Wide-band reflective films produced by side-chain cholesteric liquid-crystalline elastomers derived from a binaphthalene crosslinking agent

Article abstract of DOI:10.1016/j.polymer.2011.10.036

A series of novel side-chain cholesteric liquid-crystalline elastomers based on polysiloxane and cholesterol derivate monomer were prepared by adopting a crosslinking agent containing binaphthalene group. The chemical structures and mesomorphic properties of the monomers and elastomers were confirmed by FT-IR, ¹H NMR, DSC and POM measurements. Worthily, the elastomers exhibited unusual temperature dependence of the helical twisting power (HTP) which is demonstrated resulting from coordination of the crosslinking agent and the mesogenic units. With increase in temperature, the HTP of elastomers containing small quantity of the crosslinking unit exhibited a turning point, while that of elastomers comprising much more crosslinking unit shifted straight. Furthermore, a single layer wide-band reflective film with non-uniform pitch distribution was prepared by utilizing the HTP variety of elastomers with change in temperature. From scanning electron microscopy (SEM) investigations, the mechanism of the broadband reflection was verified.

Full text of DOI:10.1016/j.polymer.2011.10.036

Polymer 52 (2011) 5836e5845
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Wide-band reflective films produced by side-chain cholesteric liquid-crystalline
elastomers derived from a binaphthalene crosslinking agent
,
a
a
a
a
Xiaojuan Wu a, Lilong Yu c, Hui Cao a , Renwei Guo , Kexuan Li , Zihui Cheng , Feifei Wang ,
*
,
a,b,
*
Zhou Yang a , Huai Yang
*
a State Key Laboratory for Advanced Polymer Materials, Department of Materials Physics and Chemistry, School of Materials Science and Engineering,
University of Science and Technology Beijing, Beijing 100083, PR China
b Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, PR China
c Key Laboratory of Advanced Design and Intelligent Computing, Dalian University, Dalian 116622, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel side-chain cholesteric liquid-crystalline elastomers based on polysiloxane and choles-
terol derivate monomer were prepared by adopting a crosslinking agent containing binaphthalene group.
The chemical structures and mesomorphic properties of the monomers and elastomers were confirmed
by FT-IR, 1H NMR, DSC and POM measurements. Worthily, the elastomers exhibited unusual temperature
dependence of the helical twisting power (HTP) which is demonstrated resulting from coordination of
the crosslinking agent and the mesogenic units. With increase in temperature, the HTP of elastomers
containing small quantity of the crosslinking unit exhibited a turning point, while that of elastomers
comprising much more crosslinking unit shifted straight. Furthermore, a single layer wide-band
reflective film with non-uniform pitch distribution was prepared by utilizing the HTP variety of elas-
tomers with change in temperature. From scanning electron microscopy (SEM) investigations, the
mechanism of the broadband reflection was verified.
Received 11 June 2011
Received in revised form
19 October 2011
Accepted 20 October 2011
Available online 24 October 2011
Keywords:
Cholesteric liquid-crystalline elastomers
Binaphthalene
Helical twisting power
Ó 2011 Published by Elsevier Ltd.
1. Introduction
a periodic helicoidal structure [30,31]. The reflection wavelength,
l
¼ nP, where n ¼ (no þ ne)/2 is the average of the ordinary (no) and
extraordinary (ne) refractive indices of the CLC, P is the cholesteric
pitch corresponding to the length of a 2 molecular rotation. For
Liquid-crystalline elastomers (LCEs) have developed to be a new
class of liquid crystal (LC) materials because of their special optical,
mechanical and piezoelectric properties [1e3]. The polymer
network structure of LCEs is usually produced by the introduction
of crosslinking segment into LC polymer systems. And different
crosslinkings can result in materials with distinct properties [4e6].
Recently, more and more researches have focused on cholesteric
LCEs (ChLCEs) with helical structures [7e9], owing to their unusual
piezoelectricity [10e16], tunable mirrorless lasing [17,18], and
photonics [19,20]. Therefore ChLCEs exhibit potential applications
in nonlinear optical materials, electro-optical materials and are
considered as a candidate for the novel piezoelectric device.
In addition, since cholesteric liquid-crystalline polymers hold
inherent characteristics of cholesteric LC (CLC), they have applica-
tions in wide-band reflective devices [21e29]. CLC possesses the
unique properties of wavelength- and polarization- selective
reflection when the molecules are macroscopically arranged in
p
a wide-band reflective film, the reflection bandwidth Dl ¼ nePmax e
noPmin, where Pmax and Pmin are the maximal and the minimal pitch
length in the film, respectively. The pitch length is determined by
the helical twisting power (HTP) of the CLC and given by P ¼ [(HTP)
$ Xc]ꢀ1, here, Xc is the molar concentration of chiral component
[32]. Consequently, information on the HTP of ChLCEs is the base for
their application in wide-band reflective devices.
