Synthesis and mesomorphic properties of low molar mass phosphorus derivatives

Article abstract of DOI:10.1080/15421400490427421

New liquid crystalline phosphorus derivatives were synthesized as a core model compound and/or a model compound of the component for semiflexible liquid crystalline polymers. Alkoxy, oligo ethylene oxide, or chiral moieties were introduced into the terminal group of rigid moiety in order to investigate the effect of the terminal group on the thermal properties of the phosphorus derivatives. A nematic phase appears for the phosphorus derivative with alkoxy moieties or oligo ethylene oxide chain, while a mesophase is observed for the phosphorus derivative with a chiral moiety. Some phosphorus derivatives were thermally stable until more than 300°C.

Full text of DOI:10.1080/15421400490427421

This article was downloaded by: [University of Chicago Library]
On: 28 September 2013, At: 06:19
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
Synthesis and Mesomorphic
Properties of Low Molar Mass
Phosphorus Derivatives
N. Tokushige a , T. Mihara a & N. Koide a
a Department of Chemistry, Faculty of Science,
Science University of Tokyo, 1–3 Kagurazaka,
Shinjuku-ku, Tokyo, 162-8601, Japan
Published online: 18 Oct 2010.
To cite this article: N. Tokushige , T. Mihara & N. Koide (2004) Synthesis and
Mesomorphic Properties of Low Molar Mass Phosphorus Derivatives, Molecular Crystals
and Liquid Crystals, 414:1, 27-38, DOI: 10.1080/15421400490427421
To link to this article: http://dx.doi.org/10.1080/15421400490427421
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.

This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 414, pp. 27–38, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400490427421
SYNTHESIS AND MESOMORPHIC PROPERTIES
OF LOW MOLAR MASS PHOSPHORUS DERIVATIVES
N. Tokushige, T. Mihara, and N. Koide
Department of Chemistry, Faculty of Science,
Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku,
Tokyo 162-8601, Japan
New liquid crystalline phosphorus derivatives were synthesized as a core
model compound and=or a model compound of the component for semiflexible
liquid crystalline polymers. Alkoxy, oligo ethylene oxide, or chiral moieties
were introduced into the terminal group of rigid moiety in order to investigate
the effect of the terminal group on the thermal properties of the phosphorus
derivatives. A nematic phase appears for the phosphorus derivative with
alkoxy moieties or oligo ethylene oxide chain, while a mesophase is observed
for the phosphorus derivative with a chiral moiety. Some phosphorus deriva-
tives were thermally stable until more than 300ꢀC.
Keywords: liquid crystal; phosphorus derivative
INTRODUCTION
Generally, phosphorus compounds, organic bromine compounds, and
inorganic compounds are used as a flame-retardant additive. At the present
time, it is well known that phosphorus compounds are generally used as the
most effective flame retardant additive for polymers. The flame retardant
additives are used in relatively high concentration to enhance thermal stab-
ility of polymers. As a result, the use of the flame retardant additives can
lead to undesirable changes in physical and mechanical properties of the
polymers.
On the other hand, with regard to polymers without flame-retardant
groups, the liquid crystalline (LC) polyesters themselves have good flame-re-
tardant properties and exhaust carbon dioxide in the combustion process;
however, the flame-retardant properties of the LC polyesters are not ad-
equate for processability conditions such as high temperature and pressure.
Address correspondence to N. Koide, e-mail: nkoide@ch.kagu.sut.ac.jp
27

28
N. Tokushige et al.
Copolyesters (polyethylene naphthalate and polybutylene naphthalate)
containing phosphorus compounds were prepared by copolymerization of
naphthalene derivative and phosphorus compound [1]. Moreover, various
polyphosphates were reported by Kishore et al. [2–5].
Senthil et al. reported synthesis and characterization of metallomeso-
genic semiflexible liquid crystalline polymers (LCPs) containing a nonme-
sogenic phosphorus derivative [6,7]. The effect of the phosphorus
derivative in the spacer and metallomesogen on the thermal properties
were examined.
The flame-retardant properties of LCP can be greatly enhanced by intro-
ducing the phosphorus derivative to the LCP. Generally, it is necessary to
attach mesomorphic properties to phosphorus derivatives as a component
for semiflexible LCPs to enhance the liquid crystallinity of semiflexible
LCPs or to avoid reduction of the liquid crystallinity of semiflexible LCPs.
However, the phosphorus derivatives with mesomorphic properties are
little investigated. In this study, we synthesized novel low molar mass phos-
phorus derivatives as a core model compound and=or a model compound of
the component for semiflexible LCPs, as shown in Figure 1, and investi-
gated the thermal properties by polarized optical microscope, differential
scanning calorimetry (DSC) measurements, and thermogravimetric analy-
sis (TGA). Furthermore, the structure of the mesophase of the phosphorus
derivatives was examined by X-ray measurements.
FIGURE 1 Chemical structure of phosphorus derivatives.

