Synthesis and characterization of a combined main-chain/side-chain liquid-crystalline polymer exhibiting both thermotropic and lyotropic characteristics and its lyotropic phase behavior

Article abstract of DOI:10.1021/ma050656i

A combined main-chain/side-chain liquid-crystalline polymer (PSHQ4-7CNCOOH), which is composed of poly[(phenylsulfonyl-p-phenylenene 1,4-tetramethylene bis(oxybenzoate)] (PSHQ4) as the main-chain backbone and 8-[(4-cyano-4'-phenyl)oxy]octanoic acid (7CNCO

Full text of DOI:10.1021/ma050656i

9602
Macromolecules 2005, 38, 9602-9609
Synthesis and Characterization of a Combined Main-Chain/Side-Chain
Liquid-Crystalline Polymer Exhibiting Both Thermotropic and Lyotropic
Characteristics and Its Lyotropic Phase Behavior
Ming Zhou and Chang Dae Han*
Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325
Received March 29, 2005; Revised Manuscript Received June 30, 2005
ABSTRACT: A combined main-chain/side-chain liquid-crystalline polymer (PSHQ4-7CNCOOH), which
is composed of poly[(phenylsulfonyl-p-phenylenene 1,4-tetramethylene bis(oxybenzoate)] (PSHQ4) as the
main-chain backbone and 8-[(4-cyano-4′-phenyl)oxy]octanoic acid (7CNCOOH) as the side-chain mesogenic
groups, has been synthesized. It has been found that 7CNCOOH has a clearing temperature of 150 °C
and exhibits nematic mesophase, and PSHQ4-7CNCOOH exhibits both thermotropic and lyotropic
characteristics. The thermotropic characteristics of PSHQ4-7CNCOOH have been investigated using
differential scanning calorimetry (DSC), fiber and powder patterns of wide-angle X-ray diffraction (WAXD),
and polarized optical microscopy (POM). It has been found that in the thermotropic state PSHQ4-
7CNCOOH has a glass transition temperature (Tg) of 85 °C and a nematic-to-isotropic (N-I) transition
temperature (TNI) of 172 °C, and it is a glassy polymer. Interestingly, PSHQ4-7CNCOOH is found to
exhibit lyotropic characteristics, undergoing isotropic-to-nematic biphasic separation over a wide range
of concentrations in m-dibromobenzene, o-dichlorobenzene, and tetrachloroethane, enabling us to construct
a phase diagram. The thermal transitions and phase structures of the main-chain backbone of PSHQ4-
7CNCOOH, PSHQ4, were compared with those of PSHQ4-7CNCOOH. It has been found that PSHQ4
has a Tg of ca. 130 °C and a TNI of ca. 260 °C, and it undergoes, during isothermal annealing, crystallization
at about 240 °C as determined from DSC. Both the fiber and powder patterns of WAXD indicate that
PSHQ4 is a semicrystalline polymer, which is consistent with the findings from DSC. It has been found
that, in contrast to PSHQ4-7CNCOOH, PSHQ4 does not exhibit lyotropic characteristics. Both the very
large decrease (about 90 °C) in TNI after the attachment of side-chain mesogenic groups (7CNCOOH)
onto the main-chain backbone (PSHQ4) and the lyotropic characteristics of PSHQ4-7CNCOOH are
attributable to the significant increase in the bulkiness of PSHQ4-7CNCOOH, which apparently cannot
pack well, giving rise to a glassy state.
1. Introduction
During the past three decades, numerous investiga-
tors have reported on the synthesis of TLCPs. There are
too many studies to cite them all here. Some research
groups5-18 reported on the syntheses of main-chain
liquid-crystalline polymers (MCLCPs), while other re-
search groups19-30 reported on the syntheses of side-
chain liquid-crystalline polymers (SCLCPs). Several
research groups reported on the synthesis of combined
main-chain/side-chain liquid-crystalline polymers (MC-
SCLCPs). There are two different ways of synthesizing
combined MCSCLCPs: one method is to attach me-
sogenic side groups onto the flexible methylene units
of a semiflexible main-chain polymer,31-36 and another
method is to attach mesogenic side groups as lateral
substituents onto the mesogenic moieties of a semiflex-
ible main-chain polymer.37-40 Some research groups41-47
reported on the structural analysis of MCSCLCPs, while
others48-50 investigated the phase behavior of MCS-
CLCPs.
