The effects of para-, meta- and ortho-monosubstituted azobenzene moiety in the side chain on phase behavior of mesogen-jacketed liquid crystalline polymers

Article abstract of DOI:10.1016/j.polymer.2013.04.023

Three different mesogen-jacketed liquid crystalline polymers with monosubstituted azobenzene moiety in the side-chain have been studied. These are poly(2,5-bis{[para-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (denoted as PPABCS),

Full text of DOI:10.1016/j.polymer.2013.04.023

Polymer 54 (2013) 3238e3247
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The effects of para-, meta- and ortho-monosubstituted azobenzene
moiety in the side chain on phase behavior of mesogen-jacketed liquid
crystalline polymers
,
,
Chang-an Yang a b, Helou Xie a, Guanqun Zhong a, Hailiang Zhang a
*
a Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key Laboratory of Advanced Functional Polymer Materials of Colleges
and Universities of Hunan Province, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China
b Department of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, Hunan Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three different mesogen-jacketed liquid crystalline polymers with monosubstituted azobenzene moiety
in the side-chain have been studied. These are poly(2,5-bis{[para-(40-methoxy-4-oxyhexyloxy azo-
benzene) benzyl] oxycarbonyl} styrene) (denoted as PPABCS), poly(2,5-bis{[meta-(40-methoxy-4-
oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (denoted as PMABCS) and poly(2, 5-bis
{[ortho-(40-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (denoted as POABCS).
The chemical structures of the monomers were confirmed by 1H NMR, 13C NMR spectroscopy and
elemental analysis. The structure characterization of the polymers was performed by 1H NMR spec-
troscopy and gel permeation chromatography (GPC), and the phase structures and transitions of the
polymers were studied using differential scanning calorimetry (DSC), polarized light microscopy (PLM),
and one- and two-dimensional (1D, 2D) wide-angle X-ray diffraction. The effects of monosubstituted
azobenzene moiety in different positions on the liquid crystalline behaviors of the polymers were also
investigated. The results show that the phase transitional behaviors of mesogen-jacketed liquid crys-
talline polymers containing monosubstituted azobenzene moiety depend strongly on the position of the
substituent on the azobenzene moiety. We identify that PPABCS can form a hierarchically ordered
structure with double orderings on both the nanometer and subnanometer length scales. Most likely, the
thick main-chains of PPABCS obtained by “jacketing” the central rigid portion of side-chain to the
polyethylene backbone construct a 2D centered rectangular scaffold. The azobenzene-containing side-
chains pack inside the main-chain scaffold form smecitc A (SmA)-like structure and are perpendicular to
the main-chains. We compared PPABCS with PMABCS and POABCS, and found that the hierarchically
ordered structure of PMABCS was similar to that of PPABCS. It is surprising that the main-chains of
POABCS also construct a 2D centered rectangular scaffold, however, the packing of azobenzene-
Received 8 November 2012
Received in revised form
16 March 2013
Accepted 11 April 2013
Available online 18 April 2013
Keywords:
Mesogen-jacketed liquid crystalline
polymer
Azobenzene moiety
Substituent position
containing side-chains inside the main-chain scaffold develops smecitc
Furthermore, the glass transition temperature (Tg), isotropic temperature (Ti) and liquid crystalline range
T, from Tg to Ti) of the polymers decrease in the order, para > meta > ortho. It is very interesting
C (SmC)-like structure.
(
D
phenomenon that the associated enthalpy changes of these polymers are also the same order,
para > meta > ortho, which is different from those of MCLCPs and SCLCPs.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
liquid crystalline systems, and he revealed that the formation of
liquid crystalline depended on the chain rigidity, aspect ratio,
Liquid crystalline polymers have attracted much attention for
their potential applications in many fields, including optical infor-
mation storage, nonlinear optical material, holographic memories,
and so on [1e11]. In the 1950s, Flory postulated that the lattice
model was a highly versatile approach for various problems in
chemical structure, conformation and configuration, and other
factors [12e14].
In recent years, the effect of the position of the substituent in the
mesogenic moiety on the properties of the polymers has been
widely studied. Most of the research works has focused on the ef-
fect of its substituent position on photochemical behaviors [15e17],
photoelectron properties of polymers [18,19], non-linear optical
properties [20], liquid crystalline behavior [21e28]. Owing to the
very high melting point (Tm) and low solubility of main-chain liquid
* Corresponding author. Tel.: þ86 731 58293377; fax: þ86 731 58293264.
E-mail address: Hailiangzhang@xtu.edu.cn (H. Zhang).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.04.023

C.-a. Yang et al. / Polymer 54 (2013) 3238e3247
3239
crystalline polymers (MCLCPs), many studies have been devoted to
decreasing the Tm and Tg, in addition to improving the solubility by
incorporating ortho- or meta-linkages into the side chain. More-
over, the effects of para-, meta-, or ortho-linkages on the formation
of liquid crystalline have also been investigated [22,28]. The
Chung’s result has been shown that the formation of liquid crys-
tallinity of MCLCPs is found strongly dependent on the configura-
tion of the para-, meta-, or ortho-linkages in the monomeric
moieties; of these, the para-linkage is most favorable for generating
liquid crystallinity, and the ortho-linkage has a greater tendency to
form liquid crystallinity than the meta-linkage [22]. Recently, our
results have been demonstrated that the transitional behavior of
SCLCPs containing monosubstituted azobenzene moieties depends
strongly on the position of the substituent on the azobenzene
moiety; for example, the ortho-monosubstituted polymers do not
form liquid crystalline phases, but all the para- and meta-mono-
substituted polymers exhibit smectic A phase. Furthermore, the
glass transition temperature (Tg) of the polymers decreases in the
order, para > meta > ortho. For the para and meta-monosubstituted
polymers, the isotropic temperature (Ti) and liquid crystalline range
(DSC), polarized light microscopy (PLM), and one- and two-
dimensional (1D, 2D) wide-angle X-ray diffraction.
