Side-Chain Cholesteric Liquid Crystalline Elastomers Derived from a Mesogenic Cross-Linking Agent

Article abstract of DOI:10.1021/ma034915k

The synthesis of the two monomers M₁ and M₂ and a series of new side-chain cholesteric liquid crystalline elastomers P ₂-P₈ is presented. The chemical structures of the monomers and elastomers obtained were confirmed by FTIR or ¹H NMR spectroscopy. The cross-link density of the elastomers was determined by swelling experiments. The mesomorphic properties were investigated by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), polarizing optical microscopy (POM), and X-ray diffraction (XRD) measurements. Monomer M₁ showed a cholesteric phase, and M₂ revealed smectic and nematic phases. The effect of the cross-link density on the phase behavior of P₂-P₈ is discussed. Elastomers P ₂-P₆ containing less than 12 mol % of the cross-linking units displayed elasticity, reversible liquid crystalline phase transition, wide mesophase temperature ranges, and high thermal stability. Elastomer P ₇ displayed stress-induced birefringence, and P₈ showed only elasticity with no other texture. Experimental results demonstrated that the glass transition temperatures increased, and the isotropization temperatures and the mesophase temperature ranges of P₂-P ₆ decreased with increasing cross-link density.

Full text of DOI:10.1021/ma034915k

9060
Macromolecules 2003, 36, 9060-9066
Side-Chain Cholesteric Liquid Crystalline Elastomers Derived from a
Mesogenic Cross-Linking Agent
J ia n -Sh e Hu , Ba o-Ya n Zh a n g,* Yin g-Ga n g J ia , a n d Son g Ch en
The Center for Molecular Science and Engineering, Northeastern University,
Shenyang 110004, P. R. China
Received J uly 1, 2003; Revised Manuscript Received September 15, 2003
ABSTRACT: The synthesis of the two monomers M1 and M2 and a series of new side-chain cholesteric
liquid crystalline elastomers P 2-P 8 is presented. The chemical structures of the monomers and elastomers
obtained were confirmed by FTIR or 1H NMR spectroscopy. The cross-link density of the elastomers was
determined by swelling experiments. The mesomorphic properties were investigated by differential
scanning calorimetry (DSC), thermogravimetric analyses (TGA), polarizing optical microscopy (POM),
and X-ray diffraction (XRD) measurements. Monomer M1 showed a cholesteric phase, and M2 revealed
smectic and nematic phases. The effect of the cross-link density on the phase behavior of P 2-P 8 is
discussed. Elastomers P 2-P 6 containing less than 12 mol % of the cross-linking units displayed elasticity,
reversible liquid crystalline phase transition, wide mesophase temperature ranges, and high thermal
stability. Elastomer P 7 displayed stress-induced birefringence, and P 8 showed only elasticity with no
other texture. Experimental results demonstrated that the glass transition temperatures increased, and
the isotropization temperatures and the mesophase temperature ranges of P 2-P 6 decreased with
increasing cross-link density.
In tr od u ction
knowledge, research on ChLCEs is little reported.21-29
It has therefore been necessary to synthesize various
kinds of side-chain ChLCEs to explore their potential
applications.
In this study, new side-chain ChLCEs derived from
cholesteryl 4-allyloxybiphenyl-4′-carboxylate (M1) and
4-allyloxybenzoyl-4′-(6-acryloyloxyhexyloxy)benzoyl-p-
benzenediolbisate (M2) were prepared. The mesomor-
phic properties of the monomers and elastomers ob-
tained were characterized by DSC, TGA, POM, and
XRD. The influence of the cross-link density on the
phase behavior of the elastomers is discussed.
Today, liquid crystalline science is directed toward the
development of multifunctional and oriented structures.