Binaphthol has been usually used as a chiral center for the
synthesis of high HTP chiral dopant [33e35]. And studies on the
synthesis of side-chain cholesteric liquid-crystalline polymers and
networks derived from binaphthol are less reported. In this paper,
a crosslinking agent containing binaphthalene group, a cholesteric
monomer, and the corresponding side-chain ChLCEs were synthe-
sized and characterized. The effect of the content of the crosslinking
unit on the phase behavior and properties of the elastomers was
discussed. Remarkably, the elastomers exhibited special and
reversible HTP temperature dependence. And it is demonstrated
that the ChLCE has great potential in making wide-band reflective
films with non-uniform pitch distribution. Additionally, SEM was
used to illustrate the mechanism of the broadband reflection.
* Corresponding authors. State Key Laboratory for Advanced Polymer Materials,
Department of Materials Physics and Chemistry, School of Materials Science and
Engineering, University of Science and Technology Beijing, Beijing 100083, PR China.
E-mail addresses: caohui@mater.ustb.edu.cn (H. Cao), yangz@ustb.edu.cn
(Z. Yang), yanghuai@mater.ustb.edu.cn (H. Yang).
0032-3861/$ e see front matter Ó 2011 Published by Elsevier Ltd.
doi:10.1016/j.polymer.2011.10.036

X. Wu et al. / Polymer 52 (2011) 5836e5845
5837
2. Experimental section
hydrozy benzoate, 3-bromoprop-1-ene, cholesterol and H2Pt6Cl
were obtained from Sinopharm Chemical Reagent Co.Ltd. All
solvents and reagents were purified by standard methods. Besides
a nematic LC, SLC1717 (no ¼ 1.519, ne ¼ 1.720, Slichem Liquid Crystal
Material Co.Ltd.), a photo-polymerizable LC monomer, 1,4-di-[4-(6-
acryloyloxy) hexyloxy benzoyloxy]-2-methyl benzene (C6M) and
2.1. Materials
In this study, poly(methylhydrosiloxane) (PMHS, Mn ¼ 390) was
purchased from SigmaeAldrich. 1,10-binaphthyl-2,20-diol, ethyl 4-
Fig. 1. (a) Chemical structures of the materials used; (b) Synthetic routes of the mesogenic monomer and the crosslinking agent.

5838
X. Wu et al. / Polymer 52 (2011) 5836e5845
a photo-initiator, 2,2-dimethopxy-1,2-diphenyl-ethanone (IRG651,
TCI Co.Ltd.) were used. C6M was synthesized according to the
method suggested by Broer [36]. Fig. 1a shows the chemical
structures of these materials.
2.2. Synthesis of cholesteryl-4-allyloxybenzoate (M1)
M1 was synthesized according to the method reported in [37].
Yield: 78.5%. IR (KBr):
y
¼ 3080 cmꢀ1 (¼CꢀH), 2933, 2865 cmꢀ1
(eCH3 and eCH2-), 1706 cmꢀ1 (C]O), 1645 cmꢀ1 (C]C), 1608,
1509, 1458 cmꢀ1 (Ar-), 1258 cmꢀ1 (CeOeAr), 1173 cmꢀ1 (CeOeC).
1H NMR (600 MHz, CDCl3, TMS):
d
¼ 8.00e7.98 (d, J ¼ 8.4 Hz, 2H;
Ar-H), 6.93e6.92 (d,
J
¼
9.0 Hz, 2H; Ar-H), 6.06e6.03 (q,
J ¼ 6.6 Hz, 1H; CH2 ¼ CH-), 5.44 (d, J ¼ 1.2 Hz, 1H; CH2 ¼ CH-),
5.41 (d, J ¼ 1.2 Hz, 2H; ¼ CH- in cholesteryl), 5.33e5.31 (q,
J ¼ 1.2 Hz, 1H; CH2 ¼ CH-), 4.84e4.82 (t, J ¼ 4.8 Hz, 1H; eCH2-
CHO-), 4.60e4.59 (d, J ¼ 5.4 Hz, 2H; ¼ CH-CH2O-), 0.93e2.46 (m,
43H; cholesteryl-H).
2.3. Synthesis of (S)-(ꢀ)-1, 10-binaphthyl-2,20-diyl bis(4-(allyloxy)
benzoate (M2)
The synthetic route of crosslinking agent M2 is shown in Fig. 1b.
