriliak-Negami
(Eq. (1))11). The activation behavior of the side-chain
fluctuation for the different samples could be described
by the Vogel-Fulcher-Tammann law (Eq. (2))12):
e99 ˆ
r0 a
De
ÿ
Im
a
c
e0 xs
1 ‡ …ixsHN † †
s ˆ so e
Fig. 1. (A) Primary and secondary structure of poly(c-benzylL-glutamate) (PBLG). (B) Sketch of contacting the sample by
use of a solid mica-electrode. An aluminium stripe (b) is evaporated on a sheet of freshly cleaved mica (a), thus forming the
upper electrode. For the base plate, aluminium stripes (d) are
evaporated on cleaned glass plates (e). PBLG-films (c) are
grafted subsequently on the aluminium stripes and are contacted
by applying the evaporated side of the mica sheet, thus composing one sample capacitor. Wires are electrically connected to
contact pads by use of silver paint (f)
the molecular dynamics of poly(glutamates) in the bulk is
well understood: It is known that PBLG has two dielectrically active relaxation processes which are assigned to
(restricted) fluctuations of the helical main chain as a
whole (chop stick motion) and to fluctuations of the side
groups8, 9).
Results and discussion
Dielectric measurements in a frequency range from
10–1 Hz to 106 Hz were performed using a SolartronSchlumberger frequency response analyzer FRA 1 260
with a Novocontrol active sample cell (BDC-S). Effects
due to a non-linear response of the sample were excluded
by setting the generator voltage to 0.4 V. Thus, electrical
fields of 1.3 6 105 V cm–1 within the sample were
applied. The sample temperature was controlled by
ÿ
1†
DT0
TÿT0
2†
In this notation, e0 is the vacuum permittivity, r0 the
DC-conductivity and De the dielectric strength. For
Ohmic behavior, s equals one; deviations (s a 1) are
caused by electrode polarization. The correct dimension
for s m 1 is obtained by the factor a: [a] = (H N z)s-1. Parameters a and c describe the symmetric and asymmetric
broadening of the relaxation time distribution function.
From fits according to Eq. (1), the relaxation rate 1/smax
can be deduced which is given by the frequency of maximum dielectric loss e99 for a certain temperature. In
Eq. (2), D is a measure for the fragility of the glass-forming substance13), T0 is the Vogel temperature and s0
denotes the limit of the relaxation time at high temperatures.
Bulk samples were prepared as films obtained from the
pressed molten PBLG and as cast films from a concentrated chloroformic solution of the polymer. Further
measurements with the latter were carried out after drying
the sample for several days. Both types of bulk samples
were contacted by placing the film between two gilded
brass electrodes. For good electric contact, a thin aluminium foil was brought between sample and electrodes.
Furthermore, bulk samples were prepared as solution cast
films on the same type of substrate which was used for
measurements on the grafted layers. The thickness of all
samples was measured with a Veeco Instruments alpha
stepper.
Independent of the preparation, all bulk samples deliver identical dielectric spectra except a rise in the loss e99
above 105 Hz due to the electrode resistance in case of
evaporated electrodes (Fig. 2). This effect has also been
observed in measurements on grafted layers and is due to
the electrode resistance R (less than 10 X) which leads to
an equivalent parallel RC circuit (C denotes the sample
capacity). A more detailed description of this effect can
be found in ref.14)
For preparation of grafted layers, glass substrates were
cut out of microscope slides, cleaned first by ultrasonification in a chloroform bath and then in a solution of a
commercial detergent (Hellmanex) in ultrapure water
816
L. Hartmann, T. Kratzmüller, H.-G. Braun, F. Kremer
Fig. 2. Dielectric loss e 99 of a bulk sample cast from solution
(j) and of a grafted layer (C = contacted by a solid top electrode,
F = evaporated aluminium stripes as top electrode; for both
cases data are corrected as explained in the text) of PBLG at
416 K and 318 K with the corresponding numerical fits. The