One example of such supermolecular systems is liquid
crystalline networks (LCNs). Recently, research into
LCNs with anisotropic properties have expanded
rapidly,1-8 and in particular, LCNs with cholesteric
structures have attracted considerable interest, because
of their unique properties and potential applications in
numerous areas, especially in the fields of nonlinear
optical materials and electrooptical materials etc.9-13
According to the cross-link density, two new classes of
cholesteric LCNs emerge: highly cross-linked choles-
teric liquid crystalline thermosets (ChLCTs); and lightly
cross-linked cholesteric liquid crystalline elastomers
(ChLCEs).
In the past decade, ChLCTs have been extensively
investigated because the helical structure of cholesteric
phase is permanently fixed and optical properties
become temperature independent.14-20 In contrast to
ChLCTs, ChLCEs combine the basic features of polymer
elastomers with the anisotropy of physical properties
of cholesteric liquid crystalline. Consequently, ChLCEs
not only hold the entropic elasticity but also show
reversible phase transitions between liquid crystalline
and isotropic phases on heating and cooling cycles.
Compared with conventional LCEs, ChLCEs show
unusual piezoelectric properties, besides electrooptical
and mechanical properties due to deformation of the
helical structure of the cholesteric phase.
Exp er im en ta l Section
Ma ter ia ls. Polymethylhydrosiloxane (PMHS) was pur-
chased from J ilin Chemical Industry Co. 4-Hydroxybiphenyl-
4′-carboxylic acid (from Aldrich) was used as received. 1,6-
Hexanediol was purchased from Beijing Hongyu Chemical
Industry Co., and cholesterol was obtained from Henan Xiayi
Medical Co. Toluene used in the hydrosilication reaction was
first refluxed over sodium and then distilled under nitrogen.
All other solvents and reagents were purified by standard
methods.
Mea su r em en ts. FTIR spectra were measured on a Nicolet
510 FTIR spectrometer (Nicolet Instruments, Madison, WI).
1H NMR spectra (300 MHz) were recorded on a Varian WH-
90PFT spectrometer (Varian Associates, Palo Alto, CA). Phase
transition temperatures and thermodynamic parameters were
determined by using a Perkin-Elmer DSC-7 (Perkin-Elmer,
Foster City, CA) equipped with a liquid nitrogen cooling
system. The heating and cooling rates were 10 °C/min. The
phase transition temperatures were collected during the
second heating and the first cooling scans. The thermal
stability of the polymers under atmosphere was measured with
a Perkin-Elmer TGA-7 thermograrimetric analyzer. A Leitz
Microphot-FX (Leitz, Wetzlar, Germany) polarizing optical
microscope equipped with a Mettler FP 82 hot stage and FP
80 central processor was used to observe phase transition
temperatures and analyze liquid crystalline properties for the
monomers and polymers through observation of optical tex-
tures. XRD measurements were performed with a nickel-
Recent theoretical and experimental studies have
suggested that piezoelectricity can be obtained from
ChLCEs.21-23 They have the potential to act as a device
that transforms a mechanical signal into an optical
signal when stress is applied parallel to the cholesteric
helix. Therefore, ChLCEs are considered as a candidate
for piezoelectric devices. However, to the best of our
* Corresponding author. E-mail: baoyanzhang@hotmail.com.
10.1021/ma034915k CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/01/2003

Macromolecules, Vol. 36, No. 24, 2003
Liquid Crystalline Elastomers 9061
Sch em e 1. Syn th etic Rou te of Ch olester ic Mon om er
cholesteryl); 6.04 (m, 1H, CH2dCH-); 7.03-8.06 (m, 8H, Ar-
H).
4-Allyloxyben zoyl-4′-(6-a cr yloyloxyh exyloxy)ben zoyl-
p-ben zen ed iolbisa te (M2). First, 2.92 g (0.01 mol) of 4-(6-
acryloyloxyhexyloxy)benzoic acid was reacted at 40 °C with
15 mL of thionyl chloride containing a few drops of DMF and
a trace of p-hydroxyanisole for 4 h, and then the excess thionyl
chloride was removed under reduced pressure to give the acid
chloride.
