Synthesis and self-assembly behaviours of side-chain smectic thiol-ene polymers based on the polysiloxane backbone

Article abstract of DOI:10.1039/c5tc04331j

A series of polysiloxane side chain liquid crystal polymers (PSCLCPs) with chiral and achiral substitutions in the side chains, denoted as PMMS-X_chol-n (n = 0, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5 0.6, 0.7. 0.8, 0.9, and 1.0, respectively, the molar content of the chiral cholesteric unit (X_chol) in a specific polymer), were successfully synthesized via thiol-ene click chemistry. The molecular structures of the polymers were confirmed by ¹H-NMR, FT-IR, gel permeation chromatography (GPC) and thermogravimetric analysis (TGA). Their liquid crystalline (LC) properties and self-assembling behaviors were investigated in detail by a combination of various techniques, such as differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction. The results demonstrated that the phase transition behaviour and the self-assembly structure of the polymers were significantly influenced by X_chol and temperatures. With increased X_chol, the clearing points increased significantly, their mesogenic temperature ranges greatly widened, and abundant mesophases developed. Generally, two different types of LC phase structures and three different molecular arrangements were observed, depending on the two LC building blocks. Polymers with X_chol below 0.3 could self-assemble into a smectic E (SmE)-like structure and a single layer smectic A (SmA_s) structure upon heating. When X_chol was between 0.4 and 0.7, a single phase structure of a SmA_s or a bilayer smectic A (SmA_d) could be observed. While for polymers with X_chol over 0.8, a SmA_d phase structure was self-organized, further heating led to a SmA_s structure. Moreover, when the molar ratio of the chiral group or achiral group was about 0.1, a microphase-separated smectic morphology could be found, indicating that the introduction of a small amount of any components in the copolymers might destroy the well-ordered structures.

Full text of DOI:10.1039/c5tc04331j

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Synthesis and self-assembly behaviours of
side-chain smectic thiol–ene polymers based on
Cite this:DOI: 10.1039/c5tc04331j
the polysiloxane backbone†
Wenhuan Yao,‡a Yanzi Gao,‡b Xiao Yuan,b Baofeng He,b Haifeng Yu,bc
Lanying Zhang,*bc Zhihao Shen,*cd Wanli He,*a Zhou Yang,*a Huai Yang*abc and
Dengke Yange
A series of polysiloxane side chain liquid crystal polymers (PSCLCPs) with chiral and achiral substitutions in the
side chains, denoted as PMMS-Xchol-n (n = 0, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5 0.6, 0.7. 0.8, 0.9, and 1.0, respectively,
the molar content of the chiral cholesteric unit (Xchol) in a specific polymer), were successfully synthesized via
thiol–ene click chemistry. The molecular structures of the polymers were confirmed by 1H-NMR, FT-IR, gel
permeation chromatography (GPC) and thermogravimetric analysis (TGA). Their liquid crystalline (LC) properties
and self-assembling behaviors were investigated in detail by a combination of various techniques, such as
differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction. The results
demonstrated that the phase transition behaviour and the self-assembly structure of the polymers were
significantly influenced by Xchol and temperatures. With increased Xchol, the clearing points increased
significantly, their mesogenic temperature ranges greatly widened, and abundant mesophases developed.
Generally, two different types of LC phase structures and three different molecular arrangements were
observed, depending on the two LC building blocks. Polymers with Xchol below 0.3 could self-assemble into a
smectic E (SmE)-like structure and a single layer smectic A (SmAs) structure upon heating. When Xchol was
between 0.4 and 0.7, a single phase structure of a SmAs or a bilayer smectic A (SmAd) could be observed.
While for polymers with Xchol over 0.8, a SmAd phase structure was self-organized, further heating led to a
SmAs structure. Moreover, when the molar ratio of the chiral group or achiral group was about 0.1, a
microphase-separated smectic morphology could be found, indicating that the introduction of a small amount
of any components in the copolymers might destroy the well-ordered structures.
