Thermo-physical investigation of PDLC materials prepared by polymerization induced phase separation

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Thermo-physical investigation of PDLC materials
prepared by polymerization induced phase separation
LEILA BENKHALED1, FATIMA ZOHRA ABDOUNE1, 2, LAHCENE MECHERNENE2,
XAVIER COQUERET1, and ULRICH MASCHKE1*
1) Laboratoire de Chimie Macromoléculaire, UMR CNRS N°8009, Université des Sciences
et Technologies de Lille, F-59655 Villeneuve d'Ascq Cedex, France
2) Laboratoire de Recherche sur les Macromolécules, Faculté des Sciences, Université
Aboubakr Belkaïd, Bel Horizon, 13000 Tlemcen, Algeria
Introduction
PDLCs or Polymer Dispersed Liquid Crystals are made of micron sized droplets dispersed in
a solid polymer matrix [1-3]. They exhibit interesting electro-optical properties since they can
switch between an opaque light scattering off-state to a transparent on-state by application of
an electric field. PDLCs are very useful in various applications including optical shutters and
privacy windows. Preparation of these films is often based on the polymerization induced
phase separation (PIPS) process using ultra-violet (UV) [1,2] and Electron Beam (EB) [4-5]
curing techniques.
Investigation of the phase behavior and thermophysical properties leads to important
parameters that bear a strong impact upon electro-optical responses and film performance
under practical conditions. It is useful to underline that the small molecules of liquid crystal
(LC) remaining within polymer matrix act as a plasticizer reducing substantially the
mechanical moduli of the film.
In this work thermophysical parameters of UV-cured Tripropyleneglycoldiacrylate
(TPGDA)/E7 PDLC films are determined as functions of the composition of the initial
mixture. The LC solubility limit in the polymer matrix and the fractional amount of LC
contained in LC domains are obtained by Differential Scanning Calorimetry (DSC). The
analysis of DSC thermograms allows to obtain the glass transition temperatures of the LC and
the polymer matrix as well as the nematic + isotropic / isotropic transition temperature.

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Experimental
a) Materials
The nematic LC used in this work is the eutectic nematic mixture E7 (Merck KGaA,
Germany) containing four cyanoparaphenylene derivatives. It exhibits a single nematic-
isotropic transition temperature TNI (LC) = 61°C. The monomer is Tripropyleneglycol-
diacrylate (TPGDA) (UCB, Belgium). 2wt-% (of the acrylate mixture) of a conventional
photoinitiator (LucirinTPO, BASF) was added to the initial mixtures prior to UV-curing.
b) Sample preparation
For UV-curing, a Minicure Model MC4-300 (Primarc UV technology) equipped with a
medium pressure mercury arc lamp rated 80 W per cm was used. The mixtures were placed in
a tray which was passed under the irradiation source using a conveyor belt. The curing dose
was 150 mJ/cm2.
c) DSC measurements
DSC measurements were performed on a Perkin Elmer Pyris Diamond calorimeter equipped
with an Intracooler 2P system allowing cooling experiments. A rate of 10°C/min (heating and
cooling) was used in the temperature range -70 to +100°C. The program consists first in
cooling the sample followed by three heating and cooling cycles. The thermograms presented
in this work were obtained from the first heating ramps. In each case, at least five duplicate
samples having the same composition and prepared independently were used to check the
reproducibility of results. The polymer glass transition temperature was determined from the
midpoint of the transition range of the thermograms.
Results and discussion
Figure 1 gives DSC thermograms of pure E7 (upper part 100 wt-%) and the cured polymer
matrix TPGDA (lower part 0 wt-%) at a rate of 10°C/min. The polymer shows a single
transition (glass transition T
g (P)) at 40°C in the temperature range explored by the
experiments whereas the low molecular weight LC shows 2 transitions: A glass transition at
Tg (LC)=-62°C followed by a nematic-isotropic transition T
NI (LC) = 61°C. The other
thermograms displayed on Figure 1 correspond to TPGDA/E7 mixtures at several
compositions spanning the range from 0 wt-% E7 to pure LC. An important question resolved
by these thermograms is the fact that the transitions characteristic of the polymer and the LC
do not overlap. The thermograms in Figure 1 were normalized by the weight of the samples

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allowing comparison of the results obtained as a function of composition. One clearly
observes the absence of a nematic-isotropic transition for LC concentrations lower than 40 wt-
% in relation to the solubility limit of E7 in the polymer matrix. Indeed, the LC does not
phase separate from the polymer up to this concentration and form a homogenous phase with
the polymer matrix. The same tendency can be observed by looking to the appearance of the
Tg (LC) transition.
100 % E7
0 % E7
-
60
20
40
60 80 -40
-20
0 100
Temperature (°C)
Tg TNI
Figure 1 DSC thermograms of TPGDA/E7 precursor mixtures exposed to UV curing with
150 mJ/cm2 ranging from 0 wt-% (pure TPGDA) to 100 wt-% of E7 (pure LC), in
steps of 10 wt-%.
Starting from roughly 40 wt-%, increasing nematic-isotropic enthalpies ∆HNI can be detected
which correspond to the increasing fraction of segregated LC molecules.
Figure 2 displays the variation of the polymer glass transition and TNI (LC) as a function of E7
concentration indicating a small decrease of Tg (P) from T=36.5 °C (Tg(P) of pure TPGDA)
to about T=32°C for the sample at 30 wt-% E7. Above this composition the glass transition
temperature of the mixture remains almost constant due to the solubility limit of E7 in the
cured TPGDA matrix. This decrease has been observed frequently and is due to the fact that
the LC plays the role of a plasticizer for the polymer. Another parameter that can be extracted
from these thermograms is the nematic-isotropic transition temperature of the LC confined in

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domains. TNI (LC) increases sharply from 40 wt-% to 50 wt-% then a plateau was obtained
until 90% where we can see a slight increase again to reach TNI of the pure E7.
30
31
32
33
34
35
36
37
38
39
40
010 20 30 40 50 60 70 80 90 100
E7 Concentration (wt-%)
Tg (°C)
0
10
20
30
40
50
60
70
TNI (°C)
TNI (LC)
Tg (P)
Figure 2 Glass transition temperature of the polymer matrix T
g (P) and nematic-isotropic
transition temperature TNI (LC) versus E7 concentration for UV-cured PDLC films.
Conclusion
Thermophysical properties of TPGDA/E7 PDLC films prepared by UV-radiation were
determined by DSC. The thermograms obtained show three clearly distinguishable transitions.
A single glass transition temperature corresponding to the polymer matrix was observed for
sample concentrations lower than 30 wt-% of E7. At higher composition, two LC transitions
appeared, a glass transition and a nematic-isotropic transition, indicating the presence of
phase separated LC domains. The nematic-isotropic transition temperature decrease when the
monomer is added to the pure LC.
References
[1] J. W. Doane, in Liquid Crystals - Applications and Uses, edited by B. Bahadur, World
Scientific, Singapore (1990).
[2] P. S. Drzaic, Liquid Crystal Dispersions, World Scientific, Singapore (1995).
[3] Liquid Crystals in Complex Geometries, edited by G. P. Crawford and S. Zumer,
Taylor&Francis, London (1996).
[4] U. Maschke, X. Coqueret, C. Loucheux, J. Appl. Polym. Sci. 56, 1547 (1995).
[5] L. Benkhaled, L. Méchernène, A. Traisnel, M. Benmouna, J.-M. Gloaguen, X. Coqueret,
U. Maschke, Mol. Cryst. Liq. Cryst. 375, 651 (2002).