Novel PDLC films based on A photoactive polymeric binder

Article abstract of DOI:10.1080/15421400390221835

We report the synthesis and characterization of a novel polymeric material which has been used as binder in the fabrication of PDLC films. A monomer bearing a carbazolyl moiety has been copolymerized, using azobisisobutyrronitrile (AIBN) as initiator, with a monomer containing a mesogenic core. PDLC films have been prepared using copolymers with different composition in the two monomers. A. detailed morphological analysis has been carried out to evaluate a correlation between the copolymer composition and the PDLC film morphologies.

Full text of DOI:10.1080/15421400390221835

Mol. Cryst. Liq. Cryst., Vol. 398, pp. 281–291, 2003
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390221835
NOVEL PDLC FILMS BASED ON A PHOTOACTIVE
POLYMERIC BINDER
Alessandro Manni, Luigi Gobbi, and Francesco Simoni
Dipartimento di Fisica e Ingegneria dei Materiali e del
Territorio and Istituto Nazionale per la Fisica della Materia,
Universit a` di Ancona, Via Brecce Bianche,
I-60131 Ancona, Italy
Markus Klapper and Wolfgang Knoll
Max-Planck Institute for Polymer Research, Postfach 3148,
D-55201 Mainz, Germany
We report the synthesis and characterization of a novel polymeric material
which has been used as binder in the fabrication of PDLC films. A monomer
bearing a carbazolyl moiety has been copolymerized, using azobisisobutyrro-
nitrile (AIBN) as initiator, with a monomer containing a mesogenic core. PDLC
films have been prepared using copolymers with different composition in the
two monomers. A detailed morphological analysis has been carried out to
evaluate a correlation between the copolymer composition and the PDLC film
morphologies.
Keywords: PDLC film; optical storage; radical copolymerization
INTRODUCTION
PDLC (Polymer Dispersed Liquid Crystal) films are inhomogeneous com-
posite materials in which low molar mass liquid crystals are randomly
dispersed in a polymeric matrix [1].
Interest on these systems has arisen because of their employment in the
fabrication of optical devices such as switchable windows, light valves,
displays.
In recent years the possibility of using PDLC films as optical storage
media has faced out as well, making these materials even more attracting [2].
The combination of photoconducting and electro-optic properties,
achievable through the use of PDLCs, seems to be promising for the
development of new optical memories [3].
281

2
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A. Manni et al.
In these materials an inhomogeneous illumination of the sample under
the application of an electric field generates a spatial redistribution of the
charges and consequently a refractive index modulation due to the reor-
ientation of the liquid crystal domains throughout the sample. This effect
can then be used to store an optical information in the PDLC film.
In this article we describe the synthesis and characterization of a novel
polymeric material which has been used in PDLC fabrication with the aim
of exploiting further these devices as optical storage media.
We used a methacrylate bearing a carbazolyl unit (1) and a methacrylate
with a mesogenic core (2) as comonomers (Scheme 1). Monomer 1 is
responsible for the peculiar electric properties of the matrix since carba-
zolyl unit can be photo-oxidized and, under the application of an electric
field, the electrons are allowed to move across the bulk of the film.
A key-point in designing a new polymeric material for PDLC preparation
is the stabilization of the liquid crystal domains. In fact the electro-optical
properties and hence the possible marketing of such devices are deeply
influenced by the integrity of the films over a long period of time.
This stabilization can be ensured using a comonomer with the same
mesogenic unit of the liquid crystal mixture used for the preparation of
PDLC films. In this way it is possible to get areas in the polymeric matrix
towards which the liquid crystal molecules should have a greater chemical
affinity and then the tendency to segregate themselves there. This means
that it should be possible to control the sizes of liquid crystal domains
simply by varying the amount of mesogenic monomer in the copolymer.
Some attempts have already been tried to prepare PDLC using a bire-
fringent matrix [3,4]. In fact using a birefringent matrix it is possible a
better index matching between polymer and liquid crystal domain with the
consequence of reducing the haze phenomenon.
Accordingly we used as comonomer a methacrylate with a cyanobi-
phenyl group.
Several PDLC samples were prepared using as a binder copolymers with
different composition in the two comonomers. A detailed investigation has
been carried out on them using scanning electron microscopy (SEM)
connected with an image analysis system to establish a correlation between
the copolymer composition and the PDLC film morphologies.
EXPERIMENTAL PART
Materials
Methacryloyl chloride (Fluka), triethylamine and THF (Fluka) were dis-
tilled. Acetone was refluxed overnight on K CO and then distilled. 1,2-
dibromoethane was distilled. Benzene (Fluka), N-hydroxyethylcarbazole
2
3

