Influence of shorter backbone and cholesteric monomer percentage on the phase structures and thermal-optical properties of linear siloxane tetramers containing cholesterol and benzene methyl ether groups

Article abstract of DOI:10.1039/c6ra17692e

A series of new linear polysiloxane tetramers containing cholesterol and benzene methyl ether groups as side chains was successfully synthesized via a typical hydrosilylation reaction. Their chemical structures were confirmed by ¹H-NMR, FT-IR and thermogravimetric analysis (TGA). A combination of analysis methods such as differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction was carried out to systematically investigate their phase transition behaviours and phase structures. The results revealed that the mesophase structures of all the polymers were greatly dependent on the molar content of the chiral cholesteric unit (X_chol). Polymers with X_chol below 0.60 or over 0.70 could develop a stable cholesteric phase or smectic A phase, while both smectic A and cholesteric phases could be formed for polymers with X_chol between 0.60 and 0.70. Besides, polymers with X_chol below 0.60, which developed a cholesteric phase with sufficiently wide temperature ranges in the entire mesomorphic state (minimum over 50 °C and maximum over 130 °C), could also exhibit tunable selective reflection wavelengths between 470-560 nm upon heating and arbitrarily regulated glass transition temperatures between 0-40 °C. As a result, a thermochromism liquid crystalline material for temperature sensors was successfully obtained. Surprisingly, different from the results previously reported, an unusual phenomenon of the centre selective light reflection wavelength (λ_m) vs. X_chol, which decreased dramatically (blue shift) at first, then had a remarkable red shift with increasing X_chol, was first observed.

Full text of DOI:10.1039/c6ra17692e

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PAPER
Influence of shorter backbone and cholesteric
monomer percentage on the phase structures and
thermal-optical properties of linear siloxane
tetramers containing cholesterol and benzene
methyl ether groups
Cite this: RSC Adv., 2016, 6, 87502
a
b
c
bd
a
*
*
Zhou Yang
Wenhuan Yao,† Yanzi Gao,† Fasheng Li, Lanying Zhang,
and Huai Yang
abd
*
A series of new linear polysiloxane tetramers containing cholesterol and benzene methyl ether groups as side
chains was successfully synthesized via a typical hydrosilylation reaction. Their chemical structures were
confirmed by 1H-NMR, FT-IR and thermogravimetric analysis (TGA). A combination of analysis methods
such as differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction
was carried out to systematically investigate their phase transition behaviours and phase structures. The
results revealed that the mesophase structures of all the polymers were greatly dependent on the molar
content of the chiral cholesteric unit (Xchol). Polymers with Xchol below 0.60 or over 0.70 could develop
a stable cholesteric phase or smectic A phase, while both smectic A and cholesteric phases could be
formed for polymers with Xchol between 0.60 and 0.70. Besides, polymers with Xchol below 0.60, which
developed a cholesteric phase with sufficiently wide temperature ranges in the entire mesomorphic state
(minimum over 50 ꢀC and maximum over 130 ꢀC), could also exhibit tunable selective reflection
wavelengths between 470–560 nm upon heating and arbitrarily regulated glass transition temperatures
between 0–40 ꢀC. As a result, a thermochromism liquid crystalline material for temperature sensors was
successfully obtained. Surprisingly, different from the results previously reported, an unusual phenomenon
of the centre selective light reflection wavelength (lm) vs. Xchol, which decreased dramatically (blue shift)
at first, then had a remarkable red shift with increasing Xchol, was first observed.
Received 11th July 2016
Accepted 25th August 2016
DOI: 10.1039/c6ra17692e
www.rsc.org/advances
laterally attached and end-on SCLCPs with mesogens terminally
attached. Usually, the most studied polymer backbones of
SCLCPs were polysiloxane,5–7 polyacrylate,8,9 polymethacrylate,10,11
1. Introduction
Side chain liquid crystalline polymers (SCLCPs), which combine
the electro-optical properties of low molecular mass weight liquid
crystals with excellent mechanical properties and easy processing
characteristics of polymers, were one of the most fantastic
models of so matter and have received considerable interest
and extensive research in polymer chemistry and physics for both
fundamental and applied reasons in the past few decades.1–4
According to the different attachment strategies, SCLCPs can be
divided into two main categories: side-on SCLCPs with mesogens
polyacetylene,12,13 polynorbornene14,15 and so on.
In the eld of basic and applied research, the study of molec-
ular design and the structure–properties–performance relation-
ship of SCLCPs is one of the most fundamental and important
parts. There is no doubt that the properties of SCLCPs can be
regulated by changing of molecular structures of the polymer
backbone and mesogenic groups. Previous work has also revealed
that phase transition behaviours and phase structures of SCLCPs
can be drastically altered by small changes such as the length of
exible spacers, the size and architecture of the terminal exible
substituents in mesogenic core, the linkage groups between the
rigid core in the mesogenic side chain, and so on. For example,
Yang's work16,17 has revealed that the increased exible spacer
length decreased the mesophase range along with the improved
glassy transition temperatures. Imrie et al.18 and Yonetake et al.19
found that the increased exible spacer length could also induce
SCLCPs to achieve more ordered liquid crystalline (LC) phases. By
adjusting the length as well as the architecture and size of the
aDepartment 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
bDepartment of Materials Science and Engineering, College of Engineering, Peking
University, Beijing 100871, P. R. China. E-mail: zhanglanying@pku.edu.cn
cCollege of Medical Laboratory, Dalian Medical University, Dalian 116044, Liaoning
Province, P. R. China
dKey Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking
University, Beijing 100871, P. R. China
† Joint rst author, these authors contributed equally.