The acid chloride obtained was dissolved in 5 mL of
chloroform and added dropwise to a cold solution of 2.7 g (0.01
mol) of 4-hydroxyphenyl-4′-allyloxybenzoate in 15 mL of
chloroform and 1.4 mL of triethylamine. After reacting for 14
h at room temperature, the crude product was precipitated
by adding ethanol to the filtrate and recrystallized from
ethanol. Yield 47%, mp 75 °C. IR(KBr), µ (cm-1): 3076 (dC-
H); 2938, 2863 (-CH3, -CH2-); 1736 (CdO); 1643 (CdC);
1
1610, 1513 (Ar-). H NMR (CDCl3), δ (ppm): 1.51-1.87 [m,
8H, -(CH2)4-]; 3.75 (t, 2H, -CH2CH2O-); 4.18 (t, 2H,
-COOCH2-); 4.62 (t, 2H, dCHCH2O-); 5.31-5.48 (m, 4H,
CH2dCHCH2- and CH2dCHCOO-); 6.01 (m, 1H, CH2d
CHCH2-); 6.20 (m, 1H, CH2dCHCOO-); 6.96-8.17 (m, 12H,
Ar-H).
Syn th esis of th e Ela stom er s. For the synthesis of the
elastomers P 2-P 8, the same method was adopted. The syn-
thesis of P 3 is given as an example. Monomers M1, M2, and
P MHS were dissolved in dry, freshly distilled toluene. The
mixture was heated to 65 °C under nitrogen and anhydrous
conditions, and then a proper amount of THF solution of
hexchloroplatinate hydrate catalyst was injected with a sy-
ringe. After holding the reaction mixture at 65 °C for 48 h,
the networks were obtained with methanol, and then dried
under vacuum. IR (KBr), µ (cm-1): 3000-2800 (-CH3, -CH2-
); 1735, 1711 (CdO); 1606, 1512 (Ar-); 1200-1000 (Si-O-
Si).
Sch em e 2. Syn th etic Rou te of Cr oss-Lin k in g
Mon om er
Resu lts a n d Discu ssion
Syn th eses. The synthetic routes for the target mono-
mers are shown in Schemes 1 and 2. The structural
characterization of the monomers and elastomers ob-
tained were in agreement well with the prediction.
4-Allyloxybiphenyl-4′-carboxylic acid (1), 4-hydroxyphe-
nyl-4′-allyloxybenzoate (3), and 4-(6-acryloyloxyhexy-
loxy)benzoic acid (5) were synthesized according to a
route described by Hu et al.26-28 The synthetic methods
for M1 and M2 are the same. First, activation of
compounds 1 and 5 was obtained with thionyl chloride
and then reacted, respectively, with cholesterol and 3
in chloroform to prepare cholesteric monomer M1 and
cross-linking monomer M2. IR spectra of M1 and M2
showed characteristic bands at 1741-1736, 1643-1636,
and 1610-1512 cm-1 attributed to ester CdO, olefinic
CdC, and aromatic CdC stretching band. 1H NMR
spectra of M1 and M2 showed multiplets at 0.61-4.62,
4.72-6.04, and 6.96-8.17 ppm corresponding to methyl
and methylene protons, olefinic protons, and aromatic
protons, respectively.
The elastomers were prepared by a one-step hydro-
silication reaction between Si-H groups of P MHS and
olefinic CdC of M1 and M2 in toluene, using hexchlo-
roplatinate hydrate as catalyst at 65 °C. Yields and
detailed polymerization are summarized in Table 1.
