Optical properties of chiral nematic side-chain copolymers bearing cholesteryl and azobenzene building blocks

Article abstract of DOI:10.1002/pola.24490

Thermotropic chiral nematic (N*) side-chain copolymers (CPs) bearing cholesteryl and azobenzene units were synthesized to investigate the structure-property relationships of the acrylates of the chiral, achiral, and photochromic monomers of free radical polymerization-derived polymers. The polar effect of chlorine substitution on the benzene ring of the chiral monomer (M3*) widened the mesophase transition temperature only at the monomer level, but no remarkable effect on the mesomorphic, optical or thermochromism of the corresponding CPs was observed. An examination of the CPs prepared using differential scanning calorimetry and hot-stage polarizing microscopy showed that all the CPs exhibited a cholesteric nematic phase (N*), and increasing the content of the cholesteryl units in the CPs displayed only the N* phase over a much wider temperature range. On cooling from the isotropic melt of N* CPs, selective reflections of visible light that changed from short to long wavelengths were observed. The photolysis of CPs revealed that CP1-CP4 undergo reversible photoisomerization and that CP5 and CP6 undergo irreversible photoisomerization. The rate of isomerization depends on the type (-N=N-, -CH=CH-, and both) and content of photochromic units in the CPs.

Full text of DOI:10.1002/pola.24490

Optical Properties of Chiral Nematic Side-Chain Copolymers Bearing
Cholesteryl and Azobenzene Building Blocks
JUI-HSIANG LIU, YEN-LING CHOU, RATHINAM BALAMURUGAN, KAI-HSIN TIEN, WEN-TUNG CHUANG, MING-ZU WU
Department of Chemical Engineering, National Cheng Kung University, Tainan, 70101 Taiwan
Received 22 October 2010; accepted 11 November 2010
DOI: 10.1002/pola.24490
Published online 7 December 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Thermotropic chiral nematic (N*) side-chain copoly-
mers (CPs) bearing cholesteryl and azobenzene units were syn-
thesized to investigate the structure–property relationships of
the acrylates of the chiral, achiral, and photochromic mono-
mers of free radical polymerization-derived polymers. The
polar effect of chlorine substitution on the benzene ring of the
chiral monomer (M3*) widened the mesophase transition tem-
perature only at the monomer level, but no remarkable effect
on the mesomorphic, optical or thermochromism of the corre-
sponding CPs was observed. An examination of the CPs pre-
pared using differential scanning calorimetry and hot-stage
polarizing microscopy showed that all the CPs exhibited a cho-
lesteric nematic phase (N*), and increasing the content of the
cholesteryl units in the CPs displayed only the N* phase over a
much wider temperature range. On cooling from the isotropic
melt of N* CPs, selective reflections of visible light that
changed from short to long wavelengths were observed. The
photolysis of CPs revealed that CP1–CP4 undergo reversible
photoisomerization and that CP5 and CP6 undergo irreversible
photoisomerization. The rate of isomerization depends on
the type (AN¼NA, ACH¼CHA, and both) and content of
C
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photochromic units in the CPs.
2010 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 49: 770–780, 2011
KEYWORDS: chiral; copolymerization; isomer/isomerization; liquid
crystalline polymer; photoisomerization; UV–vis spectroscopy
INTRODUCTION Thermotropic chiral nematic polymers are
represented by a class of macromolecular structures in
which chiral moieties contribute to the formation of a cho-
lesteric mesophase.1,2 From the molecular structure point of
view, one would expect that a copolymer (CP) comprised
nematogenic- and chiral-monomers that should exhibit cho-
lesteric mesomorphism in view of the common practice in
which a chiral nematic material is routinely prepared by
mixing a low molar mass nematogen with a chiral dopant.3,4
The development of materials exhibiting both liquid crystal-
linity and photochromism is indeed a challenging task. Often,
such attempts result in the loss of one or both of these prop-
erties. Azo-dye liquid crystals have attracted much attention
for their potential use in optical storage applications because
of their easy availability and reasonable photo-fatigue resist-
ance as well as the marked transformation of their molecular
shape between E and Z photoisomerization.5–8
aspect that generates much interest in the field of liquid
crystals,16–18 and polymers such as chiral side-chain liquid
crystal polymers have rapidly grown in popularity in recent
years because of their potential in emerging optical technolo-
gies. Many exhibit piezo and pyroelectric effects, some are
ferroelectric, and others are capable of the selective reflec-
tion of electromagnetic radiation, particularly in the visible
region.20–23 This latter property can be a function of both
structure and temperature, and in this work, we used
copolymerization as an additional means of fine tuning this
behavior. This work deals with the synthesis and characteri-
zation of thermotropic side-chain liquid crystalline CPs from
a nematogenic monomer with two different photochromic
(AN¼¼NA and ACH¼¼CHA) monomers to investigate their
structure–property relationships. The effect of the feed ratio
of photochromic units/chiral units on mesomorphic optical
and thermal properties is discussed in detail.