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terminal tail in mesogenic core, Zhou et al.20–22 and Wan et al.23–25
The FT-IR spectra were recorded on a PerkinElmer spectrum
achieved the articial regulation of phase structures and phase 100 spectrophotometer by the KBr method.
behaviours. Additionally, Zhou et al.26–29 and Zhang et al.30–32 re-
A PerkinElmer DSC8000 with a mechanical refrigerator was
ported that the phase transition temperatures and LC properties used to obtain the phase transitions of the monomers and
were signicantly inuenced by changing the linkage groups polymers under dry nitrogen at a heating and a cooling rate of
between the rigid mesogenic core. Furthermore, Percec33 and other 10 ꢀC minꢁ1, the temperature and heat ow scale were cali-
researchers34–36 found that the mesophase thermal stability was brated by zinc and indium as standard.
strongly affected by the exibility of the polymer backbone.
In our previous work,37 we have synthesized and characterized Q600 at a heating rate of 20 C min under nitrogen.
a series of SCLCPs based on poly[3-mercaptopropylmethylsiloxane] Polarized optical microscope (POM) was carried out to observe
Thermogravimetric analysis (TGA) was performed on a TA
ꢁ1
ꢀ
(PMMS) backbone with chiral cholesterol group and achiral the liquid crystal textures of the samples on a Carl Zeiss Axio Vision
benzene methyl ether group as the side chains. Contributing to the SE64 polarized optical microscope with a Linkam LTS420 hot stage.
increased length of the polysiloxane backbone and spacer as well
One-dimensional wide-angle X-ray diffraction (1D-WAXD),
as the cooperative interaction between them, a more ordered LC small angle X-ray scattering (SAXS), and two-dimensional
phase of smectic E (SmE) developed spontaneously at low wide-angle X-ray diffraction (2D-WAXD) were used to study the
temperatures for polymers with chiral cholesteric unit (Xchol) below self-assembly structures and transitions of the binary copoly-
0.3 and a strong dependence of LC phase structures on the molar mers. All the background patterns were collected and sub-
content of the chiral group was observed. Additionally, based on tracted from the sample patterns.
cyclic siloxane backbone with chiral cholesterol group and achiral
1D-WAXD experiments were performed on a Philips X'Pert Pro
biphenyl ester group as the side chains,16 a series of tetramers was diffractometer with a 3 kW ceramic tube as the X-ray source (Cu
designed and synthesized. Results revealed that owing to the Ka and the wavelength l is 0.1542 nm), an X'celerator detector,
increased rigidity of the cyclic backbone, abundant LC phase and a Paar Physica YCU 100 hot stage in the reect mode at 40 kV
structures with blue phase (BP) and relatively high glass transition and 40 mA and protected by nitrogen atmosphere. The heatiꢁn1g
temperatures were discovered. Here in the present work, we re- and cooling rates in all 1D-WAXD experiments were 10 ꢀC min
.
ported the synthesis, characterization, and properties of a series of The samples were prepared by dissolving about 30 mg powder
SCLCPs based on linear polysiloxane backbone with chiral samples in tetrahydrofuran (THF, ꢂ1 mL) and drop-casting into
cholesterol group and achiral benzene methyl ether group as the a lm on a monocrystalline silicon substrate.
side chains. We anticipate that by choice of the linear polysiloxane
SAXS experiments were carried out on an Anton Paar SAX-
backbone with relatively smaller rigidity and lower degree of Sess high-ux small-angle X-ray scattering instrument equipped
polymerization, the resulting SCLCPs can develop stable chole- with Kratky block-collimation system and a Philips PW3830
steric LC phase over 50 ꢀC with a combination of adjustable glass sealed-tube X-ray generator with Cu target (Cu Ka and the
transition temperature between 0–40 ꢀC and sensitive thermo- wavelength l is 0.1542 nm) in the transmission mode at 40 kV
chromism characteristics, thus a series of LC materials with and 40 mA. The samples were sandwiched about 3–5 mg powder
fascinating and practical applications can be obtained.
samples between aluminium lms.
2D-WAXD experiments were executed on a Bruker D8
Discover. An unpolarized UV/VIS/IR spectrophotometer (Perkin-
Elmer Lambda 950) was performed for the spectral character-
ization in diffractometer with a 2D detector of GADDS in the
transmission mode at 40 kV and 40 mA. The samples were
oriented by mild mechanical shearing at an appropriate
temperature of the liquid crystal phase and the point-focused X-
ray beam was aligned perpendicular to the shear direction.
An unpolarized UV/VIS/IR spectrophotometer (Perkin-Elmer
Lambda 950) was performed for the spectral characterization in
transmission mode at normal incidence. The samples were
parallel oriented in a cell.
2. Experimental section
2.1 Materials
ꢀ
Polymethylhydrosiloxane (PMHS, Mn ¼ 390, Sigma-Aldrich) eth-
ylparaben (A.R. grade, Sinopharm), cholesterol (A.R. grade,
Sinopharm), acetonitrile (A.R. grade, Sinopharm), 3-bromopro-
pene (98%, Beijing Dominant Technology Co.), dimethylamino-
pyridine (DMAP) (99%, Energy Chemical), N,N0-dicyclohexyl-
carbodiimide (DCC) (98%, Energy Chemical), 4-methoxyphenol
(A.R. grade, Beijing Dominant Technology Co.), anhydrous
potassium carbonate (A.R. grade, Beijing Chemical Reagents Co.),
H2PtCl6$6H2O (TCI) was used without any further purication.
