Ferroelectric liquid crystalline polyoxetanes bearing chiral dimesogenic pendants

Article abstract of DOI:10.1039/b202594a

Two new polyoxetanes bearing dimesogenic pendants were prepared and their liquid crystalline properties were studied by differential scanning calorimetry (DSC), polarizing microscopy, and X-ray diffractometry. The dimesogens consist of cholesterol and bip

Full text of DOI:10.1039/b202594a

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Ferroelectric liquid crystalline polyoxetanes bearing chiral
dimesogenic pendants
Jun-Woo Lee,a Dong Keun Oh,a C. V. Yelamaggad,b S. Anitha Nagamanib and Jung-Il
Jin*a
aDivision of Chemistry and Molecular Engineering, and Center for Electro- and
Photo-Responsive Moleculer, Korea University, Seoul 136-701, Korea
bCenter for Liquid Crystal Research, Bangalore 560013, India
Received 15th March 2002, Accepted 22nd May 2002
First published as an Advance Article on the web 28th June 2002
Two new polyoxetanes bearing dimesogenic pendants were prepared and their liquid crystalline properties were
studied by differential scanning calorimetry (DSC), polarizing microscopy, and X-ray diffractometry. The
dimesogens consist of cholesterol and biphenyl moieties connected through polymethylene spacers. Their
biphenyl ends were attached to the oxetane monomer through a hexamethylene unit and the dimesogenic
monomers were subjected to cationic ring-opening polymerization. The molecular weights of the polymers were
very low and the degree of polymerization was only about 6. The two polymers were found to be thermotropic
and formed only the chiral smectic C (Sc*) phase and, thus, their ferroelectricity was studied. Their
spontaneous polarization values at the Sc* phase were about 20 nC cm22 with responsive times of around
0.3 ms at 7 uC below the clearing temperature.
It should be noted that all of the SCLCPs reported up to now
Introduction
bear monomesogenic pendants. As alluded to above, we have
been studying the LC properties of dimesogenic compounds
and also SCLCPs in order to establish their structure–property
relationships.
In this investigation, we prepared two new SCLCPs based on
the polyoxetane backbone that bears pendants consisting of
two different mesogenic units. For the sake of brevity, we call
the two polymers Pox-4Dm and Pox-5Dm. Pox stands for
polyoxetane, 4 and 5 indicate the number of the methlyene
groups in the spacer between the two mesogenic units and Dm
stands for the ‘dimesogenic’ pendant. The LC properties of the
polymers were studied by DSC, polarizing microscopy, X-ray
analysis, and also by the measurements of their ferroelectricity.
In recent years, there has been much interest in the liquid
crystallinity (LC) of twin dimeric1–6 and dimesogenic7–17
compounds that contain two identical or different mesogenic
units. They are not only used as model compounds for the main
chain liquid crystalline polymers (LCPs), but are also known to
reveal very interesting mesophase behavior that is unexpected
on the basis of the behavior of the corresponding monomeric
LC compounds. We found that some of them formed incom-
mensurate and twisted grain boundary (TGB) phases.10,13,15,16
On the other hand, many side-chain liquid crystalline
polymers (SCLCPs) based on poly(meth)acrylate,18–21 poly-
oxetane,22–28 and polysiloxane29–39 backbones, with and with-
out chiral centers, have been reported. In particular, some of
the SCLCPs bearing chiral centers exhibit the chiral smectic C
(Sc*) phase and, thus, ferroelectric properties.21,26,28,29,31,32
Those derived from polysiloxanes appear to show high
spontaneous polarization values and fast response times. One
of the examples31 was reported to reveal spontaneous
polarization as high as 420 nC cm22. Polyoxetane SCLCPs
also are actively being investigated because of the relative
flexibility of their main chain when compared, for example,
with poly(meth)acrylates. Hsu et al.26 reported that some of the
Sc* SCLCPs based on polyoxetanes showed very fast switching
times in the range of microseconds. We have also reported
previously the LC properties of the Sc* polyoxetanes.28 We,
however, could not measure the values of spontaneous
polarization (Ps), although a weak bistability was observed.