2. Experimental section
2.1. Materials
The compound of vinylterephthalic acid was synthesized using
the method previously reported [52]. Azobisisbutyronitrile (AIBN)
was purified by recrystallization from ethanol. Chlorobenzene was
washed with H2SO4, NaHCO3, and distilled water separately and
was distilled from calcium hydride. THF (AR; Beijing Chemical Co.)
and Triethylamine (Acros, 99%) were heated under reflux over
calcium hydride for at least 8 h and distilled before use. All other
reagents were used as received from commercial sources.
2.2. Synthesis of monomers
The synthetic route of the monomers, 2, 5-bis{[para-(40-
methoxy-4-oxy-hexyloxy azobenzene) benzyl] oxycarbonyl} sty-
rene) (denoted as PABCS), 2, 5-bis{[meta-(40-methoxy-4-oxy-hex-
yloxy azobenzene) benzyl] oxycarbonyl} styrene) (denoted as
MABCS) and 2, 5-bis{[ortho-(40-methoxy-4-oxy-hexyloxy azo-
benzene) benzyl] oxycarbonyl} styrene) (denoted as OABCS), and
their corresponding polymers were illustrated in Scheme 1. The
experimental details of the monomers synthesis and character-
ization are described below using PABCS as an example.
(DT, from Tg to Ti) are found to be in the order, para > meta, and the
associated enthalpy changes in these polymers is the opposite or-
der, meta > para [28].
Mesogen-jacketed liquid crystalline polymers (MJLCPs) have
been attracting increasing interests for its novelties in the struc-
ture and properties since the idea was first proposed by Zhou
et al. in the 1987 [29]. Although the mesogen-jacketed liquid-
crystalline polymers whose mesogenic units are laterally attached
to the polymer backbone without or with very short spacers is a
type of SCLCPs, which leads to the extended main chain confor-
mation and some properties similar to main chain liquid crys-
talline polymers (MCLCPs) because of the spatial requirement of
the bulky and rigid mesoenic units [30e32]. Presently, most of
the studies are concentrated on the synthesis of MJLCPs and the
investigation of the chemical structures of rigid or flexible side
chain, for example, the type of the mesogen, the length or type of
the terminal flexible substituents, nature of the polymer back-
bone, etc. effect on the phase structure [33e48]. However, there
have been few publications devoted to the effect of the position of
the substituent on the properties of MJLCPs. We have assumed
that the presence of mesogenic para-, meta-, or ortho-mono-
substituted will not only influence the glass transition and
isotropic temperatures of MJLCPs, but also their liquid crystalline
behaviors, which shall be different from those of MCLCPs and
SCLCPs. In the present study, we have compared the effects of
para-, meta-, or ortho-monosubstituent on the liquid crystalline
behavior of MJLCPs.
Azobenzene mesogen is one of the typical functional units,
which can undergo photon-driven reversible cis-trans isomeriza-
tion [49]. Liquid crystalline polymers containing azobenzene
mesogens have been widely studied for potential applications in
specific areas, such as reversible optical data storage, optical
switching, polarization holography, and so forth [50]. Moreover,
their liquid crystalline behaviors have been investigated and they
may form low-ordered LC phase, i.e. nematic (N), smectic A (SmA)
and smectic C (SmC) phases [51].
Three different mesogen-jacketed liquid crystalline polymers
containing para-, meta-, or ortho-monosubstituted azobenzene
moiety in the side-chains, respectively, were synthesised using
route shown in Scheme 1. The chemical structures of the monomers
were confirmed by 1H NMR, 13C NMR spectroscopy and elemental
analysis. The structure characterization of the polymers was per-
formed by 1H NMR spectroscopy and gel permeation chromatog-
raphy (GPC), and the phase structures and transitions of the
polymers were studied using differential scanning calorimetry
2.2.1. Synthesis of 4-(6-bromohexyloxy)-40-methoxy azobenzene (1)
4-(6-Bromohexyloxy)-40-methoxy azobenzene was prepared by
the method quoted in the literature [53]. 1H NMR (400 MHz, CDCl3,
d, ppm): 7.89e7.86 (m, 4H, AreH), 7.01e6.97 (m, 4H, AreH), 4.06e
4.02 (m, 2H, eCH2e), 3.89 (s, 3H, eOCH3), 3.45e3.42 (m, 2H, e
CH2Br), 1.93e1.82 (m, 4H, eCH2e), 1.55e1.51 (m, 4H, eCH2e).
