Electro-optical property and stability of liquid crystal physical gels controlled by copolymerization

Article abstract of DOI:10.1016/j.reactfunctpolym.2018.07.008

A series of side chain liquid crystal copolymers with different copolymerization ratios have been successfully synthesized by copolymerization of (4′-cyanobiphenyl-4-oxy) acrylate] (M0ACB, rigid component) and (4-cyanobiphenyl-4′-oxy) hexyl acrylate (M6ACB, flexible component). The liquid crystal physical gel (LC-PGel) can be prepared by blending the copolymer with 5CB. The introduction of the flexible component improves the compatibility of the copolymers and 4-pentyl-4′-cyanobiphenyl (5CB), so that the transparent gel can be obtained above the 5CB clear point. The gelation ability of the copolymer and the mechanical properties of the liquid crystalline physical gels formed by the side chain liquid crystal copolymers can be effectively regulated by changing copolymerization ratios. Further studies have found that the surface anchoring between the copolymers and 5CB also can be regulated by changing the proportion of two components in the copolymer and the composition of copolymer with the maximum surface anchoring is P0ACB (0.80)-co-P6ACB (0.20). The copolymerization ratios can further regulate the electro-optical properties of the LC-PGel by changing the surface anchoring. The stronger surface anchoring can lead to higher driving voltage and contrast ratio of LC-PGel. However, after orientation in an electric field, the increased surface anchoring can help the LC-PGel recover faster to its original state.

Full text of DOI:10.1016/j.reactfunctpolym.2018.07.008

Reactive and Functional Polymers 131 (2018) 44–55
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Electro-optical property and stability of liquid crystal physical gels
controlled by copolymerization
⁎
Jianhang Zhao, Lei Chen, Yongjie Yuan, Ye Li, Mengfan Gu, Hailiang Zhang
Key Laboratory of Polymeric Materials and Application Technology of Hunan Province, Key Laboratory of Advanced Functional Polymer Materials of Colleges, Universities
of Hunan Province, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of side chain liquid crystal copolymers with different copolymerization ratios have been successfully
synthesized by copolymerization of (4′-cyanobiphenyl-4-oxy) acrylate] (M0ACB, rigid component) and (4-cya-
nobiphenyl-4′-oxy) hexyl acrylate (M6ACB, flexible component). The liquid crystal physical gel (LC-PGel) can be
prepared by blending the copolymer with 5CB. The introduction of the flexible component improves the com-
patibility of the copolymers and 4-pentyl-4′-cyanobiphenyl (5CB), so that the transparent gel can be obtained
above the 5CB clear point. The gelation ability of the copolymer and the mechanical properties of the liquid
crystalline physical gels formed by the side chain liquid crystal copolymers can be effectively regulated by
changing copolymerization ratios. Further studies have found that the surface anchoring between the copoly-
mers and 5CB also can be regulated by changing the proportion of two components in the copolymer and the
composition of copolymer with the maximum surface anchoring is P0ACB (0.80)-co-P6ACB (0.20). The copo-
lymerization ratios can further regulate the electro-optical properties of the LC-PGel by changing the surface
anchoring. The stronger surface anchoring can lead to higher driving voltage and contrast ratio of LC-PGel.
However, after orientation in an electric field, the increased surface anchoring can help the LC-PGel recover
faster to its original state.
Liquid crystalline physical gel
Copolymerization regulation
Surface anchoring
Electro-optical property
Side chain liquid crystal polymer
1. Introduction
and the preparation process has been greatly simplified [9]. Compared
with the traditional polymer dispersed liquid crystals, LC-PGel also has
The nematic liquid crystal possesses best fluidity because of the
lots of incomparable advantages, such as thermo-reversibility, very low
dosage of gelators and easy synthesis.
completely disordered state of its molecular centroid, and it can make
quick response to external field. When the electric field is applied,the
alignment of nematic liquid crystal molecules will change, which leads
to the change of optical properties of liquid crystals. Thus, nematic li-
quid crystals, such as 5CB, are widely used as display materials [1–4].
The fluidity of the nematic liquid crystals endows its sensitive re-
sponse to the external field, but it also loses mechanical stability. As a
result, liquid crystals are usually injected into liquid crystal cells when
used as liquid crystal displays. In recent years, research study found
that introducing gelators into nematic liquid crystal to form stable li-
quid crystalline physical gel (LC-PGel) is another effective way to pre-
pare liquid crystal displays [5–7]. LC-PGel is a novel class of stimuli
responsive and thermally reversible soft solid materials which comprise
a three-dimensional fiber network formed by self-assembly of gelators
via non-covalent interactions in liquid crystals [8]. Compared with the
traditional liquid crystal displays, LC-PGel abandon the use of liquid
crystal cell, which greatly simplify the liquid crystal displays structure,
LC-PGel mainly consists of two parts. One is the three-dimensional
fiber networks form by the gelators. Another one is the working sub-
stance, such as 5CB. The properties of LC-PGel are closely related to the
chemical structure of gelators, and therefore the selection of gelators is
critical to obtain high performance LC-PGel. According to the relevant
research, gelators used for LC-PGel can be divided into two major ca-
tegories: low-molecular weight gelators and polymer gelators. Low-
molecular weight gelators have many advantages, such as easy pre-
paration, large variety of gelators and low dosage. A large number of
low-molecular weight gelators such as amides [10, 11], amino acids
and their derivatives [12–15], urea compounds [16], cholesterol com-
pounds [17, 18], phenanthrene and its derivatives [19], sorbitol and
their derivatives [20] have been designed and synthesized to gelling
liquid crystal since Kato et al. [21] first reported LC-PGel using low-
molecular weight gelators gelling room-temperature nematic liquid
crystal early in 1998. Xie et al. [20] reported a LC-PGel with high
⁎
Corresponding author.
