Photoresponsive side-chain liquid crystalline polymers with amide group-substituted azobenzene mesogens: Effects of hydrogen bonding, flexible spacers, and terminal tails

Article abstract of DOI:10.1039/c2sm25163a

The synthesis of a series of new photoresponsive side-chain liquid crystalline polymethacrylates with amide group-substituted azobenzene (azo) mesogens and different length of flexible spacers and terminal tails via conventional free radical polymerization is described. The resulting azo polymers proved to have high thermal stability and good solubility in common organic solvents (e.g., tetrahydrofuran and chloroform). Differential scanning calorimetry, polarizing optical microscopy, and small angle X-ray scattering studies confirmed the presence of obvious enantiotropic smectic C liquid crystalline phases (with a bilayer lamellar structure) for all these polymers. The introduction of an amide group into the azo mesogen led to the formation of strong hydrogen bonding among the side chains of the polymers (as revealed by variable temperature FT-IR), which played a decisive role in forming and stabilizing the liquid crystalline mesophases of the polymers. In addition, the length of the flexible spacers and terminal tails also significantly influenced their phase transition behaviors. Furthermore, the photoresponsivity of the polymer solutions was verified and the effects of the molecular structures of the polymers on their photoresponsive properties were also studied.

Full text of DOI:10.1039/c2sm25163a

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PAPER
Photoresponsive side-chain liquid crystalline polymers with amide group-
substituted azobenzene mesogens: effects of hydrogen bonding, flexible
spacers, and terminal tails†
*
Xinjuan Li, Liangjing Fang, Leigang Hou, Lirong Zhu, Ying Zhang, Baolong Zhang and Huiqi Zhang
Received 21st January 2012, Accepted 13th March 2012
DOI: 10.1039/c2sm25163a
The synthesis of a series of new photoresponsive side-chain liquid crystalline polymethacrylates with
amide group-substituted azobenzene (azo) mesogens and different length of flexible spacers and
terminal tails via conventional free radical polymerization is described. The resulting azo polymers
proved to have high thermal stability and good solubility in common organic solvents (e.g.,
tetrahydrofuran and chloroform). Differential scanning calorimetry, polarizing optical microscopy,
and small angle X-ray scattering studies confirmed the presence of obvious enantiotropic smectic C
liquid crystalline phases (with a bilayer lamellar structure) for all these polymers. The introduction
of an amide group into the azo mesogen led to the formation of strong hydrogen bonding among
the side chains of the polymers (as revealed by variable temperature FT-IR), which played
a decisive role in forming and stabilizing the liquid crystalline mesophases of the polymers. In
addition, the length of the flexible spacers and terminal tails also significantly influenced their phase
transition behaviors. Furthermore, the photoresponsivity of the polymer solutions was verified and
the effects of the molecular structures of the polymers on their photoresponsive properties were also
studied.
with azo units located in the main chains,2,10 and photo-
responsive LCP networks.5,8,10,11 Among them, side-chain LCPs
with pendant azo side groups have been the most extensively
investigated system.1,3,4,6,7,12 One of the main focuses in this
rapidly developing area is the rational design and efficient
synthesis of novel azo-containing side-chain LCPs in order to
provide advanced functional materials with potential applica-
tions in desired areas.
Introduction
Azobenzene (azo)-containing liquid crystalline polymers (LCPs)
have received great attention in recent years because of their
combined properties of anisotropic liquid crystals, photo-
responsive materials, and flexible polymers.1–12 The liquid crys-
talline properties of azo polymers are related to the rod-like
trans-azo mesogens, and the photoinduced fast and reversible
isomerization between the trans and cis isomers of the azo groups
upon exposure to UV or visible light provides them with fasci-
nating photoresponsive properties, which can lead to photoin-
duced reorientation of azo groups in the polymers and triggers
significant changes in their physicochemical properties. Many
potential applications have been proposed for these polymers,
such as optical data storage, liquid crystal displays, molecular
switches, nonlinear optical devices, and photomechanical
systems. So far, a great number of azo-containing LCPs have
been developed for different purposes, including side-chain LCPs
with azo mesogens as side groups,1,3,4,6,7,12 photoresponsive LCPs
In general, the molecular structure of an azo-containing side-
chain LCP can be divided into three main parts, i.e., the polymer
backbone, the pendant photoactive azo mesogens, and the flex-
ible spacer connecting them. A variety of azo-containing side-
chain LCPs with different molecular structures have been
prepared via various polymerization approaches.1,3,4,6,7,12–22
Systematic studies on the structure–property relationship of the
azo-containing side-chain LCPs have revealed that both their
phase transition behaviors and photoresponsive properties are
strongly dependent on their polymer backbones,13,14 the azo
mesogens,15–18 and the length of the flexible spacers.19–22 Among
these influencing factors, the molecular structures of the azo
mesogens, especially the terminal substitutent of the azo groups,
have been recognized as one of the most important primary
factors influencing the properties of the azo polymers because the
azo groups act as both the mesogenic and photoresponsive units
and their substituents can have significant influence on the dipole
Key Laboratory of Functional Polymer Materials (Nankai University),
Ministry of Education, Department of Chemistry, Nankai University,
Tianjin 300071, P. R. China. E-mail: zhanghuiqi@nankai.edu.cn; Fax:
+86-2223507193; Tel: +86-2223507193
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2sm25163a
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moment of the azo chromophores and thus their phase transition
and photoresponsive behaviors.15–18 Therefore, many side-chain
LCPs bearing pendant azo mesogens with various substituents
such as hydrogen,23 alkyl,24,25 perfluoroalkyl,17 alkyloxy,15,26,27
chiral alkyloxy,21,28 nitro,19,29,30 cyano,31,32 carboxyl,33 cinna-
moyl,34,35 and N-hydroxysuccinimide carboxylate groups36 have
been developed for different purposes.
polymethacrylates with amide group-substituted azo mesogens
and different lengths of flexible spacer and terminal tails
(Scheme 1). The effects of the chemical structures of the azo
polymers (including the length of the flexible spacers and
terminal tails as well as the amide linkages) on their mesophase
properties were studied in detail. The results showed that all the
above factors showed great influence on the liquid crystalline
properties of the polymers. In particular, the incorporation of an
amide group into azo mesogens resulted in the formation of
strong hydrogen bonding among the side chains of the azo
polymers, which proved to play a decisive role in forming and
stabilizing their smectic C liquid crystalline phases. In addition,
the photoresponsive properties of the azo polymer solutions were
also studied.