Received 20th December 2015,
Accepted 28th December 2015
DOI: 10.1039/c5tc04331j
www.rsc.org/MaterialsC
photoactive materials, electro-optic or nonlinear optic materials,
high-strength and high-modulus fibers, engineering plastics or
1. Introduction
Side Chain Liquid Crystal Polymers (SCLCPs), which combine the other functional materials, have drawn the scientists’ extensive
anisotropy of liquid crystalline mesogens with the mechanical attention for the past few decades.1–4 Polysiloxane Side Chain
performance of polymers showing wide potential applications as Liquid Crystal Polymers (PSCLCPs), distinguishing themselves
from traditional polyacrylate based liquid crystal polymers with
the characteristics of much lower glass transition temperature
(Tg), viscosities, surface energy, and good mechanical and thermal
stabilities,5 have received considerable interest and have been
applied in various fields ranging from optics, electrics to coatings.6–9
Generally, PSCLCPs can be obtained by traditional hydro-
silylation reaction between polymethylhydrosiloxane (PMHS)
and unsaturated carbon–carbon bond based monomers such as
alkene monomers or alkyne monomers, or by grafting mesogenic
monomers to poly[3-mercaptopropylmethylsiloxane] (PMMS) via
thiol–ene click chemistry. Since the first publication of PSCLCPs
in 1979,10 following the traditional hydrosilylation reaction which
was invented by Finkelmann, a large amount of PSCLCPs with
different structures and unique properties has been designed,
a Department of Materials Physics and Chemistry, School of Materials Science and
Engineering, University of Science and Technology Beijing, Beijing 100083,
P. R. China. E-mail: yanghuai@pku.edu.cn, yangz@ustb.edu.cn,
hewanli@mater.ustb.edu.cn
b Department of Materials Science and Engineering, College of Engineering,
Peking University, Beijing 100871, P. R. China. E-mail: zhanglanying@pku.edu.cn
c Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
Peking University, Beijing 100871, P. R. China
d Beijing National Laboratory for Molecular Sciences, Center for Soft Matter Science
and Engineering, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, P. R. China. E-mail: zshen@pku.edu.cn
e Chemical Physics Interdisciplinary Program, Liquid Crystal Insititute,
Kent State University, Kent, Ohio 44241, USA
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5tc04331j
‡ Joint first author, these authors contributed equally.
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synthesized, and characterized in detail. Furthermore, the relation- 0.3, 0.4, 0.5 0.6, 0.7. 0.8, 0.9, to 1.0 mol%, respectively, was
ships between their molecular structures and phase transition successfully synthesized and characterized. The resultant PSCLCPs
behaviours as well as phase structures have also been studied are therefore a model system to investigate the relationship
systematically. Percec et al.,11–15 Smith et al.16 and Gasparoux between a single composite change of Xchol and the variable in
et al.17 synthesized a series of end-on (mesogens are terminally the self-assembly behaviours. Furthermore, different from the work
attached) PSCLCPs bearing methoxy groups, biphenyl groups, with PMHS-based copolymers and LCEs reported previously in our
terphenyl groups and cyano groups, and found that the meso- group,50–52 we anticipate that the longer flexible spacer in PMMS
morphic and thermodynamic properties of these polymers were may offer the polymers more abundant LC phase structures,
dramatically influenced by the increasing content of benzene. interesting properties, and potential applications.