Toluene (A.R. grade, Beijing Chemical Reagents Co.) was reuxed
over sodium and distilled under nitrogen atmosphere before use.
Other chemical reagents were used as received.
2.3 Synthesis of the precursors and monomers
The synthetic routes of the precursors and monomers are
shown in Scheme 1, and the experimental details are in accor-
dance with the literature.37
2.2 Measurements
2.4 Synthesis of polymers
All 1H-NMR spectra were collected by a Bruker HW400 MHz
spectrometer (ADVANCE III-400) using deuterated chloroform All the polymers were prepared according to the synthetic routs
(CDCl3) or dimethylsulfoxide (DMSO) with tetramethylsilane similar to those reported in the literature with minor modi-
(TMS) as the internal standard at room temperature.
cation.38 Taking the preparation of PS4-Xchol-0.50 (0.50 means
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etherication reaction and ending up with Steglich esterica-
tion reaction. All the target polymers were synthesized by the
traditional hydrosilylation reaction of PMHS with cholesterol
derivates and methoxyphenyl derivates using H2PtCl6$6H2O as
catalyst. The structures of precursor, monomers, and target
1
polymers were conrmed by H-NMR and FT-IR spectroscopic
methods.
With PS4-Xchol-0.50 as an example, FT-IR spectra of M1, M2,
PMHS, and PS4-Xchol-0.50 are shown in Fig. 1, which indicated
that all the Si–H bonds were completely reacted as a result of the
disappearance of the sharp vibrational band at 2165 cmꢁ1
(assigned as the Si–H stretching) in the polymers. Furthermore,
the appearance of the vibrational bands at 2965–2836, 1713,
1609–1464, and 1101–1042 cmꢁ1, which were attributed to
vibrations from methylene (–CH2–), ester carbonyl (C]O),
aromatic (Ar–), and Si–O–Si, respectively, and the occurrence of
the signals at 8.16–8.13, 7.13–7.10, 7.01–6.99, 6.95–6.92, 3.82,
and 2.04–0.86 ppm (corresponding to aromatic (Ar–), alkoxy
(–OCH3), and cholesteryl moiety, respectively) in the 1H-NMR
spectra of all the polymers in Fig. 2, suggested the successful
preparation of the copolymer. Additionally, the complete
disappearance of the vibrational band at 1648 cmꢁ1 which
corresponded to olenic C]C stretching in the partial enlarged
spectra between 1800 and 1500 cmꢁ1 as shown in Fig. 1b, and
the vanish of the representative signals of the vinyl group in
monomers at 6.05, 5.43, and 5.31 ppm in the 1H-NMR spectra of
all the polymers in Fig. 2, indicated the completely removal of
the unreacted monomers.
3.2 Thermal and liquid-crystalline properties of polymers
DSC, TGA, and POM were used to investigate the thermal and
liquid-crystalline properties of polymers. As shown in Table 1
and Fig. 3, the temperatures at 5% weight loss of all the poly-
mers in nitrogen were above 328 ꢀC, indicating that all the
polymers have good thermal stabilities. Surprisingly and inter-
estingly, there were two inection points for chiral polymers
upon heating as shown in Fig. 3. The rst inection at around
Scheme 1 Synthetic route of the monomers and polymers.
Xchol in the polymer) as a typical procedure. M1 (302.00 mg, 0.55
mmol), M2 (156.30 mg, 0.55 mmol), and 0.3 ml H2PtCl6$6H2O/
THF (1.00 g of hexachloroplatinic acid hydrate dissolved in 100
ml of tetrahydrofuran) were dissolved in toluene (40.00 ml),
then PMHS (97.50 mg, 0.25 mmol) was added drop-wise slowly
to the stirred mixture solution and heated under nitrogen and
anhydrous conditions at 65 ꢀC for 36 h. Monitored the reaction
by FT-IR until the disappearance of the sharp vibrational band
at 2165 cmꢁ1 assigned as the Si–H stretching. The coarse
polymer was precipitated by pouring the reaction mixture into
methanol, and further puried by dissolving in chloroform,
precipitating from methanol several times in order to remove
the excessive unreacted monomers and drying in vacuum.
White powder polymer was obtained. Yield: 53%.
ꢀ
350 C could be attributed to the decomposition of the choles-
teryl mesogenic group, followed by a plateau. Then the weight
ꢀ
loss has a sharp jump at about 475 C, which corresponded to
the carbide of the samples. Aer 550 ꢀC the weight loss
remained almost unchanged, indicating completely carbon-
ation of the sample. The actual molar fraction of M1 based on
(M1 + M2) in a specic polymer can be calculated based on the
above analysis, and the evolution tendency was in good agree-
ment with that of the feed.
DSC experiment was carried out to investigate the phase
transitions of the homopolymers and copolymers. Fig. 4
exhibits the rst cooling and subsequent second heating traces
ꢁ1
ꢀ
at a rate of 10 C min under nitrogen. Besides the apparent
glass transition, one small endothermic/exothermic peak at
higher temperature in the whole heating/cooling processes was
observed for all the polymers except samples PS4-Xchol-0.60 and
PS4-Xchol-0.70, suggesting that there existed at least one ordered
3. Results and discussion
3.1 Synthesis and characterization of monomers and
polymers
As shown in Scheme 1, the monomers were synthesized phase, possibly an LC phase. Comparing the glass transition
successfully and efficiently by starting from the Williams temperatures (Tg) of the linear polysiloxane-based polymers
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Fig. 1 FT-IR spectra of monomers and polymer PS4-Xchol-0.50.