LC Phosphorous Derivatives
29
EXPERIMENTAL
Materials
The phosphorus derivatives were prepared according to Schemes 1 and 2.
Typical procedures are described below.
4-(4-Hydroxyphenyl)benzoic acid methyl ester (1)
4-(4-Hydroxyphenyl)benzoic acid (5.0 g, 23.3 mmol) was dissolved in
methanol (150 ml). Sulfuric acid (5.0 ml) was added dropwise to the
methanol solution, and then the reaction mixture was refluxed for 6 h.
After methanol was evaporated to dryness under reduced pressure, the
residue was washed with water until a neutral aqueous solution was
obtained. The white solid was obtained in a 97.0% yield (5.2 g).
IR (nujol) m cmꢁ1: 3400 (OH, phenol), 1737 (C¼O, ester), and 1605 and
1508 (aromatic group).
4-(4-Octyloxyphenyl)benzoic Acid (3)
The compound 1 (5.0 g, 21.9 mmol) was dissolved in 2-butanone
(150 ml), and then potassium carbonate (9.0 g, 65.7 mmol) was added to
the 2-butanone solution. The mixture was refluxed and then 1-bromooc-
tane (12.6 g, 65.7 mmol) was added dropwise to the mixture. The reaction
mixture was refluxed for 6 h. The filtrate was evaporated to dryness under
reduced pressure and the ethanol (100 ml) was added to the residue.
Potassium hydroxide (3.7 g, 65.7 mmol) aqueous solution (30 ml) was
SCHEME 1 Synthesis of phosphorus derivatives (6O-PD and 8O-PD).

30
N. Tokushige et al.
SCHEME 2 Synthesis of phosphorus derivatives (2MB-PD and 2EO-PD).
added to the ethanol solution and then the reaction mixture was refluxed
for 30 min. Ethanol was evaporated and then HCl aqueous solution was
added to the residue. The mixture was refluxed for 3 h. The precipitate
obtained was washed with water. The solid was dried in vacuo and then
washed with hexane. The product was obtained in a 74% yield (5.3 g).
The product was used without further purification.
IR (nujol) m cmꢁ1: 2723, 2678 (OH, carboxylic acid), 1685 (C¼O,
carboxylic acid), 1604 and 1504 (aromatic group), and 1252 (ArꢁOꢁCH2,
1
ether). H-NMR (DMSO) d ppm: 0.9(t, 3H, CH3), 1.2–1.8(m, 12H, CH2),
4.0(t, 2H, OꢁCH2), 7.0(d, 2H, ArꢁH), 7.6–7.8(q, 4H, ArꢁH), 8.0(d, 2H,
ArꢁH).
(2-Methoxyethoxy)ethyl Tosylate (7: 2EOTs)
Diethylene glycol monomethyl ether (20 g, 121 mmol) was dissolved in
pyridine (200 ml). p-Toluenesulfonyl chloride (27 g, 141 mmol) was added
to the pyridine solution in ice bath. The reaction mixture was stirred for
6 h. A large quantity of water was added to the reaction mixture in ice bath
and then stirred for 6 h. The aqueous mixture was extracted with
chloroform, and the chloroform solution was dried over magnesium sulfate.
The chloroform was evaporated. The residual liquid was obtained in a 90%
yield (35.2 g). The liquid was used without further purification.