2.2.2. Synthesis of methyl 4-(40-methoxy-4-oxy-hexyloxy
azobenzene) benzoate (2)
Compound 2 was obtained by the etherification of methyl 4-
hydroxybenzoate with 4-(6-Bromohexyloxy)-40-methoxy azo-
benzene (compound 1). In a 1000 mL three-neck round bottom
flask equipped with a condenser, N2 inlet-outlet and mechanical
stirrer, a mixture of K2CO3 (16.56 g, 120 mmol), acetone (400 mL)
was thoroughly degassed with N2. Then, methyl 4-hydroxybe-
nzoate (6.08 g, 40 mmol) and compound 1 (15.64 g, 40 mmol)
were added, and the mixture was heated to 60 ꢀC. After 48 h, the
reaction was found to be complete by thin-layer chromatography
(TLC) and 1H NMR analyses. The reaction mixture was filtrated and
washed with acetone to remove excessive compound 1. The pre-
cipitate was dissolved in CH2C12, then, the potassium salt was
filtered off. The solvent was evaporated under reduced pressure to
obtain yellow solid. Afterwards, the crude product dissolved in
CH2C12 was purified by column chromatography (silica gel,
CH2Cl2). 1H NMR (400 MHz, CDCl3,
d ppm): 8.02e7.99 (m, 2H, Are
H), 7.92e7.89 (m, 4H, AreH), 7.04e7.00 (m, 4H, AreH), 6.94e6.92
(m, 2H, AreH), 4.10e4.04 (m, 4H, eOCH2e), 3.91 (s, 3H, eOCH3),
3.90 (s, 3H, eOCH3), 1.88e1.85 (m, 4H, eCH2e), 1.61e1.59 (m, 4H,
eCH2e).
2.2.3. Synthesis of 4-(40-methoxy-4-oxy-hexyloxy azobenzene)
benzyl alcohol (3)
Compound 3 was prepared by the reduction of methyl 4-(40-
methoxy- 4-oxyhexyloxy azobenzene) benzoate (compound 2)
with LiAlH4. Into a three-neck round-bottom flask, equipped with a
condenser, ice bath, N2 inlet-outlet, and magnetic stirrer, LiAlH4
(0.76 g, 20 mmol) was suspended in dry THF (20 mL), and the

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C.-a. Yang et al. / Polymer 54 (2013) 3238e3247
Scheme 1. Synthetic Route of the Monomers and Polymers.
solution of compound 2 (9.24 g, 20 mmol) in dry THF (200 mL) was
added dropwise very slowly. After the addition was complete, the
mixture was reacted for further 2 h at room temperature. The re-
action was found to be complete by TLC and 1H NMR analyses.
Water was then added slowly with vigorous stirring to terminate
the reaction, and then diluted HCl was added to dissolve the pre-
cipitate. The product was extracted with CH2Cl2. The extracts were
dried over MgSO4, and condensed to give a yellow solid. Afterward,
the crude product dissolved in dichloromethane was purified by
column chromatography (silica gel, CH2Cl2/acetone, 50/1). 1H NMR
chloride was removed by evaporation under reduced pressure.
The residue was washed twice by petroleum ether, then, dis-
solved in dried THF (20 mL). Compound 3 (8.68 g, 20 mmol),
DMAP (3.67 g, 30 mmol) and triethylamine (4 mL) were dissolved
in dried THF (100 mL). Under intense stirring at the temperature
of the ice bath, the solution of vinylterephthal chloride was slowly
dropped into the solution over a 0.5 h time period. The mixture
was further stirred at room temperature for 24 h, and then most
of the THF was evaporated under reduced pressure. Then, water
and dilute HCl was added to dissolve the precipitate, the product
was extracted with CH2Cl2. The extracts were dried over anhy-
drous MgSO4, and condensed to give a solid. Afterward, the raw
product dissolved in dichloromethane was purified first by col-
umn chromatography (silica gel, CH2Cl2/acetone, 100/1), and
recrystallised from THF/diethyl ether to give the monomer
(PABCS, 4).
(400 MHz, CDCl3,
d ppm): 7.89e7.85 (m, 4H, AreH), 7.29e7.27 (m,
2H, AreH), 7.01e6.97 (m, 4H, AreH), 6.89e6.88 (m, 2H, AreH), 4.62
(s, 2H, eOCH2e), 4.07e3.97 (m, 4H, eOCH2e), 3.79 (s, 3H, eOCH3),
1.86e1.83 (m, 4H, eCH2e), 1.58e1.55 (m, 4H, eCH2e).
2.2.4. 2,5-Bis{[para-(40-methoxy-4-oxy-hexyloxy azobenzene)
benzyl] oxycarbonyl} styrene (4, PABCS)
1H NMR (400 MHz, CDCl3,
d ppm): 8.22e7.87 (m, 3H, AreH);
An amount of vinyl terephthalic acid (1.92 g, 10 mmol) was
mixed with thionyl chloride (40 mL), and the mixture was stirred
for about 4 h at 50 ꢀC till get a clear solution. The excess thionyl
7.86e7.84 (m, 8H, AreH); 7.40e7.38 (q, 1H, ]CHe); 7.38e7.37 (m,
4H, AreH); 7.01e6.97 (m, 8H, AreH); 6.91e6.89 (m, 4H, AreH);
5.74e5.69 (dd, 1H, ]CH2); 5.40e5.37 (dd, 1H, ]CH2); 5.30, 5.28 (s,

C.-a. Yang et al. / Polymer 54 (2013) 3238e3247
3241
4H, eOCH2e); 4.05e3.96 (m, 8H, eOCH2e); 3.87 (s, 6H, eOCH3);
1.84e1.83 (m, 8H, eCH2e); 1.56e1.55 (m, 8H, eCH2e).