Schematic representation of the elastomers obtained is
presented in Figure 1. IR spectra of the elastomers
showed the complete disappearance of the Si-H stretch-
ing band at 2166 cm-1 and the olefinic CdC stretching
band at 1643-1636 cm-1. Characteristic Si-O-Si
stretching bands appeared at 1200-1000 cm-1. In
addition, the absorption bands of ester CdO and aro-
matic still existed.
filtered Cu KR radiation with a DMAX-3A Rigaku powder
diffractometer (Rigaku, J apan).
Syn th esis of th e Mon om er s. The synthesis of the olefinic
monomers is shown in Schemes 1 and 2. 4-Allyloxybiphenyl-
4′-carboxylic acid, 4-hydroxyphenyl-4′-allyloxybenzoate, and
4-(6-acryloyloxyhexyloxy)benzoic acid were prepared according
to the literature procedures reported.30-32
Ch olester yl 4-Allyloxybip h en yl-4′-ca r boxyla te (M1).
First, 2.54 g (0.01 mol) of 4-allyloxybiphenyl-4′-carboxylic acid
was reacted at 60 °C with 15 mL of thionyl chloride containing
a few drops of N,N-dimethylfomamide (DMF) for 3 h, and then
the excess thionyl chloride was removed under reduced pres-
sure to give the corresponding acid chloride. The acid chloride
obtained was dissolved in 5 mL of dry chloroform, and added
dropwise to a solution of 3.87 g (0.01 mol) of cholesterol in 0.8
mL of pyridine and 15 mL of chloroform. The reaction mixture
was refluxed for 10 h. The crude product was precipitated by
adding ethanol to the filtrate, and was recrystallized from ethyl
acetate/ethanol (1:1). Yield 75%, mp 111 °C. IR (KBr), µ (cm-1):
3063 (dC-H), 2930, 2852 (-CH3, -CH2-), 1741 (CdO), 1636
(CdC), 1606, 1512 (Ar-), 1152 (C-O-C). 1H NMR (CDCl3), δ
(ppm): 0.66-2.07 (m, 43H, cholesteryl-H); 4.55 (t, 2H, -OCH2-
); 4.72-5.29 (m, 2H, CH2dCH-); 5.51 (m, 1H, dCH- in

9062 Hu et al.
Macromolecules, Vol. 36, No. 24, 2003
Ta ble 1. P olym er iza tion a n d Solu bility
feed (mmol)
solubilityb
M1 M2 (mol %) (%) toluene xylene DMF
Ta ble 2. P h ysica l P r op er ties of P olym er s
density
(g/cm3)
Mh c
(g/mol)
degree of
a
M2
yield
polymer
q
swellinga (wt %)
polymer
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
1.056
1.064
1.063
1.068
1.076
1.075
1.082
1.098
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
7.000 0.000
6.861 0.069
6.726 0.137
6.462 0.269
5.963 0.519
5.500 0.750
5.069 0.966
4.667 1.167
0
1
2
4
8
12
16
20
82
84
89
90
86
85
84
79
+
-
-
-
-
-
-
-
+
-
-
-
-
-
-
-
+
-
-
-
-
-
-
-
4.07
3.82
3.46
3.23
2.74
2.37
2.01
11 039
9923
8453
7594
5768
4558
3515
350
330
300
280
240
210
180
a
Toluene uptake in weight % at room temperature, referring
to the weight of the extracted polymer.
a
b
Molar fraction of M1 based on M1 + M2. Key: (+) dissolve;
(-) insolubility or swelling.
in a constant temperature insulated. Swelling experi-
ment was accomplished in several days at room tem-
perature to achieve equilibrium. Swollen elastomers
removed from solvents at regular intervals were dried
superficially with filter paper, weighed and placed in
the same condition. The measurements were continued
until a constant weight was reached for each sample.
Table 2 shows swelling properties of the elastomers. As
seen from the data listed in Table 2, with increasing
the concentration of cross-linking units in the net-
works,Mh c and the degree of swelling reduced, that is to
say, the cross-link density increased.