LC Phosphorous Derivatives
31
2-Methylbutyl tosylate (6)
The compound 6 (2MBTs) was prepared according to the synthetic
procedure of compound 5 (2EOTs). The liquid was obtained in an 84%
25
yield. ½aꢂD ¼ þ2.0ꢀ (C¼3, CHCl3).
4-[4-(2-methylbutoxy)phenyl] Benzoic Acid (8: 2MBBCA)
The compound 1 (4.0 g, 17.5 mmol) was dissolved in 2-butanone
(150 ml), and then potassium carbonate (7.3 g, 52.5 mmol) was added to
the 2-butanone solution. The 2-butanone solution was refluxed and then
the compound 4 (8.5 g, 35.0 mmol) was added dropwise to the 2-butanone
solution. The reaction mixture was refluxed for 12 h. The filtrate was
evaporated to dryness under reduced pressure, and then ethano (100 ml)
was added to the residue. Potassium hydroxide (9.0 g, 157 mmol) aqueous
solution (30 ml) was added to the ethanol solution and then the reaction
mixture was refluxed for 30 min. Ethanol was evaporated and the HCl aque-
ous solution was added to the residue. The mixture was refluxed for 3 h.
The precipitate obtained was washed with water. The solid was dried in
vacuo, and then washed with hexane. The product was obtained in a
1
48% yield (2.4 g). The product was used without further purification. H-
NMR (DMSO) d ppm: 0.95–1.1(m, 6H, CH3), 1.2(m, 1H, CH2), 1.3(m, 1H,
CH2), 1.9(m, 1H, CH), 3.9(m, 2H, OꢁCH2), 7.1(d, 2H, ArꢁH), 7.7–7.8(q,
4H, ArꢁH), 8.0(d, 2H, ArꢁH).
4-{4-[2-(2-Methoxyethoxy)ethoxy]phenyl}
(9: 2EOBCA)
The compound 9 (2EOBCA) was prepared according to the synthetic
Benzoic
Acid
1
procedure of the compound 8. H-NMR (CDCl3) d ppm: 3.3(s, 3H, CH3),
3.6(d, 2H, OꢁCH2), 3.7(d, 2H, OꢁCH2), 3.8(d, 2H, OꢁCH2), 4.1(d, 2H,
OꢁCH2), 6.9(d, 2H, ArꢁH), 7.5(d, 2H, ArꢁH), 7.6(d, 2H, ArꢁH), 8.1(d,
2H, ArꢁH).
80-PD (5)
The compound 3 (1.8 g, 4.3 mmol) and 2-(6-oxide-6H-dibenz hc,ei h1,2i
oxaphosphorin-6-yl]-1,4-benzenediol (0.7 g, 2.2 mmol) were dissolved in
tetrahydrofuran (THF, 150 ml) under nitrogen atmosphere. 4-Dimethylami-
nopyridine (0.07 g, 0.55 mmol) was added to the THF solution, and then a
THF solution (30 ml) of N,N0-dicyclohexylcarbodiimide (2.7 g, 13.2 mmol)
was added dropwise to the THF solution in ice bath. The reaction mixture
was stirred for 6 h. After filtration, THF was evaporated to dryness under
reduced pressure. The residue was washed with water and methanol. The
crude product was purified by recrystallization from mixed solvent of
chloroform and methanol (chloroform=methanol ¼ 3=1). The white solid
was obtained in a 28% yield (0.5 g).