Recent years have also witnessed considerable interest in the
use of hydrogen bonding interaction for the design of various
advanced functional polymeric materials.37–39 In particular, the
hydrogen bonding interaction has proven to be of crucial
importance in the formation and stabilization of the liquid
crystalline phases for many polymers.40–44 For example, the
liquid crystalline mesophases of the aromatic–aliphatic poly-
(ester amides)40,41 and poly(ester urethanes)42 proved to be
largely affected by their intermolecular hydrogen bonding
among amide–amide and amide–ester moieties. In addition, the
intramolecular hydrogen bonding formed between the amide
groups in the side chains of poly(N-propargylamides) bearing
pendant azo groups was found to promote the formation of
a helical structure and liquid crystalline phases for the polymer
solutions.43 Very recently, the significant influence of hydrogen
bonding on the mesophase structures of mesogen-jacketed LCPs
with amide side-chain linkages was also reported.44 Based on the
above results, it can be envisioned that the introduction of an
amide group into azo mesogens might lead to photoresponsive
side-chain LCPs with better stabilized liquid crystalline meso-
phases and some interesting properties, which should be positive
for many potential applications. To our knowledge, however,
side-chain LCPs with amide group-substituted azo mesogens
have rarely been reported up to now.
Experimental
Materials
Tetrahydrofuran (THF, Tianjin Jiangtian Chemicals, China,
99%) was refluxed over sodium and then distilled. N,N-Dime-
thylformamide (DMF, Tianjin Jiangtian Chemicals, 99.5%) was
dried with anhydrous magnesium sulfate (MgSO4) and then
distilled under vacuum. Triethylamine (Tianjin Jiangtian
Chemicals, 99%) was dried with anhydrous sodium sulfate
(Na2SO4) and then distilled. Thionyl chloride (Tianjin Jiangtian
Chemicals, 99.5%) was purified by distillation prior to use.
Azobisisobutyronitrile (AIBN, Tianjin Jiangtian Chemicals,
chemical purity (CP)) was recrystallized from ethanol and stored
at ꢀ18 ꢁC before use. 4-((4-Hydroxy)phenylazo)benzoic acid,
4-((4-u-hydroxyalkyloxy)phenylazo)benzoic acid (HAzoA-m,
m ¼ 2, 6, 10), and 4-((4-u-methacryloyloxyalkyloxy)phenylazo)
benzoic acid (MAzoA-m, m ¼ 2, 6, 10) (Scheme 1) were prepared
Herein, we report the successful synthesis and characterization
of a series of new photoresponsive side-chain liquid crystalline
Scheme 1 Synthetic route and chemical structures of the azo monomers Mm/n and their corresponding homopolymers Pm/n (m ¼ 2, 6, or 10, n ¼ 4, 6, 8,
12, or 18).
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Soft Matter, 2012, 8, 5532–5542 | 5533

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according to our previously reported procedure.36 All the other
reagents were commercially available and used as received.
available due to the overlap of the absorption bands); 1H NMR
(CDCl3): d (ppm) ¼ 7.97–7.84 (m, 6H, Ar–H), 7.04–6.96 (d, 2H,
Ar–H), 6.17 (s, 1H, –NH), 6.10 (s, 1H, CH]C–), 5.55
(s, 1H, CH]C–), 4.19–4.14 (t, 2H, –OCH2–), 4.09–4.01 (t, 2H,
–CH2O–), 3.50–3.43 (q, 2H, –NCH2–), 1.94 (s, 3H, –CH3), 1.89–
0.83 (m, 23H, –(CH2)4, –(CH2)6CH3)).
Synthesis of the azo monomers
For convenience, the azo monomers with the amide substitutent
were named as Mm/n, where M refers to monomer, m the number
of the methylene unit in the flexible spacer, and n the number of
carbon in the terminal alkyl tail (Scheme 1). A series of azo
monomers Mm/n (m ¼ 2, 6, or 10; n ¼ 4, 6, 8, 12, or 18) were
synthesized as shown below:
N-Lauryl 4-((4-(u-methacryloyloxyhexyloxy))phenylazo)ben-
zamide (M6/12). Prepared as for M2/6, but the product was
purified with silica gel column chromatography by using
a mixture of ethyl acetate and petroleum ether (1/2 v/v) as the
eluent instead of being directly precipitated into water due to the
insolubility of lauryl amine in water (yield: 90%). UV-vis (THF):
lmax/nm (3/L molꢀ1cmꢀ1) ¼ 358 (26 380), around 445 (not
available due to the overlap of the absorption bands); 1H NMR
(CDCl3): d (ppm) ¼ 7.94–7.86 (m, 6H, Ar–H), 7.04–6.96 (d, 2H,
Ar–H), 6.17 (s, 1H, –NH), 6.10 (s, 1H, CH]C–), 5.55
(s, 1H, CH]C–), 4.21–4.14 (t, 2H, –OCH2–), 4.09–4.02 (t, 2H,
–CH2O–), 3.52–3.43 (q, 2H, –NCH2–), 1.95 (s, 3H, –CH3), 1.89–
0.83 (m, 31H, –(CH2)4–, –(CH2)10CH3).
N-Hexyl 4-((4-(u-methacryloyloxyethyloxy))phenylazo)benza-
mide (M2/6). MAzoA-2 (0.5 g, 1.20 mmol), DMF (two drops),
2,6-di-tert-butylphenol (trace amount), and thionyl chloride
(2.5 mL) were added into a three-neck round-bottom flask
(25 mL) successively. After the reaction mixture was stirred at
room temperature for 4 h, the excessive thionyl chloride was
removed at reduced pressure by using a rotoevaporator. The
resulting solid was dissolved in dried THF (5 mL) and
this solution was added dropwise into a cold mixed solution of
n-hexamine (0.24 g, 2.40 mmol) and triethylamine (0.8 mL) in
dried THF (15 mL). The reaction was allowed to proceed at
room temperature for 24 h under stirring and the reaction
mixture was filtered. The filtrate was then precipitated into
a large amount of water and the resulting solid was fiꢁltered,
washed with water, and then dried under vacuum at 30 C for
48 h, leading to the orange-yellow pure product (yield: 90%).
UV-vis (THF): lmax/nm (3/L molꢀ1cmꢀ1) ¼ 356 (27 980), around
445 (not available due to the overlap of the absorption bands);
1H NMR (CDCl3): d (ppm) ¼ 7.98–7.85 (m, 6H, Ar–H), 7.06–
7.01 (d, 2H, Ar–H), 6.22–6.12 (s,s, 2H, –NH and CH]C–), 5.61
(s, 1H, CH]C–), 4.57–4.51 (t, 2H, –OCH2–), 4.36–4.29 (t, 2H,
–CH2O–), 3.53–3.43 (quartet (q), 2H, –NCH2–), 1.97 (s, 3H,
–CH3), 1.75–0.83 (m, 11H, –(CH2)4CH3).
