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ORIGINAL PAPER
Influence of the multi-functional epoxy monomers
structure on the electro-optical properties
and morphology of polymer-dispersed liquid crystal
films
Mujtaba Ellahi •Fang Liu •Ping Song •Yanzi Gao •
Hui Cao •M. Y. Rafique •Murad Ali Khaskheli •
M. Zubair Iqbal •Huai Yang
Received: 30 January 2013 / Accepted: 18 July 2013 / Published online: 1 August 2013
ÓSpringer-Verlag Berlin Heidelberg 2013
Abstract Polymer dispersed liquid crystal (PDLC) films were prepared by poly-
merization-induced phase separation method with nematic LC content as low as
40 wt%, and the electro-optical properties were carefully investigated. To accom-
plish this, the structure of multi-functional curable epoxy monomers with different
composition feed ratios and the weight percentages of the two groups were
examined in this study. The combined effects of heat-curable monomers’ structure
on the conspicuous morphology of polymer network of PDLC films formed small
holes and suitably distributed coin-like networks in both groups A and B, respec-
tively. The detailed characteristics and morphology of polymer network of PDLC
films were analyzed by employing liquid crystal device parameter tester, UV-Vis-
M. Ellahi F. Liu P. Song Y. Gao H. Cao (&)H. Yang (&)
Department of Materials Physics and Chemistry, School of Materials Science and Engineering,
University of Science and Technology Beijing, Beijing 100083, People’s Republic of China
e-mail: caohui@mater.ustb.edu.cn
H. Yang
e-mail: huaiyang@mater.ustb.edu.cn; ellahimujtaba@yahoo.com
M. Y. Rafique
Department of Physics, University of Science and Technology Beijing, Beijing 100083,
People’s Republic of China
M. A. Khaskheli
School of Mathematics and Physics, University of Science and Technology Beijing,
Beijing 100083, People’s Republic of China
M. Zubair Iqbal
Department of Physics, School of Applied Science, University of Science and Technology Beijing,
Beijing 100083, People’s Republic of China
H. Yang
Department of Materials Science and Engineering, College of Engineering, Peking University,
Beijing 100871, People’s Republic of China
123
Polym. Bull. (2013) 70:2967–2980
DOI 10.1007/s00289-013-1000-6
NIR spectrophotometer and scanning electron microscope. Meanwhile, the
enhanced curing temperature effects on the alkyl chain length, short flexible chain
length, and rigid chain segment containing epoxy monomers structure on the
increasing morphology of polymer network as well as electro-optical properties of
PDLC films were also studied. It was found that the LC domain size of the polymer
network could be regulated by adjusting the structure and composition ratio of
curable epoxy monomers, and then the electro-optics of the PDLC films could be
optimized, which is beneficial for decreasing the total LC content in PDLC devices.
Keywords Polymer-dispersed liquid crystal Epoxy monomers
Electro-optical properties Morphology
Introduction
Polymer-dispersed liquid crystal (PDLC) films have been studied for many years.
PDLC films constitute a high active research area in polymer science, a
comparatively new technology of materials that consist of micron-sized LC
droplets dispersed in a solid polymer matrix [1–3]. Optical and electro-optical (E-O)
properties of PDLC films make them perfect for many applications in different
fields, such as flexible displays, switchable windows, botanical garden, and other
technological display devices [4,5]. The instant transition of the PDLC film is
helpful for controlling shade and privacy to completely block the view or adjust
degree of its opaque-based film. When an electric field is applied to the PDLC film,
the nematic LC in the droplets reorients so that the director is parallel (k) to the
field, and therefore perpendicular (\) to the plane of the film. If the ordinary
refractive index of the liquid crystal (n
o
) is matched with the refractive index of the
polymer (n
p
), then light incident normal to the film does not encounter any variation
in refractive index, and passes through the film without being scattered. Such PDLC
films are therefore opaque in the off state, but become clear when voltage is applied
[6–8]. The transparency of the PDLC films is greatly improved because no polarizer
is needed [9]. On the other hand, on removal of the electric field, the anchoring
forces between the liquid crystal and the polymer droplet walls act to restore the LC
molecules to their original orientation, and the film once again becomes scattered.