Experimental
The synthetic route to the polymers is shown in Scheme 1. The
numbers given in this section for the intermediates and
monomers are the same as given in the scheme.
Synthesis of monomers
3-(6-Bromohexyloxymethyl)-3-methyloxetane, 1. This com-
pound was prepared following the literature method.28
Cholesteryl 5-(4’-hydroxybiphenyl-4-yloxy)pentanoate, 2(4).
Into a 100 mL two-necked round-bottomed flask equipped
with a water condenser and an argon gas inlet were placed
DOI: 10.1039/b202594a
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This journal is # The Royal Society of Chemistry 2002
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Scheme 1 Synthetic routes to the monomers and polymers.
N,N-dimethylformamide (50 mL), anhyd. potassium carbonate
(2.9g, 21.3mmol), cholesteryl5-bromopentanoate(3.9g, 7.1mmol)
and 4,4’-dihydroxybiphenyl (3.2 g, 17.08 mmol) sequentially
through the second neck of the flask and flushed with argon gas.
After closing the neck with a septum, the reaction mixture was
heated at 85 uC for 12 h. The reaction mixture was cooled to
room temperature and poured into ice-cold water (200 mL). The
solid formed was collected by filtration and was dissolved in
a mixture of ethyl acetate : methylene chloride (1 : 1). The
resultant solution was washed twice with brine (100 mL 62) and
dried over anhyd. Na2SO4. Evaporation of solvent furnished a
white solid that was purified by column chromatography using
silica gel (60-120 mesh). Elution with a mixture of ethyl acetate :
hexanes (1 : 9) afforded a white solid compound. Rf ~ 0.43 (ethyl
acetate : hexanes ~ 1 : 4). Phase transition: Cr1 108.3 uC (DH ~
3.5 J g21) Cr2 157 uC (DH ~ 61.4 J g21) I; yield: 3.8 g (82%).
IR (KBr, cm21): 3385 (O–H stretching), 3031 (aromatic C–H
stretching), 2949 and 2867 (aliphatic C–H stretchings), 1735
(CLO stretching), 1603 and 1500 (aromatic CLC stretchings),
and 1251, 1177, and 979 (C–O stretchings); 1H-NMR (CDCl3,
d): 7.44 (d, 2H, Ar), 7.41 (d, 2H, Ar), 6.93 (d, 2H, Ar), 6.87 (d,
2H, Ar), 5.37 (br d, 1H, olefinic), 4.85 (s, 1H, -OH), 4.61 (m,
1H, -CH-O-CO-), 4.0 (t, 2H, -OCH2-), 2.31 (m, 4H, 2 6 allylic
methylene), 2.25–1.05 (m, 30H, CH2, CH), 1.02 (s, 3H, CH3),
0.92 (d, 3H, CH3), 0.87 (d, 3H, CH3), 0.86 (d, 3H, CH3), and
0.67 (s, 3H, CH3); elemental analysis: calc. for C44H62O4: C
80.69, H 9.54; found C 80.44, H 9.61%.
2H, Ar), 5.37 (br d, 1H, olefinic), 4.95 (s, 1H, OH), 4.61 (m, 1H,
CH-O-CO), 3.98 (t, 2H, OCH2), 2.33 (m, 4H, 2 allylic
methylene), 2.25–1.05 (m, 32 H, CH2, CH), 1.01 (s, 3H,
CH3), 0.91 (d, 3H, CH3), 0.87 (d, 3H, CH3), 0.86 (d, 3H, CH3),
and 0.67 (s, 3H, CH3); elemental analysis: calc. for C45H64O4: C
80.79, H 9.64; found C 80.75, H 9.70%.
3-{6-[4’-(4-Cholesteryloxycarbonylbutyloxy)biphenyl-4-yloxy]-
hexyloxymethyl}-3-methyloxetane, M(4). 3-(6-Bromohexyloxy-
methyl)-3-methyloxetane, 1 (0.57 g; 2.1 6 1023 mol) and
compound 2(4) (1.4 g; 2.1 6 1023 mol) were dissolved in
150 mL of acetone containing 1.4 g of K2CO3 and 0.01 g of
tetra-n-butylammonium bromide. The mixture was refluxed
overnight under a nitrogen atmosphere. Insoluble precipitates
were removed by filtration and acetone was distilled off using
a rotary evaporator. The residue was dissolved in methylene
chloride and the solution poured into excess hexane. The
precipitated solid was collected by filtration and purified by
column chromatography using silica gel (60–120 mesh).