E-mail address: hailiangzhang@xtu.edu.cn (H. Zhang).
https://doi.org/10.1016/j.reactfunctpolym.2018.07.008
Received 18 April 2018; Received in revised form 4 July 2018; Accepted 10 July 2018
Available online 11 July 2018
1381-5148/ © 2018 Published by Elsevier B.V.

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
modulus based on low-molecular weight gelators 1, 3:2, 4-di-O-ben-
2. Experimental
zylidene-D-sorbitol, 1, 3:2, 4-di-O-p-methylbenzylidene-D-sorbitol and
1
,3:2,4-di-O-m, p-dimethylbenzylidene-D-sorbitol. Its storage moduli
2.1. Materials
4
were able to exceed 10 Pa when the gelators content is increased to
.5 wt%. However, the saturation voltage (Vsat) and off state response
1
4-pentyl-4′-cyanobiphenyl (5CB) was purchased from Yantai Derun
liquid crystal material Co. Ltd. Anhydrous tetrahydrofuran (THF) was
distilled from sodium and used immediately. 2, 2′-azobisisobutyroni-
trile (AIBN) was recrystallised from methanol. 4-(4-hydroxyphenyl)
benzonitrile and acryloyl chloride were used as received without fur-
ther purification. Dibromohexane was purchased from aladdin Co. Ltd.
The purity of all the reagents is AR.
time (τoff) were greatly increased compared with 5CB. Chen et al. [22]
reported stretchable light scattering display based on LC-PGel with poss
based dendrimer as gelator. Meanwhile, the transparency of LC-PGel
can be controlled by UV irradiation time. But unfortunately, the author
did not characterize the electro-optical response performance. Polymer
is also a common gelator used for LC-PGel. Triblock copolymers [23],
polymer particles [24] and SCLCPs [25] are usually used to prepare LC-
PGel. Kornfield et al. [23] used triblock copolymers as gelator to pre-
pare LC-PGel. However, the synthetic method of triblock copolymers
was complex and the storage moduli of triblock copolymers/5CB LC-
2.2. Synthesis of monomers and its corresponding polymers
For convenience, the monomer (4′-cyanobiphenyl-4-oxy) acrylate
was named M0ACB, the monomer (4-cyanobiphenyl-4′-oxy) hexyl ac-
rylate was named M6ACB, and the corresponding copolymers were
named P0ACB(x)-co-P6ACB(y). The synthetic route of monomers is
shown in Scheme 1. The experimental route is described as follows:
2
PGel was lower than 10 Pa. Wood et al. [24] reported a LC-PGel based
4
on PMMA particle. Its storage moduli were able to exceed 10 Pa,
however, the volume fraction of PMMA particle must be > 50%. Alexey
Bobrovsky et al. [25] for the first time prepared photochromic LC-PGel
using SCLCPs containing azobenzene group as gelator. However, the
relationship between the structure of SCLCPs and the properties of LC-
PGel has not been well studied. Based on the above analysis, we found
that the addition of the gelators will affect the electro-optical response
properties of the 5CB itself, or after the withdrawal of the electric field,
the gel network affect the recovery process of 5CB, which may cause the
off state response time of LC-PGel is greater than pure 5CB. Thus, it is
still huge challenge to prepare a class of gelators which does not affect
2.2.1. Synthesis of (4′-cyanobiphenyl-4-oxy) acrylate (M0ACB)
To a mixture of 4-hydroxy-4′-cyanobiphenyl and triethylamine in
THF, cooled to 0 °C, was added dropwise acryloyl chloride. The solvent
was stirred for 4 h. The reaction solvent was precipitated into ice water,
and was filtered and washed repeatedly with deionized water. The re-
1
sulting solid was dried to obtain products as a white solid. H NMR (δ,
3
ppm, CDCl ):7.74–7.66 (dd, 4H, pH-H), 7.60 (d, 2H, pH-H), 7.36 (d,
5
5
CB's electro-optical response properties and can increase the speed of
CB recovery.
In recent years, our team found that SCLCPs can form stable phy-
2H, pH-H), 6.66–6.62 (m, 1H, =CH
6.07–6.04 (m, 1H, =CH ). Mass Spectrometry (MS) (m/z) [M] Calcd for
11NO , 249.08; [M + H] found 250, yield: 98%.