N-Octadecyl 4-((4-(u-methacryloyloxyhexyloxy))phenylazo)
benzamide (M6/18). Prepared as for M6/12 (yield: 95%). UV-vis
(THF): lmax/nm (3/L molꢀ1cmꢀ1) ¼ 358 (26 700), around 445 (not
available due to the overlap of the absorption bands); 1H NMR
(CDCl3): d (ppm) ¼ 8.07–7.84 (m, 6H, Ar–H), 7.07–6.97 (d, 2H,
Ar–H), 6.16 (s, 1H, –NH), 6.10 (s, 1H, CH]C–), 5.55
(s, 1H, CH]C–), 4.26–4.13 (t, 2H, –OCH2–), 4.12–4.01 (t, 2H,
–CH2O–), 3.54–3.42 (q, 2H, –NCH2–), 1.95 (s, 3H, –CH3), 1.91–
0.81 (m, 43H, –(CH2)4–, –(CH2)16CH3).
N-Hexyl 4-((4-(u-methacryloyloxydecyloxy))phenylazo)benza-
mide (M10/6). Prepared as for M2/6 (yield: 95%). UV-vis (THF):
lmax/nm (3/L molꢀ1cmꢀ1) ¼ 358 (28 320), around 445 (not
available due to the overlap of the absorption bands); 1H NMR
(CDCl3): d (ppm) ¼ 8.16–7.75 (m, 6H, Ar–H), 7.13–6.87 (d, 2H,
Ar–H), 6.40–5.98 (s,s, 2H, –NH and CH]C–), 5.55 (s, 1H,
CH]C–), 4.38–3.88 (m, 4H, –OCH2– and –CH2O–), 3.65–3.43
(q, 2H, –NCH2–), 1.94 (s, 3H, –CH3), 1.89–0.77 (m, 27H,
–(CH2)8–, –(CH2)4CH3).
N-Butyl 4-((4-(u-methacryloyloxyhexyloxy))phenylazo)benza-
mide (M6/4). Prepared as for M2/6 (yield: 95%). UV-vis (THF):
lmax/nm (3/L molꢀ1cmꢀ1) ¼ 358 (27 580), around 445 (not
available due to the overlap of the absorption bands); 1H NMR
(CDCl3): d (ppm) ¼ 8.02–7.85 (m, 6H, Ar–H), 7.01–6.97 (d, 2H,
Ar–H), 6.17 (s, 1H, –NH), 6.10 (s, 1H, CH]C–), 5.55
(s, 1H, CH]C–), 4.19–4.15 (t, 2H, –OCH2–), 4.07–4.02 (t, 2H,
–CH2O–), 3.53–3.44 (q, 2H, –NCH2–), 1.94 (s, 3H, –CH3), 1.91–
0.92 (m, 15H, –(CH2)4, –(CH2)2CH3).
N-Lauryl 4-((4-(u-methacryloyloxydecyloxy))phenylazo)ben-
zamide (M10/12). Prepared as for M6/12 (yield: 95%). UV-vis
(THF): lmax/nm (3/L molꢀ1cmꢀ1) ¼ 358 (27 020), around 445 (not
available due to the overlap of the absorption bands); 1H NMR
(CDCl3): d (ppm) ¼ 8.32–8.21 (d, 2H, Ar-H), 8.04–7.86 (m, 4H,
Ar-H), 7.13–6.87 (d, 2H, Ar-H), 6.24–6.07 (s, s, 2H, –NH and
CH]C–), 5.55 (s, 1H, CH]C–), 4.21–4.13 (t, 2H, –OCH2–),
4.11–4.01 (t, 2H, –CH2O–), 3.62–3.40 (q, 2H, –NCH2–), 1.95
(s, 3H, –CH3), 1.91–0.84 (m, 39H, –(CH2)8–, –(CH2)10CH3).
In addition to the above azo monomers (Mm/n), another azo
monomer named hexyl 4-((4-(u-methacryloyloxyhexyloxy))phe-
nylazo)benzoate (M6/6-ester) (Scheme S1, Fig. S1a†), which has
a similar chemical structure to that of M6/6, but with an ester
linkage instead of an amide linkage in the terminal substitutent
of the azo mesogen, was also synthesized similarly (yield: 86%).
UV-vis (THF): lmax/nm (3/L molꢀ1cmꢀ1) ¼ 362 (23 960), around
450 (not available due to the overlap of the absorption bands);
1H NMR (CDCl3): d (ppm) ¼ 8.22–8.14 (d, 2H, Ar–H), 7.98–7.87
N-Hexyl 4-((4-(u-methacryloyloxyhexyloxy))phenylazo)ben-
zamide (M6/6). Prepared as for M2/6 (yield: 92%). UV-vis
(THF): lmax/nm (3/L molꢀ1cmꢀ1) ¼ 358 (26 480), around 445 (not
available due to the overlap of the absorption bands); 1H NMR
(CDCl3): d (ppm) ¼ 7.97–7.85 (m, 6H, Ar–H), 7.04–6.97 (d, 2H,
Ar–H), 6.16 (s, 1H, –NH), 6.10 (s, 1H, CH]C–), 5.55
(s, 1H, CH]C–), 4.21–4.14 (t, 2H, –OCH2–), 4.10–4.02 (t, 2H,
–CH2O–), 3.53–3.43 (q, 2H, –NCH2–), 1.95 (s, 3H, –CH3), 1.89–
0.86 (m, 19H, –(CH2)4, –(CH2)4CH3).
