Synthesis, structure, and properties of chiral liquid crystal monomers and polymers based on menthol

Article abstract of DOI:10.1002/pola.26350

To study structure-mesomorphism relationships of the monomers and polymers based on menthol, four new chiral monomers (M₁-M₄) and the corresponding homopolymers (P₁-P₄) with menthyl group were synthesized. Their chemical structures, formula, phase behavior, and thermal stability were characterized by FTIR, ¹H NMR, ¹³C NMR, elemental analyses, differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and thermogravimetric analysis. The selective reflection of light was investigated with ultraviolet/visible spectrometer. The influence of the mesogenic core rigidity, spacer length, and menthyl steric effect on the mesomorphism of M₁-M₄ and P ₁-P₄ was examined. By inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, four target monomers and polymers could form the expected mesophase. Moreover, their melting temperature (T_m), glass transition temperature (T_g), clearing temperature (T_i), and mesophase range (δT) increased with increasing the mesogenic core rigidity; whereas the T_m and T _g decreased, T_i and δT increased with an increase of the spacer length. M₁ and M₂ showed monotropic and enantiotropic cholesteric phase, respectively, whereas M₃ and M ₄ all revealed chiral smectic C (SmC*), cholesteric and cubic blue phases. In addition, with increasing temperature, the selective reflection of light shifted to the long wavelength region at the SmC* phase range and to the short wavelength region at the cholesteric range, respectively. P ₁ and P₂ only showed a smectic A (SmA) phase, whereas P₃ and P₄ exhibited the SmC* and SmA phases. All the obtained polymers had very good thermal stability.

Full text of DOI:10.1002/pola.26350

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Synthesis, Structure, and Properties of Chiral Liquid Crystal Monomers
and Polymers Based on Menthol
Jian-She Hu, Dan Li, Wen-Chang Zhang, Qing-Bao Meng
Center for Molecular Science and Engineering, College of Science, Northeastern University, Shenyang 110004,
People’s Republic of China
Correspondence to: J.-S. Hu (E-mail: hujs@mail.neu.edu.cn)
Received 26 April 2012; accepted 16 August 2012; published online
DOI: 10.1002/pola.26350
ABSTRACT: To study structure–mesomorphism relationships of
the monomers and polymers based on menthol, four new chiral
monomers (M1–M4) and the corresponding homopolymers
(P1–P4) with menthyl group were synthesized. Their chemical
structures, formula, phase behavior, and thermal stability were
characterized by FTIR, 1H NMR, 13C NMR, elemental analyses,
differential scanning calorimetry, polarizing optical microscopy,
X-ray diffraction, and thermogravimetric analysis. The selective
reflection of light was investigated with ultraviolet/visible spec-
trometer. The influence of the mesogenic core rigidity, spacer
length, and menthyl steric effect on the mesomorphism of
M1–M4 and P1–P4 was examined. By inserting a flexible spacer
between the mesogenic core and the terminal menthyl groups,
four target monomers and polymers could form the expected
mesophase. Moreover, their melting temperature (Tm), glass
transition temperature (Tg), clearing temperature (Ti), and
mesophase range (DT) increased with increasing the mesogenic
core rigidity; whereas the Tm and Tg decreased, Ti and DT
increased with an increase of the spacer length. M1 and M2
showed monotropic and enantiotropic cholesteric phase, respec-
tively, whereas M3 and M4 all revealed chiral smectic C (SmC*),
cholesteric and cubic blue phases. In addition, with increasing
temperature, the selective reflection of light shifted to the long
wavelength region at the SmC* phase range and to the short
wavelength region at the cholesteric range, respectively. P1 and
P2 only showed a smectic A (SmA) phase, whereas P3 and P4
exhibited the SmC* and SmA phases. All the obtained polymers
C
V
had very good thermal stability.
2012 Wiley Periodicals, Inc.
J. Polym. Sci. Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: chiral; liquid crystalline polymers (LCPs); liquid
crystal polymer; menthol; phase behavior; synthesis
INTRODUCTION From a scientific and commercial point of
view, chiral liquid crystal (LC) materials are fascinating
because they possess excellent electro-optical properties,
including selective reflection of light, thermochromism, fer-
roelectricity, and circular dichroism, and many important
potential applications in various areas such as nonlinear op-
tical devices, flat-panel displays, thermal imaging, rewritable
full-color image recording, and photostable UV screens.1–20
For side-chain LC polymers, their mesomorphism or phase
behavior mainly depends on the nature of polymer back-
bone, the type of mesogen, the flexible spacer and its length,
and the nature of terminal groups. The mesogens are usually
attached to the polymer backbone through a flexible spacer,
which is generally an aliphatic hydrocarbon chain containing,
normally, more than two methylene units. In fact, depending
on the odd/even number of carbon atoms on the spacer, the
mesogens couple to the polymer backbone and induce a
local anisotropy of the polymer chains (oblate or prolate
conformation). This effect vanishes when the spacer is too
long (more than seven carbon atoms), and no coupling is
observed.21 Depending on the chemical structure, it can
achieve a macroscopic alignment of the chiral mesophase
domains. In recent years, many chiral side-chain LC poly-
mers, mostly adopting commercially available chiral com-
pounds such as cholesterol and (S)-(þ)-2-methyl-1-butanol,
have been extensively studied.22–34
Menthol has three chiral center and strong optical activity,
so the LC materials based on menthol are expected to obtain
excellent mesomorphism and wide applications. At present,
menthol has been used to synthesize chiral monomers and
side-chain polymers.35–42 In these articles, the cholesteric
phase was induced by the copolymerization of the nematic
LC monomer and nonmesogenic chiral monomer containing
menthyl group. However, to the best of our knowledge, no
research on the LC monomers and homopolymers based on
menthol has been reported until now. Liu and Yang39
reported the synthesis and characterization of novel mono-
mers and homopolymers containing menthyl groups.