32
N. Tokushige et al.
IR (nujol) m cmꢁ1: 1726 (COO, ester group), 1604 and 1496 (aromatic
1
group). H-NMR (CDCl3) d ppm: 0.9(m, 6H, CH3), 1.3–1.6(m, 4H, CH2),
1.8(m, 4H, CH2), 4.0(m, 4H, OꢁCH2), 7.0–8.2(m, 27H, ArꢁH).
60-PD, 2MB-PD, and 2EO-PD
60-PD, 2MB-PD, and 2EO-PD were prepared according to the synthetic
procedure of 8O-PD.
2MB-PD was obtained in a 58% yield. IR (nujol) m cmꢁ1: 1733 (COO,
ester group), and 1604 and 1496 (aromatic group). 1H-NMR (CDCl3)
d ppm: 0.9–1.0(m, 12H, CH3), 1.3(m, 2H, CH), 1.5(m, 2H, CH2), 1.8(m,
2H, CH2), 3.8(m, 4H, OꢁCH2), 6.9–8.2(m, 27H, ArꢁH).
25
½aꢂD ¼ þ0:05ꢀ ðC¼3; CHCl3Þ:
2EO-PD was obtained in a 27% yield. IR (nujol) m cmꢁ1: 1731 (COO, ester
group), and 1604 and 1496 (aromatic group). 1H-NMR (CDCl3) d ppm:
3.3(d, 6H, CH3), 3.5(q, 4H, OꢁCH2), 3.7(q, 4H, OꢁCH2), 3.8(q, 4H,
OꢁCH2), 4.1(q, 4H, OꢁCH2), 7.0–8.2(m, 27H, ArꢁH).
Characterization
1H-NMR was carried out with a JEOL JNM-LA 400 spectrometer using CDCl3
as the solvent. Infrared spectra were recorded on JEOL JIR 7000 spec-
trometer. Spectra were collected at 4 cmꢁ1 resolution. DSC measurements
were conducted with a Mettler DSC821e. Entropy changes from mesophase
to isotropic phase (DSMI) of phosphorus derivatives were obtained by the
equation, DSMI ¼ DHMI=T. TGA was carried out using a Mettler TG50 system
in air. Optical microscopy measurements were performed on a Nikon polar-
izing optical microscope, OPTIPHOTO-POL, equipped with a Mettler FP80
controller and a FP82 hot stage. X-ray diffraction patterns were recorded
with a RIGAKU RINT2500 with Ni-filtered CuKa radiation. The sample in
quartz capillary (diameter 1 mm) was held in a temperature-controlled cell
(RIGAKU LC high temperature controller). Optical rotation was recorded
with a HORIBA SEPA-300 optical rotatory dispersion spectrometer. The
sample was prepared using chloroform as a solvent.
RESULTS AND DISCUSSION
Synthesis of the phosphorus derivatives is described in Schemes 1 and 2.
Thermal properties of the phosphorus derivatives are summarized in
Table 1. A schlieren texture or a thread texture is observed on both heating
and cooling runs for 6O-PD, 8O-PD, and 2EO-PD, as shown in Figures 2(a),
2(b), and 2(d). The schlieren texture of 2EO-PD is preserved at room

LC Phosphorous Derivatives
33
TABLE 1 Thermal Properties of Phosphorus Derivatives
Phosphorus derivatives Phase transition temperatures=ꢀC DSMI=(Jmol ꢁ1K ꢁ1
Td=ꢀCb
a
)
185
235
ƒƒ! ƒƒ!
60-PD
Cr
Cr
Cr
N
I
I
5.2
3.1
3.9
365
295
358
ƒƒ ƒƒ
164
176
234
220
ƒƒ! ƒƒ!
80-PD
N
ƒƒ ƒƒ
162
220
207
ƒƒƒƒƒƒƒƒ!
2MB-PD
I
M
ƒƒ ƒƒ
178
202
43
204
ƒƒ! ƒƒ!
2EO-PD
g
N
I
4.2
322
ƒƒ ƒƒ
39
203
g, glassy; Cr, crystal; N,nematic; M, mesophase; I, isotropic phase.
a Entropy change at clearing points by DSC measurements.
b Temperatures at which 10% weight loss was recorded by TGA measurements.
temperature. On the other hand, a focal conic texture appears for
2MB-PD on cooling run as shown in Figure 2(c). Generally, a focal conic
texture appears in a smectic phase and a cholesteric phase with the
FIGURE 2 Optical textures of phosphorus derivatives; (a) 6O-PD at 230ꢀC, (b) 8O-
PD at 210ꢀC, (c) 2MB-PD at 198ꢀC, and (d) 2EO-PD at 200ꢀC.