relaxation process at 416 K corresponds to the restricted motion
of the helical main chains (chop-stick-motion), that one at 318 K
corresponds to the motion of the side chains. The second process
at low frequencies in the grafted layer at 416 K is assigned to an
electrode polarization process. Data of the relaxation processes
are fitted to the Havriliak-Negami function ( N N N polarization
process, — conductivity). The solid lines indicate the two different relaxation processes while the thick solid lines represent the
sum of all contributions. Two typical sets of fit parameters are
given for the bulk sample: 318 K: De = 0.66, s = 1.4 N 10–5 s, a =
0.65, c = 0.496; 416 K: De = 2.496, s = 0.0245 s, a = 1, c = 1, r0
= 3.93 N 10–10 S N m–1, s = 0.82
drawn from a Millipore water system. After thoroughly
rinsing with ultrapure water, the substrates were blown
dry with nitrogen and subsequently put into the recipient
of a Leybold metal coater for evaporation of three parallel
aluminium electrodes of about 70 nm thickness. The
(ultra-) thin grafted PBLG layers were obtained by a surface initiated polymerization process of the corresponding N-carboxy anhydride (NCA)7). First the cleaned aluminium surfaces were modified by adsorption of 1-phosphoric acid, 12-N-ethylaminododecane which serves as
initiator layer for the polymerization. In a second step,
the so altered surface reacts with a solution of the NCA
of c-benzyl-L-glutamate in THF. After 24 h, the substrates
were removed from the solution and washed with chloroform. The thickness (30 l 5) nm of the layer was determined by means of ellipsometry. Before further treatment, the samples were dried in an oil-free vacuum
(10–5 mbar) at 60 8C. Subsequently, the top electrode has
been established either by a second evaporation of aluminium or by application of the mica method as described
in the following:
The straight-forward approach to contact (ultra-) thin
polymer layers is to evaporate a metal counterelectode4, 14). This technique has the serious drawback that it
often results in electrical shortcuts since (ultra-) thin
polymer layers are never free of defects. Moreover, in
order to ensure an acceptable electric conductivity of the
electrode, one has to evaporate a metal layer (e. g. Au, Ag
or Al) of about 70 nm thickness on the underlying polymer film having a thickness of about 50 nm. Taking this
into account, one has to envisage artifacts for the structure and for the dynamics of the grafted layer after this
treatment. These difficulties are circumvented by our new
technique to contact (ultra-) thin polymer layers.
To prepare the grafted polymer in a capacitor arrangement, the grafted layer is covered with a freshly cleaved
lamella of mica (thickness 5 lm) which was previously
evaporated with aluminium stripes having a thickness of
70 nm. Thus, a sample capacitor with an area of 2 mm2 is
formed (Fig. 1 B). Mica offers the advantage that it shows
atomically flat surfaces after being cleaved off. To maintain this purity of the surfaces, the mica was cleaved in a
flow box and placed in a small box immediately afterwards where it could be kept under protection gas (pure
nitrogen). This box was mounted in the evaporation apparatus (Leybold Univex 300) where it was opened after
evacuation of the evaporation chamber by means of a
remote control. Thus, contamination of the mica surface
and, after evaporation, of the electrode surface was
avoided.
A comparison between bulk sample, grafted layer with
evaporated electrodes and grafted layer prepared with the
novel mica method delivers similar results for the
dynamic glass transition of the side chains (in contrast to
the chop stick motion at higher temperatures; see below).
In case of the side chain fluctuation, the spectra of e 99
obtained on the grafted layer are broadened in comparison to the bulk data (for both preparation techniques) and
the relaxation time smax determined by the maximum position of e 99 is shifted to longer times (Fig. 2 and Fig. 3).