Although these chiral monomers contained two or three phe-
nyl rings, their mesogenic cores were directly linked to ter-
minal menthyl groups, and the existence of the bulky steric
menthyl groups prevented the orientation of the mesogenic
C
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molecular, so they showed no mesophase. However, our
recent research showed that the monomers with three phe-
nyl rings and their polymers derived from menthol could
form and exhibit LC phase when a flexible linkage chain was
inserted between the mesogenic core and the bulky terminal
menthyl fragments by reducing the steric effect.42 Therefore,
it is necessary to design and synthesize a series of novel chi-
ral LC monomers and polymers derived from menthol to
study their structure–property relationships and explore
their potential applications.
In this study, we reported on the synthesis, structure, and
properties of four new chiral LC monomers and homopoly-
mers based on menthol. Their phase behavior and thermal
stability were characterized with differential scanning calo-
rimetry (DSC), polarizing optical microscopy (POM), X-ray
diffraction (XRD), and thermogravimetric analysis (TGA). The
structure–mesomorphism relationships of these obtained
monomers and polymers were discussed in detail. The effect
of the mesogenic rigidity and flexible spacer length on the
phase behavior of the monomers and homopolymers was
investigated.
SCHEME 1 Synthetic route of the intermediate compounds
1–10.
EXPERIMENTAL
Materials
All chemicals were obtained from the indicated sources and
used as received. L-Menthol was purchased from Shanghai
Kabo Chemical (Shanghai, China), chloroacetic acid from
Tianjin Bodi Chemical (Tianjin, China), 4-hydroxybenzoic
acid from Shanghai Wulian Chemical Plant (Shanghai, China),
ethylene chlorohydrin from Tianjin Dagu Chemical Plant
(Tianjin, China), hexamethylene chlorohydrin from Zhouping
Mingxing Chemical Engineering (Zhouping, China), and
undec-10-enoic acid from Beijing Jinlong Chemical Reagent
(Beijing, China). 4,40-Dihydroxybiphenyl (from Aldrich) was
used as received. Polymethylhydrosiloxane (PMHS) was pur-
chased from Jilin Chemical Industry (Jilin, China). All other
solvents and reagents used were purified by standard
methods.
THMSE-600 (Linkam, UK) cool and hot stage. XRD measure-
ments were performed with a nickel-filtered Cu-Ka (k ¼
1.542 Å) radiation with a DMAX-3A Rigaku (Rigaku, Japan)
powder diffractometer.
Synthesis of the Intermediate Compounds
The synthetic route of the intermediate compounds is shown
in Scheme 1. Menthyloxyacetic acid (1), 4-menthyloxyacetox-
ybenzoic acid (3), 4-(2-hydroxy-ethoxy)benzoic acid (5),
4-(6-hydroxyhexyloxy)benzoic acid (6), and 4-(2-undec-10-
enoyloxyethoxy)benzoic acid (7) were prepared according to
the method reported by Hu et al.43–45
4-(6-Undec-10-enoyloxyhexyloxy)benzoic Acid (8)
The synthesis of 8 is similar to that for 7 as described above.
White crystal 8 was obtꢀained by recrystallization from etha-
nol. Yield: 80%, mp: 89 C.
Characterization
FTIR spectra were measured on a PerkinElmer spectrum
1
One (B) spectrometer (PerkinElmer, Foster City, CA). H NMR
IR (KBr, cmꢁ1): 2928, 2853 (ACH2A); 1728, 1688 (C¼¼O);
1641 (C¼¼C); 1607, 1514 (ArA), 1255 (CAOAC).
spectra were obtained with a Bruker ARX400 spectrometer
(Bruker, Swiss). 13C NMR (75.4 MHz) spectra were obtained
with a Varian Gemini 300 spectrometer (Varian Associates,
Palo Alto, CA). The elemental analyses were carried out with
an Elementar Vario EL III (Elementar, Hanau, Germany). The
special optical rotations were obtained on a PerkinElmer
341 polarimeter. The average molecular weight of the poly-
mer was obtained with Waters GPC 1515. The selective
reflection wavelength was measured using a PerkinElmer
950 ultraviolet/visible (UV/VIS) spectrometer with hot stage.
The phase behavior was determined with a Netzsch DSC 204
(Netzsch, Hanau, Germany) equipped with a cooling system.
The thermal stability of the polymers under nitrogen atmos-
phere was measured with a Netzsch TGA 209C thermogravi-
metric analyzer. The mesomorphism was observed with a
Leica DMRX POM (Leica, Germany) equipped with a Linkam
1H NMR (d, ppm from TMS in CDCl3): 1.35–1.82 [m, 20H,
CH2¼¼CHCH2(CH2)6CH2A and ACOOCH2(CH2)4CH2OA]; 2.10
[m, 2H, CH2¼¼CHCH2(CH2)7A]; 2.65 [t, 2H, J ¼ 7.2, CH2¼¼
CH(CH2)7CH2A]; 4.05 [t, 2H, J ¼ 6.5, ACOO(CH2)5CH2OA];
4.13 [t, 2H, J ¼ 6.6, ACOOCH2(CH2)5OA]; 4.95–5.03 (m, 2H,
CH2¼¼CHA); 5.77–6.90 (m, 1H, CH2¼¼CHA); 7.11 (d, 2H, J ¼
8.5); 8.12 (d, 2H, J ¼ 8.5); 10.91 (s, 1H, ACOOH).
ELEM. ANAL. calcd for C24H36O5: C, 71.26%; H, 8.97%. Found:
C, 71.41%; H, 9.06%.
4-Hydroxybiphenyl-40-(2-undec-10-enoyloxyethoxy)
benzoate (9)
4-(2-Undec-10-enoyloxyethoxy)benzoyl chloride was pre-
pared through the reaction of 7 with excess thionyl chloride.