34
N. Tokushige et al.
random alignment of helical axis. To assign the exact structure of the meso-
phases, the shear strain method offers a promising identification of a focal
conic texture. A focal conic texture of a smectic phase appears again very
soon after removing shear strain, while for a cholesteric phase, a planar tex-
ture instead of a focal conic texture is usually observed after removing
shear strain. We imposed shear strain by shifting the glass substrate to con-
firm the structure of the mesophase for 2MB-PD. After removing shear
strain for 2MB-PD, a planar texture of 2MB-PD forms and then a focal conic
texture reappears.
Typical DSC curves of the phosphorus derivatives are shown in Figure 3.
Two phase-transition peaks are observed on both heating and cooling runs
for the phosphorus derivatives with terminal alkoxy chains (6O-PD and 8O-
PD). Each peak is assigned to the melting and clearing points, respectively,
based on the polarized microscope measurements. Mesomorphic tempera-
ture range of 6O-PD is wider than that of 8O-PD. With regard to 2MB-PD,
which has chiral terminal moieties, a single peak appears on a heating run
while two peaks are observed on a cooling run. This result indicates that
2MB-PD exhibits a monotropic mesophase. Glass transition temperature
FIGURE 3 DSC curves of phosphorus derivatives; (a) 6O-PD, (b) 2EO-PD, and (c)
expansion in the temperature range of 25–70ꢀC for 2EO-PD.

LC Phosphorous Derivatives
35
and clearing point are observed in DSC curves of 2EO-PD. The structure of
the mesophase for 2EO-PD is preserved at room temperature.
Only a broad peak in the wide angle region is observed in the X-ray
pattern of 6O-PD at 200ꢀC on cooling run as shown in Figure 4(a). The
X-ray pattern and observation of a schlieren texture indicates that the
structure of the mesophase for 6O-PD is a nematic phase. On the other
hand, a peak in the small angle region and a broad peak in the wide angle
region are observed in the X-ray pattern of 8O-PD at 190ꢀC as shown in
Figure 4(b). The X-ray pattern of 8O-PD would be similar to that of a smec-
tic phase. The d-spacing between smectic layers based on the peak in the
˚
small angle region is 31.5 A. The calculated molecular length of 8O-PD with
˚
extended methylene conformation is 43 A. Therefore we consider that
director of 8O-PD should be tilted in the layer (smectic C phase, tilt angle:
43ꢀ), based upon the X-ray pattern and the observation of a schlieren tex-
ture or a thread texture. However, the phase transition entropy changes
from mesophase to an isotropic liquid (DSMI) for 8O-PD is 3.1 Jmolꢁ1Kꢁ1
FIGURE 4 X-ray patterns of phosphorus derivatives in the mesomorphic tempera-
ture range; (a) 6O-PD at 200ꢀC, (b) 8O-PD at 190ꢀC, (c) 2MB-PD at 190ꢀC, and (d)
2EO-PD at 150ꢀC.

36
N. Tokushige et al.
as shown in Table 1. The value of DSMI for 8O-PD is smaller than that for
6O-PD, which is displaying a typical nematic phase. If 8O-PD exhibited a
smectic C phase, the value of DSMI for 8O-PD would be about 10 times lar-
ger than that of conventional nematic phase based on the higher orienta-
tional order of the mesogenic group in the smectic C phase [8].
Therefore, the structure of the mesophase for 8O-PD would be a cybotactic
nematic phase due to the observation of a schlieren texture or a thread tex-
ture and the X-ray pattern similar to the smectic phase. The cybotactic ne-
matic phase has the molecular array with smectic C order (cybotactic
group) in nematic molecular array. The cybotactic group in the nematic
structure is a cluster of LC molecules that form the molecular array similar
to the smectic C phase. Both of the conventional nematic array of LC mole-
cules and the cybotactic groups coexist in the cybotactic nematic phase
[9–12].
Figure 4(c) shows the X-ray pattern of 2MB-PD at 190ꢀC. The X-ray
pattern of 2MB-PD is similar to that of 8O-PD. The d-spacing based on
˚
the peak in the small angle region of X-ray pattern for 2MB-PD is 24 A.
˚
The calculated length of 2MB-PD with all-trans conformation is 35.6 A.
The d-spacing value of 2MB-PD and observation of the focal conic texture
of 2MB-PD would allow us to estimate that the structure of the mesophase
is an intercalated smectic structure proposed by Diele et al. [13,16], Weiss-
´
flog and Demus [14], and Weissflog et al. On the other hand, Plate et al.
reported that small peaks were observed in the small angle region of the
X-ray pattern in the cholesteric phase [17]. In the possible model of the
cholesteric phase, mesogenic groups are arranged in layers helically
twisted along the axis normal to mesogenic groups. The structure of the
mesophase of 2MB-PD could be similar to that of the cholesteric phase
´
proposed by Plate et al. However, at the present time, we cannot exactly
determine the structure of the mesophase of 2MB-PD.
Figure 4(d) displays the X-ray pattern of 2EO-PD where a broad peak in
the wide angle region is observed. Temperature dependence of the X-ray
pattern of 2EO-PD is not remarkable. The mesophase of 2EO-PD is
assigned to a nematic phase based upon the observation of a schlieren
texture as shown in Figure 1(d) and the X-ray diffraction pattern. The
value of DSMI of 2EO-PD is smaller than that of 6O-PD. Mesomorphic tem-
perature range of 2EO-PD is wider than that of the phosphorus derivatives
with alkoxy group. Furthermore, the nematic phase of 2EO-PD is fixed at
room temperature.
Thermal degradation temperature (Td) was recorded as a temperature
at which 10% weight loss of the phosphorus derivatives occurs. Thermal
degradation temperatures are summarized in Table 1. Some phosphorus
derivatives are thermally stable until more than 300ꢀC. Thermal decompo-
sition temperature of 8O-PD is lower than that of other phosphorus