Elution with a mixture of methylene chloride : hexanes (1 : 3)
afforded a white solid compound. Rf ~ 0.40 (methylene
chloride : hexanes ~ 1 : 3). Phase transition: Cr 96.2 uC (DH ~
31.0 J g21) SA 100.5 uC (DH ~ 4.1 J g21) I; yield: 1.4 g (78%).
IR (KBr, cm21): 3037 (aromatic C–H stretching), 2935 and
2866 (aliphatic C–H stretchings), 1733 (CLO stretching), 1606
and 1499 (aromatic CLC stretchings), and 1243, 1174, and 979
(C–O stretchings); 1H-NMR (CDCl3, d): 7.46 (d, 4H, Ar), 6.94
(d, 4H, Ar), 5.38 (br d, 1H, olefinic), 4.64 (m, 1H, CH-O-CO),
4.52 and 4.36 (d, 4H, oxetane CH2), 4.01 (t, 2H, OCH2), 3.48
(t, 4H, oxetane-CH2-O-CH2), 2.32 (m, 4H, allylic methylene),
2.25–1.05 (m, 43H, CH2 and CH), 1.02 (s, 3H, CH3), 0.92
(d, 3H, CH3), 0.87 (d, 3H, CH3), 0.86 (d, 3H, CH3), and 0.67 (s,
3H, CH3); elemental analysis: calc. for C55H82O6: C 78.71, H
9.85; found C 78.63, H 9.90%.
Cholesteryl 6-(4’-hydroxybiphenyl-4-yloxy)hexanoate, 2(5). This
compound was prepared following the procedure as described for
compound2(4). A white solidcompound. Rf ~0.43 (ethyl acetate :
hexanes ~ 1 : 4). Phase transition: Cr1 133.1 uC (DH ~ 3.67 J g21
)
Cr2 164.3 uC (DH ~ 50.7 J g21) N* 182.8 uC (DH ~ 8.9 J g21) I;
yield: 3.5 g (74%).
IR (KBr, cm21): 3379 (O–H stretching), 3036 (aromatic C–H
stretching), 2927 and 2867 (aliphatic C–H stretchings), 1735
(CLO stretching), 1603 and 1500 (aromatic CLC stretchings),
and 1246, 1169, and 978 (C–O stretchings); 1H-NMR (CDCl3,
d): 7.43 (d, 2H, Ar), 7.41 (d, 2H, Ar), 6.92 (d, 2H, Ar), 6.87 (d,
3-{6-[4’-(5-Cholesteryloxycarbonylpentyloxy)biphenyl-4-yloxy]-
hexyloxymethyl}-3-methyloxetane, M(5). This compound was
prepared following the same procedure as described for
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compound M(4). A white solid compound. Rf ~ 0.40
(methylene chloride : hexanes ~ 1 : 3). Phase transition: Cr
88.2 uC (DH ~ 23.1 J g21) SA 148.1 uC (DH ~ 4.6 J g21) N*
152.1 uC (DH ~ 4.1 J g21) I; yield: 1.1 g (75%).
controller (Mettler FP-90, Germany). The X-ray diffracto-
grams of the compounds were obtained at varying tempera-
˚
tures using synchrotron radiation (1.542 A) of the 3C2 beam
line at the Pohang Synchrotron Laboratory, Korea. Polariza-
tion switching of the polymers was studied using ITO electro-
optic cells 1.5 mm thick coated with a rubbed polyimide. The
measuring instrument consists of a function and arbitrary
wave generator (DS345, Stanford Research Inc., USA), a high
voltage amplifier (FA20A, FLC Electronics, Sweden), a digital
storage oscilloscope (54600B, Agilent, USA), and a transim-
pedence amplifier with an operation amplifier chip (OPA-627)
and a 10 kV detection resister. During the measurement the
sample cell was placed on a hot stage (FP-82HT, Mettler,
Germany), which was controlled by an automatic thermal
controller (FP-90, Mettler, Germany).