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SCHEME 2 Synthetic route of the monomers M1–M4.
The acid chloride obtained (3.66 g, 10 mmol) was added
dropwise to a solution of 4,40-dihydroxybiphenyl (5.58 g, 30
mmol) in 50 mL of THF and 0.8 mL of pyridine under quick
stirring. The mixture was reacted for 6 h at room tempera-
ture and for 8 h at 65 ꢀC. After the mixture was concen-
trated, the crude product was precipitated by adding
ice-water to the residue, washed with 1.5% NaOH solution,
and then neutralized with diluted HCl. The solid 9 was
obtained by recrystallization from ethanol/acetone (3:1).
J ¼ 8.6); 7.91 (d, 2H, J ¼ 8.5); 8.11 (d, 2H, J ¼ 8.5); 9.32
(s, 1H, AOH).
ELEM. ANAL. calcd for C36H44O6: C, 75.50%; H, 7.74%. Found:
C, 75.62%; H, 7.81%.
Synthesis of the Monomers
The synthetic route of the chiral monomers M1–M4 is out-
lined in Scheme 2. They were prepared by the same method.
The synthesis of M1 is given below as an example.
ꢀ
Yield: 43%, mp: 153 C.
4-(Menthyloxyacetoxy)biphenyl-40-(2-(undec-10-enoyloxy)
ethoxy)benzoate (M1)
IR (KBr, cmꢁ1): 3415 (AOH); 2924, 2852 (ACH2A); 1734
(C¼¼O); 1641 (C¼¼C); 1609, 1500 (ArA), 1262 (CAOAC).
The compound 2 was prepared according to the reported lit-
erature.41 The acid chloride obtained (2.55 g, 11 mmol) was
dissolved in 5 mL of chloroform and then added dropwise to
a stirred solution of the compound 9 (5.16 g, 10 mmol) in
40 mL of chloroform and 1 mL of pyridine. The mixture was
reacted at room temperature for 2 h and refluxed for 24 h,
cooled to room temperature, filtered, and then concentrated.
The crude product was precipitated by adding methanol to
the filtrate and purified by column chromatography (silica
1H NMR (d, ppm from TMS in CDCl3): 1.34–1.80 [m, 12H,
CH2¼¼CHCH2(CH2)6CH2A]; 2.05 [m, 2H, CH2¼¼CHCH2
(CH2)7A]; 2.61 [t, 2H, J ¼ 7.2, CH2¼¼CH(CH2)7CH2A]; 4.25–
4.54 [m, 4H, ACOO(CH2)2OA]; 4.95–5.03 (m, 2H,
CH2¼¼CHA); 5.75–6.90 (m, 1H, CH2¼¼CHA); 6.9 (d, 2H, J ¼
8.4); 7.1 (d, 2H, J ¼ 8.6); 7.2 (d, 2H, J ¼ 8.6); 7.7 (d, 2H, J ¼
8.5); 7.92 (d, 2H, J ¼ 8.5); 8.11 (d, 2H, J ¼ 8.6); 6.92–8.03
(m, 12H, ArAH); 9.23 (s, 1H, AOH).
ꢀ
ELEM. ANAL. calcd for C32H36O6: C, 74.39%; H, 7.02%. Found:
C, 74.24%; H, 7.11%.
gel, dichloromethane). Yield: 45%. mp: 72 C.
IR (KBr, cmꢁ1): 2921, 2851 (CH3A, ACH2A); 1780, 1729
(C¼¼O); 1640 (C¼¼C); 1610–1497 (ArA); 1208 (CAOAC).
4-Hydroxybiphenyl-40-(6-undec-10-enoyloxyhexyloxy)ben-
zoate (10)
1H NMR (d, ppm from TMS in CDCl3): 0.81–2.60 [m, 34H,
A(CH2)8A and in menthylAH]; 3.28–3.36 (m, 1H, ACH< in
menthyl); 4.36–4.45 (m, 2H, AOOCCH2OA); 4.52–4.63 [m,
4H, ACOO(CH2)2OA]; 4.92–5.05 (m, 2H, CH2¼¼); 5.77–5.92
(m, 1H, ¼¼CHA); 7.03 (d, 2H, J ¼ 8.5); 7.11 (dd, 4H, J ¼ 8.6);
7.31 (d, 2H, J ¼ 8.6); 7.82 (d, 2H, J ¼ 8.5); 8.23 (d, 2H,
J ¼ 8.5).
The synthesis of 10 is similar to that for 9 as described
above. White solid 10 was obtained by recrystallization from
ꢀ
ethanol/acetone (3:1). Yield: 42%, mp: 136 C.
IR (KBr, cmꢁ1): 3461 (AOH); 2927, 2854 (ACH2A); 1732
(C¼¼O); 1641 (C¼¼C); 1608, 1498 (ArA), 1261 (CAOAC).
1H NMR (d, ppm from TMS in CDCl3): 1.36–1.83 [m, 20H,
CH2¼¼CHCH2(CH2)6CH2A and ACOOCH2(CH2)4CH2OA]; 2.09
[m, 2H, CH2¼¼CHCH2(CH2)7A]; 2.64 [t, 2H, J ¼ 7.2, CH2¼¼
CH(CH2)7CH2A]; 4.06 [t, 2H, J ¼ 6.4, ACOO(CH2)5CH2OA];
4.15 [t, 2H, J ¼ 6.5, ACOOCH2(CH2)5OA]; 4.97–5.04 (m, 2H,
CH2¼¼CHA); 5.76–6.90 (m, 1H, CH2¼¼CHA); 7.02 (d, 2H, J ¼
8.5); 7.12 (d, 2H, J ¼ 8.5); 7.23 (d, 2H, J ¼ 8.5); 7.72 (d, 2H,
13C NMR (d, ppm from TMS in CDCl3): 20.52, 21.43 (3C,
CH3); 23.72, 24.85, 28.74, 29.13, 29.45, 29.64, 29.73, 33.56,
33.66, 34.13, 40.64 (11C, CH2); 25.82, 31.54, 47.93 (3C, CH);
80.52 (1C, OCH in menthyl); 62.84, 66.15, 67.23 (3C, OCH2);
113.92, 122.09, 129.56, 130.83 (12C, aromatic tert. C);
114.89, 140.12 (2C, C¼¼C); 121.08, 136.73, 136.84, 147.29,
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149.92, 167.54 (6C, aromatic quat. C); 165.54, 166.23,
172.72 (3C, C¼¼O).