2
), 6.32–6.38 (m, 1H, =CH-),
2
C
16
H
2
sical gel in organic solvent via π-π interaction and main chain en-
tanglements with low additions (lower than 1 wt%). We prepared a
series of polymer organic gels with excellent thermal stability, good
self-supporting ability and low additions by introducing side-chain li-
quid crystal polymers into common organic solvents, such as n-heptane,
triethylamine and so on [26]. We also prepared a series of form-stable
phase change materials (FSPCMs) with high enthalpy of phase change,
excellent thermal stability and high modulus by adding side-chain li-
quid crystal polymers into paraffin, and the properties of FSPCMs can
be controlled by modification of the chemical structure of side chain
liquid crystalline polymers, such as backbone structure, length of
terminal groups and flexible spacer [27–32]. Based on the above re-
sults, we have prepared LC-PGel with stable mechanical properties and
fast electro-optical response properties by blending SCLCPs with 5CB,
and the addition of the SCLCPs does not affect the electro-optical re-
sponse of the LC-PGel under the electric field, and with the increase of
the concentration of SCLCPs, the off state response time of the LC-PGel
decreased [33]. However, transparent LC-PGel cannot be prepared be-
cause of poor compatibility between P0ACB and 5CB. Moreover, the
relationship between surface anchoring and the electro-optical prop-
erties of the LC-PGel needs to be discussed in detail.
Hence, we prepared a series of side chain liquid crystal copolymers
with different copolymerization ratios. For simplicity, the component
with flexible spacer is named flexible component. Another component
without flexible spacer is named rigid component. The molecular
structure and synthesis routes of copolymer are shown in Scheme 1. The
effect of copolymerization ratios on the gelation ability of copolymer
was measured by “tube-testing method”. The properties of LC-PGel
which formed by copolymer with different copolymerization ratios
were systematically studied including thermal stability, mechanical
properties and electro-optical properties and the relationship between
the surface anchoring and the electro-optical properties of the LC-PGel
was discussed in detail.
2.2.2. Synthesis of 4-(hexyloxybromide)-4′-cyanobiphenyl
To a mixture of 4-(4-hydroxyphenyl) benzonitrile, dibromohexane,
2 3
K CO and KI in acetone heat to 80 °C,the mixture was stirred for 8 h.
The reaction solvent was precipitated into ice water, and was filtered
and washed repeatedly with deionized water. The crude products were
purified by column chromatography on silica gel using CH
ents to give a white powder. 1H NMR (δ, ppm, CDCl
): 7.87–7.81 (dd,
4H, pH-H), 7.70–7.67 (d, 2H, pH-H), 7.05–7.03 (d, 2H, pH-H),
4.02–3.99 (m, 2H, –CH -), 3.54–3.51 (m, 2H, –CH -), 1.83–1.80 (m, 2H,
–CH -), 1.74–1.71 (m, 2H, –CH -), 1.45–1.43 (m, 4H, –CH -). Mass
20BrNO, 357.07; [M + H]
2 2
Cl as elu-
3
2
2
2
2
2
Spectrometry (MS) (m/z) [M] Calcd for C19
H
found 358, yield: 86%.
2.2.3. Synthesis of (4-cyanobiphenyl-4′-oxy) hexyl acrylate (M6ACB)
4-(hexyloxy bromide)-4′-cyanobiphenyl, acrylic acid and KHCO
were added in 300 mL DMF. The reaction solvent was heated for 12 h.
Then the reaction solvent was cooled to room temperature. The reac-
tion solvent was poured into deionized water, and then was filtered. At
3
last, the crude products were purified by column chromatograph using
1
CH
(δ, ppm, CDCl
(d, 2H, AreH), 6.42 (d, 1H, =CH
=CH ), 4.19 (m, 2H, –CH -), 4.02 (m, 2H, –CH
1.74 (s, 2H, –CH -), 1.55(dd, 4H, –CH -). Mass Spectrometry (MS) (m/
z) [M] Calcd for C22 23NO : 349; [M]found: 349.25, yield: 80%.
2
Cl
2
as eluents. The final product was a white solid powder. H NMR
3
):7.70–7.64 (dd, 4H, AreH), 7.53 (d, 2H, AreH), 6.99
), 6.15 (m, 1H, =CH ), 5.83 (d, 1H,
-),1.84 (m, 2H, –CH -),
2
2
2
2
2
2
2
2
H
3
2.2.4. Synthesis of the copolymers
The copolymers were obtained by solution free radical poly-
merization (see Scheme 1), typically carried out as described in the
following steps. In a 20 mL polymerization tube, two kinds of monomer
and AIBN were dissolved in DMF. Three units were mixed with a certain
proportion and the monomers mass concentration was 35%. After de-
2
gassed, back-filled three times with N , the tube was sealed under va-
cuum. The polymerization was stirred at 75 °C for 6 h. Then, the co-
polymer was purified by re-precipitating from DMF into ethanol.
45

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
Scheme 1. (a) synthesis route of monomer M0ACB; (b) synthesis route of monomer M6ACB; (c) synthesis route of copolymers.