Fig. 2 1H-NMR spectra of monomers and all the polymers.
in this study with those of the corresponding poly[3-
The liquid crystalline birefringence of all the polymers was
mercaptopropylmethylsiloxane] (PMMS) main chain based observed under POM with powder samples. Some typical POM
polymers,37 we found that the Tg of the linear polysiloxane- textures are shown in Fig. 5. All the polymers exhibited apparent
based copolymers were all signicantly higher, which might birefringence during the heating and cooling procedures below
be ascribed to the combined effect of the larger rigidity of the the isotropic state.
shorter main chain and the shorter spacer. Furthermore, as
As shown in Fig. 5a and b, the achiral homopolymer PS4-Xchol-
predicted, both of the Tg and the clear point temperature (Ti) 0.00 exhibited a marble texture when the sample heated to 29 ꢀC
obviously increased with the increasing content of chiral and developed a schlieren texture when the sample cooled from
monomer M1, and the mesogenic temperature range was isotropic state, which were typical characteristics of a nematic
gradually widened, possibly attributing to the gradual increase liquid crystalline phase.36,38 As a result, a nematic phase could be
of the rigidity in the side-chain structure.
conrmed for PS4-Xchol-0.00 during the whole mesomorphic state.
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Table 1 Thermal properties of polymers
M1/(M1 + M2) (MR)a
Sample
In feed
Calculatedb
Tic (ꢀC)
DTe (ꢀC)
c
Tg (ꢀC)
d
Td (ꢀC)
PS4-Xchol-1.00
PS4-Xchol-0.90
PS4-Xchol-0.80
PS4-Xchol-0.70
PS4-Xchol-0.60
PS4-Xchol-0.50
PS4-Xchol-0.40
PS4-Xchol-0.30
PS4-Xchol-0.20
PS4-Xchol-0.10
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
1.000
0.953
0.868
0.826
0.758
0.697
0.582
0.477
0.404
0.230
55.63
49.94
45.10
50.75
42.62
40.51
34.48
24.00
18.39
11.04
234.94
231.35
206.39
213.48
185.21
178.89
156.72
106.50
82.50
334
334
332
336
336
336
341
341
328
346
179.31
181.41
161.29
162.73
142.59
138.38
122.24
82.50
64.11
53.02
64.06
a
d
Molar ratio. b According to the TGA results.ꢁc 1Evaluated by DSC during the second heating at the rate of 10 ꢀC minꢁ1
.
5% weight loss temperature
e
was evaluated by TGA at a rate of 20 ꢀC min
.
Mesogenic temperature ranges determined by Ti ꢁ Tg.
Further cooling to below 190 ꢀC, a representative fan-shaped
texture could be observed, and remained unchanged upon
cooling (as listed in Fig. 5h–j), indicating that the LC phase at
low temperatures can be a typical smectic phase.39,40 The above
POM observations were in good agreement with DSC result
perfectly.
For the samples with Xchol over 0.70, when cooled to room
temperature from the isotropic state, a representative fan-
shaped texture of the smectic liquid crystalline phase could
be observed and the texture remained unchanged during cool-
ing, implying a smectic LC structure (Fig. 5k and l, taking PS4-
Xchol-0.80 as an example). These results were in accordance with
those of DSC experiments.
Fig. 3 TGA traces of polymers in nitrogen at a rate of 20 ꢀC minꢁ1
.
3.3 Smectic phase structure identication of polymers
Due to the insufficient information to the smectic phase
For sample PS4-Xchol-0.40, as shown in Fig. 5c, a fan-shaped structure identication according to the DSC and POM results,
focal conic texture was developed when the sample cooled from variable-temperature 1D-WAXD, SAXS, and 2D-WAXD experi-
the isotropic state to 142 ꢀC. Interestingly, when applying ments were performed.
a slight shear force on the sample, the oily streak texture,
Parts (a) and (b) of Fig. 6 show the 1D-WAXD patterns of PS4-
a characteristic of cholesteric phase,16,38 could be obtained Xchol-0.60 during the rst heating and subsequent cooling
immediately. Further cooling to room temperature, the oily processes, respectively. Distinctively different from the result of
streak texture could be maintained and accompanied with a red DSC experiment, the phase transition behaviour exhibited by
shi of the reection colour (Fig. 5e). Combining the results of 1D-WAXD was not apparent. Upon the rst heating in Fig. 6a,
DSC and POM, we concluded that sample PS4-Xchol-0.40 devel- only an amorphous broad halo at high-angle region could be
oped a wide Ch phase structure during the heating and cooling observed for the entire temperature region, and it became more
process.
diffused and slightly shied to the higher angles upon heating,
The DSC traces and the liquid crystalline birefringence indicating there was no molecular packing on the sub-
evolution tendency of samples PS4-Xchol-0.10, PS4-Xchol-0.20, nanometer scale. Furthermore, the intensities decreased
PS4-Xchol-0.30, and PS4-Xchol-0.50 were similar to those of PS4- slightly at rst, then a rapid decrease appeared over 175 ꢀC,
Xchol-0.40, thus a similar phase transition behaviour and phase suggesting that the sample may became isotropic. On the
structure could be conrmed.