ELEM. ANAL. calcd for C44H56O8: C, 74.13%; H, 7.92%. Found:
C, 74.21%; H 7.83%.
1H NMR (d, ppm from TMS in CDCl3): 0.81–2.64 [m, 42H,
A(CH2)8A, ACOOCH2(CH2)4CH2OA and in menthylAH];
3.28–3.36 (m, 1H, ACH< in menthyl); 4.02 [t, 2H, J ¼ 6.4,
ACOO(CH2)5CH2OA]; 4.13 [t, 2H,
J
¼
6.3, ACOOCH2
(CH2)5OA]; 4.35–4.47 (m, 2H, J ¼ 6.5, AOOCCH2OA); 4.92–
5.04 (m, 2H, CH2¼¼); 5.76–5.91 (m, 1H, ¼¼CHA); 7.03 (d, 2H,
J ¼ 8.5); 7.24 (dd, 2H, J ¼ 8.6); 7.53 (d, 2H, J ¼ 8.5); 7.94
(dd, 4H, J ¼ 8.6); 8.15 (d, 4H, J ¼ 8.5); 8.32 (d, 2H, J ¼ 8.5).
4-(Menthyloxyacetoxy)biphenyl-40-(6-(undec-10-enoyloxy)-
hexyloxy)benzoate (M2)
Yield: 49%. mp: 63 ꢀC. IR (KBr, cmꢁ1): 2929, 2869 (CH3A,
ACH2A); 1776, 1728 (C¼¼O); 1641 (C¼¼C); 1604–1492
(ArA); 1207 (CAOAC).
13C NMR (d, ppm from TMS in CDCl3): 20.51, 21.43 (3C,
CH3); 23.73, 24.85, 25.53, 25.84, 28.75, 28.86, 29.15, 29.44,
29.56, 29.66, 29.77, 33.53, 33.64, 34.16, 40.66 (15C, CH2);
25.82, 31.56, 47.93 (3C, CH); 80.53 (1C, OCH in menthyl);
62.83, 66.15, 67.25 (3C, OCH2); 113.92, 121.51, 122.13,
129.54, 130.55, 130.85 (16C, aromatic tert. C); 114.92,
140.11 (2C, C¼¼C); 121.14, 126.88, 136.75, 136.83, 147.31,
147.34, 156.39, 167.59 (8C, aromatic quat. C); 165.52,
165.55, 166.24, 172.72 (4C, C¼¼O).
1H NMR (d, ppm from TMS in CDCl3): 0.79–2.65 [m, 42H,
A(CH2)8A, ACOOCH2(CH2)4CH2OA and in menthylAH];
3.27–3.36 (m, 1H, ACH< in menthyl); 4.06 [t, 2H, J ¼ 6.3,
ACOO(CH2)5CH2OA]; 4.14 [t, 2H,
J
¼
6.3, ACOOCH2
(CH2)5OA]; 4.37–4.48 (t, 2H, J ¼ 6.5, AOOCCH2OA); 4.93–
5.04 (m, 2H, CH2¼¼); 5.76–5.90 (m, 1H, ¼¼CHA); 7.04 (d, 2H,
J ¼ 8.6); 7.12 (dd, 4H, J ¼ 8.5); 7.31 (d, 2H, J ¼ 8.6); 7.82
(d, 2H, J ¼ 8.5); 8.21 (d, 2H, J ¼ 8.5).
ELEM. ANAL. calcd for C55H68O10: C, 74.30%; H, 7.71%. Found:
C, 74.36%; H, 7.83%.
13C NMR (d, ppm from TMS in CDCl3): 20.53, 21.43 (3C,
CH3); 23.71, 24.86, 25.52, 25.82, 28.74, 28.88, 29.16, 29.43,
29.57, 29.66, 29.75, 33.55, 33.63, 34.15, 40.66 (15C, CH2);
25.82, 31.55, 47.92 (3C, CH); 80.51 (1C, OCH in menthyl);
62.85, 66.13, 67.24 (3C, OCH2); 113.93, 122.11, 129.55,
130.85 (12C, aromatic tert. C); 114.89, 140.13 (2C, C¼¼C);
121.09, 136.72, 136.83, 147.29, 149.93, 167.55 (6C, aromatic
quat. C); 165.53, 166.22, 172.73 (3C, C¼¼O).
Synthesis of the Polymers
The synthetic route of the polymers P1–P4 is outlined
in Scheme 3. The yields and average molecular weights of
P1–P4 are listed in Table 1. These polymers were prepared
by the same method, and the synthesis of P1 is given as an
example. M1 (2 mol % excess vs. the SiAH groups in PMHS)
and PMHS were dissolꢀved in dry toluene. The reaction mix-
ture was heated to 70 C under nitrogen and anhydrous con-
ditions, and then 2 mL of THF solution with the H2PtCl6 cat-
alyst (5 mg/mL) was injected into mixture with a syringe.
When the reaction was finished, P1 was obtained by precipi-
tation from toluene solution into methanol, purified by sev-
eral filtrations from hot ethanol, and then dried in vacuo.