N-Octyl 4-((4-(u-methacryloyloxyhexyloxy))phenylazo)benza-
mide (M6/8). Prepared as for M2/6 (yield: 93%). UV-vis (THF):
lmax/nm (3/L molꢀ1cmꢀ1) ¼ 358 (28 240), around 445 (not
5534 | Soft Matter, 2012, 8, 5532–5542
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(dd, 4H, Ar–H), 7.05–6.97 (d, 2H, Ar–H), 6.10 (s, 1H, CH]C–),
5.55 (s, 1H, CH]C–), 4.40–4.31 (t, 2H, –OCH2–), 4.21–4.13 (t,
2H, –OCH2–), 4.10–4.01 (t, 2H, –CH2O–), 1.95 (s, 3H, –CH3),
1.90–0.70 (m, 19H, –(CH2)4–, –(CH2)4CH3).
weights and molecular weight distributions of the polymers were
determined with a gel permeation chromatograph (GPC)
equipped with a Waters 717 autosampler, a Waters 1525 HPLC
pump, three Waters UltraStyragel columns with 5k-600k, 500–
30k, and 100–10k molecular ranges, and a Waters 2414 refractive
index detector. THF was used as the eluent at a flow rate of
1.0 mL minꢀ1. The calibration curve was obtained using poly-
styrene (PS) standards. Thermogravimetric analyses (TGA) were
performed on a Netzsch TG 209ꢁ instrument in a nitrogen
atmosphere at a heating rate of 10 C minꢀ1. Differential scan-
ning calorimetry (DSC, Netzsch 204) was utilized to study
the phase transitions of the polymers at a heating/cooling rate of
10 ꢁC minꢀ1 under nitrogen. The temperature and heat flow scale
were calibrated with standard materials including indium
(70–190 ꢁC), tin (150–270 ꢁC), zinc (350–50 ꢁC), bismuth
(190–310 ꢁC) and mercury (ꢀ100–0 ꢁC) in different temperature
ranges. The glass transition temperatures (Tg) of the polymers
were determined as the midpoints of the step changes of heat
capacity, while the phase transition temperatures were measured
from the maximum/minimum of the endothermic/exothermic
peaks. The liquid crystalline textures of the samples were
observed by using an Olympus BX51 polarizing optical micro-
scope (POM) equipped with a Linksys 32 THMSE600 hot stage
and a digital camera (Micropublisher 5.0 RTV). Small angle
X-ray scattering (SAXS) measurements were carried out on
Synthesis of the azo polymers
For convenience, the side-chain LCPs with the amide group-
substituted azo mesogens were named as Pm/n, where P refers to
polymer, m the number of the methylene unit in the flexible
spacer, and n the number of carbon in the terminal alkyl tail
(Scheme 1). A series of azo polymers Pm/n (m ¼ 2, 6, or 10; n ¼ 4,
6, 8, 12, or 18) were synthesized following the typical procedure
shown below:
The azo monomer Mm/n (0.5 g), AIBN (2 mol% with respect
to the monomer), and the freshly distilled DMF (5 mL) were
added into a two-neck round-bottom flask. A clear solution was
obtained after 10 min of stirring, which was subsequently
degassed with three freeze–pump–thaw cycles. The flask wꢁas then
sealed and immersed into a thermostatted oil bath at 60 C and
stirred for 48 h. After being cooled to the room temperature, the
polymerization mixture was added dropwise into ethanol
(100 mL) under stirring. The precipitate was collected by
filtration, washed with warm ethanol extensively until no azo
monomer was detectable by thin layer chromatography, and
then dried under vacuum at 60 ꢁC for 48 h to provide the desired
orange-yellow polymer (Table 1).
ꢀ
a Bruker NanoSTAR SAXS system using Cu Ka (l ¼ 1.542 A) as
the radiation source. The working voltage and current are 40 kV
and 35 mA, respectively. The distance between the sample and
the detector is 26.55 cm. A temperature control unit in
conjunction with the instrument was used to study the structure
evolution of liquid crystalline mesophases by heating the samples
to the desired temperatures. A Bio-Rad FTS6000 FT-IR spec-
trometer equipped with an UMA-500 microscope, an MCT
detector (cooled with liquid nitrogen), and a Linkam FTIR600
The polymer of M6/6-ester (i.e., P6/6-ester, orange-yellow
color) was also prepared similarly following the above procedure
(Table 1).
Characterization
1H NMR spectra were recorded on a Varian Unity plus-400
spectrometer (400 MHz) at ambient temperature. The molecular
Table 1 Synthetic and characterization data of the azo polymersa
b
h
Azo polymer
P2/6
Yield (%)
Mn,GPC (ꢂ 10ꢀ4
)
PDIb
1.73
1.75
1.59
1.59
1.59
1.92
1.63
2.53
1.83
Phase transitionc T/ꢁC
DHg/J gꢀ1
Td /ꢁC
327
333
333
341
342
333
332
334
316
G–SC 234 Id e
16.9
ꢀ13.2
22.4
,
56
66
65
63
70
46
66
66
47
1.95
2.19
2.97
2.56
2.56
2.74
2.87
2.87
1.64
I 222 SC–Ge f
,
P6/4
G 141 SC 159 Id
I 146 SC-Ge f
ꢀ20.1
,
P6/6
G 144 SC 179 Id
17.1
ꢀ25.7
20.0
I 168 SC-Ge f
,
P6/8
G 141 SC 196 Id
I 187 SC–Ge f
ꢀ20.6
,
P6/12
G 129 SC 206 Id
20.1
ꢀ19.7
10.1
I 196 SC–Ge f
,
P6/18
G 117 SC 206 Id
I 199 SC 110 Gf
G 136 SC 154 Id
I 147 SC 136 Gf
G 152 SC 174 Id
I 164 SC 138 Gf
ꢀ7.7
P10/6
32.2
ꢀ5.2
11.2
P10/12
P6/6-ester
ꢀ13.7
G–SA 82 Ie d
5.9
ꢀ7.3
,
e f
,
I 76 SA–G
a
All the azo polymers were prepared via the conventional free radical polymerizations of the corresponding azo monomers in DMF with AIBN (2 mol%
with respect to the monomer) as the initiator at 60 ꢁC for 48 h. b The number-average molecular weights (Mn,GPC) and polydispersity indices (PDI) of the
polymers were determined by GPC with THF as the eluent and PS standards. c G, glassy; SC, smectic C; SA, smectic A; I, isotropic. d Determined with
DSC at the second heating scan under nitrogen (10 ꢁC minꢀ1). e No Tg was detectable. f Determined with DSC at the first cooling scan under nitrogen
g
h
(ꢀ10 ꢁC minꢀ1). OH represents the enthalpy of the phase transition. The temperatures at 5% weight loss of the samples under nitrogen were
measured by TGA heating experiments at a rate of 10 ꢁC minꢀ1
.