IR (KBr, cm21) 3039 (aromatic C–H stretching), 2935 and
2870 (aliphatic C–H stretchings), 1733 (CLO stretching), 1605
and 1500 (aromatic CLC stretchings), and 1242, 1173, and 979
(C–O stretchings); 1H-NMR (CDCl3, d): 7.46 (d, 4H, Ar), 6.93
(d, 4H, Ar), 5.35 (br d, 1H, olefinic), 4.64 (m, 1H, CH-O-CO),
4.52 and 4.36 (d, 4H, oxetane CH2), 4.01 (t, 2H, -OCH2-), 3.49
(t, 4H, oxetane-CH2-O-CH2), 2.32 (m, 4H, allylic methylene),
2.25–1.05 (m, 45H, CH2 and CH), 1.02 (s, 3H, CH3), 0.92 (d,
3H, CH3), 0.87 (d, 3H, CH3), 0.86 (d, 3H, CH3), and 0.67 (s,
3H, CH3); elemental analysis: calc. for C56H84O6: C 78.83, H
9.92; found C 78.80, H 9.94%.
Synthesis of polymers Pox-4Dm and Pox-5Dm
Results and discussion
Poly(2-{6-[4’-(4-cholesteryloxycarbonylbutyloxy)biphenyl-4-yloxy]-
hexyloxymethyl}-2-methyltrimethylene oxide), Pox-4Dm. Mono-
mer M(4) (1.2 g; 1.4 6 1023 mol) was dissolved in 10 mL of
dichloromethane and to this solution boron trifluoride–diethyl
ether (0.005 mL; 2.84 6 1023 mmol) was added under an argon
atmosphere. The solution was stirred for 24 h at 0 uC, and then
poured into 200 mL of methanol. The precipitate thus obtained
was filtered. The polymer was purified by reprecipitation into
n-hexane and methanol. The resulting polymer was subjected
to Soxhlet extraction using methanol. The recovered yield was
0.95 g (68%).
IR (KBr, cm21): 3040 (aromatic C–H stretching), 2933 and
2874 (aliphatic C–H stretchings), 1732 (CLO stretching), 1606
and 1501 (aromatic CLC stretchings), and 1270, 1243, and 1106
(C–O stretchings); 1H-NMR (CDCl3, d): 7.43 (d, 4H, Ar), 6.93
(d, 4H, Ar), 5.37 (br d, 1H, olefinic), 4.63 (m, 1H, CH-O-CO),
3.20–3.54 (m, 8H, OCH2), 2.32 (m, 4H, allylic methylene),
2.25–0.67 (m, 60H, CH2, CH and CH3); elemental analysis:
calc. for C55H82O6: C 78.71, H 9.85; found C 78.63, H 9.96%.
Liquid crystallinity of monomers, M(4) and M(5)
Since the two monomers, M(4) and M(5), bear mesogenic
structures, we have examined their liquid crystallinity by DSC
and polarizing microscopy. Both compounds exhibited focal
conic optical textures with some parts being homeotropic. We
examined their miscibility with Pox-4Dm and Pox-5Dm and
found that they are immiscible with the polymers that, as will
be described below, form only the chiral smectic C phase.
Although M(4) and M(5) differ only in the length of the central
spacer between the cholesterol and biphenyl moieties, we found
that they reveal an interesting difference in the LC phase they
form. The M(4) monomer melts at 96 uC into the Smectic A
phase which became isotropic at 100.5 uC. Therefore, the
mesophase temperature range was only 4.5 uC. In contrast, the
M(5) monomer melted at 88 uC to form the Smectic A phase
which was transformed to the cholesteric phase at 148 uC. The
cholesteric phase underwent isotropization at 152 uC. We note
that the mesophase temperature range (64 uC) of M(5) is much
greater than that (4.5 uC) of M(4). Moreover, M(5) forms
the cholesteric phase in addition to the smectic phase. The
slightly longer spacer between the two mesogenic units in M(5)
lowers the melting temperature and stabilizes the thermal
stability of the mesophases. It is rather surprising to observe
such differences in phase behavior between the two monomers.