ELEM. ANAL. calcd for C48H64O8: C, 74.97%; H, 8.39%. Found:
C, 74.86%; H, 8.48%.
4-(Menthyloxyacetoxybenzoyloxy)biphenyl-40-(2-(undec-10-
enoyloxy)ethoxy)benzoate (M3)
Yield: 54%. mp: 108 ꢀC. IR (KBr, cmꢁ1): 2925, 2853 (CH3A,
ACH2A); 1781, 1733 (C¼¼O); 1641 (C¼¼C); 1606–1495
(ArA); 1212 (CAOAC).
IR (KBr, cmꢁ1): 2923, 2853 (ACH3, ACH2A); 1780, 1737
(C¼¼O); 1607, 1496 (ArA); 1275, 779 (SiAC); 1210
(CAOAC); 1200–1006 (SiAOAS).
1H NMR (d, ppm from TMS in CDCl3): 0.17–2.53 [m, 59H,
CH3SiA, A(CH2)10A and in menthylAH]; 3.30 (m, 1H, ACH<
in menthyl); 4.41 (m, 2H, AOOCCH2OA); 4.55–4.62 [m, 4H,
ACOO(CH2)2OA]; 7.09–8.31 (m, 12H, ArAH).
1H NMR (d, ppm from TMS in CDCl3): 0.83–2.64 [m, 34H,
A(CH2)8A and in menthylAH]; 3.27–3.35 (m, 1H, ACH< in
menthyl); 4.35–4.48 (m, 2H, AOOCCH2OA); 4.54–4.65 [m,
4H, ACOO(CH2)2OA]; 4.93–5.05 (m, 2H, CH2¼¼); 5.78–5.90
(m, 1H, ¼¼CHA); 7.13 (d, 2H, J ¼ 8.5); 7.33 (dd, 2H, J ¼ 8.6);
7.54 (d, 2H, J ¼ 8.5); 7.93 (dd, 4H, J ¼ 8.6); 8.14 (d, 4H, J ¼
8.5); 8.31 (d, 2H, J ¼ 8.5).
13C NMR (d, ppm from TMS in CDCl3): 20.51, 21.45 (3C,
CH3); 23.71, 24.84, 28.75, 29.11, 29.42, 29.65, 29.72, 33.56,
33.65, 34.12, 40.66 (11C, CH2); 25.83, 31.55, 47.92 (3C, CH);
80.51 (1C, OCH in menthyl); 62.85, 66.13, 67.24 (3C, OCH2);
113.91, 121.48, 122.11, 129.55, 130.56, 130.83 (16C, aro-
matic tert. C); 114.91, 140.12 (2C, C¼¼C); 121.13, 126.88,
136.74, 136.81, 147.29, 147.31, 156.38, 167.57 (8C, aromatic
quat. C); 165.52, 165.54, 166.23, 172.73 (4C, C¼¼O).
RESULTS AND DISCUSSION
Synthesis and Characterization
Until now, research on the design and synthesis of new chi-
ral LC materials has all long attracted considerable interest.
It is well known that menthol has three chiral centers and
strong optical activity, so the LC materials based on menthol
are expected to obtain a chiral smectic C and cholesteric
phases and show excellent mesomorphism. To investigate
the effect of spacer length and rigid mesogenic core on phys-
ical properties and phase behavior of target monomers and
polymers, we designed and prepared four new chiral LC
monomers (with same terminal group, different mesogenic
rigidity, and spacer length) and their corresponding homo-
polymers containing menthyl group.
ELEM. ANAL. calcd for C51H60O10: C, 73.53%; H, 7.26%. Found:
C, 74.02%; H 7.37%.
4-(Menthyloxyacetoxybenzoyloxy)biphenyl-40-(2-(undec-10-
enoyloxyhexyloxy)-benzoate (M4)
Yield: 56%. mp: 82 ꢀC. IR (KBr, cmꢁ1): 2927, 2855 (CH3A,
ACH2A); 1787, 1731 (C¼¼O); 1641 (C¼¼C); 1604–1492
(ArA); 1205 (CAOAC).
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SCHEME 3 Synthetic route of the polymers P1–P4.
For the purification of 9, the byproduct for biphenyl-4,40-diyl
bis(4-(2-undec-10-enoyloxy)ethoxy)benzoate) was removed
by washing with 1.5% NaOH solution, and the compound 9
was obtained by recrystallization from ethanol/acetone (3:1)
to remove the unreacted material 4,40-dihydroxybiphenyl.
The purification process of 10 is same as 9. The synthesis of
the monomers M1–M4 is performed by the same method.
They were prepared by reacting 2 or 4 with 9 and 10,
respectively, in chloroform in the presence of pyridine. The
chemical structures of the monomers M1–M4 were character-
ized by FTIR and 1H NMR. IR spectra of M1–M4 showed
characteristic stretching bands at 1787–1776 cmꢁ1 attrib-
uted to ester C¼¼O in menthyloxyacetate, 1733–1728 cmꢁ1
attributed to ester C¼¼O in undecylenate and substituted
benzoate, 1641 cmꢁ1 attributed to olefinic C¼¼C, and 1606–
1495 cmꢁ1 attributed to aromatic C¼¼C. 1H NMR spectra of
M1–M4 multiplet at the chemical shift values nearby 0.83–
4.65, 4.93–5.90, and 7.00–8.28 ppm correspond to methyl
and methylene protons, olefinic protons, and aromatic pro-
tons, respectively. 1H NMR spectra of M2 as an example are
shown in Figure 1. The spectra suggested that the purity of
these monomers was high, and this was confirmed by the
elemental analyses.
The polymers P1–P4 were prepared through the hydrosilyla-
tion reaction using an excess amount of olefinic monomers.