Changes in a material’s color can arise from exposure to
heat or light.9–12 These color-change phenomena are known
as thermochromism and photochromism, respectively. Ther-
mochromism is a well-known and useful property that is fre-
quently observed with many different materials.13–15 For
example, by quenching the aligned cholesteric phase to a
glass state by cooling to room temperature, films exhibiting
selective reflection from UV to near-IR have been
observed.16–19 The change in color with temperature is an
EXPERIMENTAL
Materials
The 4-hydroxybenzoic acid (99.0%), 6-chloro-1-hexanol
(95.0%), acryloyl chloride (98.0%), 4-butoxyphenol (99.0%),
(ꢀ)-2-methyl-1-butanol (99.0%), (ꢀ)-cholesteryl chlorofor-
mate (97.0%, [a]D ¼ ꢀ28ꢁ, c ¼ 2 in CHCl3), hydroquinone
(99.5%), 4,40-biphenol (97.0%), triethylamine (Et3N; 99.0%),
N,N0-dicyclohexyl-carbodiimide (DCC; 99.0%), and 2,20-
Correspondence to: J.-H. Liu (E-mail: jhliu@mail.ncku.edu.tw)
C
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 770–780 (2011) 2010 Wiley Periodicals, Inc.
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TABLE 1 Chemical Structures and Phase Transition Temperature of Monomers
Phase Transition
Monomer Structure
Temperaturea (ꢁC)
M1
K 62.8 I
I 57.1 N 33.3 K
M2*
M3*
K 110.3 N* 204.9 I
I 200.6 N* 76.6 K
K 81.7 N* 238.8 I
I 236.5 N* 75.7 K
M4
M5
M6
a
K 127.7 SmA 187.7 N 220.0 I
I 175.2 K
K 136.6 I
I 106.6 K
K 137.7 I
I 108.3 K
Estimated by DSC thermograms at a scanning rate of 5 K minꢀ1.K, crystal; N, nematic; N*, chrial nematic;
SmA, smectic A; I, isotropic phase, heating and cooling cycles.
azobisisobutyronitrile (AIBN; 98.0%) were purchased from
in these columns, with a rate of elution of 1.0 mL minꢀ1
.
Acros Chemical. The 4-dimethylaminopyridine (DMAP;
99.0%) was purchased from Lancaster Chemical. All organic
solvents were purchased from Aldrich Chemical. Dichloro-
methane (CH2Cl2) was distilled over calcium hydride under
argon immediately before use, and other solvents were
purified by standard methods. Analytical thin-layer chroma-
tography was performed on Merck aluminum plates with 0.2
mm of silica gel 60F-254. Anhydrous sodium sulfate was
used to dry all organic extracts. The AIBN was freshly
recrystallized from methanol.
The instrument was calibrated with a polystyrene standard.
Differential scanning calorimetry (DSC) was conducted with
a PerkinElmer DSC 7 at heating and cooling rates of 10 K
minꢀ1 under a nitrogen atmosphere. The phase transitions
were investigated with an Olympus BH-2 polarized light
microscope (POM) equipped with a Mettler hot stage FP-82,
and the temperature scanning rate was determined to be 10
K minꢀ1. Thermal decomposition temperature data were
recorded under a nitrogen atmosphere at a heating rate of
20 K minꢀ1 with a PerkinElmer TGA 7 thermogravimetric
analyzer (TGA). UV/Vis absorption spectra were measured
with a Jasco V-550 spectrophotometer.
Instrumentation
All new compounds were identified by 1H NMR and Fourier
Transform Infrared Spectroscopy (FTIR) spectroscopy and
elemental analysis (EA). FTIR spectra were recorded with a
KBr disk on a Jasco VALOR III (Tokyo, Japan) FTIR spectro-
photometer. 1H NMR (400 MHz) and 13C NMR (100.6 MHz)
spectra were obtained on a Bruker AMX-400 (Darmstadt,
Germany) high-resolution NMR spectrometer, and chemical
shifts are reported in ppm relative to tetramethylsilane as an
internal standard. Optical rotations were measured at 30 ꢁC
in chloroform using a Jasco DIP-370 polarimeter with the D-
line of sodium (k ¼ 589 nm) and a precision of 60.001ꢁ.
The measurements were performed using 1 wt % solutions
of the substances in chloroform. The elemental analyses
were carried out on a Heraeus CHN-O (Darmstadt, Germany)
rapid elemental analyzer. The gel permeation chromatogra-
phy (GPC) measurements were performed at 40 ꢁC with a
Hitachi L-4200 (Osaka, Japan) instrument equipped with TSK
gel GMH and G2000H columns. CHCl3 was used as the eluent
Synthesis of Monomers
The monomers M1–M4 in Table 1 were synthesized accord-
ing to previously reported methods.24–26 The synthesized
monomers and compounds were identified using both 1H
NMR and FTIR. The phase transition temperatures of the
above monomers are summarized in Table 1. Synthesis
details are as follows: M1, yield ¼ 67%; M2, yield ¼ 52%,
[a] ¼ ꢀ20.7ꢁ; M3, yield ¼ 52%, [a]D ¼ ꢀ18.6ꢁ; and M4,
yield ¼ 71%. The remaining monomers, M5 and M6, were
prepared as shown in Scheme 1 from 4-(6-acryloyloxyhexy-
loxy)cinnamic acid and 4-(6-acryloyloxyhexyloxy) benzoic
acid, respectively, as described below.