Usually, the E-O properties of PDLC films can be affected by the LC concentration,
film thickness, separation degree and dimension, composition ratio of monomers
and morphology of the LC domain size [10]. Four general methods have been
developed for the formation of PDLC films, including encapsulation, polymeriza-
tion-induced phase separation (PIPS), thermal-induced phase separation (TIPS) and
solvent-induced phase separation (SIPS) methods [11–13]. However, the PIPS
method with heat curing technique is more reliable to provide homogeneity and
forms a uniform morphology of polymer networks that renders it insensitive to
temperature changes. In addition, the PIPS method also allows better control over
the E-O behaviors of the PDLC films and enables to produce the large stretchable
panels [14]. The main advantages of PIPS method of heat curing processes are that,
it requires no evaporation; the fabrication process is relatively simple, clean, and
2968 Polym. Bull. (2013) 70:2967–2980
123
solvent free. It also has advantages over ultraviolet (UV) curing PIPS methods
because it avoids both the changes in liquid crystal properties produced by UV
irradiation resulting in the contamination of radical initiator reagents [15]. Epoxy
resins are widely utilized for coatings and structural applications, adhesives, and
composites for microelectronic encapsulates. Among the various families of
crosslinking resins, epoxies are widely used due to their exceptional performance,
coupled with very easy usage methods and limited cost. Epoxy resins are well
known for forming PDLC films of extraordinary adhesion, flexibility, thermal
stability, environmental stability, mechanical strength, electrical insulating proper-
ties, and chemical resistance behavior. Epoxy resins easily mix with polyamine
hardener with the proper refractive index to produce a matrix with superior optical
properties and determination of curing reactions of epoxy resins with diamines [16,
17]. In our group, Ding et al. [18] investigated the E-O properties of PDLC films
obtained by dual UV curing and heat curing process, which was performed by PIPS
method. Meng et al. investigated the effects of the structures of epoxy monomers on
the electro-optical properties of thermal-cured PDLC films [19]. Song et al. studied
a UV polymerization temperature dependence of PDLCs based on epoxies/acrylates
hybrid polymer matrix components by the PIPS method [20]. In the present
experiments, PDLC films with LC content as low as 40 wt% were prepared by
multi-functional epoxy monomers with different composition feed ratios and weight
percentages, to investigate briefly the synergistic effects of two groups of epoxy
monomers’ structures on conspicuous morphology of polymer network and the E-O
properties of PDLC films.
The aim of the present contribution is to describe the fabrication method using
epoxy monomers which reinforce the stability and strength of the PDLC films.
Furthermore, this paper also addresses the enhanced curing temperature effects
with alkyl chain length, short flexible chain length, rigid chain segment containing
epoxy monomers structure and their combined effects on E-O properties of PDLC
films.
Experimental detail
Materials
The heat curable epoxy monomers used were a mixture of bisphenol a diglycidyl
ether (DGEBA, Alfa Aesar, A Johnson Matthey Company), 1,4-cyclohexanedi-
methanol diglycidyl ether (1,4-CHDMDE; Meryer (Shanghai) Chemical Technol-
ogy Co., Ltd), pentaerythritol tetraglycidyl ether (PTTGE, Synasia (SuZhou) Co.,
Ltd.), poly propylene glycol diglycidyl ether (PPGDE *380, Sigma Aldrich
company), trimethylol propane triglycidyl ether (TMPTGE, Nanjing Chemlin
Chemical Industry), ethylene glycol diglycidyl ether (EGDE) resin (XY 669, Anhui
Hengyuan Chemical Co., Ltd.) and 2,20-(ethylene di oxy)bis (ethylamine) (EDBEA,
Alfa Aesar, A Johnson Matthey Company). EDBEA is a polyamine hardener for
epoxy resins.
Figure 1shows the chemical structures of these materials.
Polym. Bull. (2013) 70:2967–2980 2969
123
The nematic liquid crystal used in this study was SLC-1717 [(Nematic-isotropic
temperature) T
NI
=92.05 °C, (ordinary refractive index) n
o
=1.519, (extraordi-
nary refractive index) n
e
=1.720)], Shijiazhuang Yongsheng Huatsing Liquid
Crystal Co. Ltd.). It is a Class a, longitudinal liquid crystal polymer [21]. All of the
above materials were used without further purification.
The compositions of multi-functional curable monomers/hardener/LC mixtures
are listed in Table 1.