The unreacted monomers were removed by several repreci-
pitation from toluene solutions into methanol and filtrations
from hot ethanol. The progress of the hydrosilylation reac-
tion, monitored by the SiAH stretch intensity, went to com-
pletion, as indicated by IR when the complete disappearance
of SiAH stretching band at 2165 cmꢁ1 and olefinic C¼¼C
stretching band at 1640 cmꢁ1. Characteristic SiAC bands at
about 1269 and 782 cmꢁ1, and SiAOASi bands at about
1203, 1166, 1117, 1071, and 1005 cmꢁ1 appeared.
Specific Optical Rotations
The specific optical rotations of M1–M4 and P1–P4 were eval-
uated at 25 ꢀC in chloroform and toluene, respectively, and
the obtained results are summarized in Table 1. As can be
seen from the data listed in Table 1the specific optical rota-
tions of M1–M4 and P1–P4 were all left handed; moreover,
the specific optical rotation absolute values decreased with
increasing the aryl number in the mesogenic units; for exam-
25
ple, compared with M1 (½aꢂD ¼ ꢁ38.4ꢀ), M3 with more aryl
TABLE 1 Specific Optical Rotation, Yields, and Average
Molecular Weight of Sample
Yield
a
b
25
25
Monomer
½aꢂD
Polymer
½aꢂD
(%)
Mn ꢃ 10ꢁ3
M1
M2
M3
M4
a
ꢁ38.4
ꢁ37.9
ꢁ22.5
ꢁ20.4
P1
P2
P3
P4
ꢁ32.4
ꢁ32.0
ꢁ18.1
ꢁ13.7
86
88
83
82
8.2
9.3
10.1
10.9
Specific optical rotation, about 0.21 g in 100 mL of CHCl3.
Specific optical rotation, about 0.15 g in 100 mL of toluene.
FIGURE 1 1H NMR spectrum of M2.
b
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TABLE 2 The Phase Transition Temperatures and Mesophase
of Monomers
Mesophase, Phase Transition Temperature (ꢀC),
and Enthalpy Changes (J/g)
Monomer Heating Cycle
Cooling Cycle
M1
M2
M3
K 72.3 (70.7) I
I 66.2 (4.7) Ch 43.9 (19.9) K
K 63.0 (38.8) Ch 81.1 (3.5) I I 79.6 (3.5) Ch 35.1 (12.9) K
K 108.1 (7.7) SmC* 147.0 I 173.0 (1.1) Ch 141.1 (0.6)
(1.0) Ch 175.4 (0.4) I
K81.5 (20.3) SmC* 148.9 I 180.0 (0.4) Ch 148.0 (2.4)
(2.2) Ch 180.8 (0.5) I SmC* 74.1 (12.0) K
SmC* 96.1 (5.8) K
M4
K, solid; SmC*, chiral smectic C phase; Ch, cholesteric phase; I, isotropic.
segments showed lower specific optical rotation absolute
values. The results suggested that the existence of more phe-
nyl segments affected the molecular polarity leading to the
decrease of the specific rotation absolute values, which is in
agreement with the results reported by Liu and Yang.39
FIGURE 2 DSC curves of M4.
SmC* phase transition, and a SmC* to crystallization transi-
tion, respectively.
The mesogenic rigidity and flexible spacer length had a con-
siderable influence on the phase behavior of the monomers.
The different phase ranges of M1–M4 are presented in
Figure 3. M1 only showed the melting temperature (Tm) on
heating process, and the isotropic temperature (Ti) was not
seen. Although the same mesogenic core as M1, M2 with lon-
ger flexible spacer showed good mesomorphism, similar
results could be seen for M3 and M4. With increasing the
number of the flexible methylene, the corresponding Tm
decreased from 108.1 ꢀC for M3 to 81.5 ꢀC for ꢀM4, whereas
The specific optical rotation of P1–P4 showed the same tend-
ency as the above monomers. In addition, the specific optical
rotation absolute values of P1–P4 were less than those of the
corresponding monomers. This indicated that the concentra-
tion of optically active sites decreased because these poly-
mers consisted of optically inactive polysiloxane. This of
course affected the amount of optically active sites and this
in turn decreased the optical rotation absolute values.
Thermal Properties
ꢀ
The thermal properties of M1–M4 and P1–P4 were studied
with DSC. Their phase transition temperatures and corre-
sponding enthalpy changes, obtained on the second heating
and cooling scans from DSC, are summarized in Tables 2 and
3. Representative DSC curves of M4 are shꢀown in Figure 2.
The DSC heating and cooling rates were 10 C/min.
the Ti increased from 175.4 C for M3 to 180.0 C for M4, so
the mesophase range (DT) widen. In addition, it was clearly
seen in Figure 3 that the SmC* phase temperature range
increased more than the cholesteric phase temperature
range. With increasing the number of aryl rings or meso-
genic rigidity, the two mesophase and higher phase transi-
tion temperatures were seen. Different from M1 and M2, M3
and M4 showed the SmC* besides cholesteric phase.
According to DSC curves, M1 showed an endothermic peak
on heating and two exothermic peaks on cooling process,
and exhibited monotropic cholesteric phase. M2 showed two
endothermic peaks and two exothermic peaks, and exhibited
enantiotropic cholesteric phase. M3 and M4 showed three
endothermic peaks, which represent a melting transition, a
chiral smectic C (SmC*) to cholesteric phase transition, and a
cholesteric to isotropic phase transition, respectively. On
cooling, three exothermic peaks were seen, which represent
an isotropic to cholesteric phase transition, a cholesteric to
TABLE 3 The Phase Transition Temperatures of Polymers
Polymer
Phase Transition Temperature (ꢀC)
P1
P2
P3
P4
g 28.3 SmA 116.2 (2.8) I
g 16.5 SmA 124.1 (3.0) I
g 41.1 SmC* 101.5 (0.8) SmA 226.3 (1.4) I
g 27.2 SmC* 105.2 (1.1) SmA 228.2 (1.6) I
FIGURE 3 Different mesophase ranges of M1–M4. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
g, glass state; SmA, smectic A phase; SmC*, chiral smectic C phase;
I, isotropic.