Synthesis of 4-(6-Hydroxyhexyloxy)cinnamic Acid (1)
The above compound was prepared according to an earlier
reported procedure.27 Trans-4-hydroxycinnamic acid (7.50 g,
45.7 mmol) was dissolved in EtOH (60 mL). KOH (7.73 g,
CHIRAL NEMATIC SIDE-CHAIN COPOLYMERS, LIU ET AL.
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SCHEME 1 Synthesis of mono-
mer M4–M6.
137.5 mmol) and trace KI dissolved in H2O (20 mL) were
added dropwise to the solution. The solution was heated at
reflux for 10 min and 6-chloro-1-hexanol (7.47 g, 54.9
mmol) was then added. After the reaction mixture was
stirred for 48 h at reflux temperature, the resulting mixture
was poured into water and extracted with ethyl ether. The
water phase solution was acidified with HCl and diluted with
water until weakly acidic. The white precipitate was
collected and washed several times with water. The crude
product was recrystallized from EtOH in a yield of 46.8%.
Synthesis of 4-Nitrophenylazophenyl
4-(6-acryloyloxyhexyloxy) Cinnamate (M5)
Compound 2 (2 g, 6.29 mmol), 4-hydroxy-40-nitro-azoben-
zene (1.68 g, 6.9 mmol), and DMAP (ꢁ0.15 g, 1.26 mmol)
were dissolved in CH2Cl2 (40 mL) at 30 C. N,N0-Dicyclohexyl
carbodiimide (DCC; 1.56 g, 7.55 mmol) was dissolved in
CH2Cl2 (30 mL) and then added to the solution. The reaction
ꢁ
mixture was stirred for 48 h at 30 C. The resulting solution
was filtered, and the precipitated urea was removed. The fil-
trate was washed twice with 0.5 N HCl, saturated with aque-
ous NaHCO3 solution, and evaporated and dried in a vacuum.
The crude product was purified by column chromatography
(silica gel, ethyl acetate/hexane ¼ 1/5) to give a 68.5%
yield.
The following FTIR bands were measured (cmꢀ1): 3254
(OH), 2928, 2850 (CH2), 1689 (C¼¼O in ArACOOA), 1605,
1520 (CAC in Ar), 1285, 1254 (COC), 2681, 2561 (COOH).
The 1H NMR results were (CDCl3, d in ppm): 1.10–1.52 (m,
8H, CH2), 3.38–3.42 (t, 2H, OCH2CH2), 4.01–4.03 (t, 2H,
CH2OPh), 6.26 (dd, 1H, CH¼¼CH), 6.57–7.53 (d, 4H, aromatic),
7.65 (dd, 1H, CH¼¼CH), 12.50 (s, 1H, COOH).
The following FTIR bands were measured (cmꢀ1): 2939,
2857 (CH2), 1600 (C¼¼C in Ar), 1517 (NO2). The following
1
are the H NMR results (CDCl3, d in ppm): 1.27–2.17 (m, 8H,
CH2), 3.40–4.04 (t, 2H, OCH2), 4.16–4.20 (t, 2H, OCH2), 5.81–
5.84 (dd, 1H, CH2¼¼CH), 6.08–6.17 (dd, 1H, CH2¼¼CH), 6.38–
6.44 (dd, 1H, CH¼¼CH), 6.48–6.53 (dd, 1H, CH¼¼CH), 6.92–
6.95 (d, 2H, aromatic), 7.37–7.39 (d, 2H, aromatic), 7.54–
7.57 (d, 2H, aromatic), 7.84–7.89 (dd, 1H, CH¼¼CH), 8.02–
8.05 (d, 4H, aromatic), 8.38–8.40 (d, 2H, aromatic). Anal.
Calcd. for C30H29N3O7: C, 66.23; H, 5.39; N, 7.73. Found: C,
66.43; H, 5.25; N, 7.81. The phase transition temperature
was: K 127.7 SmA 187.7 N 220.0 I.
Synthesis of 4-(6-Acryloyloxyhexyloxy)cinnamic Acid (2)
Compound 1 (4 g, 15 mmol), N,N-dimethylaniline (2.02 g,
16.7 mmol), and trace 2,6-di-tert-butyl-4-methylphenol were
dissolved in distilled 1,4-dioxane (80 mL). The solution was
cooled with an ice/salt bath and then acryloyl chloride (2.65
mL, 33.4 mmol) dissolved in distilled 1,4-dioxane (5 mL)
was added dropwise with vigorous stirring. After the reac-
tion mixture was stirred for 24 h at room temperature, the
solution was poured into cold water and the precipitate was
filtered. The crude product was washed several times with
water and recrystallized twice from EtOH, resulting in a yield
of 66.2%.
Synthesis of 4-(6-Acryloyloxyhexyloxy) Benzoic Acid (3)
Compound 3 was synthesized in a similar manner to com-
pound 2, but using 4-hydroxybenzoic acid for a 69.0% yield.