Fig. 1 Chemical structures, names and abbreviations of the materials used
Table 1 The compositions of
the samples studied (group A
and group B)
a
Monomer mixture 1 (MM1)
PTTGE/DGEBA/EDBEA =1/
2/2 (molar ratio)
b
Monomer mixture 2 (MM2)
EGDE/TMPTGE/EDBEA =1/
2/2 (molar ratio)
c
Monomer mixture 3 (MM3)
PPGDE/1,4-CHDMDE/
EDBEA =1/2/2 (molar ratio)
Sample Monomers
(total 60 wt%)
SLC-1717
(wt%)
Curing
temperature (°C)
MM1
a
/MM2
b
/MM3
c
Group A
A1 50/10/– 40 80
A2 40/20/– 40 85
A3 30/30/– 40 90
Group B
B1 –/10/50 40 80
B2 –/20/40 40 85
B3 –/30/30 40 90
2970 Polym. Bull. (2013) 70:2967–2980
123
Preparation of the samples
The samples were prepared consisting of multi-functional heat curable epoxy
monomers/hardener and the less amount of nematic LC content (wt 40 %). Firstly,
the compounds were mixed in different percentages and stirred for 2 h until they
had been homogenized perfectly. Then, the mixtures were sandwiched between two
pieces of indium tin oxide (ITO)-coated glass substrates utilizing 20.0 ±1.0 lm
polyester spacers to control the thickness and then cured in an oven at different
temperatures for 7 h. The compositions of the samples are listed in Table 1. The
functionality of multi-functional curable epoxy resins is calculated by Fav ¼PUifi,
where F
av
is the average functionality of composite monomer,Uiand f
i
stand for the
relative percentage and functionality, respectively [22]. In this experiment, samples
were divided into two groups: one is group A for MMI and MM2 monomers, the
other is group B for MM2 and MM3 monomers with different composition ratios
and weight percentages. The theoretical molar ratio values of PTTGE/DGEBA/
EDBEA, EGDE/TMPTGE/EDBEA and PPGDE/1,4-CHDMDE/EDBEA are 1/2/2,
1/2/2 and 1/2/2, respectively.
Morphology analysis
The morphology of the polymer networks of the samples was observed by a
scanning electron microscope (SEM) (EVO 18, Zeiss, Germany). The samples were
separated and dipped into cyclohexane (C
6
H
12
) for 4 days at room temperature to
extract the LC molecules, and then the films were dried for 12 h under vacuum.
After the polymer network had been sputtered with carbon, the microstructure of the
polymer network was observed under SEM [18].
Electro-optical measurement
The electro-optical properties of the samples were studied using a liquid crystal
device parameters tester (LCT-5016C, Changchun Liancheng Instrument Co. Ltd.).
A halogen tungsten lamp beam was used as the incident light source, and the
incident wavelength (k) through the samples was fixed with the help of a
wavelength (k) filter (632.8 nm). The transmittance of the PDLC films was recorded
by a photodiode, and the response of the photodiode was monitored by a digital
storage oscilloscope. A square wave modulated electric field (100 Hz) was applied,
and the distance between the PDLC film and photodiode was 300 mm. The
transmittance of air was normalized as 100 %.
Wave length (k) measurement
The transmittance of the off-state samples was studied using a UV-Vis-NIR
spectrophotometer (V-570, Jasco Corp., Tokyo, Japan). The wavelengths were
measured in the range 300–800 nm, and the results were recorded with an incident
angle, d=0.