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hot stage and temperature control system was utilized to carry
out the variable temperature FT-IR measurements. The polymer
samples were sandwiched between two KBr slides and their
temperatures were changed at a rate of 10 ꢁC minꢀ1 unless
otherwise stated. 16 scans were conducted for each spectrum
at the selected temperatures in the wavelength range of 700–
4000 cmꢀ1 with a resolution of 8 cmꢀ1. A UV-vis scanning
spectrophotometer (TU1900, Beijing Purkinje General Instru-
ment Co., Ltd) was utilized to obtain the UV-vis spectra of the
monomer and polymer samples at 25 ꢁC. The photochemical
isomerization of the azo polymer solutions in THF was investi-
gated by irradiating them first with a 365 nm UV lamp (12 W)
until the photostationary state was reached and then with
a 450 nm visible light lamp (18 W, wavelength range: 400–
550 nm, lmax ¼ 450 nm, a filter was put between the samples and
the lamp during the study in order to block the light with
wavelength l < 430 nm). Note that all the above photoresponsive
studies were performed by using the same quartz sample cell
(with 1 cm pathlength) for the polymer solutions (i.e., the sample
thickness is same for all the polymer solutions) and the same
incident light intensity (by fixing the distance between the lamps
and the samples) in order to accurately evaluate the effects of the
molecular structures of the polymers on their photoresponsive
properties.
Fig. 1 1H NMR spectra of M6/6 (a) and P6/6 (b) in CDCl3.
The conventional free radical polymerizations of the azo
monomers Mm/n were then carried out with AIBN ꢁas the initi-
ator and freshly distilled DMF as the solvent at 60 C for 48 h
with continuous stirring under an argon atmosphere. All the
resulting azo polymers (namely Pm/n) were purified by precipi-
tation into ethanol and subsequent thorough washing with warm
ethanol until the monomer could no longer be detected by
thin layer chromatography . A reasonable polymerization yield
of 46–70% was obtained in each case (Table 1). The obtained azo
polymers were easily soluble in common organic solvents such as
THF, chloroform, and dichloromethane.
Results and discussion
Synthesis of the amide group-substituted azo monomers and their
polymers
A series of new methacrylate azo monomers with the amide
substitutent and different lengths of flexible spacer and terminal
alkyl chain (i.e., Mm/n: m ¼ 2, 6, or 10; n ¼ 4, 6, 8, 12, or 18)
were synthesized according to the synthetic route illustrated in
Scheme 1. 4-((4-Hydroxy)phenylazo)benzoic acid, 4-((4-u-
hydroxyalkyloxy)phenylazo)benzoic acid (HAzoA-m, m ¼ 2, 6,
10), and 4-((4-u-methacryloyloxyalkyloxy)phenylazo)benzoic
acid (MAzoA-m, m ¼ 2, 6, 10) were prepared according to the
previously reported procedure.36 MAzoA-m (m ¼ 2, 6, 10) were
reacted with thionyl chloride and the resulting carboxyl chlorides
were subsequently coupled with various alkyl amines to provide
the desired products Mm/n (m ¼ 2, 6, or 10; n ¼ 4, 6, 8, 12, or 18)
in quite good yields (typically above 90%). The purity of the azo
monomers was satisfactory, as verified with both thin layer
The above-obtained azo polymers were firstly characterized
1
1
with H NMR. Fig. 1(b) shows the H NMR spectrum of one
representative polymer P6/6. It can be seen clearly that the vinyl
proton signals of the methacryloyl group from M6/6 have
disappeared completely from the spectrum, further confirming
that the unreacted azo monomer has been fully removed from the
purified polymer. In addition, the chemical shifts and the peak
integrations of all the protons in the polymer are in good
consistence with its expected structure. Similar results were also
obtained for other polymers.
1
chromatography and H NMR.
1
The H NMR spectrum of one representative azo monomer
M6/6 is shown in Fig. 1(a), from which the characteristic vinyl
proton signals of the methacryloyl group at 5.55 and 6.10 ppm
can be seen clearly. In addition, the chemical shifts and
peak integrations of all the protons in the azo monomer are
in excellent agreement with its expected structure, demonstrating
the successful preparation of pure M6/6. The UV-vis spectrum
of the solution of M6/6 in THF exhibits one strong
absorption band around 358 nm and another very weak one
around 445 nm, respectively (Fig. S2 in the ESI†), which are
typical for the azo compounds and can be ascribed to the
p / p* and n / p* electron transitions of the –N]N– bond,
respectively.45 Similar results were also obtained for other
azo monomers.
The molecular weights and polydispersity indices (PDI) of the
obtained azo polymers Pm/n were determined with GPC and the
results are presented in Table 1. It can be seen that the number-
average molecular weights Mn,GPC of Pm/n are in the range of
19 500–29 700 and their PDI range from 1.59 to 2.53, which is
reasonable for the conventional free radical polymerization of
azo-containing methacrylate monomers.19,21,29,30
Thermal and phase transition behavior of the azo polymers
The thermal and phase transition behavior of the azo polymers
Pm/n were first examined with a combination of TGA, DSC, and
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POM. The temperatures at 5% weight loss determined by TGA
under nitrogen were found to be $327 ꢁC for all these azo
polymers, indicating their good thermal stability (Table 1). Fig. 2
shows the DSC curves of the azo polymers with the same length
of flexible spacer but with different terminal alkyl chain length
(i.e., P6/n, n ¼ 4, 6, 8, 12, 18). It can be seen clearly that all the azo
polymers exhibit one phase transition peak in both the second
heating and first cooling scans. In addition, a glass transition at
141, 144, 141, 129, 117 ꢁC was observed for P6/4, P6/6, P6/8, P6/
12, and P6/18 in their DSC second heating scan, respectively,
revealing that the glass transition temperatures (Tg) of the
polymers at first slightly increased with an increase in the length
of the terminal alkyl chains from n ¼ 4 to 6 and then decreased
with a further increase in the terminal tail length, just as reported
previously by others.46 This phenomenon could be ascribed to
the combined effects of the steric hindrance and plasticization
function of the terminal alkyl chains on the Tg of the polymers.