subsequent cooling as shown in Fig. 6b, the broad halo at high-
ꢀ
However, polymers PS4-Xchol-0.60 and PS4-Xchol-0.70 angle region could be reconstructed at 175 C and retained to
behaved quite differently. Taking PS4-Xchol-0.70 as an example, room temperature, with the concentrated central position
when cooled from isotropic state to 210 C, a fan-shaped focal slightly shiing to the higher-angle simultaneously. However,
ꢀ
conic texture was developed at rst (as shown in Fig. 5f), then it there were no other diffraction peaks presented at low-angle
transferred into oily streak texture under mechanically sheared region in Fig. 6a and b, even repeating experiments with
force (see Fig. 5g), and also with a red shi of reection colour larger sample amounts and larger exposure times were per-
upon cooling, thus a cholesteric phase could be conrmed. formed. Because the calculated length of the chiral and achiral
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process of 1D-WAXD, SAXS experiments were carried out to
conrm this conjecture. As expected, one sharp diffraction peak
with d-spacing of 3.06 nm was observed and the intensity
decreased gradually as shown in Fig. 6c, indicating the existence
of an ordered phase, possibly an LC phase. When the temper-
ature reached 175 ꢀC, the diffraction peak disappeared, sug-
gesting that the sample entered a low ordered phase (nematic
phase or cholesteric phase, for example) or became isotropic.
Combining the results of DSC, POM, 1D-WAXD and SAXS, the
phase behaviour and phase structure of sample PS4-Xchol-0.60
could be summarized as follows: simple smectic phase struc-
ture of SmA or SmC at low temperatures as a result of the
representative fan-shaped texture under POM as well as the
sharp diffraction peak at low-angle region and the diffused
high-angle scattering halo under SAXS and 1D-WAXD, then
a cholesteric phase at high temperatures on account of the
characteristic oily streak texture observed under POM.
The DSC traces, POM textures, 1D-WAXD and SAXS patterns
of polymer PS4-Xchol-0.70 were similar to those of PS4-Xchol-0.60,
thus a similar phase transition behaviour and phase structures
could be conrmed.
For the polymers with Xchol above 0.70, their DSC results
showed only an endothermic/exothermic peak besides Tg, and
no other typical textures were observed under POM except
smectic fan-shaped texture. Thus variable-temperature SAXS and
1D-WAXD experiments were carried out to investigate their
phase transitions and phase structures. Taking PS4-Xchol-0.80 as
an example, as shown in Fig. 7a and b, there was a broad scat-
tering halo in high-angle region during the whole heating scan,
indicating the absence of a molecular packing in the sub-
nanometer scale. A diffraction peak with q value of 2.06 nmꢁ1
(corresponding to a d-spacing of 3.05 nm) in low-angle region
could be observed and the intensity decreased gradually upon
heating, indicating the development of an ordered phase,
possibly a LC phase. Aer 195 ꢀC, the low-angle diffraction peak
was vanished, accompanied with more diffusion of the high-
angle broad halo, suggesting that the sample might become
isotropic. Combining the results of DSC, POM, 1D-WAXD and
SAXS, the phase structure of sample PS4-Xchol-0.80 in the entire
mesomorphic state could be a simple smectic phase of SmA or
SmC as a result of the representative fan-shaped texture under
POM as well as the sharp diffraction peak at low-angle region
and the diffused high-angle scattering halo under SAXS and 1D-
WAXD.
Since the 1D-WAXD and SAXS patterns lack dimensionality,
2D-WAXD experiments were carried out to identify the smectic
structures of polymers with Xchol over 0.50. The samples were
mechanically sheared and annealed at different temperatures
aer cooling from the isotropic state. Fig. 8a shows the repre-
sentative 2D-WAXD pattern of PS4-Xchol-0.60 recorded at room
temperature with the X-ray incident beam perpendicular (along
Z direction) to the shear direction (X direction). A pair of strong
Fig. 4 DSC thermo grams of polymers during the first cooling (a) and
subsequent second heating (b) scans at a rate of 10 ꢀC minꢁ1, and the
partial enlarged traces of PS4-Xchol-0.60 (c).
groups from the Si atom in the backbone to the last C atom of low-angle diffraction arcs appeared on the equator, which was
the side chain with the alkyl chains in all-trans conformation perpendicular to the shear direction, indicating that the normal
were 2.72 nm and 1.72 nm, respectively, as well as the insen- of the smectic layers was perpendicular to the shear direction.
sitive of the ID-WAXD test, we presumed that there might be Meanwhile, the high-angle scattering halo was more or less
some diffraction peaks at even lower angles. Aer the cooling concentrated on the meridian with rather broad azimuthal
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Fig. 5 Representative textures of samples PS4-Xchol-0.00 (a and b), PS4-Xchol-0.40 (c–e), PS4-Xchol-0.70 (f–j), and PS4-Xchol-0.80 (k and l) at
different temperatures cooled from isotropic state.
Fig. 6 1D-WAXD patterns of PS4-Xchol-0.60 during the first heating (a) and the subsequent cooling process (b), and SAXS pattern (c) of PS4-
X
chol-0.60 during the first heating process.
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Fig. 7 SAXS (a) and 1D-WAXD (b) patterns of PS4-Xchol-0.80.