3-bromoprop-1-ene (220 mmol, 26.6 g) was added dropwise to
a mixture of ethyl 4-hydroxybenzoate (200 mmol, 33.2 g), 200 ml of
butanone, NaOH (220 mmol, 8.8 g) and KI (3 g) at room tempera-
ture and heated to 80 ꢁC under magnetic stirred for 16 h. After
filtration and removal of solvent, the residue was dissolved in
500 ml 10% NaOH and extracted twice by ether. After removal of
ether, ethyl 4-(allyloxy)benzoate (1) was obtained. Then, a solution
of 300 ml water and 20 g KOH were added to compound 1 at room
temperature. The mixture was heated to 110 ꢁC, refluxed for 10 h
and acidified to PH 3.0 with hydrochloric acid. 32.4 g of white
crystal 4-(allyloxy)benzoic acid (2) was obtained after several
recrystallizations from ethanol. Yield: 91.0%. IR (KBr):
y
¼ 3025 cmꢀ1 (¼CꢀH), 2965-2817 cmꢀ1 (eCH2-, CH2 ¼ and ¼ CH),
2678-2564 cmꢀ1 (eOH in eCOOH), 1680 cmꢀ1 (C]O), 1605,
1510 cmꢀ1 (Ar-), 1252 cmꢀ1 (CeOeC). 1H NMR (600 MHz, CDCl3,
TMS):
d
¼ 8.07e8.05 (q, J ¼ 1.8 Hz, 2H; Ar-H), 6.97e6.96 (q,
J ¼ 1.8 Hz, 2H; Ar-H), 6.09e6.03 (m, 1H; CH2 ¼ CH-), 5.46e5.42 (m,
1H; CH2 ¼ CH-), 5.34e5.32 (m, 1H; CH2 ¼ CH-), 4.63e4.61 (q,
J ¼ 3.0 Hz, 2H; ¼ CH-CH2O-).
Fig. 2. Synthetic route and schematic representation of elastomers.
Compound 2 (31 mmol, 5.5 g), and thionyl chloride (65 mmol,
7.7 g) were added into a round flask equipped with an absorption
instrumentof hydrogenchloride. Themixturewasheatedto90ꢁCand
refluxed for 6 h. After removal of the solvent, 4-(allyloxy)benzoyl
chloride(3) wasobtained. (S)-(ꢀ)-1,10-binaphthyl-2,20-diol (15 mmol,
4.3 g) and pyridine (1.0 ml) were dissolved in 50 ml of dry THF to form
asolution.Thesolutionwasaddeddropwisetocompound3, heatedto
90ꢁC and refluxedfor 10 h. The mixturewas cooled, filtered, poured in
200 ml of cold water and extracted twice by CH2Cl2. After removal of
the solvent, the residue was purified by column chromatography
(silica gel, ethyl acetate/petroleum ether ¼ 1/1) to obtain 5.8 g of M2.
2.4. Synthesis of the elastomers
For synthesis of elastomers E1-E4, the same method was
adopted, and the synthetic route and schematic representation of
elastomers are shown in Fig. 2. The content of crosslinking agent
M2 increases gradually from E1 to E4. The polymerization experi-
ments are summarized in Table 1. The synthesis of elastomer E2
Yield: 63.8%. IR (KBr):
y
¼ 3065 cmꢀ1 (¼CꢀH), 2924, 2858 cmꢀ1
Table 1
(eCH2-, CH2 ¼ and ¼ CH),1743, 1778 cmꢀ1 (C]O), 1645 cmꢀ1 (C]C),
Polymerization, molecular weights and polydispersity indexes of elastomers.
1602,1506 cmꢀ1 (Ar-),1254 cmꢀ1 (CeOeAr),1158 cmꢀ1 (CeOeC). 1H
Sample
Feed (mmol)
M2a (mol%)
Yield (%)
Mnb
PDIb
NMR (600 MHz, CDCl3, TMS):
d
¼ 7.97e7.96 (d, J ¼ 9.0 Hz, 2H; Ar-H),
PMHS
M1
M2
7.90e7.89 (d, J ¼ 8.4 Hz, 2H; Ar-H), 7.80e7.79 (d, J ¼ 9.0 Hz, 2H; Ar-H),
7.60e7.58 (d, J ¼ 9.0 Hz, 2H; Ar-H), 7.56e7.55 (d, J ¼ 9.0 Hz, 2H; Ar-H),
7.44 (s, 2H; Ar-H), 7.37 (s, 2H; Ar-H), 7.32 (s, 2H; Ar-H), 6.74e6.73 (d,
J ¼ 9.0 Hz, 4H; Ar-H), 6.00e5.99 (m, 2H; CH2 ¼ CH-), 5.39 (d, J ¼ 1.8 Hz,
1H; CH2 ¼ CH-), 5.36 (d, J ¼ 1.8 Hz,1H; CH2 ¼ CH-), 5.29 (d, J ¼ 1.2 Hz,
1H; CH2 ¼ CH-), 5.28 (s, 1H; CH2 ¼ CH-), 4.51e4.50 (d, J ¼ 4.8 Hz,
4H; ¼ CH-CH2O-).
E1
E2
E3
E4
1.0
1.0
1.0
1.0
2.850
2.280
1.900
0.950
0.475
0.760
0.950
1.425
14.3
25.0
33.3
60.0
90
88
91
89
3744
3356
3171
2975
1.44
1.43
1.40
1.38
a
Molar fraction of M2 based on (M1 þ M2).
b
The apparent number-average molecular weight (Mn) and polydispersity index
(PDI) were measured by GPC using PS standards.