Polym. Bull. (2013) 70:2967–2980 2971
123
Results and discussion
Morphology of polymer network of the samples
Figure 2shows the morphology of the polymer network of the samples A1–A3 and
B1–B3 at different temperatures. It can be observed that the domain size of the LC
is greatly affected due to the presence of the short flexible chain length (EGDE),
alkyl chain length (PPGDE), multi-functional monomers (TMPTGE and PTTGE),
rigid chain segment containing curable monomer (DGEBA) and 1,4-CHDMDE. As
shown in Fig. 2the domain size of the polymer network of both groups A and B
increased and the crosslinking density chain of the polymer network decreased in
sequence. However, the polymer network of group A is different from that of group
B due to the various curable monomer structures. The polymer network of PDLC
films of group A has small holes, while the polymer network of group B has a
suitably distributed coin-like network with small meshes around large domains. The
LC domain size and morphology of PDLC films are determined by the LC droplet
nucleation and the polymer gelation. According to the polymer liquid crystal theory,
rigid LC sequences and flexible spacers are alternately connected in a linear chain
forming a longitudinal order given by Brostow and Hess [23]. That is, the LC
droplet size is mainly controlled by the rate of polymerization, the relative ratios of
materials composition and types of LCs as well as physical parameters such as
viscosity, rate of diffusion and solubility of LCs in the polymer [24,25]. In this
study, the LC domain size was influenced by the existence of the short flexible chain
length EGDE and tri-functional TMPTGE monomers in both groups. It is well
known that the C–C and C–O bonds are much more flexible than the rigid chain
segment monomer [19]. In other words, in group B, the PPGDE monomer is more
flexible and shows a more rapid rate of polymerization than monomer DGEBA in
group A, but with increasing temperature the viscosity of the rigid chain segment
monomer decreases. Usually, the viscosity of the rigid chain segment containing
Fig. 2 SEM micrographs of the polymer networks of the samples A1–A3 and B1–B3 with different
curing temperature
2972 Polym. Bull. (2013) 70:2967–2980
123
monomer was larger than the flexible chain curable monomer. Thus, the LC domain
size of group B was different from that of group A. Normally, the introduction of
multi-functional epoxy monomers resulted in an increased amount of crosslinking
points in the polymer network; this also resulted in a more rapid rate of
polymerization. Therefore, the LC domain size of sample A1 was smaller than that
of sample B1. As shown in Fig. 2, the domain size of the polymer network of
epoxy-curable monomers synergistically combines the properties of both groups (A
and B) increases with increasing the temperature respectively due to the one key
factor. In heat curing PIPS method the rate of diffusion of the LC domain size is
directly proportional to the curing temperature, and it is given by the Fick’s second
law [26,27].
D¼D0expðQ=RTÞð1Þ
Where, D=diffusion coefficient, D
0
=diffusion coefficient constant, Q=acti-
vation energy, T=absolute temperature, R=ideal gas constant.
Additionally, the initial increase of the curing temperature could particularly
decrease the viscosity of the LC. The decrease of the viscosity of the LC was helpful
to the diffusion of the LC, and then developed the accumulation of the LC and the
growth of the domain size of LC. Although, the polymerization rate coefficient (k)
of the epoxy heat curable monomers depend exponentially on temperature
according to Arrhenius equation [28–30]:
k¼AexpðEact=RT Þð2Þ
Where E
act
is the activation energy, Ris the ideal gas constant, Ais the frequency
(collisions) factor and Tis the absolute temperature.
Analysis of Eqs. (1) and (2) indicates that when the epoxy-curable monomers
were cured from 80 to 90 °C, the increase in the diffusion of the LC quickened
moderately, and the accumulation of the LC became easier, relatively, leading to
increase of the LC domain size in the polymer network, with multi-functional
composition of curable monomers. As shown in Fig. 2, the LC domain size of the
polymer network increased with the growth of the LC content. This was related to
the relative content of the heat-curable epoxy monomers and the nematic LC as low
as 40 wt%. By increasing the curing temperatures, the polymerization rate of the
epoxy monomers collectively increased. The diffusion rate of the LC molecules and
the polymerization rate of the epoxy monomers depend exponentially on the heat-
curing polymerization at various temperatures. It plays a vital role during the
formation of the polymer network and corresponds to the generation of macro-
molecular polymer structures in both groups (A and B). Consequently, we have
observed that the curable monomers EGDE and TMPTGE showed a crucial
function during the formation of the polymer network of PDLC films in both groups.
Our findings show that the group B contained PPGDE and 1,4-CHDMDE epoxy
monomers whose polymerization rates are different, thus causing suitably coin-like
polymer network, with smaller meshes around larger meshes, while group A
contained DGEBA and PTTGE monomers; have hole like polymer network because
enhanced curing temperature faster the polymerization rate of the monomers. The
Polym. Bull. (2013) 70:2967–2980 2973
123
results show that the composition ratios of epoxy monomers drastically influence
the mesh size of the polymer network of the LC domain size in both groups.
Electro-optical properties of the samples
The E-O properties are very elemental and significant in the evaluation of PDLC
films. In addition, the influences of epoxy monomer structures on PDLC films can
be obtained by analyzing the E-O properties of samples. The transmittance-applied
voltage curves of groups (A and B) are shown in Fig. 3a and b, respectively. It can
be seen that increase in the applied voltage increases the transmittance. We can
clearly examine that threshold voltage (V
th
) and saturation voltage (V
sat
) decreases
with respect to the different composition ratio of epoxy monomers essentially. The
V
th
and driving V
sat
are defined as the electric voltage required for the transmittance
to reach 10 and 90 %, respectively.