Zhang et al.18–20 studied a series of end-on PSCLCPs containing
methyl groups and found the similar evolution tendency as
described in Percec’s work. Yang et al. synthesized a series of
2. Experimental
2.1 Materials and methods
PSCLCPs by using alkyne hydrosilylation.21 Tschirner et al.22 and
Ringsdorf et al.23 synthesized a series of side-on (mesogens are
laterally attached) PSCLCPs which formed nematic phases or
2.1.1 Materials.
Poly[3-mercaptopropylmethylsiloxane]
smectic phases. Zhou et al.24 prepared a series of mesogen- (PMMS, SMS-992, M.W. 4000–7000, 95 cst, Gelest Inc.), ethyl-
jacketed liquid crystal polymers (MJLCPs) with a polysiloxane paraben (A.R. grade, Sinopharm), cholesterol (A.R. grade, Sino-
backbone and found that the mesogen-jacketed effect could still pharm), acetonitrile (A.R. grade, Sinopharm), 3-bromopropene
force the polymers to self-organize into supramolecular columnar (98%, Beijing Dominant Technology Co.), dimethylaminopyridine
nematic or smectic liquid crystalline phases. However, for the (DMAP) (99%, Energy Chemical), N,N0-dicyclohexylcarbodiimide
method of traditional hydrosilylation, the purification difficulty (DCC) (98%, Energy Chemical), 4-methoxyphenol (A.R. grade,
derived from the usage of expensive noble metal platinum Beijing Dominant Technology Co.), potassium hydroxide (A.R.
catalysts, the complexity of the chemical structures and performance grade, Beijing Chemical Reagents Co.), and anhydrous potassium
of the products attributing to the coexistence of Markovnikov carbonate (A.R. grade, Beijing Chemical Reagents Co.) were used
and anti-Markovnikov addition reaction have limited their wide without any further purification. Toluene (A.R. grade, Beijing
applications.25
Chemical Reagents Co.) was refluxed over sodium and distilled
The thiol–ene addition reaction, namely hydrothiolation of a under a nitrogen atmosphere before use. Azodiisobutyronitrile
CQC bond, as a representative click reaction with features of (AIBN) (A.R. grade, Beijing Dominant Technology Co.) was purified
high efficiency, strong stereo-selectivity, simple reaction conditions by recrystallization from ethanol. Other chemical reagents were
with no side products, etc., has been well known since 1905.26 Since used as received.
1
then, it has sparked an enormous amount of interest in the
2.1.2 Methods. All H-NMR spectra were collected using a
development of organic synthetic chemistry, polymer synthetic Bruker HW400 MHz spectrometer (ADVANCE III-400) using
chemistry, biological chemistry, and materials science. By virtue of deuterated chloroform (CDCl3) or dimethylsulfoxide (DMSO)
the thiol–ene reaction, functional polymer materials with linear,27–29 with tetramethylsilane (TMS) as the internal standard at room
crosslinked,30–32 or net-worked33,34 structures were designed and temperature.
synthesized. Reviews about the mechanism of thiol–ene click chem-
The FT-IR spectra were recorded on a PerkinElmer spectrum
istry and various applications in polymers and materials synthesis 100 spectrophotometer by the KBr method.
were also reasonably elaborated.35,36 Recently, Yang et al. prepared
A PerkinElmer DSC8000 with a mechanical refrigerator was
a series of main chain liquid crystal polymers (MCLCPs),37,38 used to obtain the phase transition of the monomers and
SCLCPs,39–41 and liquid crystal elastomers (LCEs)42,43 by thiol–ene polymers under dry nitrogen at a heating and a cooling rate
click chemistry, in which only anti-Markovnikov addition products of 10 1C minꢀ1; the temperature and heat flow scale were
were obtained. The self-assembly properties of the SCLCPs as well calibrated using zinc and indium as standard.
as the effect of the molecular structure of the mesogen on the
mesophase were also investigated.
Thermogravimetric analysis (TGA) was performed on a TA
Q600 at a heating rate of 20 1C minꢀ1 under nitrogen.
Gel permeation chromatography (GPC) was performed on a
In this paper, inspired by the accurate control of the molar
ratio between the thiol group and the mesogenic monomer as Waters 2410 instrument equipped with a Waters 2410 refractive
well as the region-selectivity of thiol–ene click chemistry, and index detector and three waters m-styragel columns (103, 104, and
the outstanding performances such as electro-optics,44,45,49 105 Å). The column packing allowed the separation of polymers
piezoelectricity,46 ferroelectricity47,48 and pyroelectricity49 over a wide molecular weight range of 2200–600 000. All GPC
derived from the symmetry breaking by the introduction of data were gathered by using tetrahydrofuran (THF) (HPLC grade,
the molecular chirality to MCLCPs/PSCLCPs, the first example Fisher Scientific) as the eluent at a flow rate of 1.0 ml minꢀ1 at
of PSCLCPs based on the PMMS main chain with different 35 1C and calibrated using polystyrene standards.
compositions of chiral and achiral mesogenic groups has been
Polarized optical microscopy (POM) was carried out to
presented. A battery of PSCLCPs (PMMS-Xchol-n, n means the observe the liquid crystal textures of the samples using a Carl
molar ratio in a specific polymer) with the molar content of the Zeiss Axio Vision SE64 polarized optical microscope with a
chiral cholesteric group (Xchol) ranging from 0, 0.1, 0.15, 0.2, Linkam LTS420 hot stage.