X. Wu et al. / Polymer 52 (2011) 5836e5845
5839
was given as an example. LC monomer M1 (2.28 mmol, 1.24 g) was
dissolved in 50 ml of dry, fresh distilled toluene. To the stirred
solution, crosslinking agent M2 (0.76 mmol, 0.46 g), PMHS
(1.0 mmol, 0.39 g) and 2 ml of H2PtCl6/THF (0.50 g of hexa-
chloroplatinic acid hydrate dissolved in 100 ml of tetrahydrofuran)
were added and heated under nitrogen and anhydrous conditions
at 80.0 ꢁC for 36 h. After removal of the solvent, the residue was
dissolved in THF and precipitated in methanol, and dried under
induced to arrange as the direction of the oriented polymeric film
[38,39]. 40- m-thick PET (polyethylene terephthalate) films were
used as spacer of the cells. The monomers and elastomers were
filled into the cells by capillary action at their mesophase
temperature ranges.
The wide-band reflective films were prepared by carrying out
the following procedure. The cell containing mixture 7 was irra-
diated with UV light (5 mW cmꢀ2, 365 nm) for 30 min at 100 ꢁC,
during which polymer network was formed from the photo-
polymerization of C6M. Then it was cooled slowly to 70 ꢁC and
quenched below the melting temperature of mixture 7. Finally
properties of the cell were tested at room temperature.
m
vacuum to obtain 2.1 g of elastomer E2. IR (KBr):
y
¼ 2949-
2870 cmꢀ1 (eCH3, eCH2-), 1736, 1711 cmꢀ1 (C]O), 1604, 1510 cmꢀ1
(Ar-), 1077-1025 cmꢀ1 (SieOeSi).
2.5. Characterization and measurements
The synthesized compounds were characterized by FT-IR (Per-
kin Elmer Spectrum One) in solid state by the KBr method with
wavenumber ranging from 400 to 4000 cmꢀ1 and 1H NMR (Bruker
DMX-600 spectrometer) with CDCl3 as the solvent. The molecular
weights of elastomers were estimated using gel permeation chro-
matography (GPC). The optical textures were observed by POM
(Olympus BX51) with a hot stage calibrated to an accuracy of
ꢂ0.1 ꢁC (Linkam THMS-600). The phase transition temperatures
were investigated by DSC (Perkin Elmer Pyris 6) with a heating and
a cooling rate of 10 ꢁC minꢀ1 under a dry nitrogen purge. The
morphology of the pitch distribution was observed by scanning
electron microscopy (SEM, Zeiss EVO18).
2.6. Preparation of the mixtures and cells
The preparation of 1e7 mixtures is according to the way
described bellow. The components of every mixture were dissolved
in dichloromethane, and then sonicated for at least 2.0 h to ensure
that an even mixture was obtained after removing the solvent.
Mixtures 1e6 were prepared to characterize the HTP of monomers
and elastomers. And mixture 7 was prepared to fabricate wide-
band reflective films.
To obtain homogeneous alignment of the mesogens, a 2.0 Wt%
polyvinyl alcohol (PVA) aqueous solution was coated onto the
inner surfaces of the substrates of the cells by spin casting (3000
rminꢀ1 for 30s). The deposited film was dried at 80.0 ꢁC for about
1 h, and subsequently rubbed with a textile cloth under a pressure
of 2.0 g cmꢀ2 along one direction to make the polymer oriented.
As the liquid crystal mesogens are club-shaped, they can be
Fig. 3. FT-IR spectra of monomers and elastomers.
Fig. 4. 1H NMR spectra of elastomers E1eE4.

5840
X. Wu et al. / Polymer 52 (2011) 5836e5845
(1706e1738 cmꢀ1) and aromatic (1608-1506 cmꢀ1) still existed.
Table 2
Thermal properties of monomers and elastomers.
These characteristics suggested the successful incorporation of
monomers onto the polysiloxane chains for all the elastomers. 1H
NMR can provide an effective method for verification in the
synthesis of LCEs [40]. As can be seen in Fig. 4, the augment of the
peak area during 6.0e8.5 ppm corresponding to aromatic CeH
proton and the abatement of the peak area corresponding to
methyl, methylene proton in 0.0e3.0 ppm well reflected the
changes with increasing the concentration of crosslinking agent M2
from E1-E4.
Compound
Tm1 (ꢁC)
Tm2 (ꢁC)
Ti (ꢁC)
D
Ta
M1
M2
E1
E2
E3
115.2
7.0b
51.1
45.8
42.7
43.0
e
235.1
e
119.9
e
54.3
e
145.0
138.0
e
93.9
92.2
e
e
135.0
130.0
E4
e
e
a
Mesophase temperature ranges.
Glass transition temperature of M2.
b
3.2. Thermal and texture analysis
3. Results and discussion
The thermal properties of the monomers and elastomers listed
in Table 2 were determined by DSC and POM measurements. From
DSC results, a melting transition and a cholesteric-isotropic phase
transition for M1 appeared at 115.2 and 235.1 ꢁC; a glass transition
and a melting transition for M2 occurred at 7.0 and 54.3 ꢁC,
respectively. POM results showed that M1 exhibited enantiotropic
cholesteric phase, while M2 revealed no texture and birefringence
on heating and cooling cycles. As M1 was filled into a homogeneous
treated cell, it presented planar texture of cholesteric phase when
heated upon 115.2 ꢁC, and focal-conic texture of cholesteric phase
when cooled from its isotropic state as shown in Fig. 5a and b.