Novel PDLC Films Based on a Photoactive Polymeric Binder
283
SCHEME 1

2
84
A. Manni et al.
0
Aldrich), 4-hydroxy-4 -cyanobiphenyl (Aldrich), hydroquinone (Aldrich),
(
methacrylic acid (Fluka) and N,N-dimethylformamide (Merck) were used
as received. AIBN was recrystallized by methanol.
5
Synthesis of N-(2-hydroxyethyl)carbazolyl Methacrylate (1)
The amount of 3.96 g (18.6 mmol) of N-hydroxyethylcarbazole was dis-
solved in 100 ml of benzene in a three-neck round bottom flask, equipped
with dropping funnel and under inert atmosphere. 3.10 ml (22.2 mmol) of
triethylamine were added. The solution was cooled by mean of an ice-bath
and, under vigorous stirring conditions, 2.10 ml (22 mmol) of metha-
cryloyl chloride were added dropwise over a time of 20 min. After one hour
the ice-bath was removed and the reaction let for two hours at room tem-
perature.
Then ammonium chloride was filtered off and the organic phase was
washed with NaOH 5% and water. The organic layer was dried over Na SO
and the solvent was evaporated. The product was recrystallized by
2
4
methanol to yield 2.5 g (8.9 mmol) of 1 (yield: 48%).
1
ꢀ
IR (KBr) n/cm : 1719.2 (C ¼ O).
1
H-NMR (CDCl ): d 7.90 (2H, aromatic), 7.25–7.04 (6H, aromatic), 5.71
3
(
1H,vinylic),5.26(1H,vinylic),4.41–4.32(4H,-CH CH -),1.59(3H,methyl).
1
2
2
3
C (CD Cl ): d 167.37, 140.83, 136.26, 126.23, 126.11, 123.27, 120.60,
2
2
1
19.53, 109.13, 62.90, 18.32.
0
6
Synthesis of 1-bromo-2-(4 -cyanobiphenyl-4-yloxy)ethane
0
The amount of 2.72 g (13 mmol) of 4-hydroxy-4 -cyanobiphenyl and
1.8 g (116 mmol) of 1,2-dibromoethane were added to a well-stirred
suspension of K CO (13.47 g, 97.5 mmol) in 150 ml of dry acetone in
2
2
3
a three-neck round bottom flask, under inert atmosphere. The solution
was refluxed overnight. The solution was filtered hot and the
residue washed with acetone; solvent and unreacted 1,2-dibromo-
ethane were removed under reduced pressure. The residue was cromato-
graphated on silica gel (eluant: petroleum ether/ethylacetate ¼ 8/2) and
the second collected fraction was recrystallized by ethanol to give
0
1
4
.697 g (5.6 mmol) of 1-bromo-2-(4 -cyanobiphenyl-4-yloxy)ethane(yield:
3.2%).
1
H-NMR (CD Cl ): d 7.73–7.56 (6H, aromatic), 7.04–7.0 (2H, aromatic),
2
2
4
1
.35 (2H, -CH O), 3.69 (2H, -CH Br).
1
2 2
3
C (CD Cl ): d 159.09, 145.13, 132.89, 132.41, 128.76, 127.37, 119.26,
2
2
15.53, 110.57, 68.37, 29.82.

Novel PDLC Films Based on a Photoactive Polymeric Binder
285
0
Synthesis of 2-(4-cyanobiphenyl-4 -yloxy)ethyl
Methacrylate (2)
The amount of 0.183 g (2.13 mmol) of methacrylic acid and 0.183 g
2.04 mmol) of KHCO were stirred to form the salt, in a double-neck round
(
bottom flask, under inert atmosphere. 0.437 g (1.44 mmol) of compound 3
3
were diluited in 4 ml of N,N-dimethylformamide and added to the salt. Other
0 ml of DMF and a small amount of hydroquinone were added and the
1
solution was refluxed. The reaction was monitored by thin layer cromato-
graphy and after two hours the starting material had completely reacted. On
cooling the mixture was poured into water (100 ml) and well stirred to
coagulate the precipitate, which was dissolved in dichloromethane.
The organic solution was washed with NaOH 5% and then with water.
The organic layer was dried on Na SO and the solvent removed under
2
4
reduced pressure. The residue was purified by column cromatography
eluant: petroleum ether/ethylacetate ¼ 8/2) to give 0.375 g (1.22 mmol) of
compound 2 (yield: 84.8%).
(
ꢀ1
IR (KBr) n/cm : 1720.4 (C ¼ O), 1599.9 (C ¼ C).
1
H-NMR (CDCl ): d 7.69–7.50 (6H, aromatic), 7.02–6.98 (2H, aromatic),
3
6
(
.13 (1H, vinylic), 5.58 (1H, vinylic), 4.51 (2H), 4.25 (2H), 1.93
3H, methyl).
1
3
C (CD Cl ): d 167.40, 159.59, 145.25, 136.56, 132.91, 132.19, 128.72,
2
2
1
27.38, 125.92, 119.30, 115.52, 110.56, 66.54, 63.30, 18.38.
Synthesis of Copolymers Containing Carbazolyl and
Cyanobiphenyl Moieties (3)
All copolymeric samples were prepared following the same procedure
(Scheme 2).
Weighted amounts of monomer 1 and monomer 2 were dissolved in
distilled THF and put in a shlenkerohr. The total concentration of the
monomer was held constant at 0.4 M in each reaction. AIBN (0.25% mol
with the respect to the monomers) was added. The solution was degassed
using the ‘‘freeze and thaw’’ technique (four times) and then let under
ꢁ
argon atmosphere. The schlenkrohr was placed in an oil bath at 65 . The
reaction was stopped after four hours freezing the solution in liquid
nitrogen. The solution was diluted with THF and precipitated in a large
amount of methanol. Precipitation was repeated twice. The collected
polymer was dried in vacuum overnight (for the conversions see Table 1).
Preparation of the PDLC Films
The PDLC samples were prepared by means of the TIPS tecnique [8]. A
cell, which is composed by two conductive glasses (3 cm61.5 cm60.1 cm)