J. Mater. Chem. C
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Paper
One-dimensional wide-angle X-ray diffraction (1D-WAXD),
2.2.2 Synthesis of cholesteryl-4-(allyloxy) benzoate (M1). A
Small Angle X-ray scattering (SAXS), and Two-dimensional wide- solution of N,N0-dicyclohexylcarbodiimide (DCC) (12.36 g, 0.06 mol)
angle X-ray diffraction (2D-WAXD) were used to study the self- in dichloromethane was added drop-wise slowly to a mixture of
assembly structures and transitions of the binary copolymers. All 4-(allyloxy) benzoic acid (8.40 g, 0.05 mol), cholesterol (11.60 g,
the background patterns were collected and subtracted from the 0.03 mol), dimethylaminopyridine (DMAP) (0.92 g, 7.50 mmol), and
sample patterns.
200 ml of dichloromethane, and then stirred at room temperature
1D-WAXD experiments were performed on a Philips X’Pert for 24 h. After filtration and removal of dichloromethane, the
Pro diffractometer with a 3 kW ceramic tube as the X-ray source concentrated solution was purified by silica gel column chromato-
(Cu Ka and the wavelength l is 0.1542 nm), an X’celerator detector, graphy with dichloromethane as the eluent to give M1 as a white
and a Paar Physica YCU 100 hot stage in the reflection mode at 40 kV crystal. Yield: 91%. FT-IR (KBr, cmꢀ1): 2946–2884 (–CH3, –CH2–,
and 40 mA and protected by a nitrogen atmosphere. The heating H2CQ, andQC–H), 1712 (CQO), 1648 (CQC), 1606, 1580, 1509,
and cooling rates in all 1D-WAXD experiments were 10 1C minꢀ1
.
1464 (Ar–), 1274, 1249 (C–O–C). 1H-NMR (400 MHz, CDCl3, TMS, d,
The samples were prepared by dissolving about 30 mg of powder ppm): 8.00–7.98 (2H, dt, Ar–H), 6.93–6.91 (2H, dt, Ar–H), 6.09–6.02
ꢀ
ꢀ
samples in tetrahydrofuran (THF, B1 ml) and drop-casting into a (1H, m, CH2QCH–CH2–), 5.45–5.42 (1H, dq, one of CH2QCH–
ꢀ
ꢀ
film on a monocrystalline silicon substrate.
CH2–), 5.40 (1H, t,QCH– in the cholesteryl moiety), 5.33–5.30 (1H,
ꢀ
SAXS experiments were carried out on an Anton Paar SAXSess dq, one of CH2QCH–CH2–), 4.84–4.80 (1H, m, –O–CH– in the
high-flux small-angle X-ray scattering instrument equipped with a cholesteryl moiety), 4.60–4.59 (2H, d, CH2QCH–CH2–), 2.46–2.44
ꢀ
ꢀ
ꢀ
Kratky block-collimation system and a Philips PW3830 sealed-tube (2H, d, –O–CH–CH2–C– in the cholesteryl moiety), 2.04–0.86 (38H,
ꢀ
X-ray generator with the Cu target (Cu Ka and the wavelength l is m, H in the cholesteryl moiety), 0.69 (3H, s, –CH3 in the cholesteryl
ꢀ
ꢀ
0.1542 nm) in the transmission mode at 40 kV and 40 mA. The moiety).
samples were sandwiched with about 3–5 mg of powder samples
between aluminium films.