It is immediately clear that one can not assume to have
contact between the upper electrode and the polymer
layer over the whole contact area. Consequently, one has
to expect distinct differences between the measured capacity and the calculated value according to the geometry
of the sample capacitor. The data of the dielectric loss e 99
Molecular dynamics of grafted PBLG in the swollen and in the dried state
Fig. 3. Activation plot for the two relaxation processes in dried
and swollen state. The solid symbol f denotes both processes of
the bulk measurement for an unswollen sample. The corresponding measurements on the grafted layer are denoted by the open
symbols: C and D are used to mark the fluctuation of the helical
main chains and the electrode polarization process at high temperatures (mica technique), F- and G indicate the data of the side
chain motion obtained by use of evaporated electrodes and of
the mica method, respectively. * is used to denote the side chain
fluctuation after the swelling procedure. In this case, S denotes
the starting point and E denotes the end of the measurement of
the swollen sample. Fits according to the VFT law are shown by
solid lines for the bulk and for the (ultra-) thin film before the
swelling procedure. The following fit parameters were obtained
for the bulk: log (1/s0) = 10.3, D N T0 = 670.8 K, T0 = 264.5 K
and for the grafted layer: log (1/s0) = 11.9, D N T0 = 1 270.2 K, T0
= 240.3 K. Arrows indicate the dielectrically determined values
for the glass transition temperature Tg, i. e. the temperature
which corresponds to a relaxation rate of smax = 100 s: Tgbulk =
(280 l 2) K, Tglayer = (287 l 2) K
show this discrepancy as well, but the spectra can be
brought into quantitative accordance by multiplying the
measured values of e 99 with the ratio evbulk/evgrafted, where ev
denotes the high frequency values of the real part e 9 of
the dielectric function:
e99corrected ˆ
ebulk
v
e99measured
egrafted
v
3†
Corrected values for e 99 of the grafted layer obtained by
following this procedure are shown in Fig. 2. The remaining differences between the bulk spectra and those of the
grafted layer with evaporated electrodes can be explained
by errors in the determination of the layer thickness and
of the contact area of the electrodes due to the surface
roughness of the polymer film. The introduced corrections are important for the discussion of the dielectric
strength De; the frequency position of the maximum loss,
corresponding to smax, is not influenced by these effects.
One has to state that, as an advantage of the new technique of contacting (ultra-) thin polymer films, the observation of the chop stick motion became only possible if a
solid upper electrode was used. In this case, a second
relaxation process occurred which is assigned to an electrode polarization effect because of its comparatively
high dielectric strength and the fact that it is the slowest
of all observed processes. In contrast, the evaporation of
metal electrodes led to noisy spectra at high temperatures
which did not permit the analysis of the dielectric loss
using Eq. (1). Consequently, information concerning the
chop stick motion within the grafted layer is lost in case
of evaporated electrodes.
The spectra of e 99 of the dynamic glass transition of the
side chains within the grafted layer are much broader
than those of the bulk sample. This effect of a broadening
of the relaxation time distribution is in accordance with
findings of similar measurements on thin films4, 14), as
well as on low molecular weight glass forming liquids
confined to a nanoporous matrix15–18). While in the cited
publications, this result was estimated as an indication for
the existence of cooperative rearranging regions (CRR)
which determine the dynamics in vicinity of the glass
transition temperature Tg, this explanation does not hold
in our case. Since side chain fluctuations should not be
affected by the geometric confinement, we explain our
findings by the assumption that the potential governing
the dynamics of the side-chain dipoles is more heterogeneous in the grafted layer than in the bulk, thus leading to
a broadening and a slowing down of this process.
All spectra of the side chain fluctuation were fitted to
the equation of Havriliak and Negami and thus the activation plot (Fig. 3) has been obtained. Concerning the thermal activation, one has to point out that the local fluctuation of the side chains would be expected to have an Arrhenius-like activation behavior. This was indeed found for
glutamates with methyl side chains only19). However, for
PBLG or similar substances with longer side chains, the
thermal activation of the corresponding relaxation can be
well described by the VFT law8, 14), thus indicating that in
these cases the side chains undergo a glass transition.
As a further difference between bulk and grafted layer
besides the broadening of the relaxation time distribution,
it is found that the dynamic glass transition of the side
chains in the (ultra-) thin grafted layer is slowed down in
817
818
L. Hartmann, T. Kratzmüller, H.-G. Braun, F. Kremer
comparison to the bulk (Fig. 3). Since these differences
between bulk and grafted layer were obtained independently of the preparation of the bulk sample and of the way
of contacting the (ultra-) thin layer, they have to be attributed either to the different structure of both films or to the
decreased thickness of the grafted layer. To decide this
question, further investigations into grafted films with
different thicknesses and spin-coated films of PBLG have
to be done. The chop stick motion in the grafted layer is
found to be faster than in the bulk reference. This can be
explained by the assumption that the molecular weight of
the grafted PBLG is smaller than in the bulk. The determination of both values is in progress.
To improve the contact between layer and upper electrode, PBLG was swollen in a chloroform atmosphere for
several hours. As result of this procedure, a higher capacity was achieved which was in better agreement with
calculated values. The calculated capacity is 8 6 10–10 F,
while the measured value was around 3 6 10–10 F.