Fig. 8 2D-WAXD pattern of PS4-Xchol-0.60 (a) and the shear geometry (b).
distributions, proving that a typical SmA phase structure was over 130 ꢀC), for polymers with Xchol between 0.60 and 0.70,
developed for sample PS4-Xchol-0.60 at low temperatures.21,41 a single layer smectic A phase could be observed at low
Similar patterns were obtained for samples PS4-Xchol-0.70, PS4- temperatures and a cholesteric phase was developed at high
Xchol-0.80, PS4-Xchol-0.90, and PS4-Xchol-1.00.
temperatures. Only a single layer smectic A phase was self-
organized for polymers with Xchol over 0.70. Interestingly, the
phase structures of the linear polysiloxane-based polymers in
this study was largely different from their counterparts with
cyclic siloxane backbone,16 although they had the almost iden-
tical side chain mesogens, the similar compositions between
the two mesogens, and exactly the same degree of polymeriza-
tion. For example, cyclic siloxane polymers with high Xchol could
develop an additional blue phase during the heating and cool-
ing procedures, suggesting that the existence of ring tension
play a decisive role on the formation of a blue phase. In other
words, the linear polysiloxane, which was lack of a ring tension,
might be easier to induce the molecules to adopt a more
ordered arrangement, thus result in the disappearance of blue
phase and cholesteric phase in high Xchol polymers. Compared
3.4 Dependence of the phase structure on the two LC
building blocks and interplay between them
The thermotropic LC properties and the composition depen-
dence of all the polymers are shown in Fig. 9 and the phase
transition behaviours are summarized in Table 2. Table 2 also
lists the calculated d-spacing and the minimum layer spacing of
the periodic arrangement of the polymers exhibiting smectic
phase. As shown in Fig. 9, the achiral homopolymer PS4-Xchol
-
0.00 formed a nematic phase upon heating since there was no
chiral centre. Polymers with Xchol below 0.60 developed
a cholesteric phase with very broad temperature ranges in the
entire mesomorphic state (minimum over 50 ꢀC and maximum
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selective light reection,42,43 special electro-optic effects,44,45
and optical rotation effect,46 which consequently endows them
with various functions and widely applications in the elds of
temperature and pressure sensors,47,48 stealth technologies,49
information storage,50 energy saving9,51,52 and so on. Polymers
with Xchol below 0.60 in this work could developed a chole-
steric phase with sufficiently wide temperature ranges in the
entire mesomorphic state (minimum over 50 ꢀC and
maximum over 130 ꢀC), and their glass transition tempera-
tures could arbitrarily regulate between 0–40 ꢀC, therefore, we
anticipate that they will have a wide variety of practical
applications as a kind of functional LC materials. Fig. 10
shows a typical transmission spectrum under normal inci-
dence for sample PS4-Xchol-0.20 at different temperatures (the
Fig.
polymers.
9 Composition dependence of transition temperature for minimum light transmission corresponds to the maximum
selective light reection of circularly polarized light). The
centre selective reection wavelength (lm) was about 472 nm at
room temperature, a blue shi of about 90 nm could be
observed and the colours of the polymer altered from azure-
blue to deep-purple with the increasing of temperatures.
Accordingly, a thermochromism material for sensor was
successfully obtained. The lm of other samples in Ch phase,
which was as a function of temperature, was also presented
a similar evolution rule upon heating as shown in Fig. 11 (the
black dash lines represent the SmA–Ch transition tempera-
tures); the values of lm decreased sharply or slightly until the
temperatures were close to the Ch–I transitions.
with the PMMS based polymers we previously reported,37 results
revealed that a longer backbone and spacer were more bene-
cial to stabilize the higher ordered LC phase. In a word, subtle
changes in the molecular structure could bring about tremen-
dous variation of the LC phase properties.
3.5 Thermal-optical properties of polymers
It is well known that cholesteric LC with unique helical
structures exhibits particular optical properties such as
Table 2 d-Spacing, phase transition, and lm of polymers
Phase transitionsb (ꢀC)
Heating
Cooling
a
Sample
d(100) (nm)
cc (nm)
3.05
3.05
3.05
3.05
3.05
—
lmd (nm)
—
PS4-Xchol-1.00
PS4-Xchol-0.90
PS4-Xchol-0.80
PS4-Xchol-0.70
PS4-Xchol-0.60
PS4-Xchol-0.50
PS4-Xchol-0.40
PS4-Xchol-0.30
PS4-Xchol-0.20
PS4-Xchol-0.10
PS4-Xchol-0.00
3.05
3.05
3.05
3.05
3.05
—
G55.63SmA234.94I
I234.13SmA56.74G
G49.94SmA231.35I
I230.32SmA56.51G
G45.10SmA206.39I
I206.98SmA43.35G
G50.75SmA187.32Ch213.48I
I211.63Ch187.25SmA45.37G
G42.62SmA141.65Ch185.21I
I183.69Ch141.39SmA39.57G
G40.51Ch178.89I
I177.38Ch39.01G
G34.48Ch156.72I
I155.20Ch32.59G
G24.00Ch106.50I
I105.40Ch23.00G
G18.39Ch82.50I
I81.42Ch15.99G
G11.41Ch64.06I
—
—
466
440
408
389
356
380
525
—
—
—
—
—
—
—
—
—
I62.53Ch8.98G
G6.97N32.09I
—
—
I30.04N4.80G
a
Calculated at the temperature of 80 ꢀC for all the polymers. b According to the DSC results; G: glass state; Ch: cholesteric phase; SmA: smectic A
phase; I: isotropic state. c The minimize layer spacing of the periodic arrangement. d Selective reection wavelength measured near Ch–I transition.
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controlling the ratio between chiral and achiral side chains so
as to satisfy different practical application requirement.