2
86
A. Manni et al.
SCHEME 2
TABLE 1 Characterization of Copolymeric Samples
Conversion
(%)
Sample
M
w
M
n
Dispersion
T
g
C1
C2
70.6
68.3
63.2
64
63.600
79.300
70.000
79.800
27.000
37.400
35.700
42.200
2.36
2.12
1.95
1.88
139.16
137.99
135.93
135.55
C2.5
C3

Novel PDLC Films Based on a Photoactive Polymeric Binder
287
TABLE 2 Mixtures Used in PDLC Preparation
Sample code
Polymer (%)
Liquid crystal (%)
TNF (%)
1
M1
M2
M3
M4
M5
49 (H/1)
50
50
50
50
50
1
1
1
1
1
49 (C1)
49 (C2)
49 (C2.5)
49 (C3)
1
Homopolymer obtained by radical polymerization of monomer 1.
and kept to a defined thickness (23 mm) by Mylar spacers, was filled by
capillarity at 170 , using the mixtures listed in Table 2.
ꢁ
The mixtures, composed by the eutectic liquid crystal mixture E7
Merck Ltd.), copolymer 3 and 2,4,7-trinitrofluorenone (TNF) in the ratio
(
ꢁ
5
0:47:1, were stirred at 170 before use. After filling, the samples were put
ꢁ
in a oven at 170 for 10 min and then were allowed to reach room tem-
perature slowly. On cooling the mixture enters a region of the phase dia-
gram where the homegenous solution is not stable and phase separation
takes place. The obtained PDLC are opaque and strongly scatter the light.
RESULTS AND DISCUSSIONS
Synthesis and Characterization of the Copolymer
Four samples of copolymer 3, with different compositions in the comono-
mers, were synthesized using radical polymerization.
The amount of monomer 2 was sistematically varyed in the feed with the
aim of clarifying what is the role it plays in determining the film mor-
phology. Its amount in the feed ranged from 5% to 20% by weight. Copo-
lymeric samples were identified, with the respect to the amount of
monomer 2 in the feed, as C1 (5% by weight of 2 in the feed), C2 (10%),
C2.5 (15%), C3(20%).
Conversions, weight-average molecular weight (M ), number-average
w
molecular weight (M ) and glass transition temperature (T ) for all the
n
g
copolymeric samples are listed in Table 1.
As far as it can be seen, rising the concentration of monomer 2 in the
feed causes an increase in polymer masses, which can be explained sup-
posing an higher reactivity of that monomer compared to the one of
monomer 1.
In fact, usually increasing the amount of the more reactive monomer in
the feed the propagation step is faster and consequently it is possible to get
polymers with higher masses. Considering the chemical structures it can be