Fig. 6 shows the DSC thermograms of elastomers E1-E4. In the
DSC curves, a melting temperature (Tm) can be detected. The value
of Tm decreases with increasing crosslinking content from E1 to E4.
3.1. 1H NMR and FT-IR characterization of monomers and
elastomers
As shown in Fig. 1b, mesogenic monomer M1 and crosslinking
agent M2 were synthesized in four steps. The structures of mono-
mers were confirmed by FT-IR and 1H NMR spectra. The FT-IR
spectra of M1 and M2 showed characteristic olefinic C]C at
1645 cmꢀ1, ester C]O band at 1706 cmꢀ1 for M1, 1743 and
1778 cmꢀ1 for M2 as can be seen from Fig. 3. Yields, detailed
polymerization and molecular characteristics of elastomers are
summarized in Table 1. The apparent number-average molecular
weight (Mn) of elastomers decreases gradually with the increase of
the concentration of the crosslinking unit. Elastomers E1-E4 were
synthesized by a one-step hydrosilylation reaction between the Si-
H bond of PMHS and the olefinic C]C bond of monomers to form
SieCeC bond. After the reaction finishing, the olefinic C]C bond is
consumed and its stretching band should disappear in the FT-IR
spectra of elastomers. The FT-IR spectra of E1-E4 are shown in
Fig. 3. They showed the complete disappearance of Si-H stretching
From POM measurements,
a melting temperature (Tm) and
a clearing temperature (Ti) were observed for E1 and E2. During the
temperature range between Tm and Ti, they exhibited cholesteric
phase textures as shown in Fig. 5c and d. Two kinds of melting
temperatures were seen by POM observations for E3 and E4. The
first melting temperature (Tm1) is consistent with the melting
temperature detected on the DSC curves. Both E3 and E4 changed to
be isotropic state when heated to the second melting temperature
(Tm2). All the thermal information for the elastomers obtained
at 2160 cmꢀ1 and olefinic C]C stretching band at about 1645 cmꢀ1
.
Characteristic SieOeSi stretching bands appeared at 1200-
1000 cmꢀ1. In addition, the absorption bands of ester C]O
Fig. 5. Optical textures of monomers and elastomers: (a) oily streak texture of M1 on heating to 130.3 ꢁC; (b) focal-conic texture of M1 on cooling to 224.9 ꢁC from its isotropic state;
(c) planar texture of E1 at 117.0 ꢁC; (d) planar texture of E2 at 127.0 ꢁC.

X. Wu et al. / Polymer 52 (2011) 5836e5845
5841
Table 3
from DSC and POM measurements are summarized in Table 2. As
seen from the data in Table 2, adopting small quantity of cross-
linking units in the elastomers did not markedly affect their
mesomorphic behaviors [41], and reversible LC phase transitions
can be observed as a result of enough molecular motion. For
example, both E1 and E2 exhibited wide mesophase temperature
ranges. However, when the content of the crosslinking agent is up
to about 30.0 mol%, such as for E3 and E4, LC phase disappears
owing to the depression of LC orientational order.
The compositions, weight ratios, phase transition temperatures and corresponding
enthalpy changes of mixtures 1e7.
d
Mixture (type)
Weight ratio
TCh-I (ꢁC)c
D )
H (J gꢀ1
1
2
3
4
5
6
7
2.0/-/-/-/-/-/98.0a
-/2.0/-/-/-/-/98.0a
-/-/2.0/-/-/-/98.0a
-/-/-/2.0/-/-/98.0a
-/-/-/-/2.0/-/98.0a
-/-/-/-/-/2.0/98.0a
95.99/4.0/0.01b
94.6
84.0
91.1
93.0
89.8
88.0
116.5
0.71
0.68
0.86
0.67
0.93
0.50
3.63
a
3.3. HTP analysis of monomers
Weight ratio: M1/M2/E1/E2/E3/E4/SLC1717.
Weight ratio: E1/C6M/IRG651.
TCh-I: Clearing temperature from cholesteric to isotropic phase.
b
c
It is well known that a CLC can also be formed by doping a chiral
dopant into a nematic LC. As mentioned before, the pitch length of
CLC, P ¼ [(HTP) $ Xc]ꢀ1, where HTP and Xc are the helical twisting
power and molar concentration of the chiral dopant, respectively.
Therefore, HTP of monomers and elastomers could be characterized
by measuring the pitch lengths of CLCs induced by them, when the
Xc is unchangeable. The pitch lengths were measured by the Cano
wedge technique [42]. In this measurement, a wedge-shaped cell
d
DH: Delta H from cholesteric to isotropic phase.
the increase of temperature. From above discussion, it is clear that
with increase in temperature the HTP of M1 increases, while that of
M2 decreases; and the HTP of M2 is greatly larger than that of M1.
In other words, monomer M1 and crosslinking agent M2 have
opposite temperature dependence of HTP. Hence the HTP of elas-
tomers grafting both M1 and M2 is possible to exhibit distinct
phenomenon.