4. Conclusions
In summary, by conventional hydrosilylation reaction, a series
of new linear polysiloxane tetramers based on cholesterol and
benzene methyl ether groups as side groups was successfully
designed and synthesized. Their thermal properties, phase
transition behaviours, and selective reective behaviours were
systematically investigated in details. Results indicated that all
the polymers exhibited thermotropic LC properties with great
wide mesomorphic state temperature range and their LC phase
structures were strongly dependent on the molar content of
chiral groups. Compared with their counterparts based on
cyclic siloxane backbone and PMMS backbone, respectively, the
introduction of the linear polysiloxane backbone with relatively
smaller rigidity and lower degree of polymerization generated
a new series of SCLCPs with rather wide temperature ranges of
cholesteric or smectic A phase, indicating that a small change in
the molecular structure could bring about great variation of the
LC phase behaviours. Furthermore, polymers with Xchol below
0.60 also exhibited the distinct colour changes with the
increasing of temperatures. It is believed that the newly
synthesized SCLCPs with a combination of characteristics of
wide temperature ranges of cholesteric phase, thermochrom-
ism, as well as arbitrarily regulated glass transition tempera-
tures between 0–40 ꢀC will have practical applications in
temperature sensors and so on. Future work will be focused on
the preparation and performance investigation of device and
the further enhancement of the thermal-optical properties of
the polymers.
Fig. 10 Transmission spectra of PS4-Xchol-0.20 as a function of
temperature.
Fig. 11 The centre selective light reflection wavelength (lm) as
a function of temperature for some samples.
Acknowledgements
Table 2 lists the lm values near the Ch–I transition temper- Financial support from the National Natural Science Founda-
ature. Surprisingly, different from the results previously re- tion (Grant No. 51173003, 51573003, 51372029, 51373024), the
ported, an unusual phenomenon of lm vs. Xchol was rst Fok Ying Tung Education Foundation (Grant No. 142009), and
observed. It decreased dramatically (blue shi) at rst, then had the Major Project of Beijing Science and Technology Program
a remarkable red shi with increasing Xchol, which might (Grant No. Z151100003315023) is gratefully acknowledged.
attribute to the competition between space steric hindrance and
helical twisting power (HTP). The increasing of chiral content of
M1 not only enlarged the value of HTP, but also accompanied
Notes and references
with the increasing of large space steric hindrance and rigidity
which would restrict the exercise of HTP. When Xchol # 0.3, the
restriction of space steric hindrance on HTP was small and
negligible, which resulted in the dominant status of HTP. Due
to the inverse proportion relationship between HTP and the
cholesteric pitch (P) (HTP ¼ (PXc)ꢁ1, Xc is the chiral content),
according to the Bragg equation (lm ¼ nP sin q, n is the average
index, q is the incidence angle), a blue shi of lm was observed
with the increasing Xchol. However, when Xchol $ 0.4, space
steric hindrance occupied the leading position, and the
restriction of space steric hindrance on the performance of HTP
could not be balanced by the increased value of HTP with
increasing Xchol, which lead to a red shi of the lm. In addition,
the lm of a specic polymer could be adjusted by nely
1 V. P. Shibaev, I. V. Yakovlev and S. G. Kostromin, Vysokomol.
Soedin., Ser. A, 1990, 32, 1552.
2 V. P. Shibaev, S. G. Kostromin and S. A. Ivanov, Polymers as
Electroactive and Photooptical Active Media, Springer, Berlin,
1996.
3 V. P. Shibaev, Polym. Sci., Ser. A, 2014, 56, 727–762.
4 C. S. Hsu, Prog. Polym. Sci., 1997, 22, 829–871.
5 H. Finkelmann and G. Rehage, Makromol. Chem., Rapid
Commun., 1980, 1, 31–34.
6 F. B. Meng, Z. Y. Wang, G. W. Chai, H. G. Wang, Y. Chen and
B. Y. Zhang, J. Appl. Polym. Sci., 2010, 116, 12.
7 H. Yang, M. X. Liu, Y. W. Yao, P. Y. Tao, B. P. Lin, P. Keller,
X. Q. Zhang, Y. Sun and L. X. Guo, Macromolecules, 2013, 46,
3406–3416.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 87502–87512 | 87511

View Article Online
RSC Advances
Paper
8 A. S. Angeloni, D. Caretti, C. Carlini, E. Chiellini, G. Galli, 29 Z. G. Zhu, J. G. Zhi, A. H. Liu, J. X. Cui, H. Tang, W. Q. Qiao,
A. Altomare, R. Solaro and M. Laus, Liq. Cryst., 1989, 4,
513–527.
X. H. Wan and Q. F. Zhou, J. Polym. Sci., Part A: Polym. Chem.,
2007, 45, 830–847.
9 Y. Z. Gao, W. H. Yao, J. Sun, H. M. Zhang, Z. D. Wang, 30 J. S. Hu, Z. X. Zhao, B. Kong and D. Li, Colloid Polym. Sci.,
L. Wang, D. K. Yang, L. Y. Zhang and H. Yang, J. Mater.
Chem. A, 2015, 3, 10738–10746.
10 J. Cesarano, I. A. Aksay and A. Bleier, J. Am. Ceram. Soc., 1988,
71, 250–255.
11 K. Ichimura, Y. Akita, H. Akiyama, K. Kudo and Y. Hayashi,
Macromolecules, 1997, 30, 903–911.
12 J. W. Y. Lam and B. Z. Tang, J. Polym. Sci., Part A: Polym.
Chem., 2003, 41, 2607–2629.
13 Z. Q. Yu, J. W. Y. Lam, C. Z. Zhu, E. Q. Chen and B. Z. Tang,
Macromolecules, 2013, 46, 588–596.
14 H. Yang, F. Zhang, B. P. Lin, P. Keller, X. Q. Zhang, Y. Sun
and L. X. Guo, J. Mater. Chem. C, 2013, 1, 1482.
15 Z. Y. Zhang, Q. Wang, P. P. Hou, Z. H. Shen and X. H. Fan,
Polym. Chem., 2015, 6, 7701–7710.