2.3. Preparation of LC-PGel
obtained by a TA Q10 DSC instrument. The temperature and heat flow
were calibrated using standard materials (indium and zinc) at a cooling
−
1
The binary mixture of gelators and 5CB (the mixture was weighed in
and heating rates of 10 °C min
.
at 1.0 g in total) were sealed in a screw-capped bottle and heated in
magnetic heating sleeve. After the gelators were dissolved completely,
the mixture was allowed the bottle to stand at 25 °C and then a white
opaque gel was obtained and stable for several months. All other LC-
PGel was prepared via the same method.
3.4. Minimum gelation concentration (MGC) and gel-to-sol transition
temperature (TGS) measurements
The MGC and TGS of LC-PGel were tested by a “tube-testing
method”: A weighted gelators were mixed with 5CB in a screw-capped
bottle (the samples were weighed at 1.0 g in total) and the mixture was
heated until a transparent solution was formed. The solution was cooled
at room temperature for 1 h. When upon inversion of the screw-capped
bottle no fluid ran down the walls of the bottle, we judged it “successful
gelation”. The minimum concentration of gelators necessary for gela-
tion is defined as MGC. Immerse the gel test tube into the oil bath, and
3. Instruments and measurements
3
.1. 1H NMR measurements
1H NMR spectra was recorded on a Bruker ARX400 MHz spectro-
meter. CDCl
3
or DMSO‑d
6
were used as solvent and tetramethylsilane
−
1
(
TMS) was used as the internal standard.
the oil was heat up at 2 °C min . The temperature that gel turn into sol
is named TGS
.
3.2. Gel permeation chromatography (GPC)
3.5. Polarized optical microscope (POM) observation
The number average molecular weight (Mn) and polydispersity
index (PDI = Mw/Mn) of copolymer were measured by a GPC (PL-GPC
Leica DMLM-P microscope equipped with a Mettler FP82 hot stage
1
1
20) instrument. DMF was used as eluent and the flow rate was
and a digital camera was used to observe the network structure of LC-
PGel.
−1
.0 mL min
at 80 °C.
3.3. Differential scanning calorimetry (DSC) measurement
3.6. Scanning election microscope (SEM) observation
Phase transition temperature of P0ACB(x)-co-P6ACB(y) was
Scanning electron microscope (JEOL JSM-6610) was used to
46

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
observe the microstructures and morphology of xerogel. Samples of
P0ACB(x)-co-P6ACB(y)/5CB were prepared by dropping the gels on
Aluminum foil and were immersed in petroleum ether to extract 5CB
300 °C (Fig.2a and Table 1). Co-polymers show two distinct peaks
which corresponding to glass transition temperature (T
point temperature (T ). With increase of the flexible component, the T
and T of the copolymer decreased (Fig.2b and Table 1). Because the
g
) and clear
i
i
(
Each gram of gel is soaked in ten milliliters of petroleum ether) for 1 h
g
and then evacuated by a vacuum pump at 25 °C for 10 h to remove
petroleum ether. To minimize sample charging, a thin layer of Au was
deposited onto the samples before SEM examination.
decoupling effect of flexible components improves the flexibility of the
copolymers [34].
4.2. Gelation ability of copolymers, thermal stability and thermal phase
3.7. Rheological measurements
transition of LC-PGel
Rheological measurements were carried out with a stress-controlled
Most of the copolymers synthesized in this paper can gelate 5CB. In
order to study the effect of flexible components of copolymers on the
gelation ability and LC-PGel thermal stability, P0ACB (1.00:0) and
P0ACB(x)-co-P6ACB(y) were tested by “tube-testing method” [35].
As shown in Table 2, P0ACB can form stable gels in 5CB and the
MGC is 1 wt%. The copolymerization ratio of P0ACB(x)-co-P6ACB(y) is
0.95:0.05 and 0.70:0.30 and the MGC is 0.5 wt%. The MGC is 0.2 wt%
when the proportion of the flexible component in the copolymer is
between 5%~30%. Even the addition of copolymer > 10 wt%, the co-
polymer is still unable to form gel in 5CB when the contents of the
flexible component are over 40% in the copolymer. In general, the
formation of the gel is divided into three steps: the recognition of
molecules, the uniaxial aggregation to form fibrillar objects and the
interconnection of fibrillar objects [36].This process is related to gela-
tor–gelator and gelator–solvent interactions. From previous studies
[33], we know that SCLCPs without flexible spacers (P0ACB) can form
stable gel in 5CB, while SCLCPs with flexible spacers (P6ACB) have
good compatibility with 5CB. The copolymer in this article consists of
two components: One part is the rigid component, which provided
gelator–gelator interaction to form stable gel supporting structure. The
other part is the flexible component, which improved the copolymer
compatibility with 5CB and offer gelator–solvent interaction. With the
increase of the flexible component, the compatibility of copolymer with
5CB is increased and the rigid component still maintains stable gel
network structure. Hence, the gelation ability of copolymer enhanced
significantly and the MGC is reduced. However, when the flexible
component is > 30% in the copolymer, the self-assembly of rigid
component in the copolymer is affected so that the gelator–gelator in-
teraction is less than gelator–solvent interaction. Thus, the gelation
ability of copolymer weakened and the MGC is increased. When the
flexible component is > 40%, the copolymer unable to form gel in 5CB
since the self-assembly of rigid component in the copolymer is com-
pletely disappeared.
rheometer (TA ARES rheometer) equipped with steel parallel-plate
geometry (25 mm diameter). All measurements were tested at 25 °C.