2
88
A. Manni et al.
noticed that the polymerizable units are the same for both monomers. The
different reactivity could then be explained claiming the steric demand of
the two monomers. The steric hindrance of the cyanobiphenyl group,
probably because of its rod-like shape, should be lower than the carbazolyl
group’s one. Then, addiction of monomer 2 to the propagating species has a
lower activation energy and it takes place faster. Compositional analysis of
copolymeric samples, performed using NMR an UV spectroscopy, has
confirmed this finding.
The molar percentage of monomer 2 in the copolymers has been
determined using NMR spectroscopy by the ratio between the signal at
7
6
.0 ppm, due to two protons of the carbazolyl moiety, and the signal at
.7 ppm belonging to two protons of the cyanobiphenyl group. The
compositional analysis was carried out also using UV spectroscopy to
get a confirmation of the data obtained by NMR spectroscopy. Poly-(2-N-
carbazolylethylmethacrilate) was choosen as a reference compound. This
polymer and copolymer 3 show an absorption peak at 342 nm. UV spectra
were run for a set of solutions of poly-(2-N-carbazolylethylmethacrilate) in
methylene chloride to get a calibration curve plotting absorbance values
against percent concentration of the homopolymer.
Then the molar percentage of monomer 1 has been determined from the
UV spectra of different kinds of copolymer 3 using the calibration curve
and, by difference, the amount of monomer 2 has been derived.
The data, obtained using the two different spectral tecniques, are con-
sistent and they both confirm the preference in the addiction of monomer 2
with the respect to the addiction of monomer 1 in the copolymer. This
behaviour can be observed in Figure 1 where the molar percentage of
monomer 2 is plotted versus its molar percentage in the feed. The dotted
line refers to an ideal behaviour according to which monomer 2 is incor-
porated in the copolymer in the same proportion as in the feed.
Morphologic Analysis of The PDLC Films
PDLC films were prepared under the same conditions using the mixtures
listed in Table 2. The only difference among the mixture is in the compo-
sition of the copolymer.
All the samples were investigating by scanning electron microscopy
SEM). In our work a Philips XL 20 SEM operating at 7 kV was used to
(
study the morphology of the samples. A professional Image Analysis Soft-
ware Package (Image Pro-Plus by Mediacybernetics) was used to perform
spatial calibrations of the acquired SEM pictures and statistical analysis of
the droplet diameter distributions. All the observed samples show the
typical cheese-like morphology of PDLC films with droplet diameters ran-
ging from 0.5 to 3.2 mm.

Novel PDLC Films Based on a Photoactive Polymeric Binder
289
FIGURE 1 Analysis of copolymer composition.
The statistical analysis performed on a large quantity of samples pre-
pared using different mixtures put in evidence that the average domains
diameter grows as the amount of monomer 2 in the copolymer 3 gets
bigger. This trend is clearly shown in Figure 2 where the mean domains
diameter is plotted as a function of monomer 2 molar concentration in the
feed. A comparison between sample M1 and sample M5 allows us to do the
following considerations. We used poly-(2-N-carbazolylethylmethacrylate)
to prepare mixture M1, while we used copolymer C3 to prepare mixture
M5. It is then possible to evaluate the effect of the introduction of a
monomer with a mesogenic core in the matrix on the morphology of the
PDLC films.
SEM micrographs and the respective droplet diameter distributions for
samples M1 and M5 are shown in Figure 3.
It has been found that the mean diameter of liquid crystal domains is 1.1
mm for sample M1 and 2.7 mm for sample M5. It is possible to explain this
evidence supposing that the mesogenic units in the copolymer form cavities,
whose dimensions grow bigger as the concentration of monomer 2 increases.
Liquid crystal molecules, considering the similarity among their structure
and the one of the mesogenic core present in the copolymer, diffuse in those
cavities during phase separation process before polymer gelation occurs.
Moreover, the two distributions shown in Figure 2 clearly prove that the
liquid crystal domains number decreases passing from sample M1 to sample
M5. The increase of the amount of monomer 2 in the copolymer could lead

2
90
A. Manni et al.
FIGURE 2 Average domains diameter as a function of the concentration of
monomer 2 in the feed.
FIGURE 3 SEM micrographs and droplet diameter distribution for samples M1
and M5.

Novel PDLC Films Based on a Photoactive Polymeric Binder
291
to an higher solubility of the liquid crystal in the polymer matrix, explaining
the decrease in domains number.
On the contrary even if the total number of liquid crystal droplets (for an
2
analyzed area of 6400 mm ), is 315 for sample M1 and 108 for sample M5, it
2
was found that the total area of the liquid crystal domains is about 412 mm
2
for sample M1 and about 682 mm for sample M5. This simply means that
the liquid crystal volume fraction separated from the polymeric matrix is
higher for sample M5 than for sample M1.
As a consequence it can be stated that the phase separation process is
better accomplished increasing the amount of monomer 2 in the copolymer
used as matrix in PDLC fabrication.
CONCLUSIONS
In this article we have reported the synthesis and the compositional
characterization of a novel polymeric material which shows both photo-
conductive and mesogenic cores as pendant groups. PDLC films have been
prepared using this copolymeric material with the aim of understanding the
relationship between the copolymer’s composition and the morphology of
the obtained PDLC films. A morphological analysis, performed by means of
scanning electron microscopy, has clarified the role played by the copoly-
mer composition in controlling the average domains size and in favouring
the phase separation process.
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