2.2.3 Synthesis of 4-methoxyphenyl 4-(allyloxy) benzoate
(M2). The synthesis of M2 used the same method as described
2D-WAXD experiments were executed on a Bruker D8 Discover for the preparation of M1. FT-IR (KBr, cmꢀ1): 3081–2836 (–CH3,
diffractometer with a 2D detector of GADDS in the transmission –CH2–, H2CQ, and QC–H), 1733 (CQO), 1648 (CQC), 1604,
mode at 40 kV and 40 mA. The samples were oriented by mild 1579, 1502, 1460 (Ar–), 1269, 1246 (C–O–C). 1H-NMR (400 MHz,
mechanical shearing at an appropriate temperature of the liquid CDCl3, TMS, d, ppm): 8.16–8.13 (2H, dt, Ar–H), 7.13–7.10 (2H,
ꢀ
crystal phase and the point-focused X-ray beam was aligned dt, Ar–H), 7.01–6.99 (2H, dt, Ar–H), 6.95–6.92 (2H, dt, Ar–H),
ꢀ
ꢀ
ꢀ
perpendicular to the shear direction.
6.11–6.03 (1H, m, CH2QCH–CH2–), 5.47–5.43 (1H, dq, one of
ꢀ
An unpolarized UV/VIS/IR spectrophotometer (Perkin-Elmer CH2QCH–CH2–), 5.35–5.32 (1H, dq, one of CH2QCH–CH2–),
Lambda 950) was used for the spectral characterization in 4.64–4.62 (2H, dt, CH2QCH–CH2–), 3.82 (3H, s, –OCH3).
ꢀ
ꢀ
ꢀ
ꢀ
transmission mode at normal incidence, or in reflection mode
with a reflectance accessory (150 nm Int. Sphere).
2.2.4 Synthesis of polymers. All the polymers were prepared
according to the synthetic routes similar to those reported in the
literature with minor modifications.40 The preparation of PMMS-
Xchol-0.50 (0.50 means Xchol in the polymer) was taken as a typical
procedure. M1 (302.00 mg, 0.55 mmol), M2 (156.30 mg,
2.2 Synthetic procedures
2.2.1 Synthesis of 4-(allyloxy) benzoic acid. 3-Bromopropene 0.55 mmol), PMMS (137.60 mg, 1.00 mmol –SH), AIBN (16.40 mg,
(7.26 g, 0.06 mol) was added drop-wise to a mixture of ethyl 0.10 mmol), and toluene (2.20 ml) were transferred into a polymer-
4-hydroxybenzoate (8.30 g, 0.05 mol), anhydrous potassium ization tube. The polymerization tube was sealed under the condi-
carbonate (6.90 g, 0.05 mol), and 150 ml of acetonitrile at room tions of vacuum after degassing and exchanging with nitrogen via
temperature and refluxed under magnetic stirring for 10 h. After three freeze–thaw cycles. The reaction was carried out at 65 1C for
filtration and removal of solvent, the buff liquid was obtained as 24 h. Then the coarse polymer was precipitated by pouring the
the intermediate product.
reaction mixture into methanol, and further purified by dissolving in
Then the buff liquid and potassium hydroxide (8.40 g, 0.15 mol) chloroform, precipitating from methanol several times in order to
were dissolved in the ethanol/water (1/5) mixture and refluxed remove the excessive unreacted monomers and drying in a vacuum.
at 105 1C for 5 h. After removal of ethanol, the unhydrolyzed A white powder polymer was obtained. Yield: 91%.
reactants were extracted three times with ether. The aqueous
phase was acidified to pH 2.0 with hydrochloric acid. After
filtration, the residue was recrystallized from ethanol to give
3. Results and discussion
3.1 Synthesis and characterization of monomers and
4-(allyloxy) benzoic acid as a white needle crystal. Yield: 93%.