Beginning in the early 1970s, the synthesis of liquid-
crystalline polymers (LCPs) for industrial applications
emerged. One notable example is the commercial de-
velopment of Kevlar fiber,1 which is based on poly(p-
benzamide) (PBA) or poly(p-phenylene terephthalamide)
(PPTA). Kevlar fiber is produced from lyotropic (aniso-
tropic) spinning dopes prepared with sulfuric acid as a
solvent because PBA and PPTA are thermally unstable
at elevated temperatures. The anisotropic nature of the
spinning dope is believed to be a key factor in achieving
the unusually strong mechanical properties of Kevlar
fiber. Also in that period, the synthesis of rigid-rod-like
thermotropic LCPs (TLCPs) for industrial applications
emerged, with several having been commercialized.2-4
One of the great challenges in this area has been the
synthesis of TLCPs having reasonably low melting
temperature (Tm). When the Tm of a TLCP is too high,
a processing difficulty arises in that thermal degrada-
tion may occur before reaching Tm. TLCPs having a high
Tm would also have a high isotropization (or clearing)
temperature (Tiso), at and above which they become
isotropic liquids and consequently have low melt viscos-
ity. Thus, in the past considerable efforts were made
on the synthesis of TLCPs that would have a reasonably
low Tm and Tiso, by inserting flexible spacers in the
mesogenic main-chain backbone and/or by grafting
bulky pendent groups onto the main-chain backbone.
This resulted in semiflexible (segmented) TLCPs having
a much lower Tm and Tiso compared to those of rigid-
rod-like TLCPs.
Very recently, we synthesized a combined MCSCLCP
having the chemical structure (which will be hereafter
referred to as PSHQ4-7CNCOOH)
Notice that mesogenic cyanobiphenyl groups with seven
10.1021/ma050656i CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/12/2005

Macromolecules, Vol. 38, No. 23, 2005
Main-Chain/Side-Chain LC Polymer 9603
Scheme 1. Synthetic Route for PSHQ4-7CNCOOH
methylene units (which will hereafter be referred to as
7CNCOOH) are attached onto the flexible methylene
unit of the main-chain backbone containing mesogenic
units. The synthesis of PSHQ4-7CNCOOH was moti-
vated by the desire to investigate how much the pres-
ence of the side-chain mesogenic group 7CNCOOH in
PSHQ4-7CNCOOH might affect the TNI and meso-
phase structure of the main-chain backbone. For this,
we also synthesized an MCLCP, PSHQ4. Another moti-
vation of this study was to explore the possibility of
being able to prepare lyotropic solutions from the ther-
motropic PSHQ4-7CNCOOH, which was found to be a
glassy LCP as will be shown below. Successful prepara-
tion of lyotropic solutions from the thermotropic PSHQ4-
7CNCOOH would provide us with a very unique oppor-
tunity to clarify one of the unresolved issues in LCP
rheology to date, namely the sign of first normal stress
difference (N1) in steady-state shear flow.
It is worth mentioning that the preparation of lyotro-
pic solutions from a crystallizable TLCP has been found
to be very difficult in general. This is because when a
crystallizable TLCP is dissolved in a large quantity of
solvent, the dissolved TLCP first crystallizes in the
solution as the solvent is evaporated even at a very slow
rate, preventing the formation of lyotropes. Such an
observation suggests that a desirable situation to obtain
lyotropic solutions from a TLCP would be to dissolve a
glassy TLCP in an appropriate solvent, so that crystal-
lization would not take place as the solvent is evapo-
rated from a very dilute solution. In the past only a few
attempts have been reported on the preparation of
lyotropic solutions from TLCPs. Viney et al.48 reported
on the phase behavior of lyotropic solutions over a very
wide range of concentrations of a glassy MCSCLCP, and
Han and Chang51 prepared lyotropic solutions over a
very limited range of concentrations of a glassy, seg-
mented, thermotropic main-chain copolyester.