While the steric hindrance effect might be dominant for the
polymers with shorter terminal alkyl chains, the plasticization
effect might play a more important role for those with longer
terminal tails. In this context, it is worth mentioning that the Tg
values of Pm/n are obviously higher than those of the previously
reported side-chain liquid crystalline polymethacrylates with
pendant carboxylate group-substituted azo mesogens,21,36 which
is likely to be due to the formation of hydrogen bonding between
the amide groups in these azo polymers. In addition to the glass
transition and phase transition, P6/18 also exhibits a faint and
rather broad endothermic and exothermic peak around 67 and
58 ꢁC in the DSC second heating and first cooling scans,
respectively, which might be caused by the side chain crystalli-
zation due to the enhanced flexibility of long terminal tails.47,48
POM study revealed that when the azo polymers P6/n (n ¼ 4, 6,
8, 12, 18) were cooled from their isotropic states, a large number
of anisotropic entities appeared from the dark background of the
isotropic liquids and they grew upon further cooling and finally
developed into typical schlieren textures (Fig. 3). In addition,
POM observations showed that the phase transition peaks in the
DSC curves of the azo polymers represented the transition
between the liquid crystalline phases and isotropic phases and the
mesomorphism was enantiotropic. Furthermore, the liquid
crystalline textures of the azo polymers were found to be similar
for both the heating and cooling scans. As shown in Table 1, the
phase transition temperatures of the azo polymers (i.e., the
clearing points Tcl) increased markedly when the number of
carbon (i.e., n) in the terminal alkyl chain increased from 4 to 12,
but they levelled off when n was further increased. The groups of
Jansen46 and Picken49 also found that the phase transition
temperatures increased with increasing the tail length for a series
of side-chain liquid crystalline polycarbonates and polyethers,
respectively. In particular, it is noteworthy that the increase in
the tail length led to a broader range of liquid crystalline phase
(i.e., Tcl–Tg) for P6/n, which is positive for potential applications.
The DSC curves of the azo polymers with the same length of
terminal alkyl chains but different length of flexible spacers (i.e.,
Pm/6, m ¼ 2, 6, 10) are shown in Fig. 4. It can be seen clearly that
the DSC curves of P2/6 and P6/6 exhibit one phase transition
peak in both the second heating and first cooling scans. On the
other hand, P10/6 shows one phase transition peak in the second
Fig. 3 POM images upon cooling: P6/4 at 144 ꢁC (annealed for 31 min)
(a), P6/6 at 166 ꢁC (annealed for 95 min) (b), P6/8 at 185 ꢁC (annealed for
30 min) (c), P6/12 at 194 ꢁC (annealed for 35 min) (d), and P6/18 at 196 ꢁC
(annealed for 48 min) (e).
Fig. 2 DSC curves of the azo polymers P6/n (n ¼ 4, 6, 8, 12, 18) from
the second heating scan (a) and from the first cooling scan (b)
(ꢃ10 ꢁC minꢀ1).
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Fig. 4 DSC curves of the azo polymers P6/6-Ester, Pm/6 (m ¼ 2, 6, 10),
and P10/12 from the second heating scan (a) and from the first cooling
scan (b) (ꢃ10 ꢁC minꢀ1).
Fig. 5 POM images upon cooling: P2/6 at 220 ꢁC (annealed for 120 min)
(a), P6/6 at 166 ꢁC (annealed for 95 min) (b), P10/6 at 144 ꢁC (annealed
for 30 min) (c), P10/6 at 134 ꢁC (annealed for 94 min) (d), P10/12 at
160 ꢁC (annealed for 94 min) (e), and P6/6-Ester at 75 ꢁC (annealed for
30 min) (f).
heating scan and two transition peaks in the first cooling scan.
No noticeable glass transition was detected for P2/6 in both the
second heating and first cooling scans, whereas a glass transition
ꢁ
was observed at 144 and 136 C for P6/6 and P10/6 in the DSC
second heating scan, respectively, indicating that the Tg of the
polymer series decreased with an increase in the spacer length.
This effect has often been observed in side-chain LCPs and has
proven to be caused by the increased internal plasticization
action with the increase in the spacer length.21,30 The POM
characterization revealed that there existed enantiotropic liquid
crystalline phases for all the azo polymers Pm/6 (m ¼ 2, 6, 10)
with their mesophases showing schlieren textures (Fig. 5(a–d)).
Moreover, the phase transition peaks in the DSC curves of P2/6
and P6/6 in both the second heating and first cooling scans,
together with the phase transition peak in the DSC curve of P10/
6 in the second heating scan and the higher temperature transi-
tion peak in the DSC curve of P10/6 in the first cooling scan,
proved to represent the transition between the liquid crystalline
phases and isotropic phases. Note that the liquid crystalline
textures of P10/6 did not change around the lower temperature
transition peak in its DSC curve in the first cooling scan, even
after rather long annealing time, indicating that the lower
temperature peak did not represent the transition between
different liquid crystalline phases (this was further confirmed
by the SAXS results in the following part) and it might stem
from the glass transition, just as observed by Zhou and
coworkers in a series of azo-containing side-chain liquid crys-
talline poly(1-alkyne)s.16 To study more about this phenomenon,
another azo polymer P10/12 was also synthesized, which has the
same length of flexible spacer as P10/6 but with a longer terminal
tail. The DSC investigation showed that there existed two peaks
in both the first cooling and second heating scans for P10/12
(Fig. 4). A careful study with POM also demonstrated that the
higher temperature peaks represented the transition between the
liquid crystalline phases and isotropic phases, while the lower
temperature ones might be glass transition peaks.
It is well known that SAXS can provide useful information
concerning molecular arrangement, packing mode of the meso-
gen, and type of order in the mesophase of a liquid crystalline
polymer and it has thus been widely utilized for accurately
assigning the exact nature of the liquid crystalline phases.36,50,51
Therefore, the liquid crystalline mesophases of some represen-
tative azo polymers (i.e., P6/6, P6/12, and P10/6) were further
studied with in situ variable temperature SAXS. Fig. 6 shows the
SAXS patterns of the studied azo polymers, which were obtained
at their liquid crystalline temperatures (with a certain time of
annealing) upon cooling from the isotropic states. Three scat-
tering peaks at q1 ¼ 1.23, q2 ¼ 2.47, and q3 ¼ 3.70 nmꢀ1 were
discernible in the SAXS pattern of P6/6 with the ratio of q1 to q2
to q3 being 1 : 2 : 3 (Fig. 6(b)), which demonstrated the presence
of a long-range ordered lamellar structure.51 This, together with
its schlieren texture (as observed with POM) and the general rule
‘‘schlieren texture is frequently found in the smectic C phase
instead of smectic A phase’’,44 revealed the presence of a smectic
C liquid crystalline phase for P6/6. In addition, three scattering
peaks with q1 ¼ 1.04, q2 ¼ 2.06, and q3 ¼ 3.10 nmꢀ1 were
observed in the SAXS pattern of P6/12 with the ratio of q1 to q2
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Fig. 6 SAXS patterns of the azo polymers upon cooling from the
isotropic states: P6/6-ester at 74 ꢁC with an annealing time of 40 min (scan
time: 30 min) (a), P6/6 at 166 ꢁC with an annealing time of 10 min
(scan time: 10 min) (b), P6/12 at 194 ꢁC with an annealing time of 10 min
(scan time: 10 min) (c), P10/6 at 144 ꢁC with an annealing time of
10 min (scan time: 10 min) (d), and P10/6 at 133 ꢁC with an annealing
time of 10 min (scan time: 10 min) (e).