Th er m a l P r op er ties. In general, the phase behavior
of side-chain LCEs mainly depends on the nature of the
polymer backbone, the rigidity of the mesogenic unit,
the length of the flexible spacer, and the cross-link
density. The polymer backbones are primarily polyacry-
lates, polymethacrylates and polysiloxanes. However,
polyacrylates and polymethacrylates show higher glass
transition temperatures because of their backbones. To
obtain mesomorphic properties and elasticity at moder-
ate temperature, polysiloxanes backbone and flexible
spacer are usually adopted. The polymer backbone and
the mesogenic units have antagonistic tendency: the
polymer backbone is driven toward a random coil-type
configuration, whereas the mesogenic units stabilize
with long-range orientation order. The mesogenic units
are usually attached to the polymer backbone by a
flexible spacer. The flexible spacer decouples the me-
sogenic side groups from the polymer backbone and
renders the mesogenic units to order.
The thermal properties of the monomers M1 and M2
and polymers P 1-P 8 were determined by DSC and POM
measurement. The phase transition temperatures and
corresponding enthalpy changes are summarized in
Tables 3 and 4, respectively. According to the data listed
in Table 3, a melting transition and a cholesteric to
isotropic phase transition for M1 appeared at 111.6 and
251.7 °C; a melting transition, a smectic to nematic
phase, and a nematic to isotropic phase transition for
M2 occurred at 76.5, 86.5, and 199.0 °C, respectively.
For the elastomers P 2-P 6, a glass transition at low
temperature and a cholesteric to isotropic transition at
high temperature appeared on DSC curves. As seen
from the data in Table 3, low cross-link density did not
significantly affect the phase behavior of the elastomers,
and reversible phase transitions were observed because
of enough molecular motion. In contrast, higher cross-
link density had strong influence on the phase behavior,
it could cause liquid crystalline phase to disappear due
to the disturbance of mesogenic order. Therefore, DSC
curves of P 7 and P 8 only showed a glass transition.
Above all, the phase transitions are reversible and do
not change on repeated heating and cooling cycles.
F igu r e 1. Schematic representationof side-chain liquid crys-
talline elastomers.
Sw ellin g P r op er ties. The degree of swelling and the
cross-link density of the elastomers could be determined
by swelling experiments or by stress-strain measure-
ments. From swelling experiments, the cross-link den-
sity or the average molar mass of the network chains
between two cross-links, Mh c, was calculated according
to the Flory swelling theory
Mh c ) Fe‚V1 (0.5-ø1)-1‚q5/3
(1)
where Fe is the density of the elastomer, V1 is the molar
volume of the swelling solvent, q is the equilibrium ratio
of the volumes of the swollen and unswollen elastomers,
and ø1 is the Flory-Huggins parameter or the interac-
tion parameter between the solvent and the elastomer.
The parameter ø1 was calculated from the second
virial coefficient A2 according to eq 2
2
ø1 ) 0.5 - A2Fu V1
(2)
where Fu is the density of the un-cross-linked polymer.
From eqs 1 and 2, Mh c was obtained.
The equilibrium ratio of the elastomers was deter-
mined through the following equation
q ) 1 + (W2/W1 - 1)Fp/Fs
(3)
where W1 and W2 are the weights of the elastomers
before swelling and at equilibrium swelling, and Fp and
Fs are the densities of the polymer and solvent, respec-
tively.
Swelling experiments were measured in 10 mL of
toluene with 0.5 g of samples. The samples were placed
into glass vial closed hermetically. They were subse-
quently homogenized at room temperature by strong
shaking. Each vial was weighed before and after ho-
mogenization. After the vial was sealed, they were left

Macromolecules, Vol. 36, No. 24, 2003
Liquid Crystalline Elastomers 9063
Ta ble 3. P h a se Tr a n sition Tem p er a tu r es (°C) of Mon om er sa
transition tempb
(corresponding enthalpy changes/J ‚g-1
heating/cooling
)
c
d
monomer
∆T1
∆T2
M1
M2
K111.6(35.6)Ch251.7(1.3)I/I234.2(0.8)Ch81.5(29.7)K
K76.5(22.2)SA86.5(3.7)N199.0(2.7)I/I194.7(1.5)SA77.1(2.2)N36.3(17.2)
140.1
122.5
152.7
158.4
a
b
Key: K, solid; Ch, cholesteric; SA, smectic A; N, nematic; I, isotropic. Peak temperatures were taken as the phase transition
temperature. c Mesophase temperature ranges during heating cycle. Mesophase temperature ranges during cooling cycle.