It must be mentioned that, the size of the LC droplets has a strong association
with E-O performance in a PDLC system. Usually, the V
th
inversely proportional to
the radius of LC droplet (R) as shown below [31].
Vth ¼d
3aqpþ2
qLC
Kl
21ðÞ
Dee0
1
2
ð3Þ
Where d=thickness of the PDLC film, l=a/b, the ratio of the length of the semi-
major axis, a, to the length of the semi-minor axis, b,q
p
=resistivity of the
polymer, q
LC
=resistivity of the liquid crystal, K=elastic constant of liquid
crystal, De =dielectric anisotropy of the liquid crystal e
0
=vacuum permittivity.
The voltage-transmittance values of samples A1–A3 and B1–B3 are shown in
Fig. 4. Significantly, as we have observed Fig. 4, the V
th
and V
sat
decreases with
increasing the same ratio of TMPTGE and EGDE curable monomers in both groups.
It is directly related to the increase of LC domain size, which indicates that LC
domain size has an important effect on anchoring energy. It is well known that the
driving voltages can be significantly influenced by the microstructures of the
polymer network. The Eq. (3) indicates that the electro-optical properties of PDLC
Fig. 3 The transmittance-applied voltage curves of relative content of epoxy monomers samples
belonged to group A (a) and group B (b)
2974 Polym. Bull. (2013) 70:2967–2980
123
system can be controlled by LC domain size and other factors, such as film
thickness, the resistivity and the dielectric anisotropy of LC. Hence, V
th
and V
sat
decrease with increasing the LC domain sizes. As for samples in group B, from B1
to B3, with the increasing alkyl chain length of PPGDE monomer altering the values
of V
th
and V
sat
, with changes from 38.231, 30.410 to 18.234 V and 95.238, 81.807 to
56.898 V, respectively. The varying trend of driving voltages of group A is similar
as in group B due to the tetra-functional PTTGE and rigid chain segment DGEBA
monomers specially. Moreover, with the increasing of the LC domain size, the
interface between LC molecules and polymer matrix decreases (as shown the
Fig. 2), in resulting; decreasing the anchoring effects on LC molecules from
polymer matrix. Therefore, the LC molecules are much easier to orient along the
direction of the electric field, consequently; V
th
and V
sat
decreased. In contrast, we
have examined the values of driving voltage in group A are higher than group B;
because the LC mesh size of group A is smaller than the group B. In conclusion,
with increasing the alkyl chain length of PPGDE monomer the driving voltage
increased dramatically.
Contrast ratio (CR) is a key measure of the electro-optical properties in a PDLC
films. CR of PDLC films are used to characterize the differences in between a
transparent and an opaque state. It is define as,
CR ¼sR=sDð4Þ
where s
R
and s
D
are transmittance in the on and off-state of PDLC film. A high
value of CR can be obtained when the microstructure of the PDLC film is appro-
priate. Figure 5indicates the CR of the samples groups (A and B). It can be seen
that the CR of group A is higher than group B due to the tetra-functional PTTGE
and rigid chain segment DGEBA curable monomers; and initial transmittance T
0
.In
Fig. 5,T
0
for sample A1, is below 0.2 % so, it has the highest CR of 306.031. On
the other hand CR of group B is low due to T
0
being too high. It is well known that
the strong scattering in PDLC films can be enhanced by the use of highly-bire-
fringent LC, a high droplet density and thick films. Moreover, for a definite system
of PDLC films where the LC content is fixed, the number of the light scattering
mismatch centers in the samples decrease with increasing the LC domain size of the
polymer network and the off-state light scattering intensity of the samples also
decrease. As a result, the DGEBA and PTTGE monomers, which can increase the
Fig. 4 The threshold voltage and saturation voltage of the PDLC samples A1–A3 and B1–B3
Polym. Bull. (2013) 70:2967–2980 2975
123
LC domain size with enhanced temperature; decreased the light scattering intensity
and CR significantly.
Figure 6shows about the response time of the PDLC films with applied voltage
dependence of the rise time (s
R
) and decay time (s
D
)of samples (A and B) groups.