FTIR (cmꢀ1) bands were measured at: 2933, 2854 (CH2),
1733 (C¼¼O in ArACOOA), 1604, 1528 (CAC in Ar), 1291,
1246 (COC), 2672, 2566 (COOH), 1685 (C¼¼C). 1H NMR
showed the following peaks (DMSO-d6, d in ppm): 0.94–1.28
(m, 8H, CH2), 4.05 (t, 2H, CH2OPh), 4.12 (t, 2H, COOCH2),
5.44 (dd, 1H, CH2¼¼CH), 5.68 (dd, 1H, CH2¼¼CH), 5.86 (dd,
1H, CH2¼¼CH), 6.53–7.40 (d, 4H, aromatic), 12.5 (s, 1H,
ACOOH).
The FTIR results were (cmꢀ1): 2931, 2856 (CH2), 1730
(C¼¼O in ArACOOA), 1610, 1535 (CAC in Ar), 1290, 1245
(COC), 982 (¼¼CAH), 2675, 2560 (COOH), 1690 (C¼¼C). The
1H NMR results were (CDCl3, d in ppm): 1.25–2.05 (m, 8H,
CH2), 3.92–4.01 (t, 2H, CH2OPh), 4.12–4.19 (m 2H, COOCH2),
5.81 (dd, 1H, CH2¼¼CH), 6.11 (dd, 1H, CH2¼¼CH), 6.37 (dd,
2H, CH¼¼CH), 6.88–6.91 (d, 2H, aromatic), 7.47–7.50 (d, 2H,
aromatic), 7.74 (dd, 1H, CH¼¼CH).
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Synthesis of 4-Nitro-40-(6-acryloxyhexyloxybenzoyloxy)
Stilbene (M6)
Compound 3 (3 g, 10.3 mmol), 4-hydroxy-40-nitrostilbene
(2.73 g, 11.3 mmol), and DMAPꢁ(0.26 g, 2.1 mmol) were dis-
solved in CH2Cl2 (60 mL) at 30 C. N,N0-Dicyclohexylcarbodii-
mide (DCC; 2.56 g, 12.4 mmol) was dissolved in CH2Cl2 (30
mL) and then added to the solution. The reaction mixture
ꢁ
was stirred for 48 h at 30 C. The resulting solution was fil-
tered, and the precipitated urea was removed. The filtrate
was washed twice with 0.5 N HCl, saturated with aqueous
NaHCO3, and evaporated and dried in a vacuum. The crude
product was purified by column chromatography (silica gel,
ethyl acetate/hexane ¼ 1/5) to give a yield of 72.3%.
FIGURE 1 1H NMR spectrum of M6.
FTIR (cmꢀ1) bands were: 2943, 2863 (CH2), 1596 (C¼¼C in
Ar), 1502 (NO2). 1H NMR peaks were (CDCl3, d in ppm):
1.48–2.08 (m, 8H, CH2), 4.13–4.17 (t, 4H, OCH2), 5.84–5.88
(dd, 1H, CH2¼¼CH), 6.11–6.18(dd, 1H, CH2¼¼CH), 6.32–6.36
(dd, 1H, CH2¼¼CH), 7.10–7.12 (d, 2H, aromatic), 7.31–7.33 (d,
2H, aromatic), 7.40–7.44 (dd, 1H, CH¼¼CH), 7.55–7.59 (dd,
1H, CH¼¼CH), 7.76–7.78 (d, 2H, aromatic), 7.87–7.89 (d, 2H,
aromatic), 8.11–8.13 (d, 2H, aromatic), 8.34–8.26 (d, 2H, aro-
matic). The analytical calculations for C30H29NO7 were: C,
69.83; H, 5.63; N, 2.72. Found: C, 69.45; H, 5.67; N, 2.61.
according to Scheme 1. Additionally, the details of the syn-
thetic procedures are described in the experimental section.
All the monomers were structurally confirmed by FTIR, 1H
NMR, 13C NMR, and EA. The data revealed that the spectros-
copy results were in accordance with the structure. Figure 1
shows the 1H NMR spectrum of monomer M6. The results
from the spectra agreed with the proposed molecular struc-
ture of M6.
The 1H NMR spectrum of M6 showed peaks at 5.81–6.40
ppm, indicative of the vinyl protons of the terminal acrylate.
The multiplet at 1.5–1.9 ppm indicated the presence of alkyl
protons that were present in the spacer. The aromatic pro-
tons attached to the ether linkages appeared as doublets at
7.25 and 7.40 ppm. The aromatic protons attached to the
carbonyl and nitro groups appeared as doublets at 8.34 and
8.25 ppm, respectively. The vinyl protons in the stilbene
units were confirmed by the doublets at 7.65 and 7.50 ppm.
Synthesis of CPs (CP1–CP6)
CPs were obtained through the polymerization of monomers
ꢁ
in benzene in the presence of 3 mol % of AIBN at 60 C for
24 h.28 A feed molar ratio of the comonomers of 83:17 was
used to prepare the binary CPs. A typical synthetic proce-
dure is as follows. We started with a solution containing a
known amount of monomer in benzene and 3 mol % of
AIBN. The monomer solution was poured into a glass poly-
merization tube equipped with a sealing cap, which was
then degassed in a vacuum using the freeze-thaw technique
and subsequently sealed. After the completion of polymeriza-
tion, the polymers were precipitated in a solution containing
excess methanol. The crude polymers were then purified via
dissolution in benzene, reprecipitated in methanol, and
finally dried in a vacuum. The process used to synthesize
ternary CPs was very similar to the process described for bi-
nary CPs.