d
Ta ble 4. Th er m a l P r op er ties of P olym er s
polymer Tg/°C Ti/°C ∆H/J ‚g-1 ∆Ta Tclb/°C Tdc/°C λmd/nm
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
65.4 312.5
63.8 311.2
64.2 308.5
65.9 294.6
69.8 295.7
77.1 288.2
79.4
2.8
2.0
1.7
1.8
1.4
0.9
247.1
247.4
244.3
228.7
225.9
211.1
315
316
314
301
302
298
264
348.9
332.5
345.4
342.2
335.2
319.5
331.4
328.4
1215
1224
1381
1426
1459
85.6
a
b
Mesophase temperature ranges (Ti - Tg). Temperature at
which the birefringence disappeared completely. c Temperature at
which 5% weight loss occurred. Reflection wavelength measured
d
by UV/vis spectra at cholesteric phase.
The glass transition temperature (Tg) is an important
parameter in connection with structures and properties.
In general, chemical cross-linking imposes additional
constraints on the motion of chain segments, and causes
an increase in the Tg value. However, the effect may be
small for lightly cross-linked polymers, and the Tg value
is also affected by the flexible cross-linking chains
similar to the plasticization effect. Compared with the
un-cross-linked polymer, Tg values of lightly cross-linked
polymers may fall. Taking the cross-linking effect and
plasticization effect into account, Tg is given by
F igu r e 2. Effect of M2 content on phase transition temper-
atures of the elastomers. Key: (9) Tg; (b) Ti; (2) ∆T.
Tg ) Tgo - KxFx
Tg ) Tgo + KxFx
(4)
(5)
where Tg and Tgo are the glass transition temperatures
of cross-linked and un-cross-linked polymers, Kx is
constant, and Fx is the cross-link density. When the
cross-link density is less than a certain critical value,
the plasticization effect of flexible cross-linking chains
is predominant, and eq 4 is adopted. Conversely, the
cross-linking effect is predominant, and eq 5 is adopted.
Figure 2 shows the effect of the concentration of cross-
linking units on the phase transition temperatures of
P 1-P 8. It can be seen that Tg value of P 2 and P 3
decreased by 1.6 and 1.2 °C compared with P 1. However,
general tendency is toward an increased Tg value with
increasing cross-link density. For P 2-P 8, when the
concentration of cross-linking units increased from 1 to
20 mol %, the Tg value increased from 63.8 to 85.6 °C.
Similar to Tg, chemical cross-linking affected isotro-
pization temperature (Ti) in two ways. On one hand, the
flexible cross-linking chains acted as diluents and led
to a decrease in the Ti value; on the other hand,
chemical cross-linking could prevent the motion and
orientation of mesogenic molecule at and in the vicinity
of the cross-linking sites and did not favor the formation
of mesogenic orientation order in the networks with
increasing cross-link density. Consequently, Ti disap-
peared when the cross-link density was greater than a
certain critical value. According to Table 4, the Ti value
of P 1-P 6 decreased from 311.2 to 288.2 °C when the
concentration of cross-linking units increased from 1 to
12 mol %. In addition, P 1-P 6 displayed wide mesophase
F igu r e 3. TGA thermograms of the elastomers.
temperature ranges (∆T). ∆T of elastomers decreased
with increasing cross-link density because the Tg value
increased, whereas the Ti value decreased.