LC Phosphorous Derivatives
37
FIGURE 5 TGA curve of phosphorus derivatives (2MB-PD) in air.
derivatives, which have methylene chains as a terminal group of rigid moi-
ety. At the present time, we cannot explain the phenomenon concerning
the thermal decomposition of 8O-PD. Figure 5 shows a typical TGA curve
of the phosphorus derivatives. Two main thermal degradation steps are
observed in the TGA curve. The first step is detected in 350–450ꢀC region,
the other shows in the 500–650ꢀC region. The details of the thermal degra-
dation behavior are not clarified; however, the second step may be attrib-
uted to the partial degradation of phosphorus moiety [1].
REFERENCES
[1] Wang, C. S. & Lin, C. H. (1999). Polymer, 40, 747–757.
[2] Annakutty, K. S. & Kishore, K. (1988). Polymer, 29, 756–761.
[3] Kishore, K., Annakutty, K. S., & Mallick, I. M. (1988). Polymer, 29, 762–764.
[4] Annakutty, K. S. & Kishore, K. (1988). Polymer, 29, 1273–1276.
[5] Kannan, P., Gangadhara, & Kishore, K. (1991). Polymer, 32, 1909–1913.
[6] Senthil, S. & Kannan, P. (2001). J. Polym. Sci.: Part A: Polym. Chem., 39, 2396–2403.
[7] Senthil, S. & Kannan, P. J. Polym. Sci.: Part A: Polym. Chem., 40, 2256–2263.
[8] Kumar, S. Ed., Liquid Crystals: Experimental Study of Physical Properties and Phase
Transitions. Cambridge University Press: New York, (2001) p. 61.
[9] Kelker, H. & Hatz, R. Ed., Handbook of Liquid Crystals. Verlag Chemie: Weiheim, Deer-
field Beach, FL, Basel, (1980) pp. 5, 26.
[10] de Vries, A. (1970). Mol. Cryst. Liq. Cryst., 10, 219–236.

38
N. Tokushige et al.
[11] Blumstein, A., Vilasagar, S., Ponrathnam, S., Clough, S. B., & Blumstein, R. B. (1982).
J. Polym. Sci.: Polymer Physics Edition, 20, 877–892.
ꢀ
[12] Azaroff, L. V. & Schuman, C. A. (1985). Mol. Cryst. Liq. Cryst., 122, 309–319.
[13] Diele, S., Weissflog, W., Pelzl, G., Manke, H., & Demus, D. (1986). Liq. Crystals, 1,
101–105.
[14] Weissflog, W. & Demus, D. (1988). Liq. Crystals, 3, 275–248.
[15] Weissflog, W., Demus, D., & Diele, S. (1990). Mol. Cryst. Liq. Cryst., 191, 9–15.
¨
[16] Diele, S., Madicke, A., Knauft, K., Neutzler, J., Weissflog, W., & Demus, D. (1991). Liq.
Crystals, 10, 47–61.
´
[17] Plate, N. A., Talrose, R. V., Freidzon, Ya. S., & Shibaev, V. P., (1987). Polymer J., 19,
135–145.