The DGEBA and PPGDE monomers are directly proportional to the decay time (s
D
)
and inversely proportional to the rise time (s
R
) in both groups (A and B) with
increasing the rigid chain segment and alkyl chain length, respectively. These are
two most important factors to investigate the performance of PDLC films. Rise time
is explained as the time which required to go from 10 to 90 % of the maximum
transmittance of the sample upon turning on and which required to go from 90 to
10 % of the maximum transmittance of sample turning off is known as decay time.
Usually, a competition between the applied electric field and the interface elastic
forces anchoring the LC molecules governs the response time [32]. Mostly, larger
domains of the LC result shows in smaller s
R
and longer the s
D
. The results are
associated to the boundary interaction in between the polymer matrix and LC
droplet, which manage the re-orientation and orientation of LC droplet under
electrical field force. It means that if the size of the LC domain is bigger than it
contains smaller s
R,
as a result; the orientation of LC droplet needs shorter time to
contain on higher anchoring energy. On the other hand s
D
is expected to be
Fig. 5 The contrast ratio of the PDLC samples A1–A3 and B1–B3
Fig. 6 The Rise time and decay time of the PDLC samples A1–A3, B1–B3. The PDLC films were driven
by an applied field of 100 V at 100 Hz
2976 Polym. Bull. (2013) 70:2967–2980
123
independent of the voltage, because much greater distortion of the director and a
greater restoring energy of deformed leading the liquid crystal molecule go back to
original position quickly [33–35]. It can be seen that in Fig. 6s
D
increases
indefinitely with decreasing the s
R
. The s
R
in group A shows lower value than group
B due to the different compositions of epoxy monomers, micro structure of both
groups and presence of DGEBA and PPGDE monomers particularly, which effect
on response time in both groups. As shown in Figs. 2and 6, a good uniformity and
small domain size resulted in a faster response time in group A. In contrast, the
response time is very slow in group B, which has non-uniform coin-like structure.
The large domain size made the decay time very slow, and the small meshes around
the large domain size resulted in a longer rise time. Consequently, the group A s
R
lower than group B.
As is shown in Fig. 7the wave length (k) dependence of the off-state
transmittance for all samples in the UV-Vis spectra. It can be found that the
transmittances of all the samples tended to increase with increasing of wave length
(in the wave length range of 300–800 nm). The transmittance of the off-state PDLC
films in both groups for samples A1 and B1 were relatively lower than the other
samples, and there was a regular change for the samples A2, A3 and B2, B3
respectively. This was in good agreement with the results of the LC domain size of
the polymer network, as shown in Fig. 2.
Conclusions
We have investigated PDLC films by multi-functional curable epoxy monomers
using heat curing processes with different composition feed ratio, weight% and
different molecular structures. The effects of short flexible chain length, alkyl chain
length, multi-functional and rigid chain segment containing monomers structure on
the morphology and electro-optical properties have been studied. On the morphol-
ogy, in both groups the enhanced temperature favors the tri-functional, short flexible
chain and rigid chain segment containing epoxy monomers to increase large LC
Fig. 7 The wavelength (k) dependence of the off-state transmittance (T
off
) for the samples A1–A3 and
B1–B3
Polym. Bull. (2013) 70:2967–2980 2977
123
domain size and existence of PPGDE and 1,4-CHDMDE monomers form suitably
distributed coin-like polymer network in group B. However, the multi-functional
and rigid chain segments curable monomers have combined effects on lowering the
driving voltages, but causes higher response time and lower CR. The study also
shows a special association between the multi-functional epoxy-curable monomers’
structure and the electro-optical properties of PDLC films with conspicuous
morphology. Furthermore, the results of this study also suggest that it is possible to
regulate the LC domain size and optimize the electro-optical performance, by
adjusting the composition and weight ratio of heat-curable epoxy monomers to
obtain PDLC films having good electro-optical properties that are beneficial for
decreasing the total LC content in PDLC devices. The results in this paper show
significant advantages for manufacturing PDLC films and developing the PDLC
market.
Acknowledgments This work was supported by the National Natural Science Fund for Distinguished
Young Scholars (Grant No. 51025313), the National Natural Science Foundation (Grant No. 50973010),
the National Natural Science Foundation (Grant No. 51173003), the National Natural Science Foundation
(Grant No. 51143001), the Research Fund of the State Key Laboratory for Advanced Metals and
Materials, the Open Research Fund of the State Key Laboratory of Bioelectronics (Southeast University)
and the Fundamental Research Funds for t he Central Universities (Grant No.FRF-TP-12-032A).
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