The TGA was performed on a TA SDT 2960 instrument at a
heating rate of 20 ꢀC/min in nitrogen atmosphere.
The apparent number-average MW (Mn) and MW distribution
(Mw/Mn) were measured on a GPC (Waters 1515) instrument with a
13C NMR (
d, ppm, CDCl3): 25.84, 25.86, 29.16, 29.30 (eCH2e),
55.52 (eOCH3); 67.01, 67.04, 67.94, 68.15 (eOCH2e); 114.18, 114.63,
114.70, 124.31, 124.33, 127.62, 127.73, 128.13, 130.15, 130.19, 130.33,
132.52, 133.29, 139.59, 147.03, 147.17, 159.34, 161.15, 161.57 (Ar);
117.66 (CH2]); 134.87 (]Ce); 165.66, 166.63 (C]O).
Calc. for C62H64N4O10 (Mol. Wt. 1024): C, 72.64; H, 6.29; N, 5.47;
Found: C, 72.85; H, 6.32; N, 5.55.
set of HT3, HT4 and HT5. The m-Styragel columns used THF as an
eluent, and its flow rate was 1.0 mL/min at 35 ꢀC. Calibration was
made with polystyrene standards (PS).
DSC examination was carried out on a TA DSC Q100 calorimeter
with a programed heating procedure in nitrogen. The sample size
was about 5 mg and encapsulated in hermetically sealed aluminum
pans, whose weights were kept constant. The temperature and heat
flow scale at different cooling and heating rates were calibrated
using standard materials such as indium and benzoic acid.
LC texture of the polymers was examined under PLM (Leica DM-
LM-P) coupled with a Mettler-Toledo hot stage (FP82HT). The films
The other monomers were prepared in a similar manner. The
spectra data were as follows.
2.2.5. 2,5-Bis{[meta-(40-methoxy-4-oxy-hexyloxy azobenzene)
benzyl] oxycarbonyl} styrene (MABCS, 5)
1H NMR (400 MHz, CDCl3,
d ppm): 8.26e7.92 (m, 3H, AreH);
7.88e7.85 (m, 8H, AreH); 7.45e7.38 (q, 1H, ]CHe); 7.31e7.29 (m,
4H, AreH); 7.00e6.96 (m, 8H, AreH); 6.88e6.86 (m, 4H, AreH);
5.76e5.71 (dd, 1H, ]CH2); 5.42e5.39 (dd, 1H, ]CH2); 5.34, 5.31 (s,
4H, eOCH2e); 4.04e3.96 (m, 8H, eOCH2e); 3.87 (s, 6H, eOCH3);
1.84e1.82 (m, 8H, eCH2e); 1.56e1.55 (m, 8H, eCH2e).
with thickness of w10 mm were casted from THF solution and
slowly dried at room temperature.
One-dimensional (1D) wide-angle X-ray diffraction (WAXD)
experiments were performed on a Philips X0 Pert Pro diffractometer
with a 3 kW ceramic tube as the X-ray source (Cu Ka) and an X0
celerator detector. The reflection peak positions were calibrated
13C NMR (
d, ppm, CDCl3): 25.81, 25.91, 29.11, 29.21 (eCH2e),
55.54 (eOCH3); 62.92, 63.00, 67.97, 68.09 (eOCH2e); 111.33, 111.37,
114.19, 114.68, 114.70, 120.34, 124.03, 124.19, 124.35, 128.20, 128.31,
129.69, 129.72, 129.87, 130.45, 132.64, 133.39, 139.55, 146.95, 147.18,
151.17, 161.18, 161.55 (Ar); 117.63 (CH2]); 134.93 (]Ce); 165.65,
166.69 (C]O).
with silicon powder (2q q
> 15ꢀ) and silver behenate (2 < 10ꢀ). The
sample stage is set horizontally, and a temperature control unit
(Paar Physica TCU 100) in conjunction with the diffractometer was
utilized to study the structure evolutions as a function of temper-
ature. The heating and cooling rates in the WAXD experiments
were 10 ꢀC/min.
Two-dimensional wide-angle X-ray diffraction (2D WAXD) fiber
patterns were recorded on a Bruker D8 Discover diffractometer
equipped with a general area detector diffraction system (GADDS)
as a 2D detector, in a transmission mode at room temperature.
Again, calibrations were made against silicon powder and silver
behenate. Samples were mounted on the sample stage, and the
point-focused X-ray beam was aligned either perpendicular or
parallel to the mechanical shearing direction.
2.2.6. 2,5-Bis{[ortho-(40-methoxy-4-oxy-hexyloxy azobenzene)
benzyl] oxycarbonyl} styrene (OABCS, 6)
1H NMR (400 MHz, CDCl3,
d ppm): 8.26e7.92 (m, 3H, AreH);
7.89e7.84 (m, 8H, AreH); 7.48e7.40 (q, 1H, ]CHe); 7.41e7.28 (m,
4H, AreH); 7.00e6.93 (m, 8H, AreH); 6.93e6.87 (m, 4H, AreH);
5.75e5.71 (dd, 1H, ]CH2); 5.42, 5.40 (s, 4H, eOCH2e); 5.439e5.36
(dd, 1H, ]CH2); 4.02e3.94 (m, 8H, OCH2e); 3.87 (s, 6H, eOCH3);
1.84e1.74 (m, 8H, eCH2e); 1.51e1.50 (m, 8H, eCH2e).