Figure 3 presents TGA thermograms of the elas-
tomers. TGA results showed that the temperatures at
which 5% weight loss occurred (Td) were greater than
300 °C for P 2-P 8; this reveals that the synthesized
elastomers have a high thermal stability.
Op tica l P r op er ties. The unique optical properties
of cholesteric liquid crystalline are related to the helical
supermolecular structure of the cholesteric phase. The
periodic helical structure of the cholesteric phase selec-
tively reflects visible light like an ordinary diffraction
grating, the pitch of which controls the wavelength of
selective reflection of light. If the reflected wavelength
lies in the visible range of the spectrum, the cholesteric
phase exhibits brilliant colors. The transmitted light

9064 Hu et al.
Macromolecules, Vol. 36, No. 24, 2003
shows complementary color. The wavelength of selective
reflection of light λm obeys the Bragg condition
λm ) njP
(6)
where nj is the average index of refraction and P is the
pitch of the cholesteric phase, defined as the spatial
distance over which the director rotates 360°.
Because of the angular dependence of the reflection
conditions, different colors are seen depending on dif-
ferent observation angles. So λm is given by
λm ) njP cos æ
(7)
where æ is the incidence angle. When æ ) 0° (normal
incidence), λm ) λ0 ) njP. The intensity of reflected light
is a maximum at λm ) λ0, and it falls off very rapidly
on either side of λ0.
In reality, the orientation of the cholesteric helices is
not exactly perpendicular to the cell surface. Taking this
into account, eq 2 becomes
1
2
sin R
nj
sin â
nj
λm ) njP cos sin-1
+ sin-1
(8)
(
[
(
)
(
)
]}
where R and â are the incidence angle and observation
angle, respectively.
The helical pitch is an important parameter in con-
nection with optical properties of the cholesteric phase.
Although the microscopic origins of the helical pitch are
still a subject of study, it is known that the helical pitch
and optical properties of side-chain cholesteric liquid
crystalline networks mainly depend on the polymer
backbone, the rigidity of mesogenic units, the length of
the flexible spacer, the cross-link density, and the outer
condition (such as temperature, force field, electric field
and magnetic field). For each elastomer, the maximum
reflection wavelength λm was measured with UV-vis
spectra. The λm values are summarized in Table 4. As
can be seen from Table 4, the reflection color could not
be observed because λm value was greater than the
wavelength of visible light. Moreover, λm values in-
creased, that is to say, the helical pitch also increased
with an increase in the concentration of cross-linking
units.
F igu r e 4. Optical textures of monomers (320×): (a) oily
streak texture of M1 at 237 °C; (b) fan-shaped texture of M2
at 83 °C; (c) Schlieren texture of M2 at 189 °C.
The optical textures of the monomers and polymers
are studied by POM with hot stage under nitrogen
atmosphere. POM observation results showed that M1
exhibited enantiotropic cholesteric phase and M2 re-
vealed smectic and nematic phases on heating and
cooling cycles. When M1 was heated to 111 °C, the
sample began to melt, the typical cholesteric oily streak
texture gradually appeared, and the texture disap-
peared at 257 °C. When the isotropic state was cooled
to 253 °C, the focal-conic texture appeared. If a me-
chanical field was superimposed on the sample right
now, for example, slight shearing of the melt would
cause macroscopic orientation of the cholesteric do-
mains, and the focal-conic texture transformed to an oily
streak texture, which is a typical characteristic of
cholesteric liquid crystalline. When M2 was heated to
75 °C, a smectic fan-shaped texture appeared, and then
heating continued to 88 °C, nematic schlieren texture
gradually occurred, and mesomorphic behavior disap-
peared at 203 °C. Photomicrographs of M1 and M2 are
shown in Figure 4 parts a-c.
did not appear, the reason for which is that the polymer
chains hinder the formation of the helical supermolecu-
lar structure of the mesogens. Because of the introduc-
tion of the nematic units, the elastomers P 2 and P 3
exhibited cholesteric oily streak or Grandjean texture,
and the types of cholesteric textures of P 4-P 6 were not
easily identified because of the viscoeelasticity. More-
over, P 7 displayed stress-induced birefringence although
the DSC curve showed no mesophase to isotropic phase
transition. This is similar to those described by Mitch-
ell.33 P 8 showed only elasticity with no other texture;
this is consistent with the results obtained by DSC.