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cholesteric phase. For M3 and M4, they all exhibited broken
fan-shaped texture of a SmC* phase, oily streak and focal
conic textures of cholesteric phase, and platelet texture of a
cubic blue phase. The optical textures of M4, as an example,
are shown in Figure 5(a–d).
The blue phase is frustrated defect phaseꢀ, generally observed
in a temperature interval smaller than 1 C, between the iso-
tropic phase and the cholesteric phase of highly chiral meso-
gens, and reveals different color corresponding to different
lattice planes, which shows Bragg scattering at different
wavelengths, because the condition for constructive interfer-
ence depends on the distance between lattice planes and
their orientation with respect to the direction of incident
light. Because of much smaller enthalpies for subsequent
transition between the blue phase and into the cholesteric
phase,46 the transition trace of the blue phase using tradi-
tional DSC cannot be seen, but the visual appearance of the
cubic blue phase wꢀith POM is quite characteristic. On very
slow cooling of 0.2 C/min, single crystals of the blue phase
can be obtained, further cooling the blue phase results in a
transition to the cholesteric phase. The platelet texture of
FIGURE 4 Different mesophase ranges of P1–P4. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Moreover, compared with Tm and Ti of M2, those of M4
ꢀ
increased by 18.5 and 99.7 C, respectively, and DT widened
ꢀ
from 18.1 ꢀC for M2 to 99.3 ꢀC for M4 because the Ti
increased greater than Tm with increasing the mesogenic
core rigidity.
the blue phase was clearly seen on slow cooling to 173.9 C
for M3 and 179.7 ꢀC for M4, and gradually transformed to
the focal conic texture of the cholesteric phase at 172.8 ꢀC
ꢀ
for M3 and 177.9 C for M4.
The polymers P1 and P2 showed a glass transition at low
temperature and a LC to isotropic phase transition at high
temperature, whereas P3 and P4 all showed a glass transition,
SmC* to SmA phase transition, and SmA to isotropic phase
transition, respectively. For the side-chain LC polymers, it is
well known that their phase behavior mainly depended on
the polymer backbone, mesogenic core, flexible spacer, and
terminal groups. For P1–P4, because of the same polysiloxanes
backbone and terminal groups, their phase behavior mainly
depended on the mesogenic rigidity and spacer length.
According to Table 3the influence of the mesogenic rigidity
and spacer length on the phase behavior of P1–P4 showed the
same tendency as those described above for the monomers.
Namely, with increasing the mesogenic rigidity, the glass tran-
sition temperature (Tg) and Ti all increased; as the spacer
length increased, the Tg decreased while Ti increased. This
result suggested that a longer spacer tended to stabilize the
mesophase more than a shorter one because longer spacer
could give higher degree of decoupling between the meso-
genic side groups and main chains. In addition, the mesophase
ranges of P1–P4, presented in Figure 4 were greater than
those of the corresponding monomers. This indicated that the
polymerization could further stabilize and widen the meso-
phase range. In a word, a flexible polysiloxane backbone and
a long flexible spacer had a tendency toward a low glass tran-
sition temperature and wide mesophase range.
The polymers P1 and P2 exhibited an enantiotropic fan-
shaped texture of a smectic A (SmA) phase; in this case, the
smectic layers are basically perpendicular to the substrate
plane. This indicated that the more ordered SmA phase was
formed by the polymerization compared with the corre-
sponding monomers shown cholesteric phase. In addition, P3
and P4 showed broken fan-shaped texture of the SmC* phase
besides SmA phase. This behavior was attributed to an
increased density of the mesogenic units in side chain and
hence an ordered organization into the LC phases. The opti-
cal texture of P4 on cooling is shown in Figure 6.
The Selective Reflection of Light
The unique optical properties of SmC* and cholesteric LC are
related to their helical supermolecular structure. The periodic
helical structure selectively reflects visible light like an ordinary
diffraction grating, the pitch of which controls the wavelength
of the selective reflection of light. If the reflected wavelength
lies in the visible range of the spectrum, both SmC* and choles-
teric phases exhibit brilliant colors. Because of the angular
dependence of the reflection conditions, the wavelength of the
selective reflection of light k obeys the Bragg condition:
k ¼ nP cos h;
(1)
where n is the average index, n ¼ (ne þ no)/2, ne and no are
the extraordinary and ordinary indices of refraction, respec-
tively. P is the pitch of SmC* or cholesteric phase, defined as
the spatial distance over which the director rotates 360ꢀ,
and h is the incidence angle. The width of the selective
reflection band Dk is equal to pDn, where Dn ¼ ne ꢁ no is
the birefringent of a LC layer perpendicular to the helix axis.
Optical Texture
To observe and record the optical textures of M1–M4 and
P1–P4, a POM with hot stage was used. POM results showed
that M1 only exhibited focal conic texture of a cholesteric
phase during cooling process, whereas M2 exhibited enantio-
tropic oily streak texture and focal conic texture of the
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FIGURE 5 Optical textures of M4 (ꢃ200). (a) Oily streak texture of cholesteric phase on heating to 171 ꢀC; (b) platelet texture of a
cubic blue phase on cooling to 179 ꢀC; (c) focal conic texture of cholesteric phase on cooling to 175 ꢀC; and (d) broken fan-shaped
texture of SmC* phase on cooling to 138 ꢀC.