(KCRK-07, Japan) with a wedge angle, q (0.01937 rad), was used and
the inner surfaces of its two glass substrates were treated to
provide a homogeneous orientation of the LC molecules. After the
mixture was filled into the cell, a Grandjean-Cano texture formed
with disclination lines separated by a distance L. The pitch length P
3.4. HTP analysis of elastomers
is determined from P ¼ 2Ltan
q z 2Lq. When Xc is small, the pitch
Fig. 8 shows the temperature dependence of the pitch lengths of
mixtures 3e6. When heating the pitch lengths of mixtures 3 and 4
increase firstly and then decrease; while those of mixtures 5 and 6
length is long and easily measured as the disclination lines are clear
and regular. So the smaller weight ratio of the chiral dopant is
helpful for the pitch length measurement. Here, we prepare
mixtures 1e6 by doping about 2 Wt% monomer or elastomer into
the nematic LC SLC1717. The compositions, weight ratios, phase
transition temperatures and corresponding enthalpy changes of the
mixtures are listed in Table 3.
Fig. 7a shows the temperature dependence of the pitch length of
mixture 1. It can be found that the pitch length decreases from
12.24 to 9.07 m
m with increasing temperature from 20 to 93 ꢁC,
which also can be seen from the corresponding POM micrographs
of mixture 1 in the cell inside Fig. 7a. Then from mixture 1 and
combining the formula P ¼ [(HTP) $ Xc]ꢀ1, it can be concluded that
the HTP of monomer M1 increases greatly with increasing
temperature.
The temperature dependence of the pitch length of mixture 2,
a CLC induced by crosslinking agent M2, is shown in Fig. 7b. The
pitch length increases from 1.84 to 3.02
mm when heating from 20
to 80 ꢁC. The results indicated that the HTP of M2 decreases with
Fig. 7. Temperature dependence of the pitch length of mixture 1 (a) And mixture 2 (b);
the insets are the corresponding POM micrographs taken in the Cano wedges at 20 and
93 ꢁC for mixture 1, 20 and 80 ꢁC for mixture 2, respectively.
Fig. 6. DSC thermograms of elastomers on the first heating at 10 ꢁC minꢀ1
.

5842
X. Wu et al. / Polymer 52 (2011) 5836e5845
Fig. 8. Temperature dependence of the pitch lengths of mixtures 3e6.
increase all through the temperature range. This illustrates that
with increase in temperature the HTP of E1 or E2 decreases firstly
and then increases; while that of E3 or E4 exhibits a straight
increase. It is noticeable that the pitch length increases more
smoothly and the temperature for it decreasing enhances in
sequence from mixture 3 to mixture 4.
Accordingly, the HTP of E1 increases with increasing temperature
up to 70 ꢁC. As the M2 fraction increases, the HTP of elastomers
decreases at high temperature. And when it is up to about 30.0 mol
%, the turning point disappears. In other words, the HTP of E3 or E4
decreases straight and the pitch length of mixture 5 or mixture 6
increases all along with increase in temperature as seen from Fig. 8
(d) and (e). So the pitch length of mixture 6 increased from P4
A possible explanation for above phenomena is given consid-
ering both contributions of M1 and M2 segments on the HTP
temperature dependence of elastomers. On one hand, as the HTP of
M2 is larger than that of M1, the HTP of elastomers increases with
increasing the content of M2 from E1 to E4. Correspondingly, the
pitch length of mixtures decreases from mixture 3 to mixture 6 as
shown in Fig. 8a. On the other hand, owing to the opposite
temperature dependence of HTP for M1 and M2, the HTP of E1 or E2
exhibits a turning point from decreasing like that of M2 to
increasing because of the function of M1 with increasing temper-
ature. So the pitch length of mixture 3 or mixture 4 increases firstly
and then decreases with increasing temperature, as shown in Fig. 8
(b) and (c). For example, the pitch length of mixture 3 increased
(3.9 mm) to P6 (6.3 m
m) when heated from 20 to 85 ꢁC, as shown in
Fig. 10. From above discussion, it can be concluded that owing to
coordination of M1 and M2 segments, the HTP of elastomers
exhibits special thermally sensitive properties, which is important
for their application in display areas.
3.5. Application of elastomers in wide-band reflective films
As the HTP of the synthesized ChLCEs is temperature dependent
discussed above, their pitch lengths exhibit temperature depen-
dence correspondingly. So their selective reflection wavelengths
can shift with changing temperature. Fig. 11 shows that the trans-
mittance spectrum of E1 shifts to shorter wavelength with the
increase of temperature. And the reflection wavelengths are located
from P1 (18.5
to P3 (19.9
m
m
m) to P2 (20.6
m
m) from 20 to 70 ꢁC, while decreased
m) when heated to 90 ꢁC as shown in Fig. 9.
Fig. 9. Schematic representation and POM photographs of mixture 3 in the wedge-shaped cell with increasing temperature.

X. Wu et al. / Polymer 52 (2011) 5836e5845
5843
Fig. 10. Schematic representation and POM photographs of mixture 6 in the wedge-shaped cell with increasing temperature.
in 704, 580, 480 and 424 nm at 70, 90, 110, and 130 ꢁC, respectively.