Strain sweep at a constant frequency (1 rad/s) was performed in the
0
.01–10% range to measure the linear viscoelastic region (LVER) of the
gel sample. The frequency sweep was obtained from 0.1 rad/s to
00 rad/s at a constant strain of 0.5%. Dynamic temperature ramp test
1
−
1
was performed in the region of 30 °C to 200 °C at 2 °C min , using
5 mm diameter parallel-plate geometry with a frequency of 1 rad/s
and small strain amplitude.
2
3
.8. Electro-optical response test
Electro-optical properties of LC-PGel were measured with indium
tin oxide liquid crystal cell (The gap of the cell was 8 μm). LC-PGel was
filled into liquid crystal cell by capillary action above TGS. Transmission
as a function of applied voltage and/or relaxation time was character-
ized by an LCD parameter tester (LCT-5016C, North LC Engineering
Research and Development Centre, China) with 1 kHz AC electric field.
A halogen lamp was used as an incident light source. AC electric fields
(
1 kHz) were applied to the cells. The threshold voltage (Vth) and sa-
turation voltage (Vsat) were defined as the applied voltages required for
obtaining 10% and 90% of the maximum transmittance, respectively.
When a voltage is applied (100 V), the time required to achieve the 90%
maximum transmittance is defined as τon and when the voltage is re-
moved, the time required to reach the 10% maximum transmittance is
defined as τoff. The contrast ratio (CR) was deduced by calculating the
ratio of the maximum transmittance to the minimum transmittance.
4
. Results and discussion
4
.1. Synthesis and characterization of monomers and copolymers
The monomers and the corresponding copolymers were synthesized
As can be seen in Fig. 3 (a), the TGS of P0ACB/5CB and P0ACB (x)-
co-P6ACB (y)/5CB gel increase sharply with increasing the concentra-
tion of P0ACB or P0ACB (x)-co-P6ACB (y) in 5CB and cannot be im-
proved anymore until the mass fraction exceeds a certain concentration.
By comparing the TGS of the LC-PGel with a concentration of 6 wt%, we
find that the TGS of the gels decrease with the decrease of rigid com-
ponents in the copolymers. This result shows that the rigid component
can effectively stabilize the gel network system. Moreover, as shown in
referring to the synthetic route showed in Scheme 1. The 1H NMR
spectra of copolymer P0ACB (0.80)-co-P6ACB (0.20) and monomers
were shown in Fig. 1. The chemical shifts and integral area of peak of
all the protons in M0ACB and M6ACB were excellent in coincident with
their expected structure, suggesting that pure monomers were suc-
cessfully obtained. After two monomers copolymerization, the char-
acteristic vinyl proton signals of acryl group at 6.66–6.62, 6.32–6.38
and 6.07–6.04 ppm totally disappeared and the characteristic peaks of
two kinds of monomers appeared in the position of 7.71 ppm (α) and
Fig. 3 (b), by comparing the TGS of the 6 wt% LC-PGel with T
copolymers, we find that the thermal stability of the gel structure is
closely related to the flexibility of copolymers. The high T and T
i
g i
and T of
6
.65 ppm (β) indicating that the copolymers were successfully synthe-
g
1
sized. The copolymerization ratio was characterized by H NMR. The
ratio of two peak areas was calculated. This result is basically consistent
with the theoretical copolymerization ratio. The results are shown in
the Table 1.
means that the copolymer maintain stable structure at high tempera-
tures. Therefore, the gel network structure formed by copolymer gela-
tors with high T and T has high thermal stability.
g i
The phase transition behavior of LC-PGel was characterized by DSC.
Fig. 4(a) shows the DSC heating curve of pure 5CB and 6 wt% P0ACB
(x)-co-P6ACB (y)/5CB. Pure 5CB exhibit two obvious phase transition
peaks, related to crystallization-melt transition and anisotropy-isotropic
transformation. There exists single transition peak in the LC-PGel, re-
lated to anisotropy-isotropic transformation. From the test results, the
anisotropy-isotropic transformation of 5CB is independent on the ad-
dition of copolymers. Moreover, the addition of copolymers can inhibit
the crystallization of 5CB.
The number average molecular weight and polymer distribution
index of all copolymers were measured by GPC. The results are shown
in Table 1. All copolymers have high enough molecular weight and
narrow polymer distribution index to remove the influence of Mn and
PDI on the properties of LC-PGel. The thermal stability and liquid
crystal phase behavior of all copolymers were characterized by TGA
and DSC. The test curves are shown in Fig.2. All copolymers have good
thermal stability and their decomposition temperatures are higher than
47

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
Fig. 1. 1H NMR spectras of M0ACB (a), M6ACB (b) and P0ACB (0.80)-co-P6ACB (0.20) (c).