Photomicrographs of polymers are shown in Figure 5,
parts a-c.
X-r a y Diffr a ction An a lysis. XRD studies were
carried out to obtain more detailed information on the
liquid crystalline structure and type. In general, a sharp
and strong peak at low angle (1° < 2θ < 4°) in the small-
angle X-ray scattering (SAXS) curve and a broad peak
associated with lateral packing at 2θ ≈ 20° in the wide-
The un-cross-linked polymer P 1 exhibited a smectic
fan-shaped texture, and the expected cholesteric phase

Macromolecules, Vol. 36, No. 24, 2003
Liquid Crystalline Elastomers 9065
F igu r e 6. X-ray diffraction patterns of quenched samples.
ing cross-link density, which indicates the decrease of
liquid crystalline order from P 2 to P 6. Figure 6 shows
representative XRD curves of quenched samples of
elastomers. According to Bragg equation, λ ) 2d sin θ,
the corresponding d spacings were 9.8-9.1 and 5.3-
5.0Å, respectively. Therefore, the cholesteric phase
structures of P 2-P 6 were confirmed by optical texture
and XRD results.
Con clu sion s
In this study, a series of new side-chain ChLCEs
containing cholesteryl4-allyloxybiphenyl-4′-carboxylate
and 4-allyloxybenzoyl-4′-(6-acryloyloxyhexyloxy)benzoyl-
p-benzenediolbisate side groups were synthesized and
characterized. All of the obtained polymers showed very
wide mesophase temperature ranges and high thermal
stability. Light chemical cross-linking did not markedly
affect phase behavior of the elastomers. The elastomers
containing less than 12 mol % of cross-linking units
showed elasticity, reversible mesophase transition, and
cholesteric texture during heating and cooling cycles.
For P 2-P 6, the glass transition temperature increased,
and the isotropization temperature and the mesophase
temperature range decreased with increasing the cross-
link density.
F igu r e 5. Optical textures of polymers (320×): (a) fan-shaped
texture of P 1 at 213 °C; (b) oily streak texture of P 2 at 256 °C;
(c) Grandjean texture of P 3 at 184 °C.
Ack n ow led gm en t. The authors are grateful to the
National Natural Science Fundamental Committee of
China and HI-Tech Research and Development Pro-
gram (863) of China, and the Science and Technology
Research Major Project of the Ministry of Education of
China for financial support of this work.
angle X-ray diffraction (WAXD) curve can be observed
for smectic structure. For the nematic structure, no peak
appears in the SAXS curve, but in the WAXD curve, a
broad peak at 2θ ≈ 20° can also be observed. For the
cholesteric structure, no peak also appears in the SAXS
curve; however, a broad peak occurs at 2θ ≈ 17°. The
diffraction angles of the cholesteric structure at wide
angle are obviously less than those of the smectic and
nematic structures, and this is a very important char-
acteristic to judge the cholesteric structure. Moreover,
some research also shows that a weak peak at 2θ )
8-10° is usually observed for cholesteric structure.34-36
For P 2-P 6, a sharp peak associated with the smectic
layers at low angles did not appear in the SAXS curves,
and a weak peak and a broad peak were observed,
respectively, at 2θ ) 8.9-9.6 and 16.5-17.4° in WAXD
curves. Moreover, broad peaks were more and more
diffuse, and the peak intensity decreased with increas-
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