FT-IR (KBr, cmꢀ1): 2985–2825 (–CH2–, H2CQ, and QC–H),
polymers
2668–2554 (–OH in –COOH), 1680 (CQO), 1648 (CQC), 1602,
1575, 1503, 1425 (Ar–), 1251 (C–O–C). 1H-NMR (400 MHz, As shown in Scheme 1, the monomers were synthesized successfully
DMSO, TMS, d, ppm): 12.61 (1H, s, –COOH), 7.88–7.85 (2H, d, and efficiently by starting from the Williams etherification reaction
ꢀ
Ar–H), 7.02–7.0 (2H, d, Ar–H), 6.08–5.98 (1H, m, CH2QCH– and ending up with Steglich esterification reaction. All the polymers
ꢀ
ꢀ
ꢀ
CH2–), 5.41–5.36 (1H, dd, one of CH2QCH–CH2–), 5.28–5.25 were obtained with high yields over 87% by the thiol–ene addition
ꢀ
(1H, dd, one of CH2QCH–CH2–), 4.63–4.62 (2H, d, CH2QCH–CH2–). reaction using AIBN as the initiator. The chemical structures of
ꢀ
ꢀ
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from methylene (–CH2–), ester carbonyl (CQO), aromatic (Ar–),
Si–C, and Si–O–Si, respectively, indicated the successful thiol–ene
addition reaction and complete removal of the unreacted mono-
1
mers. Furthermore, in the H-NMR spectra shown in Fig. 2, the
complete disappearance of the representative signals of the vinyl
group in monomers at 6.05, 5.43, and 5.31 ppm, the signal of the
thiol group in PMMS at 1.37 ppm in the spectra of all the polymers,
and the appearance of the proton peak of the aromatic group
confirmed the success of the reaction and high purity of the
products once again. Additionally, a reciprocal relationship between
the integral area of the signals at 4.82 ppm (–O–CH– in the
ꢀ
cholesteryl moiety) in M1 and 3.82 ppm (–OCH3) in M2 (as remarked
ꢀ
by red dotted boxes in Fig. 2) as well as the vibrational band of CQO
at about 1711 cmꢀ1 in M1 and 1737 cmꢀ1 in M2 (as remarked in
Fig. 1b) can be observed and the actual molar fraction of M1 based
on (M1 + M2) in a specified polymer was calculated and is
summarized in Table 1, which was relatively large compared to that
in the feed, indicating that the substituted vinyl monomer M1 was
more reactive than M2 although the substituent of the former was
much larger and more rigid than that of the latter.53,54
As listed in Table 1 and Fig. S1 (ESI†), GPC measurements
%
showed that the number-average molecular weight (Mn) of
polymers decreased with the increasing content of M2, and the
1
%
calculated Mn according to the H-NMR results also exhibited a
similar variation tendency, which was in accordance with that
the molecular weight of M1 is greater than that of M2.
3.2 Thermal and liquid-crystalline properties of polymers
DSC, TGA, and POM were used to investigate the thermal and
liquid-crystalline properties of the homopolymers and copoly-
mers. As shown in Table 1 and Fig. S2 (ESI†), all the polymers
have good thermal stabilities with the temperatures at 5%
weight loss above 310 1C in nitrogen. And the thermal stability
decreases slightly with the increasing content of the chiral
cholesteric monomer M1, indicating that the thermal stability
of the benzene ring is superior to the cholesteryl moiety.
DSC experiments were carried out to investigate the phase
transition behaviours of the homopolymers and the copoly-
mers. Considering the influence of the thermal history, the
traces during the first cooling and subsequent second heating
were recorded at a rate of 10 1C minꢀ1 under nitrogen as shown
Scheme 1 Synthetic route of the monomers and polymers.
precursors, monomers, and target polymers were confirmed by
1
FT-IR and H-NMR spectroscopic methods.
1
Fig. 1 and 2 show the FT-IR and H-NMR spectra of chiral
M1, achiral M2, PMMS, and all the polymers, respectively. As
shown in Fig. 1a and b, the disappearance of the weak vibrational
bands at 2552 cmꢀ1 (assigned as the S–H stretching) and 1648 cmꢀ1
(assigned as the CQC stretching), and the appearance of the
vibrational bands at 2965–2836, 1737/1717, 1606–1464, 1260,
and 1071–1018 cmꢀ1, which were attributed to the vibrations
Fig. 1 FT-IR spectra of monomers, PMMS, and all the polymers.
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