The mesomorphic behaviors of all the monomers were stud-
ied by DSC and polarized optical microscopy, and the transi-
tion temperatures obtained from the DSC analysis are sum-
marized in Table 1. The results revealed that only monomers
M1–M4 exhibited liquid crystalline properties. Monomers
M2* and M3* exhibited chiral nematic liquid crystals (N*) in
which M3 exhibited a much wider temperature range for the
liquid crystal phases when compared with M2. The DSC
thermogram of monomer M2* is shown in Figure 2. This
may be due to the polar effect of chlorine29,30 because sub-
stitution with chlorine increases spontaneous polarization
when compared with nonchlorine compounds, which broad-
ens the mesophase range. In the case of monomers M5 and
M6, no significant liquid crystalline behavior was observed
by either POM or DSC analysis. This may be explained by the
fact that both M5 and M6 contain cinnamic units, which
tend to undergo thermal polymerization during heating
above their isotropic transition temperatures. On careful con-
sideration of the molecular structure of monomers M4–M6,
monomers M4 and M5 contained AN¼¼NA groups, and only
M4 exhibited liquid crystalline behavior (SmA) within the
range of 127–220 ꢁC. However, M5 and M6 contained the
ACH¼¼CHA group but did not exhibited liquid crystalline
properties. From this, we concluded that the photochromic
ACH¼¼CHA group present in both M5 and M6 affect the
liquid crystalline properties. Moreover, not only the geometry
Fabrication of Polymer Films for Thermochromism
The synthesized CPs were dissolved in chloroform and
coated on a black treated glass plate. The coated film was
dried at 80 ꢁC for 3 h. Before testing, the film was heated
above its isotropic melting temperature, followed by slow
cooling to room temperature. A mask with a ‘‘UKN’’ opening
was used to fabricate the patterned films. The polymer was
deposited on the black treated glass plate through the mask,
similar to a screen-printing process.
RESULTS AND DISCUSSION
Synthesis and Mesomorphic Properties of
Monomers (M1–M6)
The achiral (M1) and chiral (M2* and M3*) monomers were
synthesized using our previously reported procedures,24–26
and the photochromic monomers (M4–M6) were synthesized
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
However, because M1 has two ester groups, the long termi-
nal alkyl chain (butyl group) provided more flexibility to the
molecule; thus, it exhibited sharp phase transitions.
For monomer M3, an SmA mesophase was expected, but an
N* mesophase was observed, presumably because M3 has a
chloro substituent on the benzene ring, which introduces an
additional dipole moment across the long axis of the mole-
cule. This enhances the intermolecular lateral attractions
such that the smectic phase should appear for chlorine sub-
stituted monomer M3. Instead, the observed mesophase was
N*. This may result from the increase in molecular breadth,
which forces the molecules further apart, thus reducing the
strength of the intermolecular lateral attractions.32
The monomer M4 showed a smectic A (SmA) mesophase
ꢁ
FIGURE 2 DSC thermogram of M2*. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
between 127 and 187 C, and thereafter exhibited a nematic
mesophase that remained until the isotropic transition tem-
perature was reached at 220 ꢁC. Both applied methods in
heating and cooling modes indicated one mesophase with an
unchangeable fan-shaped texture that is characteristic of the
smectic A mesophase. This type of smectic phase may be
expected, as liquid crystals with a nitro group in the para
position often have a nematic mesophase and perpendicular
smectics.33
of the molecule but also the polarity of the molecule deter-
mined the liquid crystalline properties.
Generally, the polarity of a molecule is dependent on the ori-
entation of the ester linking groups.31 M4, M5, and M6 pos-
sess only two ester groups; one is connected with a flexible
spacer containing a terminal double bond, and the other acts
as a linking group to the mesogen. We introduced one addi-
tional ester group in monomers M2 and M3 to connect the
chiral group, thus contributing to the polarity of the system.
Synthesis and Characterization of CPs (CP1–CP6)
The copolymerization of a nematogenic- and chiral-monomer
was accomplished in benzene in the presence of radical ini-
tiator azobis(isobutyronitrile) at 333 K for 48 h under a
SCHEME 2 Synthesis of copoly-
mers.
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TABLE 2 Results of Copolymerization
the composition of the synthesized CPs. From the GPC analy-
sis, it was found that all the CPs synthesized had a moderate
molecular weight within the range of 0.99 ꢂ 104 and 1.59 ꢂ
104 g molꢀ1 and associated polydispersities in the range of
1.23–2.16. All the CPs were found to be soluble in organic
solvents such as dichloromethane, chloroform, and Tetrahy-
drofuran (THF).