The flexible component in the copolymer also affects the thermal
As illustrated in Fig. 5, the birefringence phenomenon is observed in
the LC-PGel formed by P0ACB or P0ACB (0.80)-co-P6ACB (0.20) at
room temperature. This result indicates that the addition of the copo-
lymers does not affect the anisotropy of the 5CB [20]. As the tem-
perature rises above the clear point of 5CB, phase separation phe-
nomenon are observed both in P0ACB (0.80)-co-P6ACB (0.20)/5CB and
P0ACB/5CB. But the phase separation of P0ACB (0.80)-co-P6ACB
(0.20)/5CB is not as obvious as that of P0ACB/5CB due to the in-
troduction of the flexible component in copolymer. This phenomenon
may be explained that the introduction of the flexible component leads
to the better compatibility of the copolymer and 5CB, which increases
the homogeneity of the copolymer with 5CB. Dispersive regions with
birefringence indicate the existence of ordered structures formed by the
side chain of polymers via π-π interaction. These ordered structures are
stable until the temperature rises to TGS. Finally, the temperature rises
above TGS, the ordered structure in the LC-PGel have been destroyed
and the birefringence phenomenon disappeared totally.
behavior of the LC-PGel. As can be seen in Fig. 4 (b), both P0ACB/5CB
and P0ACB (0.80)-co-P6ACB (0.20)/5CB are white opaque gel at room
temperature. When the temperature rises to the clear point of 5CB
(
35 °C), P0ACB/5CB turns into a semitransparent gel and P0ACB(0.80)-
co-P6ACB(0.20)/5CB becomes transparent gel. This phenomenon may
be explained that the introduction of the flexible component leads to
the better compatibility of the copolymer and 5CB, which increase the
homogeneity of the copolymer with 5CB above the clear point of 5CB.
As the temperature increases to TGS, both gels are completely destroyed
and transformed into colorless transparent liquid.
4
.3. Microstructure of LC-PGel
Polarizing microscope (POM) can be used to observe the phase
structure change process of LC-PGel during the heating process. The
P0ACB (0.80)-co-P6ACB (0.20) was selected to prepare LC-PGel with
the concentration of 6 wt%. After heating and dissolving to transparent,
a droplet solution was added to the glass sheets, then cooled to room
temperature and placed under POM for observation.
Scanning electron microscopy (SEM) is an effective method to ob-
serve the microstructures of LC-PGel. The LC-PGel with concentration
of 3 wt% was prepared by copolymers and P0ACB. Xerogels were
48

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
Table 1
Table 2
The properties of copolymers.
MGC of copolymers.
a
Mn(×10−4)b PDIb
(°C)c
(°C)d
(°C)e
P0ACB(x)-co-P6ACB
α / β
T
g
T
i
T
d
x
:y
1.00
:0
0.95
:0.05
0.90
:0.10
0.85
:0.15
0.80
:0.20
0.75
:0.25
0.70
:0.30
0.60
:0.40
(
(
y)
x:y)
MGC (wt%)
1.0
0.5
0.2
0.2
0.2
0.2
0.5
> 10
0
0
0
0
0
0
0
1
.60:0.40
.70:0.30
.75:0.25
.80:0.20
.85:0.15
.90:0.10
.95:0.05
.00:0
1.70
2.65
3.13
4.26
5.35
8.94
4.72
3.59
7.01
4.05
3.25
4.69
2.07 80
2.54 93
2.21 95
2.65 97
2.23 100
2.53 119
2.33 129
2.51 148
–
328
337
336
333
335
336
338
344
175
184
209
220
245
263
282
G' and G" of all LC-PGel decrease sharply with increased shear strain,
indicating that LC-PGel collapsed and lose its mechanical properties.
Finally, when strain value is > 3.5%, the G' is less than G" indicating
that LC-PGel show flow characteristics of liquid.
16.93 5.39
4.91
–
To further detect the resistance of LC-PGel to external stress, fre-
quency sweeps tests were conducted. Frequency sweeps were tested at
low strain (0.05%), which within the linear viscoelastic region. Test
samples are consistent with the strain test. As can be seen in Fig.7(c)
and Fig.7(d), the G' and G“ of all LC-PGel have a slight dependence on
frequency within the frequency range of 0.1-100 rad/s. G' remained
larger than G" and there is no crossover throughout the whole fre-
quency, indicating that all LC-PGel have a good tolerance to external
forces. Interestingly, with rigid component increased, the corre-
a
By calculating the ratio of the peak area of the corresponding 1H NMR
spectra of α and β.
b
Obtained from PL-GPC 120 instrument, linear PS as standards.
The glass transition temperature of polymer measured by DSC at a heating
c
−
1
rate of 10 °C min
under nitrogen atmosphere during the second heating
process.
d
The transition temperature from liquid crystalline phase to isotropic phase
−1
measured by DSC at a heating rate of 10 °C min
under nitrogen atmosphere
during the second heating process.
3
4
sponding G' of the LC-PGel increase from 10 Pa to 10 Pa. The result of
frequency sweeps test further indicate that the increase of rigid com-
ponents can improve the mechanical modulus of LC-PGel.
e
The temperatures at 5% weight loss of the samples under nitrogen were
−
1
measured by TGA heating experiments at the rate of 10 °C min
.