Fig. 7 Possible liquid crystalline mesophase structures of Pm/n (a) and
P6/6-ester (b).
to q3 being 1 : 2 : 3 (Fig. 6(c)), again suggesting the presence of
a smectic C liquid crystalline phase for P6/12. Similarly, P10/6
also showed three scattering peaks at q1 ¼ 1.04, q2 ¼ 2.03, and
q3 ¼ 3.09 nmꢀ1 in its SAXS pattern, with the ratio of q1 to q2 to q3
being 1 : 2 : 3 (Fig. 6(d)). More importantly, no change was
observedꢁfor the SAXS patterns of P10/6 at both 144 (Fig. 6(d))
and 133 C (Fig. 6(e)) (even after annealing), which was consis-
tent with the POM result and further confirmed that the lower
temperature peak in the DSC first cooling curve of P10/6 did not
represent the transition between two liquid crystalline phases.
Based on the DSC, POM, and SAXS results, we can conclude
that all the obtained azo polymers Pm/n showed enantiotropic
smectic C liquid crystalline phases. Table 2 presents the meso-
phase layer spacing values (d) of the above side-chain LCPs
determined with SAXS (d ¼ 2p/q1).51 The obtained d values
proved to be greater than the calculated molecular length (l) for
the fully extended side-chain liquid crystalline unit of Pm/n, but
smaller than 2l, suggesting a possible bilayer lamellar structure,
as schematically shown in Fig. 7(a), where the hydrogen bonding
formed among the neighboring amide groups was considered to
enhance the attractive interaction between the mesogenic groups
and stabilize the liquid crystalline mesophases.
been well established that the FT-IR spectral peak positions of
both the N–H stretching (nN–H) and C]O stretching (amide I)
bands from the amide groups can provide useful information
about their hydrogen bonding state.44,52,53 Fig. 8 shows the
variable temperature FT-IR spectra of the representative poly-
mer P6/6 obtained during both the second heating and first
cooling processes. It can be seen clearly that P6/6 exhibits two
infrared bands at 3445 cmꢀ1 (very weak shoulder) and 3317 cmꢀ1
(strong peak) for nN–H at ambient temperature, which can be
assigned to the ‘‘free’’ (higher than 3400 cmꢀ1) and hydrogen-
bonded N–H stretching modes, respectively. This, together with
the rather low-frequency peak of the amide I band (1637 cmꢀ1
,
C]O stretching), clearly demonstrates that most N–H groups
are associated with C]O groups through hydrogen bonding at
room temperature. Moreover, the N–H stretching vibration also
proved to be dependent on the sample’s temperature. The
hydrogen-bonded N–H stretching peak (with a frequency lower
than 3400 cmꢀ1) became weaker and shifted towards higher
frequencies upon increasing the sample temperature, while the
free N–H stretching peak with a frequency higher than 3400 cmꢀ1
became stronger in the meantime (Fig. 8(a)), indicating that the
hydrogen bonding among the amide groups got weaker and
some of the bonded N–H groups were transformed to free ones
with increasing temperature (it is worth mentioning here,
however, that the hydrogen bonding still existed among the
To confirm the presence of hydrogen bonding among the
amide groups in the liquid crystalline phases of Pm/n, their
variable temperature FT-IR experiments were performed. It has
Table 2 SAXS characterization conditions and results of the azo polymers
Azo polymer
Tanalysis (ꢁC)a
tannealing (min)a
l (nm)b
d (nm)c
Liquid crystalline mesophased
P6/6
P6/12
P10/6
166
194
144
133
74
10
10
10
10
40
3.12
3.87
3.62
3.62
3.12
5.11
6.04
6.04
6.04
3.27
Bilayer SC
Bilayer SC
Bilayer SC
Bilayer SC
Fully interdigitated SA
P6/6-ester
a
b
Tanalysis and tannealing refer to the SAXS analysis temperatures and annealing times for the azo polymers during their first cooling processes. The
c
calculated molecular length (l) for the fully extended side-chain liquid crystalline units of the studied azo polymers. The mesophase layer spacing
of the side-chain LCPs determined by SAXS. d SC, smectic C; SA, smectic A.
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obtained at its liquid crystalline temperature (with a certain time
of annealing) upon cooling from the isotropic state (Fig. 6(a)),
where one weak scattering peak at q1 ¼ 1.92 nmꢀ1 and another
very weak one at q2 ¼ 3.83 nmꢀ1 were detectable with the ratio of
q2 to q1 being 2, suggesting the presence of a long-range ordered
lamellar structure with a layer spacing d ¼ 3.27 nm (d ¼ 2p/q1).51
This d value is in good agreement with the calculated molecular
length (l) for the fully extended side-chain liquid crystalline unit
of P6/6-ester (l ¼ 3.12 nm), suggesting a fully interdigitated
smectic A structure (i.e., near to 100% overlap of the side-chain
units), as schematically shown in Fig. 7(b). It can thus be
concluded on the basis of the above results that the hydrogen
bonding interaction indeed plays a decisive role in the formation
and stabilization of smectic C liquid crystalline phases in the
side-chain LCPs containing amide-substituted azo mesogens.
Photoisomerization behavior of the azo polymers
The photochemical properties of one representative azo polymer
P6/6 in THF were firstly investigated. By irradiation with 365 nm
UV light, the studied polymer solution underwent trans to cis
photoisomerization until a photostationary state was eventually
reached (Fig. 9(a)). The intensity of the p / p* transition band
around 356 nm decreased, whereas the intensity of the n / p*
transition band around 445 nm slightly increased. The existence
of isobestic points at 310 and 417 nm is characteristic for the
existence of two distinct absorbing species in equilibrium with
each other and at the same time proves that no side reaction takes
place during the photoisomerization process in the range studied.
Fig. 9(b) shows the UV-vis spectral changes of P6/6 solution in
Fig. 8 FT-IR spectra of P6/6 at different temperatures during the second
heating (a) and first cooling (b) processes.
amide groups in the polymers even at 220 ꢁC). On the other hand,
the intensity and frequency of the N–H stretching vibration
were fully recovered upon decreasing the sample temperature,
suggesting that the hydrogen bonding interaction was reversible,
just as expected (Fig. 8(b)). Note that the C]O stretching band
of the amide groups in the studied polymer also showed revers-
ible change in its frequency following the change of the sample
temperatures while no frequency shift was observed for the C]O
stretching band from the ester groups in the polymer, which
further demonstrated the formation of hydrogen bonding
between C]O and N–H groups of the amide substitutents in the
azo polymer.