In this paper, we report the results of the synthesis
and characterization of an MCSCLCP, PSHQ4-7CN-
COOH, and the lyotropic characteristics of PSHQ4-
7CNCOOH. In doing so, we compare the thermal
transition in PSHQ4-7CNCOOH as investigated from
differential scanning calorimethry (DSC), the me-
sophase structure as investigated from wide-angle X-ray
diffraction (WAXD), and polarizing optical microscopy
(POM) with those of the MCLCP, PSHQ4. The purpose
of the comparison is to understand the roles that the
side-chain mesogenic groups play in influencing the
thermal transition and mesophase structure of PSHQ4-
7CNCOOH. We also present images from POM of some
representative lyotropic solutions of PSHQ4-7CN-
COOH and a phase diagram showing isotropic-to-
nematic biphasic separation over a wide range of
concentrations. Further, we present our view on the
reasons why PSHQ4-7CNCOOH exhibits both ther-
motropic and lyotropic characteristics, while other
combined MCSCLCPs reported in the literature would
not exhibit lyotropic characteristics.
Previously, Lee and Han52 synthesized 5CNCOOH, which has
two fewer methylene units than 7CNCOOH, and described the
details of synthesis procedures for 5CNCOOH. Thus, the
interested readers are referred to their paper for the synthesis
procedures for 7CNCOOH, which are nearly identical.
Synthesis of Polymers. We synthesized PSHQ4-7CN-
COOH using the reaction route shown in Scheme 1. We also
synthesized an MCLCP, PSHQ4, which forms the main-chain
backbone of PSHQ4-7CNCOOH, using the reaction route
shown in Scheme 2.
Characterization of the Polymers Synthesized. The
chemical structures of both PSHQ4-7CNCOOH and PSHQ4
were characterized using proton nuclear magnetic resonance
(1H NMR) spectroscopy and Fourier transform infrared (FTIR)
spectroscopy. 1H NMR spectra were obtained using a spec-
trometer (Varian Gemini-200, 200 MHz). Films suitable for
FTIR spectroscopy were prepared by casting 2% (w/v) solution
in tetrahydrofuran (THF) or chloroform directly on the KBr
salt plate. Film thickness was adjusted, such that the maxi-
mum absorbance of any band was less than 1.0, at which the
Beer-Lambert law is valid. It was slowly dried for 24 h in a
fume hood until most of the solvent evaporated and then dried
at 70 °C for a few days in a vacuum oven. Samples were then
stored in a vacuum oven until use. FTIR spectra were obtained
using a Perkin-Elmer spectrometer (model 16PC FTIR).
Spectral resolution was maintained at 4 cm-1. Dry argon gas
was used to purge the sample compartment in order to reduce
the interference of water and carbon dioxide in the spectrum.
Figure 1 shows the FTIR spectra for PSHQ4-7CNCOOH and
PSHQ4. The purpose of displaying Figure 1 is to demonstrate
that indeed PSHQ4-7CNCOOH has -CN and -(CH2)7 groups,
both coming from the mesogenic cyanobiphenyl groups with
seven methylene units, 7CNCOOH, attached onto the flexible
methylene units of the main-chain backbone, PSHQ4. Since
both PSHQ4-7CNCOOH and PSHQ4 dissolve in tetrachlo-
roethane, but not in THF, we could not determine their
molecular weights using gel permeation chromatography
against polystyrene standards. Thus, we measured the intrin-
2. Experimental Section
Synthesis of 8-[(4-Cyano-4′-biphenyl)oxy]octanoic Acid
(7CNCOOH). We synthesized a liquid crystalline monomer
7CNCOOH with the chemical structure

9604 Zhou and Han
Scheme 2. Synthetic Route for PSHQ4
Macromolecules, Vol. 38, No. 23, 2005
Figure 1. IR spectra for PSHQ4-7CNCOOH and PSHQ4.