Fig. 7(e) shows the dynamic temperature ramp test chart of 6 wt%
P0ACB(x)-co-P6ACB(y)/5CB. The G' of LC-PGels were measured with
the change of temperature. With temperature increase, the G' of all LC-
PGel decrease slowly, indicating that mechanical strength of all LC-PGel
are decreased slowly. When the temperature rises to TGS, the G' of LC-
PGel decrease rapidly, indicating that the structure of LC-PGel is de-
stroyed and unable to maintain stable mechanical properties. At this
time, the LC-PGel present the properties of viscous flow. This result is
basically consistent with the test result of “tube-testing method”. The
dynamic temperature ramp test shows that the increase of the rigid
component in the copolymer can not only increase the mechanical
modulus, but also improve the thermal stability of the gel.
obtained by soaking in petroleum ether. The test results of xerogels are
shown in Fig.6. The resulting network of aggregates seems to have quite
an open structure (Fig.6a), whereas with the increase of rigid compo-
nents, the network appears dense (Fig.6 b-d). However, the SEM images
may not reveal precisely the structure or the state of arrangement of the
copolymers in the LC-PGel before the removal of the 5CB, since changes
such as the collapse or stacking of aggregates may occur during the
soaking process.
4.4. Mechanical properties of LC-PGel
The dynamic rheological measurement was performed to char-
acterize the mechanical properties of the LC-PGel. The linear regime of
G' and G" was measured first as functions of shear strain. The LC-PGel
prepared by copolymers with different copolymerization ratios were
measured and the concentrations of all LC-PGel were 6 wt%. As shown
in Fig. 7(a) and Fig. 7(b), the G' and G"of all gels does not change with
strain value and the G' always larger than G“ in the strain range from
4.5. Electro-optical properties of LC-PGel
Based on the above research, we can understand that the rigid
component and flexible component could significantly affect the
properties of the LC-PGel. As a kind of light scattering material, the
electro-optical properties of LC-PGel are also an important parameter.
The copolymers of different copolymerization ratios were blended with
0.01 % to 1 %, indicating that this region is the linear viscoelastic re-
gion of all LC-PGel. When the shear strain value is greater than 1 %, the
Fig. 2. TGA curves of copolymers (a) and the second heating DSC curves of copolymers (b).
49

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
Fig. 3. (a) The TGS temperature of LC-PGel with different concentrations of P0ACB or P0ACB (x)-co-P6ACB (y); (b) The TGS of the LC-PGel with a concentration of
6
wt% and the Ti of copolymers with different copolymerization ratios.
5
CB to prepare LC-PGel with concentration of 6 wt%. After heating and
as the flexible component increased. It is because the increase of flex-
ible components leads more network-5CB interfaces and rigid compo-
nents maintain a stable gel network structure. Thus, 5CB molecules are
more difficult to orientations under an electric field. When the flexible
component is larger than 20%, the value of the Vth and Vsat decrease as
the flexible component increased. It is because the reduction of rigid
dissolving to transparent, the mixture was poured into a liquid crystal
cell with a thickness of 8μm. The electro-optic characteristic parameters
of the liquid crystal cell were tested after cooling to room temperature.
The test results are listed in Table 3 and also drawing to Fig.8. When
the flexible component is < 20%, the value of the Vth and Vsat increase
Fig. 4. (a) DSC heating curves of 6 wt% P0ACB (x)-co-P6ACB (y)/5CB; (b) Thermal transition process of P0ACB/5CB and P0ACB (0.80)-co-P6ACB (0.20)/5CB.
50

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
Fig. 5. Polarizing diagram of P0ACB (0.80)-co-P6ACB (0.20)/5CB during heating process (Compared with P0ACB/5CB).
Fig. 6. Scanning electron microscope pictures of xerogel formed by different copolymers: (a) P0ACB (0.70)-co-P6ACB (0.30); (b) P0ACB (0.80)-co-P6ACB (0.20); (c)
P0ACB (0.90)-co-P6ACB (0.10); (d) P0ACB (1.00:0).
components in copolymers makes the gel network structure soften. 5CB
molecules are easily driven under an electric field. So, only when the
content of rigid component is 20%, the strong network structure and
compatibility with 5CB enable copolymers to gain strong surface an-
choring with LC. After forming LC-PGel, 5CB can be firmly absorbed by
copolymers.
The contrast ratio of LC-PGel was calculated by the electro-optical
test curve (Fig. 9), which present the same trend as Vth and Vsat in
Fig. 8. The contrast ratio is deduced by calculating the ratio of the
maximum transmittance to the minimum transmittance. In the liquid
crystal cell, the copolymer concentration of the LC-PGel is the same,
thus, the maximum transmittance of all the LC-PGel is almost the same,
which means that the contrast ratio of the LC-PGel mainly depend on
the minimum transmittance. In the process of gel cooling, the gel net-
work was formed by self-assembly of the copolymers. 5CB was absorbed
by the network fiber and it was able to array along the fiber because of
the surface anchoring. Therefore, the copolymers with larger surface
anchoring lead to greater entropy of 5CB in the LC-PGel, and then the
51

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
Fig. 7. Rheological test of LC-PGel: Strain dependence of (a) storage modulus (G′) and (b) loss modulus (G″) for 6 wt% P0ACB (x)-co-P6ACB (y)/5CB at room
temperature; Condition: ω=1rad/s. Frequency dependence of (c) GIS and (d) G″ for 6 wt% P0ACB (x)-co-P6ACB (y)/5CB at room temperature; Condition: 0.05%
−
1
strain; (e) Temperature dependence of G' for 6 wt% P0ACB (x)-co-P6ACB (y)/5CB from 30 °C to 200 °C. Condition: 0.05% strain, 1 rad/ s frequency and 2 °C min
heating rate.