To get more insight into the effect of hydrogen bonding on the
mesophases of the above azo polymers, a new side-chain LCP
named P6/6-ester was also synthesized as a control polymer,
which has a similar chemical structure to that of P6/6, but with an
ester linkage instead of an amide linkage in the terminal sub-
stitutent of the azo mesogen (Scheme S1, Fig. S1b†). Fig. 4 shows
the DSC curves of P6/6-ester from both the second heating and
first cooling scans. It can be seen clearly that the DSC curves of
P6/6-ester exhibit one obvious phase transition peak in both the
second heating and first cooling scans, just as P6/6. However, the
phase transition temperature of P6/6-ester proved to be signifi-
cantly lower than that of P6/6 and a temperature difference of 92
(upon the first cooling scan)/97 ꢁC (upon the second heating
scan) was observed between P6/6-ester and P6/6, which is likely
to be due to the absence of hydrogen bonding among the
neighboring azo mesogens in P6/6-ester. POM observation
revealed the presence of obvious broken focal-conic textures
during both the heating and cooling processes of P6/6-ester
(Fig. 5(f)), indicating that the studied polymer had an enantio-
tropic smectic mesophase. The SAXS pattern of P6/6-ester was
Fig. 9 UV-vis spectral changes in dependence of time for the solution of
P6/6 in THF (C ¼ 0.0247 mg mLꢀ1) at 25 ꢁC upon irradiation with
365 nm light (a) and upon irradiating the polymer solution at the
photostationary state with l > 430 nm visible light (b).
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THF with time under irradiation with l > 430 nm visible light,
where the polymer solution had been irradiated with 365 nm UV
light for 300 s prior to visible light irradiation. It can clearly be
seen that the polymer solution undergoes cis to trans back-
isomerization upon visible light irradiation, but the finally
recovered absorbance of trans-azobenzene is lower than that
before UV irradiation with the recovery of the trans-isomer being
87%. A similar phenomenon was also observed previously by us
and others.36,54,55 Its real cause is not totally clear yet but might
be that, at the same wavelength where cis to trans photochemical
back-isomerization was performed, there was also a weak
absorption from the trans-isomer, which eventually led to an
equilibrium with cis to trans and trans to cis isomerizations
taking place under the same visible light after most of the cis-
isomer returned to the trans-isomer.26,36 Nevertheless, the
photochemical isomerization became completely reversible upon
the subsequent cycles of UV and visible light irradiation (Fig. not
shown).
6, 8, 12) solutions. Its real cause is not yet fully understood, but
probably due to the entanglement of the rather long terminal
alkyl chain in this polymer, thus leading to more restriction to the
azo groups during their photochemical isomerization process.
Similarly, the photochemical back-isomerization rates of the
polymer solutions under the irradiation of visible light were also
more or less the same, but the relative absorbance of the P6/18
solution in the photostationary state finally reached was again
somehow higher than those of the other P6/n (n ¼ 4, 6, 8, 12)
solutions (Fig. 10(b)). Further investigation is underway to
provide a reasonable explanation for this phenomenon.
The time dependence of the relative absorbance of the polymer
series Pm/6 (m ¼ 2, 6, 10) in THF was also studied (Fig. 11).
Fig. 11(a) shows that the polymer with a shorter flexible spacer
(i.e., P2/6) had a relatively higher relative absorbance (i.e., lower
conversion of trans-isomer to cis-isomer) under the irradiation of
UV light when the photostationary state was eventually reached,
which might stem from the more significant coupling effect
between the polymer backbones and the pendant azo groups in
the presence of a shorter flexible spacer, just as observed by
Wang and coworkers for the photoinduced mass-migration
behavior of the azo-containing side-chain LCPs with different
length flexible spacers.20 In comparison, the photochemical back-
isomerization processes of the Pm/6 solutions under the irradi-
ation of visible light were more or less the same, suggesting that
the length of the flexible spacers in the azo polymers has little
influence on their visible light-driven back-isomerization
processes (Fig. 11(b)).
The time dependence of the relative absorbance (i.e., At/A0,
where At refers to the absorbance of the polymer solution at
356 nm at time t upon UV or visible light irradiation and A0 the
original absorbance of the polymer solution at 356 nm without
UV light irradiation) of the polymer series P6/n (n ¼ 4, 6, 8, 12,
18) in THF showed that the photoinduced isomerization rates
of the P6/n solutions were rather similar under the irradiation of
365 nm UV light (Fig. 10(a)). However, the relative absorbance
of the P6/18 solution in the photostationary state finally reached
proved to be somehow higher than those of the other P6/n (n ¼ 4,
Fig. 10 Dependence of the relative absorbance (At/A0, l ¼ 356 nm) on
time upon irradiation with 365 nm UV light (a) or with l > 430 nm visible
light (b) for the P6/n (n ¼ 4 (filled square), 6 (empty circle), 8 (empty
triangle), 12 (filled circle), 18 (filled diamond)) solutions in THF
(C ¼ 0.0247 mg mLꢀ1) at 25 ꢁC.
Fig. 11 Dependence of the relative absorbance (At/A0, l ¼ 356 nm) on
time upon irradiation with 365 nm UV light (a) or with l > 430 nm visible
light (b) for the Pm/6 (m ¼ 2 (triangle), 6 (circle), 10 (square)) solutions in
THF (C ¼ 0.0247 mg mLꢀ1) at 25 ꢁC.
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ꢁ
19 S. Freiberg, F. Lagugne-Labarthet, P. Rochon and A. Natansohn,
Macromolecules, 2003, 36, 2680–2688.
A series of new photoresponsive side-chain liquid crystalline
polymethacrylates with amide group-substituted azo mesogens
and different length of flexible spacers and terminal alkyl chains
have been successfully synthesized via conventional free radical
polymerization. Hydrogen bonding interactions among the
amide groups proved to play a decisive role in forming and
stabilizing the smectic C liquid crystalline mesophase structures
of the polymers. In addition, the length of the flexible spacers and
the terminal tails also showed great influence on the phase
transition behavior of the azo polymers in terms of their glass
transition and phase transition temperatures, where the increase
in the tail length led to a broader range of liquid crystalline phase
for the azo polymers, which is positive for potential applications.
Furthermore, the photoresponsivity of the polymer solutions
was also confirmed by the occurrence of the UV and visible light-
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of the polymers opens up a new avenue for efficiently designing
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substituted azo LCPs with a broad range of stable liquid crys-
talline phases and obvious photoresponsive properties are highly
promising for many potential applications.
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