Furthermore as a result of the swelling procedure, the
side chain fluctuation becomes faster in the swollen sample (Fig. 3). With increasing temperature, the content of
chloroform decreases, and the relaxation rate tends to the
values of the unswollen sample. Measurement from high
to low values of the temperature results in the same thermal activation as the unswollen sample, thus indicating
that the swelling process is fully reversible. The chop
stick motion is not affected by the swelling procedure.
The dielectric strength De of the side chain fluctuation
can be well described by
De ˆ
nl2eff
3kTe0
Fig. 4. Relaxation strength De of the side chain fluctuation for
the grafted layer (corrected values; C – evaporated electrodes,
D – mica technique) and for the bulk (f). Calculated values
according to Eq. (4) are given by the dashed line (- - -)
4†
where n denotes the density of dipoles and leff their effective dipole moment. The calculated values in Fig. 4 were
obtained by assuming leff = 1.1 D6) and n = 5 6 1021 cm–3.
The latter value is well comparable with an estimation
taking into account approximate values for chain length
(60 nm) and grafting density (0.005 Å–2). The comparison
between calculated and measured values is shown in
Fig. 4.
While there is a good agreement between calculation
and the corresponding measurements on bulk and grafted
layer prepared according to the mica method, De is notably higher when the electrodes were evaporated. This difference is attributed to changes within the grafted layer
during the evaporation of aluminium.
Concerning the chop stick motion, it has to be considered that this motion is restricted by interactions between
neighboring chains and by fixing one end of the chains to
the electrode surface leading to a reduction of De (Fig. 5)
compared to the expected values according to Eq. (4).
This situation corresponds to the model of Wang and
Pecora20) who described the restricted rotational diffusion
Fig. 5. Relaxation strength De of the fluctuation of the helical
main chains: D denotes the corrected values of the grafted layer
contacted by the mica technique, while f is used for bulk values.
The inset illustrates the restricted chop-stick-motion according
n indicates the length axis
to the model of Wang and Pecora20): ~
of the rod-like molecule and h0 the apex angle of the cone which
restricts the fluctuation of the rod-like molecule having a tilt
angle h
Molecular dynamics of grafted PBLG in the swollen and in the dried state
of rod-like molecules. Thereby, the fluctuation of the
molecule length axis is constrained to a cone with an
apex angle h0 (inlet in Fig. 5). In the frame of this model,
it is possible to deduce a relation connecting Derod for the
free fluctuation of helical main chains according to
Eq. (4) and the values Dechopstick obtained from the fits:
1
2
Dechopstick ˆ Derod 1 ÿ …1 ‡ cosh0 †
4
5†
Application of Eq. (5) to the data delivers an angle h0
of approximately 2 8 which corresponds well to ref.8)
There, similar glutamic acid containing polymers have
been investigated by means of dielectric spectroscopy;
for evaluation of the bulk data, the model of Wang and
Pecora has been applied as well.
Acknowledgement: The authors are grateful to Dr. Iris Maege
(Institut für Makromolekulare Chemie, TU Dresden) for the
synthesis of the 1-phosphoric acid, 12-N-(ethylamino)dodecane
initiator. Financial support within the Sonderforschungsbereiche
294 “Moleküle in Wechselwirkung mit Grenzflächen” and 287
“Reaktive Polymere in nichthomogenen Systemen, in Schmelzen
und an Grenzflächen” is highly acknowledged.
1)
2)
3)
4)
5)
6)
7)
Conclusions
In summary, a novel method is introduced which to our
best knowledge enables for the first time to study the
molecular dynamics in (ultra-) thin layers of grafted
PBLG. The results are compared to measurements on
bulk samples and on grafted PBLG layers contacted by
evaporating metal electrodes. Two relaxation processes
are observed which are assigned to (restricted) fluctuations of the helical main chains as a whole (chop stick
motion) and to the dynamic glass transition of the side
chains. The latter shows a VFT-temperature dependence
which scales well with calorimetric measurements9). Its
relaxation time distribution is broadened in case of the
grafted layers with respect to the bulk, and its relaxation
rate smax is slightly shifted to longer times. The chop stick
motion within the grafted layer can only be observed
when the mica method is applied. Results obtained for
the dielectric strength De are in good agreement with calculations based on free dipole fluctuations (side-chain
motion) and on the model of Wang and Pecora (chop
stick motion). Swelling the polymer with chloroform
leads to a faster motion of the side chains. This effect is
fully reversible.
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