Then broadband reflective films may be obtained by using the
property of the synthesized elastomers.
short pitch by polymer networks formed from the photo-
polymerization of C6M after UV-curing. When cooling, as the HTP
of E1 decreases with decreasing temperature up to 70 ꢁC as
mentioned before and the anchoring effect of the polymer network
was not strong enough, the pitch length of ChLCEs in the regions far
away from the polymer network becomes long. And benefiting
from the polymer property hold by the ChLCEs, this long pitch was
frozen after quenching the cell below its melting temperature
(33 ꢁC). As a result, a non-uniform pitch distribution was formed in
the film as shown in Fig. 13b. Also, as the UV light irradiates the
whole cell, non-uniform pitch distribution presents in the whole
film.
Mixture 7 was prepared to fabricate wide-band reflective films.
Fig. 12 shows that the reflection wavelength of mixture 7 exhibits
an obvious blue shift with increasing temperature from 70 to
100 ꢁC. The reflection band broadened to about 251 nm from 430 to
681 nm after UV-curing at 100 ꢁC. A possible explanation to this
phenomenon is given considering the HTP variety of the ChLCE at
different temperatures and the anchoring effect of the polymer
network. Fig. 13 shows the schematic representation of the rear-
rangement of the ChLCE molecules in mixture 7 when temperature
changes. At high temperature, the mixture was filled into a homo-
geneously treated cell and exhibited uniform short pitch as shown
in Fig. 13a. Then ChLCEs in some local regions were fixed in this
It has been reported that SEM can be used to detect the pitch of
CLCs [30,43]. Due to the anchoring effect of the polymer network,
it can template the cholesteric order and pitch distribution of
Fig. 11. The transmission spectrum of E1 shifts to shorter wavelength as temperature
increases.
Fig. 12. The transmission spectra of mixture 7 before and after UV-curing.

5844
X. Wu et al. / Polymer 52 (2011) 5836e5845
Fig. 13. Schematic representation of the rearrangement of ChLCE molecules in the cell containing mixture 7 when temperature changes. (a) Before UV-curing at high temperature;
(b) After UV-curing at high temperature and cooled to low temperature under the melting temperature of mixture 7.
liquid crystals. The micro-structure of the helical polymer
network in a wide-band reflective film has been reported widely
[44,45]. So the non-uniform pitch distribution of mixture 7 was
observed with SEM to demonstrate the mechanism described
above. Fig. 14 shows the micro-structure of the fractured surface
of the irradiated cell containing mixture 7. The dimension of the
three bands in the fine structure on the fractured plane is P, cor-
4. Conclusion
In this paper, we described the synthesis and characterization of
a
series of side-chain ChLCEs containing cholesteryl-4-
allyloxybenzoate as mesogenic unit and (S)-(ꢀ)-1, 10-binaphthyl-
2,20-diyl bis(4-(allyloxy)benzoate as chiral crosslinking unit. The
elastomers containing less than 25.0 mol % of crosslinking units
showed reversible cholesteric phase and wide mesophase
temperature ranges. Furthermore, HTP temperature dependences
were studied. With increase in temperature, the HTP of M1
increased and that of M2 decreased, which induced that the HTP of
elastomers containing less than 25.0 mol% of the crosslinking unit
exhibited a turning point (from decreasing to increasing), while
that of elastomers comprising much more crosslinking unit
increased all along when heating. In addition, wide-band reflective
films were prepared by utilizing the HTP variety of the synthesized
ChLCEs with the change of temperature. And the SEM studies
suggested that a non-uniform pitch distribution has formed in the
film. In view of practical applications, these robust broadband
reflective films may be attractive for reflective LC displays, bright-
ness enhancement films and smart switchable reflective windows.
responding to a 2
that different pitch lengths, changing gradually from about
0.24 m (P1) to about 0.38 m (P5), distributed randomly in the
fractured plane and P1 < P2 < P3 < P4 < P5. According to the
equation,
p cholesteric molecular rotation. It can be seen
m
m
l
¼ nP, l1 should be 430 nm while l5 should be 681 nm
in theory. This demonstrates that the HTP variety of the synthe-
sized ChLCEs with the change of temperature and polymer
networks formed at high temperature can induce a non-uniform
pitch distribution and confirms above explanation of the
mechanism.
Acknowledgment
This work was supported by National Natural Science Fund for
Distinguished Yong Scholar (Grant No.51025313), National Natural
Science Foundation (Grant No.50973010, 50803005 and 51043012),
Science and Technology Project of Beijing. China (Grant
No.D09083044209001), Research Fund of State Key Laboratory for
Advanced Metals and Materials and Open Research Fund of State
Key Laboratory of Bioelectronics (Southeast University).
References
[1] Kishi R, Sisido M, Tazuke S. Macromolecules 1990;23:3779e84.
[2] Zentel R. Angew Chem Adv Mater 1989;101(10):1437e45.
[3] Loffler R, Finkelmann H. Macromol Chem Rapid Commum 1990;11:321e8.
[4] Meng FB, Zhang BY, Liu LM, Zhang BL. Polymer 2003;44:3935e43.