Compositiona
b
Mw
Copolymers
Comonomers
x/y/zc
(ꢂ104)
PDIb
CP1
CP2
CP3
CP4
CP5
CP6
a
M1/M2*/M4
M1/M2*/M5
M1/M2*/M6
M1/M3*/M4
M1/M3*/M5
M1/M3*/M6
82.5/8.5/9.0
83.0/5.0/12.0
85.3/5.1/9.6
83.0/4.6/12.4
80.2/3.2/16.6
83.2/4.1/12.7
1.55
0.99
1.59
1.11
0.69
1.28
2.16
1.79
1.44
1.23
1.32
1.32
Mesomorphic and Thermal Properties of CPs
The liquid crystalline properties of all the synthesized CPs
were studied by hot stage optical microscopy (HOPM) and
DSC analysis, and the DSC results are summarized in Table
3. The results obtained from the HOPM analysis were in ac-
cordance with the DSC analysis. The DSC traces of CP3 and
CP5 are shown in Figure 3. We observed that all the CPs
exhibited glass transition temperatures, Tg, ranging from
below to above ambient, indicating that the samples were
amorphous with no evidence of the crystallinity observed in
the monomers. The polymer backbone does enhance the
tendency of low molecular mass compounds toward meso-
morphism but does so by depressing or even cancelling the
ability of low molar mass mesogenic compounds to crystal-
lize after polymerization.34–37 From the optical microscopy
analysis, we found that all the CPs exhibited a chiral nematic
(N*) mesophase. Cowie and Hunter28 reported that CPs with
lower concentrations of the chiral monomer incorporated
into the chain do not display a smectic phase. As expected,
on cooling from the isotropic melt, all the CPs displayed a
pale bluish green color with planar textures and oily streaks,
which is characteristic of the cholesteric phase (N*). The
inset picture in Figure 3 shows the N* mesophase during the
heating cycle of CP3 at 124.6 ꢁC. The transition tempera-
tures from both DSC and POM revealed that increasing the
ratio of chiral monomer (M2*/M3*) widened the liquid crys-
tal mesophase range. On the other hand, decreasing the feed
ratio of photochromic monomers (M4–M6, in which M5 and
M6 are nonliquid crystal monomers) increased or widened
the N* mesophase range.
In benzene at 60 ꢁC for 24 h in the presence of 3 mol % AIBN.
b
Mw and PDI of the polymers were determined by gel permeation chro-
matography using polystyrene standards in CHCl3.
c
Feed molar ratio x/y/z ¼ 83/17/10; final molar ratio was estimated by
elemental analysis.
nitrogen atmosphere. The CPs were isolated and repeatedly
precipitated from acetone or methanol until the monomers
were completely removed. The CPs were then dried in a vac-
uum oven for several days before analysis. The structures of
the CPs synthesized in the present investigation are summar-
ized in Scheme 2. Each CP consisted of a higher amount (about
80%) of M1 and less chiral monomer (M2 or M3) and photo-
chromic monomer (azo, cinnamic, or both). After polymeriza-
tion, the corresponding signals of the vinyl group at 5.81–6.45
ppm in the 1H NMR spectrum of the monomers disappeared,
indicating that the polymerization was complete and that no
more monomer was present in the polymers. When compared
with the monomer 1H NMR, the peaks of the chemical shifts
corresponding to the polymers were broader, which was con-
sistent with the expected polymer structure.
The CP compositions proved difficult to determine with ac-
curacy, so the trends in behavior were expressed in terms of
the molar feed composition of the monomer. After polymer-
ization, the monomer contents of each of the repeating units
were estimated using EA. The feed molar ratio of the como-
nomers determined from the EA is given in Table 2. In com-
parison with the feed molar ratio, the relative polymerization
rate of each comonomer was estimated from the results of
From Table 3, it was observed that CP1 exhibited higher
mesophase transition (DT ¼ 133.7) than the other CPs,
TABLE 3 Thermal Properties and Specific Rotation of Copolymers
Phase Transition Temperaturea (ꢁC)
TGA
5%c
50%c
Char Yield (%)d
b
Copolymers
Heating Cycle
Cooling Cycle
[a]D
CP1
CP2
CP3
CP4
CP5
CP6
a
G 25.2 N* 158.9 I
G 31.9 N* 134.5 I
G 26.4 N* 143.5 I
G 15.1 N* 124.2 I
G 37.4 N* 117.5 I
G 20.9 N* 119.9 I
I 156.5 N* 27.6 G
I 131.3 N* 34.0 G
I 140.3 N* 29.1 G
I 122.1 N* 18.6 G
I 114.3 N* 39.3 G
I 117.3 N* 25.5 G
ꢀ3.37
ꢀ6.58
ꢀ4.90
ꢀ5.63
ꢀ6.38
ꢀ3.99
327
304
283
308
304
302
445
420
410
431
425
415
5.08
8.16
10.01
6.69
9.32
14.07
b
c
G, glassy; N*, chiral nematic; TGA, thermogravimetric analyzer; I, iso-
tropic phase; the phase transition temperature of copolymers was esti-
mated using differential scanning calorimetry, during heating and
Specific rotation of compounds, 0.1 g in 10 mL CHCl3.
Weight loss in %.
Char yield was measured at 600 ꢁC.
d
cooling at a rate of 5 K minꢀ1
.
CHIRAL NEMATIC SIDE-CHAIN COPOLYMERS, LIU ET AL.