13C NMR (
d, ppm, CDCl3): 25.86, 25.89, 29.17, 29.21 (eCH2e),
55.54 (eOCH3); 67.04, 67.09, 67.84, 68.13 (eOCH2e); 114.17, 114.39,
114.44, 114.67, 120.31, 120.36, 124.33, 124.34, 128.22, 128.48, 129.72,
130.47, 132.36, 133.20, 137.19, 137.06, 139.74, 146.97, 147.12, 159.34,
161.14, 161.54 (Ar); 117.85 (CH2]); 134.85 (]Ce); 165.55, 166.49
(C]O).
3. Results and discussion
3.1. Synthesis and characterization of the monomers and polymers
As shown in Scheme 1, each of the monomers (PABCS, MABCS
and OABCS) was synthesized in the similar manner. The monomer
PABCS will be taken as a representative example.
2.3. Polymerization
The preparation and characterization of 4-(6-bromohexyloxy)-
40-methoxy azobenzene, has been described elsewhere [53]. First,
methyl para-(40-methoxy-4-oxy-hexyloxy azobenzene) benzoate
was prepared by etherification of methyl 4-hydroxybenzoate with
4-(6-bromohexyloxy)-40-methoxy azobenzene. Then, para-(40-
methoxy-4-oxy-hexyloxy azobenzene) benzyl alcohol was syn-
thesized by reduction of methyl para-(40-methoxy-4-oxy-hexyloxy
azobenzene) benzoate with LiAlH4. Finally, the monomer PABCS
was obtained by esterification of vinyl terephthalic chloride and
para-(40-methoxy-4-oxy-hexyloxy azobenzene) benzyl alcohol. The
rude product was purified by column chromatography (silica gel,
CH2Cl2/acetone, 100/1), followed by recrystallization from THF/
diethyl ether to obtain the monomer, PABCS.
The polymers were each obtained by conventional solution
radical polymerization (see Scheme 1), typically carried out as
described in the following example.
The monomer PABCS (4; 0.5 g, 0.5 mmol), 0.01 M of AIBN
(300 mL) in chlorobenzene solution, and dry chlorobenzene (2 mL),
were placed in a 25 mL reaction tube containing a magnetic stirrer
bar. After three freezeepumpethaw cycles, the tube was sealed
under vacuum. Polymerization was carried out at 60 ꢀC for 24 h. The
sample was diluted with THF and precipitated into a large volume
of hot acetone. The sample was purified by similarly re-
precipitating three times from THF into hot acetone, and dried
overnight at room temperature in vacuo.
The structure of the monomers was confirmed by 1H NMR, 13
C
2.4. Instruments and measurements
NMR spectroscopy and elemental analysis. In order to study the
influence of Mw on the phase behavior, we attempted to synthesize
the polymers with different molecular weights via ATRP. The result
showed that the high MW polymer couldn’t be prepared, but we
were able to obtain polymers of high molecular weight by free
radical polymerization.
Elemental analysis was carried out with an Elementar Vario EL
instrument.
1H and 13C NMR spectra were recorded on a Bruker ARX400
spectrometer at room temperature, using deuterated chloroform
(CDCl3) as the solvent and tetramethylsilane (TMS) as the internal
standard.
As an example, Fig. 1a and b show the 1H NMR spectra of the
monomer, PABCS, and the corresponding polymer, PPABCS,

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C.-a. Yang et al. / Polymer 54 (2013) 3238e3247
Table 1, all the polymers were quite stable, 5% weight loss occurring
only above 330 ꢀC in nitrogen atmosphere. The phase transitions of
the polymers were studied by DSC, and the transition temperatures
and corresponding enthalpy changes were also summarized in
Table 1. To eliminate the effect of thermal history, all the samplꢁes1
were initially heated from 30 ꢀC to 240 ꢀC at a rate of 20 ꢀC min
and held at 240 ꢀC for 5 min. The DSC thermal diagrams were
recorded at 10 ꢀC minꢁ1 during the first cooling and second heating
process, of which the DSC traces are shown in Fig. 2. Each of the
polymers had two phase transitions, attributed to the glass tran-
sition and the isotropic-liquid crystalline phase transition. From
Fig. 2 and Table 1, it will be seen that the glass transition temper-
atures (Tg) of the polymers (PPABCS, PMABCS and POABCS) were
w80 ꢀC, w66 ꢀC and w64 ꢀC, respectively; and that the isotropic
temperatures (Ti) of three polymers were w219 ꢀC, w188 ꢀC and
w157 ꢀC, respectively; and that the liquid crystalline range (
from Tg to Ti) were w139 ꢀC, w122 ꢀC and w93 ꢀC, respectively. The
Tg, Ti and T for three polymers all decreased in the order of
DT,
D
para > meta > ortho. This trend was similar to that observed for
MCLCPs [22] and SCLCPs [28]. It was surprising, however, that the
associated enthalpy changes in three polymers decreased in the
same order, para (w9.69 J gꢁ1) > meta (w9.24 J gꢁ1) > ortho
(w2.8 J gꢁ1), which is different from the results for MCLCPs and
SCLCPs [22,28]. This suggests that the enthalpy changes may be
dependent on the shape of the side-chains and the flexibility of the
polymers [54,55]. The mechanism for this surprising result will be
separately studied in detail in the near future.