Poly(2-{6-[4’-(5-cholesteryloxycarbonylpentyloxy)biphenyl-4-
yloxy]hexyloxymethyl}-2-methyltrimethylene oxide), Pox-5Dm.
The recovered yield was 0.91 g (62%). IR (KBr, cm21): 3038
(aromatic C–H stretching), 2934 and 2872 (aliphatic C–H
stretchings), 1731 (CLO stretching), 1604 and 1499 (aromatic
CLC stretchings), and 1270, 1242, and 1103 (C–O stretchings);
1H-NMR (CDCl3, d): 7.43 (d, 4H, Ar), 6.93 (d, 4H, Ar), 5.37
(br d, 1H, olefinic), 4.63 (m, 1H, CH-O-CO), 3.25–3.56 (m, 8H,
OCH2), 2.32 (m, 4H, allylic methylene), 2.21–0.64 (m, 62H,
CH2, CH and CH3); elemental analysis: calc. for C56H84O6: C
78.83, H 9.92; found C 78.77, H 10.01%.
General properties of Pox-4Dm and Pox-5Dm
The two polymers considered here, Pox-4Dm and Pox-5Dm,
were prepared at 0 uC by cationic ring-opening polymerization
of the corresponding oxetane monomers, M(4) and M(5). The
cationic initiator utilized in the polymerization was BF3?OEt2,
which has been widely used in the ring-opening polymerization
of oxetane and its derivatives.22–24,27,28 Methylene chloride was
the reaction medium. Polymerization proceeded homogenously
throughout the polymerization. The resulting polymers were
subjected to Soxhlet extraction using methanol in order to
remove soluble impurities and low molecular weight products.
Table 1 summarizes the general properties of the polymers
thus obtained. The polymers were soluble in common organic
solvents such as tetrahydrofuran (THF), methylene chloride
(MC) and chloroform. The number average molecular weights
Characterization
The FT-IR and H-NMR spectra of the compounds were
recorded respectively on a Bomem MB instrument and on a
Varian Gemini 300 spectrometer, respectively. Thermal
properties were studied in a nitrogen atmosphere on a
differential scanning calorimeter (DSC; Mettler DSC 821e,
Germany) at a heating and cooling rate of 10 uC min21
.
Molecular weights were determined using a Waters 150CV
PLUS gel permeation chromatograph. The phase transitions
and optical textures were examined on a polarizing microscope
(Olympus BH-2, Japan) equipped with a hot stage (Mettler
FP-82HT, Germany) controlled by an automatic thermal
¯
(Mn) were relatively low: 5200 for Pox-4Dm and 4500 for Pox-
5Dm. These values correspond to a degree of polymerization
Table 1 General properties of Pox-4Dm and Pox-5Dm
PDIb
DPb
b
b
Tga/uC
Tma/uC
Tia/uC
¯
Mn
¯
Mw
Polymer
Pox-4Dm
Pox-5Dm
68.2
72.5
105.5
108.1
178.6
214.8
5160
4480
7930
7810
1.54
1.63
6.2
5.7
aThe values were obtained from DSC (heating rate:10 uC min21). bThe values were obtained from GPC (eluent: THF).
J. Mater. Chem., 2002, 12, 2225–2230
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Fig. 2 WideangleX-raydiffractogramsof(a)Pox-4Dmand(b) Pox-5Dm.
heat of isotropization (DHi) is much higher than the DHm value
of the low temperature melting transition. We observed the
same phenomenon in the accompanying entropy changes (DSi)
for the two transitions but their differences were much smaller.
These observations imply the polymers have a very low degree
of crystallinity. The wide angle X-ray diffractograms shown in
Fig. 2 tell us that the two polymers are only slightly crystalline
and in the light of the broad diffractions at 2h ~ 15–22u (5.9–
Fig. 1 DSC thermograms of Pox-4Dm and Pox-5Dm obtained under a
nitrogen atmosphere at the heating and cooling rates of 10 uC min21
.