In fact, the helical axis of SmC* or cholesteric phase is not
completely normal to substrate plane, so eq 1 was revised
to eq 2:
where h1 and h2 are the incidence angle and reflection
angle of light, respectively. When the normal incidence
occurred, h ¼ 0ꢀ, a maximum wavelength km is described as
follows:
ꢂ
ꢀ
ꢁ
ꢀ
ꢁꢃ
1
sin h1
sin h2
k ¼ nP cos arcsin
þ arcsin
;
(2)
km ¼ nP:
(3)
2
n
n
FIGURE 6 Optical textures of P4 (ꢃ200). (a) Broken fan-shaped texture at 100 ꢀC and (b) fan-shaped texture at 180 ꢀC.
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FIGURE 9 Variation of the d-layer spacing as a function of tem-
perature for P4.
phase transition temperatures but also the selective reflec-
tion wavelength of SmC* and cholesteric phases. With
increasing the mesogenic rigidity or reducing the spacer
length, the selective reflection of light shifted to the long
wavelength region. The reflection color of M2–M4 mainly
exhibited red at SmC* phase, and red and green at choles-
teric phase. It is well known that dP/dT > 0 for SmC* phase,
and dP/dT < 0 for cholesteric phase, so km is temperature
dependent. To describe the relationships of km and tempera-
ture, km of M4 was measured by UV/VIS spectra with hot
stage. Figure 7 shows the UV/VIS spectra of M4 with tem-
perature. km increased from 732 nm at 90 ꢀC to 859 nm at
150 ꢀC at SmC* phase, whereas km decreased from 689 nm
at 160 ꢀC to 539 nm at 180 ꢀC at cholesteric phase; this
indicated that the selective reflection shifted to the long
wavelength region (red shift) at SmC* phase and to the short
wavelength region (blue shift) at cholesteric phase with
increasing temperature.
FIGURE 7 UV/Vis spectra of M4 at SmC* (a) and cholesteric (b)
phase temperature ranges.
The selective reflection wavelength depends on the pitch,
and the pitch depends on the molecular structure and exter-
nal field. For M2–M4, the structure affected not only the
Although the polymers P3 and P4 also showed the SmC*
phase, no selective reflection color was macroscopic seen,
and no wavelength of the selective reflection at 200–800 nm
was shown in the curves measured by UV/VIS spectra.
TABLE 4 TGA Data of the Polymers
Weight Loss (%)
a
b
Td
Tmax
Polymer
(ꢀC)
(ꢀC)
300 ꢀC
400 ꢀC
500 ꢀC
600 ꢀC
P1
P2
P3
P4
a
335
338
340
345
374
376
381
380
0.5
0.5
0.4
0.3
62.1
60.3
59.1
58.6
79.9
79.5
78.6
78.2
87.9
88.1
87.7
88.4
Temperature at which 5% weight loss occurred.
b
FIGURE 8 XRD curve of P4 at different mesophase ranges.
Temperature of maximum thermal decomposition rate.
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of a biphenyl segment and the longer side chain groups may
cause intermolecular pꢁp interaction between the mesogenic
units in side chain, and interaction between the repeating
units and entanglement of the polymer chains, which are
expected to increase in the stabilization of the polymer.
CONCLUSIONS
The synthesis and characterization of four new chiral LC
monomers and the corresponding polymers based on men-
thol are described. By inserting a flexible spacer between the
mesogenic core and the bulky terminal menthyl fragments,
M1–M4 all showed mesomorphism and exhibited cholesteric
oily streak texture or focal conic texture. In addition, M3 and
M4 also exhibited broken fan-shaped of a SmC* phase and
platelet texture of a cubic blue phase. km increased at SmC*
phase and decreased at cholesteric phase with increasing
temperature. P1–P4 exhibited broken fan-shaped texture of
the SmC* phase and fan-shaped texture of SmA phase. With
increasing the number of aryl rings, the corresponding Tm,
Ti, and DT all increased. With increasing the number of the
flexible methylene, the corresponding Tm decreased, whereas
Ti and DT increased. In addition, the effect of mesogenic
core and spacer length on Tg and Ti showed the similar tend-
ency as those described for the monomers. Moreover, the
mesophase ranges of P1–P4 were greater than those of the
corresponding monomers. In addition, the experimental
results demonstrated that a flexible siloxane backbone and a
long spacer tended to exhibit a low glass transition tempera-
ture, more ordered phase, wide mesophase range, and high
thermal stability. These experimental results can supply new
chiral LC material and theoretical basis to further synthesize
and study SmC* or cholesteric elastomers with ferroelectric-
ity or piezoelectricity.
FIGURE 10 TGA curves of P1 and P4. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
XRD Studies
To identify mesophase structure, XRD studies were carried
out. Figure 8 displays the representative XRD curves of P4 at
ꢀ
65 and 110 C. A sharp reflection corresponding to the peri-
odic distance associated with the smectic layers at low angle,
and a broad peak associated with the lateral interference of
the mesogenic cores or the distance of the local arrangement
between the mesogenic side groups in the smectic layer at
wide angle were observed. To further confirm the presence of
the SmC* and SmA phases, the temperature-dependent d-spac-
ing of the smectic layers obtained from powdered samples of
P4 is shownꢀin Figure 9. On cooling from isotropic tempera-
ture to 105 C, the d-spacing was practically constant. The d-
spacing of corresponding first-order and second-order reflec-
tions was about 40.4 and 20.8 Å, respectively. This result sug-
gested the presence of a SmA phase, which is also confirmed
by POM. When the measuring temperature lowered from 100
ACKNOWLEDGMENTS
The authors are grateful to Program for Science and Technology
Bureau of Shenyang (F12-277-1-44) and Fundamental
Research Funds for the Central Universities (N110405006 and
N110705001) for financial support of this study.
ꢀ
to 45 C, the d-spacing of the first-order reflection gradually
decreased from 40.4 to 36.8 Å, which gives strong evidence
for the formation of a tilted SmC* phase.
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