The optical textures shown by the polymers (PPABCS, PMABCS
and POABCS) were observed using PLM. At temperatures below Ti,
three polymers all exhibited strong birefringence, and the PLM
textures clearly indicted the existence of LC phases. Fig. 3 shows
three typical PLM images recorded from PPABCS at 140 ꢀC, PMABCS
at 140 ꢀC, and POABCS at 120 ꢀC, respectively, corresponding to the
three polymers. When three polymers were respectively heated
above their Ti, the isotropic phases were observed.
Fig. 1. 1H NMR spectra of PABCS (a) and PPABCS (b) in CDCl3-d.
respectively. The characteristic resonance peaks of the vinyl sub-
stituent of monomer PABCS can be seen at 5.40e5.37, 5.74e5.69
and 7.40e7.38 ppm, denoted m, n and o, respectively. After poly-
merization, these signals had disappeared completely, and the
chemical shifts of the polymers were quite broad, consistent with
the expected polymer structure.
The monomers and polymers were completely soluble in com-
mon organic solvents such as THF, benzene and chlorobenzene.
GPC analysis was carried out to determine the apparent MW and
molecular weight distributions of the polymers, and the molecular
characterization of the polymers is summarized in Table 1.
To further elucidate the phase structures and transitions more
clearly, WAXD experiments were performed at different tempera-
tures. Fig. 4a represents a set of 1D WAXD patterns of the polymer
PPABCS recorded during heating from 60 ꢀC to 220 ꢀC. As shown in
Fig. 4a, the sample renders a diffuse scattering halo at 2q
of w20ꢀ (d
spacing of w0.44 nm) in the high-angle region at lower tempera-
tures, which turns into a typical amorphous halo when the tem-
perature exceeds 219 ꢀC, indicating that the sample has lose some
3.2. Phase transitions and phase structures
TGA was employed to characterize the thermal stabilities of the
polymers (PPABCS, PMABCS and POABCS). As could be seen from
Table 1
GPC, DSC and TGA results and thermotropic properties of the polymers.
a
Sample
Mn
Mw
Mn
/
Tg
TLC-I
DHLC-I Liquid crystalline Td (N2)
(310ꢁ4
)
(ꢀC)b (ꢀC)b (J/g)b
range D
T (ꢀC)b
(ꢀC)c
a
PPABCS
PMABCS 7.01
POABCS 8.31
7.75
1.65 80
1.72 66
1.62 64
219
188
157
9.69
9.24
2.80
139
122
93
337
353
354
a
Obtained from Waters 1515 instrument, linear PS as standards.
The glass transition temperature (Tg), the liquid crystal transition temperature
b
(TLC-I) and corresponding enthalpy changes (DHLC-I) were evaluated by DSC at a rate
of 10 ꢀC/min under the second heating.
c
Temperature at which the weight loss of the polymers reached 5% was obtained
Fig. 2. DSC curves of the polymers (PPABCS, PMABCS and POABCS) during the first
from TGA under nitrogen [Td(N2)] by TGA heating experiments at a rate of 20 ꢀC/min.
cooling and second heating at a rate of 10 ꢀC minꢁ1
.

C.-a. Yang et al. / Polymer 54 (2013) 3238e3247
3243
Fig. 3. PLM images of (a) PPABCS at 140 ꢀC, (b) PMABCS at 140 ꢀC, and (c) POABCS at 120 ꢀC during heating process (Magnification: ꢂ200).
Fig. 4. 1D WAXD patterns of the samples (a) PPABCS, (b) PMABCS and (c) POABCS during the heating.

3244
C.-a. Yang et al. / Polymer 54 (2013) 3238e3247
degrees of order on the subnanometer scale after the lowest tran-
sition at 80 ꢀC. As indicated by the arrows in Fig. 4a, in the low-
angle region, six diffraction peaks (indexed as peak 1, 2, 3, 4, 5
and 6 in Fig. 4a) at 2.57ꢀ (d spacing of 3.44 nm), 3.39ꢀ (d spacing of
2.61 nm), 4.44ꢀ (d spacing of 1.99 nm), 5.15ꢀ (d spacing of 1.72 nm),
5.79ꢀ (d spacing of 1.53 nm) and 7.71ꢀ (d spacing of 1.14 nm) can be
identified, and the scattering vectors of the six peaks are found to
be in the ratio 1:1.32:1.73:2:2.25:3. When temperature reaches
220 ꢀC, all the low-angle diffractions disappear, indicating that the
sample enters an isotropic state. The low- and high-angle diffrac-
tions at lower temperatures indicate that the polymer PPABCS
possesses two ordered structures on different length scales of
nanometer and sub-nanometer, respectively. We consider that the
PMABCS. However, the diffraction pattern of the polymer POABCS
at 2
of w20ꢀ (d spacing of w0.44 nm) in the high-angle region at
low temperatures is obviously different from those observed from
the polymers PPABCS and PMABCS, suggesting that the new or-
dered structure formed by the packing of azobenzene containing
side-chains.