(DP) of only about 6. Their average polydispersity index (PDI)
¯
˚
4.4 A) they form fluid mesophases that correspond to short
spacing or interchain distance.
¯
or the ratio of weight average molecular weight (Mw) to Mn
is about 1.5.
When we examined the optical textures of the two polymers,
they both revealed focal conic textures (Fig. 3 (a)). According
to the small angle X-ray analysis shown in Fig. 4, the two
polymers commonly show one sharp diffractogram peak in the
mesophase temperature range. The spacing estimated from the
Usually, cationic ring-opening polymerization of oxetane
derivatives bearing small substituents results in high molecular
weight (MW) polymers.22–24 In contrast, monomers having
bulky substituents give rise to low MW polymers, most
probably due to steric hindrance in the propagation stage.27
The same phenomenon has been observed by us previously.28
The two polymers are semicrystalline, and their glass
transition temperatures are 68.2 and 72.5 uC, respectively
(see Fig. 1). According to the DSC thermograms shown in
Fig. 1, the two polymers exhibit three endothermic peaks in
the heating cycle and the transitions were reversible as one can
see from the cooling DSC thermograms; three corresponding
exothermic peaks appeared in the cooling cycle. The low
temperature peaks represent glass transitions, while the second
peaks are associated with melting transitions and the high
temperature peaks with mesophases, i.e., isotropization.
Further discussion on the mesophase transitions will be
made in the following subsection.
˚
diffraction angle 2h ~ 2.0u and 1.9u, are 49.0 and 51.6 A
respectively for Pox-4Dm and Pox-5Dm. Since the molecular
˚
breadths of the two polymers are 46.0 and 47.3 A, when the
spacers are assumed to be in the trans,anti conformation, the
pendant groups must be interdigitated. Combining the micro-
scopic observation with the X-ray analysis and the electro-optic
behavior of the polymers, to be described below, leads to the
conclusion that the mesophase formed by the two polymers is
the chiral smectic C (Sc*) phase.
Fig. 4 also shows that the two polymers did not diffract in
the small angle region above their Tis.
Optical switching and electro-optic behavior
The electro-optic response was measured for the polymer
samples sandwiched between two indium–tin oxide (ITO)
coated glass plates having rubbed polyimide orienting layers.39
Rectangular alternating electric fields were applied to the
samples.25,30,35,40 Fig. 5 shows the dependence of response
times of Pox-4Dm and Pox-5Dm on the temperature and
applied electric field. We used a reduced temperature, T 2 Ti,
as well as the temperature in the abscissa of Fig. 5. As expected,
higher temperatures and applied electric fields result in faster
response times ranging from 0.3 ms to 0.4 ms near Ti. These
response times are faster than for common low molecular
Liquid crystalline properties
As alluded to above, both Pox-4Dm and Pox-5Dm exhibit two
reversible phase transitions involving mesophases according
to their DSC thermograms (Fig. 1). Table 2 summarizes the
thermodynamic parameters associated with the transitions.
Pox-5Dm exhibits higher transition temperatures than Pox-
4Dm. In particular, the isotropization temperature (Ti) of
Pox-5Dm, 214.8 uC, is significantly higher than the Ti (178.6 uC)
of Pox-4Dm. A similar phenomenon was observed for the Tg
and Tm values of the two monomers. For both polymers, the
Table 2 Thermodynamic data for the phase transitions of Pox-4Dm and Pox-5Dma
DHm
DHi
DSm
DSi
21
Polymer
J g21
kJ mol21
J g21
kJ mol21
J g21 K21
J mol21 K21
J g21 K21
J mol
K21
Pox-4Dm
Pox-5Dm
3.6
3.7
3.0
3.2
9.8
14.2
6.4
12.1
9.5
9.8
8.0
8.3
21.7
29.0
14.2
24.8
aHeating and cooling rate: 10 uC min21; atmosphere: N2 blow, 15 mL min21
.
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Fig. 5 Response time versus temperature and reduced temperature for
(a) Pox-4Dm and (b) Pox-5Dm at various applied electric fields.