LC-PGel has lower transmittance in the off state. To sum up, the P0ACB
0.80)-co-P6ACB (0.20) with maximum surface anchoring can form the
maximum contrast ratio.
Under the voltage of 100 V, the response time of all LC-PGel was tested
by LCD parameter tester. As shown in Fig. 10, the open response time
(τon) of all LC-PGel is almost unchanged, and the changing trend of off
state response time (τoff) is opposite to the Vth and Vsat. The structure of
(
The response time of LC-PGel is another important parameter.
52

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
Table 3
Electro-optic properties of LC-PGel formed by different copolymers.
th(V μm 1)a
−
V
sat(V μm−1)b
CRc
τ
on (ms)d
τ
off (ms)e
Sample
V
0
0
0
0
0
0
0
1
.60:0.40
.70:0.30
.75:0.25
.80:0.20
.85:0.15
.90:0.10
.95:0.05
.00:0
0.192
0.612
1.612
5.475
4.575
3.550
0.492
0.327
2.732
5.694
6.556
11.871
11.367
9.529
8.864
7.191
4.44
9.68
0.112
0.120
0.115
0.140
0.130
0.126
0.105
0.123
7.056
6.144
0.534
0.502
0.892
1.034
2.930
5.275
12.33
16.67
12.88
12.66
10.65
8.59
a
The Vth is defined as the applied voltages required for obtaining 10% of the
maximum transmittance.
b
The Vsat is defined as the applied voltages required for obtaining 90% of the
maximum transmittance,
c
The CR is deduced by calculating the ratio of the maximum transmittance
to the minimum transmittance.
d
When a voltage is applied (100 V), the time required to achieve the 90%
maximum transmittance is defined as τon
.
e
When the voltage is removed, the time required to reach the 10% max-
imum transmittance is defined as τoff
Fig. 10. Electrical response time of LC-PGel formed by different copolymers.
.
the gel network formed by copolymers is flexible and the side chain
group of the copolymer is similar to 5CB. Under the application of
100 V voltage, the flexible gel network permit the orientation of side
group, and therefore the addition of copolymer can not impede the
orientation speed of 5CB and not affect the τon of 5CB. When the vol-
tage is removed, there have surface anchoring between the side chain
group and 5CB, the recovery of oriented side group can help 5CB back
to its original state, which cause the reduction of τoff, and thus the LC-
PGel formed by stronger anchor energy copolymers gain shorter τoff.
For the sake of comparison, pure 5CB was tested with different
copolymer concentrations LC-PGel. The electro-optical parameters test
results of pure 5CB and P0ACB (0.80)-co-P6ACB (0.20)/5CB are shown
in Fig. 11 and the test results are summarized in Table 4. The addition
of copolymer improves the Vth and Vsat of 5CB but the open response
time (τon) is almost unchanged, and the off state response time (τoff) is
reduced. In addition, with the increase of copolymer concentration, the
V
th and Vsat of the LC-PGel increase. This is caused by the increase of
surface anchoring between the copolymer and 5CB. Moreover, the
contrast ratio of LC-PGel shows the same trend. This phenomenon de-
pends on two reasons. The one part is the surface anchoring like the
above explanation, which is reduce the minimum transmittance of the
LC-PGel. The other is the light scattering effect of the copolymer gela-
tors, which causes the reduction of maximum transmittance. Although
Fig. 8. The relation diagram of the electro-optical characteristic parameters
and the copolymerization ratio.
Fig. 9. Electro-optic properties curves of LC-PGel formed by different copoly-
mers (Concentration: 6 wt%).
Fig. 11. Electro-optical characteristic curve of LC-PGel formed by different
concentration P0ACB (0.80)-co-P6ACB (0.20).
53

J. Zhao et al.
Reactive and Functional Polymers 131 (2018) 44–55
Table 4
Funding
Electro-optical characteristic of LC-PGel formed by different concentration
P0ACB (0.80)-co-P6ACB (0.20).
This work was supported by National Natural Science Foundation of
China (No. 51573152, 51373148). Open Fund Project of Innovation
Platform in Colleges and Universities of Hunan Province (16K085).
th(V μm−1)a
sat(V μm−1)b
CRc
on (ms)d
off (ms)e
τ
Sample
V
V
τ
0
1
2
3
4
5
6
%
%
%
%
%
%
%
0.168
0.289
2.884
3.286
4.528
4.926
5.475
0.239
5.905
8.301
8.836
9.461
9.769
11.871
4.15
8.84
0.112
0.114
0.098
0.106
0.100
0.108
0.140
8.758
6.398
4.752
2.240
2.178
1.260
0.502
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11.11
13.90
14.91
15.67
16.67
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