q
To further identify the supramolecular structures of the poly-
mers PPABCS, PMABCS and POABCS, the 2D WAXD experiments
were carried out. The samples were obtained by mechanical shear
at temperature below the isotropic temperature. Fig. 5a schemat-
ically shows the sheared sample with x- (the shear direction) and
z-direction (shear gradient), respectively. Fig. 5b shows a 2D WAXD
pattern of the oriented PPABCS recorded at room temperature. In
this figure, the X-ray incident beam is aligned parallel to y direc-
tion (See the schematic drawing of Fig. 5a, wherein x and z di-
rections are the shear direction and shear gradient, respectively),
where a pair of the high-angle diffractions, which is mainly
resulted from the interference of side-chains which pack parallel to
each other, is located on the meridian in Fig. 5b. Considering that
the strong mechanical shearing can preferentially align the main-
chain axis parallel to the shear direction (the meridian), this ge-
ometry of high angle diffraction signifies that side-chains are
perpendicular to the main-chain axis. We conclude that the side-
chains form a SmA-like packing at low temperatures. On the
other hand, Three pairs of low-angle diffractions (spots 1, 3, and 5)
high-angle diffraction at 2q
of w20ꢀ should be contributed from the
packing of azobenzene containing side-chains, and the low-angle
diffractions reflect the existence of an ordered structure formed
by the main-chains composed of polyethylene backbone and the
center rigid portion of side-chain.
To further investigate the effects of para-, meta-, or ortho-
monosubstituent on phase transition behaviors of the polymers,
the phase transitions of the polymers PMABCS and POABCS could
also be followed by 1D WAXD experiments. Table 2 lists the
d spacings of the diffractions and smectic interlayer distances
observed from the polymers PPABCS, PMABCS and POABCS,
respectively. Fig. 4b depicts a set of 1D WAXD patterns of the
polymer PMABCS at various temperatures. As shown in Fig. 4b, in
the low-angle region, only three diffraction peaks (indexed as peak
1, 2, and 3 in Fig. 4b) can be observed at below 188 ꢀC, and the
scattering vectors of the three peaks are in the ratio 1.73:2:2.18. At
below 66 ꢀC, peak 2 is rather weak. With the increasing tempera-
ture, the intensity of the peak increases. Above 188 ꢀC, all the low-
angle diffraction peaks disappeared, and scattering could be seen in
at 2
q
of 2.57ꢀ, 5.15ꢀ and 7.71ꢀ appear on the equator, respectively,
of
and two pairs of low-angle diffractions (spots 2 and 4) at 2
q
4.44ꢀ and 5.79ꢀ locate on quadrants, with an angle of 60ꢀ and 45ꢀ
away from the equator, respectively. With the main-chain along
the meridian, the low-angle diffractions located on the equator can
be assigned as (hk0) of the main-chain ordered packing. We may
assign 1, 3, and 5 to be (200), (400) and (600) diffraction, and 2 and
4 to be (310) and (410) diffraction, respectively. Combing the re-
sults shown in Fig. 5b, we may obtain a rectangular lattice, with
a ¼ 6.88 nm and b ¼ 4.00 nm. Such a rectangular lattice has been
observed in the self-assembly of dendrons [56,57], dendronized
polymers [58,59], and MJLCPs [60e62]. The possible supramolec-
ular structure of PPABCS at low temperatures is schematically
drawn in Fig. 6a.
the low 2
q
region, indicating that the sample had become isotropic.
Moreover, a relatively sharp peak at 2
q
of w20ꢀ (d spacing of
w0.44 nm) could be seen in the high-angle region at lower tem-
peratures, which is similar to that of the polymer PPABCS. For the
polymer POABCS, four diffraction peaks (indexed as peak 1, 2, 3, and
4 in Fig. 4c) in the low-angle region can be observed from Fig. 4c
and Table 2 at below 157 ꢀC, and the scattering vectors of the four
peaks are in the ratio 1:1.23:1.87:2, indicating that the ordered
structure formed, which is also similar to those of PPABCS and
Fig. 5c shows the 2D WAXD pattern of the oriented PMABCS at
room temperature with the X-ray incident beam parallel to y di-
rection. Three pairs of low-angle diffractions appear on the equator
and are indexed as (200), (400), and (410), respectively, indicating
that a rectangular lattice, with a ¼ 6.72 nm and b ¼ 3.88 nm, had
formed. The pair of the high-angle diffractions is located on the
meridian in Fig. 5c, indicating that the side-chains of PMABCS form
a SmA-like packing and the diffraction behavior is similar to that of
PPABCS. Fig. 5d depicts a 2D WAXD pattern of the oriented POABCS
sample at room temperature. With the X-ray incident beam is
aligned parallel to z direction, three pair of the low angle diffrac-
tions appeared on the equator, and were indexed as (200), (110),
and (400), respectively, indicating that a rectangular lattice, with
a ¼ 5.54 nm and b ¼ 2.46 nm, had formed. In the high-angle region,
it is surprising, however, that four arcs of w0.44 nm are in the
Table 2
WAXD date for the samples PPABCS, PMABCS and POABCS.
PPABCS (at 140 ꢀC)
d (nm)
hkla
Low-angle regionb
PMABCS (at 140 ꢀC)
POABCS (at 120 ꢀC)
d (nm)
hkla
d (nm)
hkla
3.44
2.61
1.99
1.72
1.53
1.14
200
1.94
1.68
1.54
310
400
410
2.77
2.25
1.48
1.39
200
110
310
400
210
310
400
410
600
High-angle regionc
w0.44 nm
quadrants and the tilting angle was ꢃ60ꢀ (See Fig. 5d,
a
¼ 60ꢀ),
w0.44 nm
w6.72 nm
w0.44 nm
w5.54 nm
which shall mainly come from the interference between the
azobenzene-containing side-chains parallel to each other. There-
fore, the 2D WXRD result indicates a SmC structure [63,64]. The
side view of the packing model of POABCS at low temperatures
deduced from the WAXD results is illustrated in Fig. 6b.