Fig. 3 Optical photomicrograph of (a) Pox-4Dm taken at 169 uC and
(b) oriented Pox-5Dm taken at 167 uC (magnification 2006).
The sign of the spontaneous polarization Ps was determined
by the field reversal method through the optical observation of
the extinction direction by rotating the stage according to
Lagerwall and Dahl’s convention.41 The tilt angles (h) were
measured as a function of temperature between crossed
polarizers as half the rotation between two extinction positions
corresponding to opposite polarization orientations. The Ps
signs for the polymers reported here are negative and tilt angles
(h) are found to be relatively small, 2u and 2.5u, respectively.
We measured Ps of the polymers using the triangular wave
method and the results are graphically presented in Fig. 6. For
both polymers, the Ps values smoothly decrease with increasing
temperature as observed by many others for low molar mass as
well as polymeric FLCs.19,20,38 The faster response times and
the greater Ps values of the polymers at higher temperatures
within the Sc* mesophase temperature ranges must result from
diminished melt viscosity leading to easy molecular reorientation.
The Ps values range from 22 to 16 nC cm22 with the values
for Pox-5Dm being slightly higher than those for Pox-4Dm.
This parallels the response times of the two polymers. The Ps
Fig. 4 SmallangleX-raydiffractogramsof(a)Pox-4Dmand(b)Pox-5Dm.
weight nematic LCs and even some Sc* FLCPs from
polysiloxanes. Part of these fast responses can be ascribed to
a high measurement temperature or Sc* phase temperature and
a relatively low degree of polymerization of the two polymers.
It, of course, is well known that optical switching in
ferroelectric LCs (FLCs) requires a much shorter time than
the nematic LCs. It is also noted that the response time for Pox-
5Dm appears to be slightly faster than that for Pox-4Dm.
Suzuki and Okawa19 have reported previously the same
phenomenon for polysiloxanes FLCs: longer spacers resulted
in faster response times probably due to easier polarization of
the mesogenic groups.
Fig. 6 Temperature dependence of spontaneous polarization.
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11 A. Yoshizawa, K. Matsuzawa and I. Nishiyama, J. Mater. Chem.,
1995, 5, 2131.
12 P. J. Le Masurier and G. R. Luckhurst, Liq. Cryst., 1998, 25, 63.
13 F. Hardouin, M. F. Achard, J.-I. Jin, Y.-K. Yun and S. J. Chung,
Eur. Phys. J., 1998, B1, 47.
values for the polymers reported herein are significantly higher
than those reported for FLC poly(meth)acrylates, but lower
than for polysiloxanes. Poths et al.31 reported a Ps value as high
as 420 nC cm22 for a side-chain FLC polysiloxane.
14 C. V. Yelamaggad, A. Srikrishna, D. S. Shankar Rao and
S. K. Prasad, Liq. Cryst., 1999, 26, 1547.
15 S. W. Cha, J.-I. Jin, M. Laguerre, M. F. Achard and F. Hardouin,
Liq. Cryst., 1999, 26, 1325.
Conclusion
We have described the synthesis and LC properties of first
examples of side-chain FLCPs bearing dimesogenic pendants.
The Sc* phase could be induced simply by combining biphenyl
and cholesterol moieties through polymethylene spacers and
attaching the dimesogenic pendants onto a polyoxetane
backbone through a spacer. The two polymers (to be exact,
they should be called oligomers) form only the Sc* phase over a
rather broad temperature range. Their response times in
electro-optic switching are several hundred ms and the Ps values
are about 20 nC cm22. Both values are considered to be fairly
significant even for the development of practical applications.
Modification of the structure of the dimesogenic pendant to
increase the dipole moment and to increase in the degree of
polymerization or molecular weight of the polymers are
subjects for future research.
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Acknowledgement
The X-ray experiments at PLS were supported by the Ministry
of Science and Technology and Pohang Steel Company. Other
measurements were performed at the Center for Electro- and
Photo-Responsive Molecules, Korea University, supported by
the Korean Science and Engineering Foundation. J.-W. Lee is a
recipient of the Brain Korea 21 fellowship supported by the
Ministry of Education, Korea.
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