[5] Hu JS, Zhang BY, Zhou AJ, Yang LQ, Wang B. Eur Polym J 2006;42:2849e58.
[6] Hu JS, Zhang BY, Zheng YY, Li QY. React Funct Polym 2005;64:1e11.
[7] Sawa Y, Ye F, Urayama K, Takigawa T, Gimenez-Pinto V, Selinger RLB, et al.
PANS 2011;108:6364e8.
Fig. 14. SEM image of the fractured surface of cross-linked mixture 7. The dimension of
the three bands in the fine structure on the fractured plane corresponds to a 2
p
molecular rotation, that is, P. Different regions exhibited diverse pitch lengths, and
P1 < P2 < P3 < P4 < P5.

X. Wu et al. / Polymer 52 (2011) 5836e5845
5845
[8] Serra F, Matranga MA, Ji Y, Terentjev EM. Opt Express 2010;18:575e81.
[9] HirotaH, JiY, SerraF, TajbakhshAR, TerentjevEM. Opt Express 2008;16:5320e31.
[10] Hirschmann H, Meier W, Finkelmann H. Makromol Chem Rapid Commun
1992;13:385e94.
[30] de Gennes PG, Prost J. The physics of liquid crystals. New York: Oxford
University; 1993.
[31] Lam L. In: Kitzerow H, Bahr C, editors. Chirality in liquid crystals. New York:
Springer; 2001. p. 159.
[11] Pleiner H, Brand HR. J Phys II 1993;3:1397e409.
[12] Terentjev EM. Euro Phys Lett 1993;23:27e32.
[32] Kumar G, Neckers D. Chem Rev 1989;89:1915e37.
[33] Akagi K. Chem Rev 2009;109:5354e401.
[13] Pelcovits RA, Meyer RB. De Physique II 1995;5:877e82.
[14] Brehmer M, Zentel R. Mol Cryst Liq Cryst 1994;243:353e76.
[15] Kelly SM. J Mater Chem 1995;5:2047e73.
[16] Terentjev EM, Warner M. Eur Phys J B 1999;8:595e601.
[17] Finkelmann H. Adv Mater 2001;13:1069e72.
[18] Schmidtke J, Stile W, Finkelmann H. Phys Rev Lett 2003;90:083902.
[19] Cicuta P, Tajbakhsh AR, Terentjev EM. Phys Rev E 2002;65:051704.
[20] Bermel PA, Warner M. Phys Rev E 2002;65:056614.
[21] Mitov M, Boudet A, Sopena P. Eur Phys J B 1999;8:327e30.
[22] Xiao JM, Zhao DY, Cao H, Yang H. Liq Cryst 2007;34(4):473e7.
[23] Mitov M, Binet C, Boudet A. Mol Cryst Liq Cryst 2001;358:209e23.
[24] Zografopoulos DC, Kriezis EE, Mitov M, Binet C. Phys Rev E 2006;73:061701.
[25] Xiao JM, Cao H, He WL, Ma Z, Geng J, Wang LP, et al. J Appl Polym Sci 2007;
105:2973e7.
[34] Guo RW, Li KX, Cao H, Wu XJ, Wang GJ, Cheng ZH, et al. Polymer 2010;51:
5990e6.
[35] Guo RW, Cao H, Liu HJ, Li KX, Huang W, Xiao JM, et al. Liq Cryst 2009;36(9):
939e46.
[36] Broer DJ, Boven J, Mol GN. Makromol Chem 1989;190:2255e68.
[37] Adams NW, Bradshaw JS, Bayona JM, Markides KE, Lee ML. Mol Cryst Liq Cryst
1987;147:43e60.
[38] Geary JM, Goodby JW, Kmetz AR, Patel JS. J Appl Phys 1987;62(10):4100e8.
[39] Sato Y, Sato K, Uchida T. Jap J Appl Phys 1992;31:L579e81.
[40] Ambrogi V, Giamberini M, Cerruti P, Pucci P, Menna N, Mascolo R, et al.
Polymer 2005;46:2105e21.
[41] Noel C, Shibaev VP, Baron M, Hess M, Jenkins AD, Jin J, et al. Pure Appl Chem
2001;73(5):845e95.
[42] Cano R, Soc B, Mineral F. Crystallogr 1968;91:20e7.
[43] Hu W, Zhao HY, Song L, Yang Z, Cao H, Cheng ZH, et al. Adv Mater 2010;22:
468e72.
[26] Fan B, Vartak S, Eakin JN, Faris SM. Appl Phys Lett 2008;92:061101.
[27] Fan B, Vartak S, Eakin JN, Faris SM. J Appl Phys 2008;104:023108.
[28] Li L, Faris SM. Proceeding of the SPIE 1996;2873:202e5.
[29] Witte PVD, Brehmer M, Lub J. J Mater Chem 1999;9:2087e94.
[44] Broer DJ, Lub J, Mol GN. Nature 1995;378:467e9.
[45] Bian ZY, Li KX, Huang W, Cao H, Yang H. Appl Phys Lett 2007;91:201908.