Smectic interlayer distances
w6.88 nm
a
Indices are based on the unit cells proposed.
Below the isotropic temperatures (Ti), the low-angle diffractions came from the
b
centered rectangular scaffold of the main-chain. PPABCS at 140 ꢀC, a ¼ 6.88 nm,
b ¼ 4.00 nm and c ¼ 0; PMABCS at 140 ꢀC, a ¼ 6.72 nm, b ¼ 3.88 nm and c ¼ 0;
POABCS at 120 ꢀC, a ¼ 5.54 nm, b ¼ 2.46 nm and c ¼ 0.
3.3. The effect of para, meta and ortho mesogen monosubstitution
c
Below the isotropic temperatures (Ti), the high-angle diffractions came from the
packing of azobenzene-containing side-chains inside the main-chain scaffold.
PPABCS, SmA-like structure; PMABCS, SmA-like structure; and POABCS, SmC-like
structure.
The results indicate that the mesophase formation of the poly-
mers PPABCS, PMABCS and POABCS is different from those of

C.-a. Yang et al. / Polymer 54 (2013) 3238e3247
3245
Fig. 5. 2D-WAXD patterns of the sheared PPABCS (b), PMABCS (c) and POABCS (d) samples recorded at room temperature. (a) Schematic drawing of the sheared samples with x and
z direction are the shear direction and shear gradient, respectively. The shear direction is on the meridian and the X-ray incident beam is along the (b) y-, (c) y-, and (d) z-direction,
respectively.
MCLCPs and SCLCPs [22,28]. For MCLCPs, the formation of liquid
crystallinity is found strongly dependent on the configuration of
the ortho-, meta-, and para-linkages in monomeric units and that
the para-linkage is most favorable to form liquid crystallinity and
the ortho-linkage has a greater tendency to form liquid crystallinity
than the meta-linkage. Moreover, for SCLCPs, the ortho-linkage does
not form liquid crystallinity, the para- and meta-linkages all exhibit
obvious liquid crystalline phase. However, for the polymers
PPABCS, PMABCS and POABCS, the thick main-chains obtained by
“jacketing” the central rigid portion of side-chain to the poly-
ethylene backbone construct a 2D centered rectangular scaffold
and can form a hierarchically ordered structure with double or-
derings on both the nanometer and subnanometer length scales,
which are MJLCPs.
In comparison, the bond angle between the methoxy-
substituent on the azobenzene moiety and the first benzene ring
of the central rigid portion of side-chain increased in the order
ortho < meta < para. In the case of the ortho-monosubstituted
POABCS, the kinetic mobility of the azobenzene mesogen is very
limited, due the small bond angle, which disturbs the order
packaging of monosubstituted azobenzene moieties. The side-
chains of the polymer POABCS only forms a SmC-like packing.
Moreover, we suggest that the flexible spacer groups connecting
the first benzene ring and the monosubstituted azobenzene
mesogenic unit are also very important in the ordering process.
When the length of the flexible spacer is increased, the ortho-
monosubstituted polymer may provide a wider range of chain
conformations to align the azobenzene mesogens effectively.
However, in the polymers PPABCS and PMABCS, the azobenzene
mesogens are able to achieve the required alignment, since the
bond angle is sufficiently large and the length of the flexible
spacers connecting the first benzene ring and the monosubstituted
azobenzene moiety is now sufficient. The side-chains of the
polymers PPABCS and PMABCS therefore do in fact form SmA-like
packing.
It has been noted from Fig. 2 and Table 1 that the glass transition
temperature (Tg), isotropic temperature (Ti), liquid crystalline range
(DT, from Tg to Ti) and enthalpy change of the polymers PPABCS,
PMABCS and POABCS decrease in the order, para > meta > ortho. It
is very interesting phenomenon that the associated enthalpy
changes of these polymers are also the same order, para > meta >
ortho, which is different from those of MCLCPs and SCLCPs. We
propose that mesophase formation in mesogen-jacketed liquid
crystalline polymers containing monosubstituted azobenzene
moieties is strongly dependent on the position of the substituent on
the azobenzene moiety. Moreover, mesogen-jacketed liquid crys-
talline polymers containing para-monosubstituted azobenzene
moieties have an enhanced shape anisotropy compared with those
with in the meta- and ortho-position, and thus show greater sta-
bilisation of the mesophases.

3246
C.-a. Yang et al. / Polymer 54 (2013) 3238e3247
temperature (Tg), isotropic temperature (Ti) and liquid crystalline
range ( T, from Tg to Ti) of the polymers decrease in the order,
D
para > meta > ortho. It is very interesting phenomenon that the
associated enthalpy changes of these polymers are also the same
order, para > meta > ortho.
Acknowledgment
This work is supported by the National Natural Science Foun-
dation of China (NNSFC Grants: 21106037), the Scientific Research
Fund of the Hunan Provincial Education Department (No: 11A040),
Hunan Provincial Natural Science Foundation of China (No:
12JJ4015) and the Scientific Research Project of Hunan Institute of
Science and Technology (No: 2011Y04).
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This investigation has revealed that the formation of liquid
crystalline phases of mesogen-jacketed liquid crystalline polymers
containing monosubstituted azobenzene moiety depends strongly
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