775

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
and 325 ꢁC corresponded to the cleavage of ester linkage
present in the polymer. The second step between 450 and
ꢁ
490 C may have the outcome of cleavage of the aryl–alkyl–
ether linkages present in the polymer chain.41 The char
ꢁ
yields of all the polymers were measured at 600 C, and the
results showed that all the CPs showed a low char yield of
5–10%. The char yield of the CPs was found to increase
with the increasing ratio of photochromic monomers or
increase with a decrease in the ratio of chiral monomers.
Optical Properties of CPs
The optical properties of the photochromic monomers (M4–
M6) and all the CPs were studied in a chloroform (10ꢀ5 M)
solution irradiated with 365 nm UV light. The representative
spectral changes during the photolysis of M4 and CP4 in
chloroform are depicted in Figure 5. At room temperature,
azobenzene chromophores are expected to be in the more
stable trans state in the dark. Under these conditions, the
absorption spectra of CPs showed a major absorption peak
near 350 nm and a weak one at 450 nm, which were
assigned to the p–p* electronic transition of the trans iso-
mers and the n–p* transition of the cis isomers, respectively.
On irradiation with UV light, the CP undergoes trans–cis pho-
toisomerization. The intensity of the absorption peak at 350
nm progressively decreases with increasing irradiation time
FIGURE 3 DSC traces of CP3 and CP5. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
which is due to the higher degrees of polymerization. It is
reported that the increase of mesomorphic phase transition
temperatures with increasing degrees of polymerization is
because this transition is strongly molecular weight depend-
ant within the range of low molecular weight, whereas it is
almost independent for higher molecular weight ranges.38–40
The specific rotations [a]D of all the CPs were determined in
chloroform, and the results are summarized in Table 3. The
specific rotations were all negative values and increased
with increasing chiral monomers feed ratios. The thermal
behavior of the CPs was evaluated by TGA in a nitrogen
atmosphere at a heating rate of 10 ꢁC minꢀ1. Because the
thermograms of all the CPs were quite similar, only repre-
sentative TGA traces of CPs CP1–CP4 are shown in Figure 4,
and the data are illustrated in Table 3. The thermal stability
of the polymers was evaluated at 5 and 50% weight losses
at the corresponding minimum temperatures. The results
ꢁ
showed that all the CPs were stable up to 283 C, and degra-
dation started thereafter in nitrogen. The degradation
occurred in a two-step manner; the first step between 300
FIGURE 5 Variation of UV–Vis spectra of M4 and CP4. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 4 Representative TGA thermograms of CP1–CP4.
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ARTICLE
until an equilibrium state is reached. At the same time, the
weak peak at 450 nm becomes more pronounced. Similar
results (UV spectral pattern) were observed for the remain-
ing CPs containing azobenzene groups. A photostationary
state was attained as the cis-isomer predominated after a
suitable irradiation period.42
The time required for attaining a photostationary state for
the photochromic monomers M4, M5, and M6 in solution
was around 30 s, 5 min, and 30 min, respectively. Such varia-
tions in the time taken to reach the photostationary states in
the monomers was expected because of the varied monomer
structures; M4 possessed only the AN¼¼NA group, which
isomerizes at a faster rate and reaches the photostationary
state in a shorter irradiation time when compared with M6,
which possessed only ACH¼¼CHA, and M5, which contained
both groups. The ACH¼¼CHA group usually requires higher
energy to invert from a trans to cis configuration. For exam-
ple, the CP CP6, which contained only the ACH¼¼CHA group
caused only a 0.03 absorption variation after irradiation for
35 min at 365 nm. All the CPs required more time (>1 h) to
attain a photostationary state and exhibited less of a cis-frac-
tion in the stationary state, which was ascribed to the elec-
tron withdrawing nature of the nitro group hindering trans–
cis isomerization of the azobenzene unit. This revealed that
the nature of the substituent present on the terminal posi-
tion of the azobenzene unit plays a critical role in the revers-
ible trans–cis isomerization. After attaining a photostationary
state, the resultant polymer solution was placed in a dark
environment; the absorption band in the range of 350–360
nm increased (up to 90–95% of the initial value) slowly
because of the slow thermal cis–trans back isomerization of
the azobenzene unit at ambient temperature. In another
experiment, after attaining the photostationary state, the re-
sultant polymer solution was irradiated with 560 nm visible
light, which caused the cis–trans back isomerization to occur
rapidly when compared with ambient temperature in the
dark. CP3 and CP6, which contain only the ACH¼¼CHA
group, were expected to undergo photocrosslinking during
UV irradiation. However, only trans–cis isomerization was
observed, but it was irreversible. It was further shown by
DSC analysis before and after irradiation that with CP3 and
CP6, no differences (no shift) in transition temperatures
were observed, which was indicative of the photoisomeriza-
tion process. This may be due to the low ACH¼¼CHA group
content (9.6 and 12.7), which may result in the inability to
achieve enough conformational freedom to undergo
crosslinking.