16 Z. H. Cheng, H. Cao, D. Y. Zhao, W. Hu, W. L. He, X. T. Yuan,
J. M. Xiao, H. Q. Zhang and H. Yang, Liq. Cryst., 2011, 38, 9–
15.
17 X. F. Zhang, L. Y. Zhang, J. F. Tan, X. J. Wu, B. R. Li,
F. F. Wang, H. Yang and Z. Yang, J. Mater. Sci., 2014, 49,
4927–4937.
2008, 287, 215–224.
31 J. S. Hu, C. Liu, X. Zhang and Q. B. Meng, Eur. Polym. J., 2009,
45, 3292–3301.
32 J. S. Hu, B. Wang, C. Liu and X. Z. He, Eur. Polym. J., 2010, 46,
535–545.
33 V. Percec and D. Tomazos, Macromolecules, 1989, 22, 2062–
2069.
34 R. A. Lewthwaite, J. W. Goodby and K. J. Toyne, J. Mater.
Chem., 1993, 3, 241–245.
35 M. Ungerank, B. Winkler, E. Eder and F. Stelzer, Macromol.
Chem. Phys., 1995, 196, 3623–3641.
36 C. S. Canessa, C. Acuilera, J. L. Serrano and L. Oriol, J. Polym.
Sci., Part A: Polym. Chem., 1996, 34, 1465–1473.
37 W. H. Yao, Y. Z. Gao, X. Yuan, B. F. He, H. F. Yu, L. Y. Zhang,
Z. H. Shen, W. L. He, Z. Yang, H. Yang and D. K. Yang, J.
Mater. Chem. C, 2016, 4, 1425–1440.
38 X. J. Wu, H. Cao, R. W. Guo, K. Li, F. F. Wang, Y. Z. Gao,
W. H. Yao, L. Y. Zhang, X. F. Chen and H. Yang,
Macromolecules, 2012, 45, 5556–5566.
39 H. L. Xie, T. H. Hu, X. F. Zhang, H. L. Zhang, E. Q. Chen and
Q. F. Zhou, J. Polym. Sci., Part A: Polym. Chem., 2008, 46,
7310–7320.
18 A. A. Craig and C. T. Imrie, Macromolecules, 1995, 28, 3617–
3624.
19 K. Yonetake, J. Soga and Y. Masuko, Polym. J., 1997, 29, 500– 40 C. G. Nardele and S. K. Asha, J. Phys. Chem. B, 2014, 118,
507. 1670–1684.
20 S. Chen, L. Y. Zhang, L. C. Gao, X. F. Chen, X. H. Fan, 41 S. Chen, L. C. Gao, X. D. Zhao, X. F. Chen, X. H. Fan, P. Y. Xie
Z. H. Shen and Q. F. Zhou, J. Polym. Sci., Part A: Polym.
Chem., 2009, 47, 505–514.
and Q. F. Zhou, Macromolecules, 2007, 40, 5718–5725.
42 M. Mitov and N. Dessaud, Nat. Mater., 2006, 5, 361–364.
21 Y. H. Cheng, W. P. Chen, Z. H. Shen, X. H. Fan, M. F. Zhu and 43 M. Mitov, Adv. Mater., 2012, 24, 6260–6276.
Q. F. Zhou, Macromolecules, 2011, 44, 1429–1437.
22 Y. F. Zhu, Z. Y. Zhang, Q. K. Zhang, P. P. Hou, D. Z. Hao,
44 P. Ribiere, S. Pirkl and P. Oswald, Phys. Rev. A, 1991, 44,
8198–8209.
Y. Y. Qiao, Z. H. Shen, X. H. Fan and Q. F. Zhou, 45 R. B. Meyer, Appl. Phys. Lett., 1968, 12, 281–282.
Macromolecules, 2014, 47, 2803–2810.
23 Y. Guan, X. Chen, Z. Shen, X. Wan and Q. Zhou, Polymer,
2009, 50, 936–944.
24 X. Mei, Y. Chu, J. Cui, Z. Shen and X. Wan, Macromolecules,
2010, 43, 8942–8949.
25 X. Mei, J. Zhang, Z. H. Shen and X. H. Wan, Polym. Chem.,
2012, 3, 2857–2866.
26 Q. F. Zhou, X. L. Zhu and Z. Q. Wen, Macromolecules, 1989,
22, 491–493.
46 R. A. M. Hikmet and H. Kemperman, Nature, 1998, 392, 476–
479.
47 T. R. Wolinski, A. Jarmolik and W. J. Bock, IEEE Trans.
Instrum. Meas., 1999, 48, 2–6.
48 D. J. Mulder, A. P. H. J. Schenning and C. W. M. Bastiaansen,
J. Mater. Chem. C, 2014, 2, 6695–6705.
49 L. P. Zhang, M. Wang, L. Wang, D. K. Yang, H. F. Yu and
H. Yang, Liq. Cryst., 2016, 43, 750–757.
50 N. Tamaoki, Adv. Mater., 2001, 13, 1135–1147.
27 D. Zhang, Y. Liu, X. Wan and Q. F. Zhou, Macromolecules, 51 H. Khandelwal, R. C. G. M. Loonen, J. L. M. Hensen,
1999, 32, 4494–4496.
28 D. Zhang, Y. X. Liu, X. H. Wan and Q. F. Zhou,
Macromolecules, 1999, 32, 5183–5185.
A. P. H. J. Schenning and M. G. Debije, J. Mater. Chem. A,
2014, 2, 14622.
52 H. Yang, K. Mishima, K. Matsuyama, K. I. Hayashi,
H. Kikuchi and T. Kajiyama, Appl. Phys. Lett., 2003, 82,
2407–2409.
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