FIGURE 6 Variation of UV–Vis spectra of M4 and CP4 at 254
nm. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
effects on the reflection bands for CP1–CP6. The reflection
wavelength and reflection bandwidth can be estimated using
the following equations:
ꢀ
k0 ¼ n ꢃ p
(1)
(2)
Dk ¼ Dn ꢃ p
ˇ
where k0 is the central wavelength of the reflected light, n
is the average refractive index of the liquid crystal, p is the
pitch of the cholesteric liquid crystal, Dk is the reflection
band width, and Dn is the birefringence difference. The
reflection bands of CP1, CP2, and CP3 showed that increas-
ing the temperature caused a blue shift. Higher tempera-
tures usually increase the molecular randomness and
ˇ
decreases n, resulting in a blue shift. Furthermore, increas-
Figure 6 shows the absorption spectra of CP2 and CP5 irra-
diated with 254 nm UV. As seen in CP2, the isobestic point
was maintained during the first 5 min of irradiation, and af-
ter that, it vanished and there was a significant shift in the
UV–Vis spectrum. In addition, the CP5 isobestic point was
not observed, suggesting that 254 nm UV irradiation caused
an irreversible configurational isomerization.
ing the temperature also decreases Dk, which decreases
both the reflection light strength and the reflection band-
width. As shown in Figure 4 for CP1–CP3, the cholesteric
liquid crystals were thermally stable between 30 and 60
ꢁC. CP3 displayed a relatively broad reflection band. All the
reflection bands of the CPs were located in visible light.
Accordingly, the reflection light must be colorful. CP4
showed a thermally stable reflection between 40 and 60
ꢁC. CP5 and CP6 showed decreased thermal resistances at
The pitch of cholesteric liquid crystals is usually affected by
temperature. Figure 7 shows the results of the thermal
CHIRAL NEMATIC SIDE-CHAIN COPOLYMERS, LIU ET AL.
777

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 Dependence of tem-
perature on reflection band of
CP1–CP6.
higher temperatures because the reflection bands became
narrow and had lower reflection efficiencies.
Thermochromism of CPs
Based on the results from DSC and HOPM analyses, it was
concluded that all the CPs exhibited a chiral nematic liquid
crystalline phase (N*). This is a distinct advantage as it is
now possible to freeze a particular reflection color into the
polymer glass by quenching the CP rapidly from the appro-
priate temperatures in the N* phase. For example, CP4, on
cooling from the isotropic melt, formed a pale bluish green
and a homogeneous, planar texture with white, oily streaks,
which is a characteristic of the cholesteric phase. The slow
cooling of the same sample gave reflected color changes
(shown in Fig. 8) in the sequence:
ꢁ
ꢁ
ꢁ
110 C=blue ! 80 C=green ! 50 C=yellow
To investigate the practical thermochromism of the CP films,
FIGURE 8 Real image recorded film of CP4 at various tempera-
tures and a color scale for relative wavelength.
the synthesized CP CP4 was coated on a glass substrate. The
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ARTICLE
polymer film was heated to 88 ꢁC and then quenched with
an ice water bath. The film was reheated to 110 C and sub-
7 So, B. K.; Jang, M. C.; Park, J. H.; Lee, K. S.; Hoon, S. H.; Lee,
ꢁ
S. M. Opt Mat 2003, 21, 685–689.
sequently quenched in an ice bath. The resulting colors
quenched from 110 ꢁC (a), 88 ꢁC (b), and 50 ꢁC (c) are
shown in Figure 8 (a–c, respectively). As an additional
experiment, the CP4 material was deposited through a UKN
shadow mask onto a black substrate. The plate was heated
to 110 ꢁC and then quenched at various temperatures. As
shown in Figure 8, we were able to fabricate an UKN printed
substrate with various colors, demonstrating that the synthe-
sized CPs and the treating processes have potential for use
in the field of colorful, rewritable materials. Figure 8 (d)
shows a color scale for the comparison of wavelengths with
the reflection color.
8 Alazaroaie, S.; Toader, V.; Carlescu, I.; Kazmierski, K.; Scu-
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cules 2001, 34, 8005–8013.
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rett, C. J. Macromolecules 2006, 39, 9311–9319.
12 Sin, S. L.; Gan, L. H.; Hu, X.; Tam, K. C.; Gan, Y. Y. Macro-
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13 Wu, S.; Niu, L.; Shen, J.; Zhang, Q.; Bubeck, C. Macro-
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CONCLUSIONS
14 Kumar, G. S.; Neckers, D. C. Chem Rev 1989, 89, 1915–
It was concluded that the copolymerization of a nematogenic
monomer with the two different photochromic monomers
(AN¼¼NA and ACH¼¼CHA) made it possible to obtain an N*
CP with different responses to the light irradiation of dif-
ferent wavelengths. According to polarizing optical micros-
copy, the synthesized CPs were able to produce a chiral
nematic phase N* with an isotropization temperature of
119–158 ꢁC and glass transition temperature of approxi-
mately 15–37 ꢁC. The thermal properties of the CPs
revealed that all the CPs were thermally stable up to 283
ꢁC, and they then started to decompose in a two stage
process. Photolysis studies explained the reversible photoi-
somerization phenomena. The thermochromism of the CPs
displayed different color patterns when quenching at vari-
ous temperatures. These kinds of multifunctional CPs have
potential applications in the field of colorful, rewritable
materials.
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