X-ray generator (model XRD-6) operated at 30 kV and 30 mA
and a Rigaku imaging system with an 18 kV rotating X-ray
generator. For the General Electric X-ray generator, the X-ray
beam was monochromatized to Cu KR with a graphite crystal.
The X-ray powder and fiber diffraction patterns were recorded
with a camera, which was placed at a distance of 52.48 mm
from the specimen, and the exposure time for each measure-
ment was 4 h. Variable-temperature WAXD experiments were
also conducted on as-cast films of PSHQ4-7CNCOOH and
PSHQ4, using a Rigaku X-ray rotating anode (Cu KR) genera-
tor equipped with a diffractometer. The diffraction peak
positions and widths were calibrated with silicon crystals of
known crystal size in the high-2θ angle region and silver
behanate in the low-2θ angle region. The 2θ scanning range
employed was 1.3°-35° in reflection. A temperature controller
with an accuracy of (1 °C was used, and the diffraction
patterns were measured at various temperatures ranging from
50 to 250 °C.
sic viscosity [η] at 30 °C of each polymer dissolved in tetra-
chloroethane and found that [η] ) 0.675 dL/g for PSHQ4-
7CNCOOH and [η] ) 0.560 dL/g for PSHQ4. Thus, we conclude
that the molecular weight of PSHQ4-7CNCOOH is higher
than that of PSHQ4.
Preparation of Lyotropic Solutions of PSHQ4-7CN-
COOH. We prepared lyotropic solutions of PSHQ4-7CN-
COOH using m-dibromobenzene (mp ) -7 °C; bp ) 218 °C),
o-dichlorobenzene, (mp ) -18 °C; bp ) 178 °C), and tetra-
chloroethane (mp ) -43 °C; bp ) 147 °C) as solvents. Specific
procedures employed are as follows: (i) a predetermined
amount of polymer was first placed in a glass tube immersed
in an oil bath at 90 °C; (ii) the polymer was then dissolved by
adding an excess amount of a solvent into the glass tube, for
which the amount of the solvent used varied depending upon
the initial concentration specified; (iii) the solvent was evapo-
rated to achieve a predetermined concentration, with vigorous
stirring, for 12-24 h in the oil bath, and then the glass tube
was taken out of the oil bath followed by cooling to room
temperature; the amount of solvent evaporated was deter-
mined gravimetrically to determine the final concentration of
the solution.
Sample Preparation. Thermotropic specimens for DSC,
POM, and WAXD were prepared by casting PSHQ4-7CN-
COOH from dichloromethane and PSHQ4 from dichloroethane
in the presence of an antioxidant (0.1 wt % Irganox 1010, Ciba-
Geigy Group), yielding about 2 wt % solution. The as-cast films
of about 2-3 µm in thickness were first dried in a fume hood
for 2 weeks and then in a vacuum oven for 1 week to remove
any residual solvent. Fibers were also prepared by melt
drawing of PSHQ4-7CNCOOH and PSHQ4, which were later
employed for the WAXD experiment.
Differential Scanning Calorimetry (DSC). Thermal
transition temperatures of the solvent-cast films of each
polymer were determined by DSC (Perkin-Elmer 7 Series). All
DSC runs were made under a nitrogen atmosphere with
heating and/or cooling rates of 20 °C/min, and the thermal
histories (namely, annealing temperature and the duration of
annealing) of the specimens were varied.
Wide-Angle X-ray Diffraction (WAXD). Two-dimensional
WAXD experiments were conducted at room temperature on
both melt-drawn fibers and as-cast films of PSHQ4-7CN-
COOH and PSHQ4, respectively, using a General Electric
Polarizing Optical Microscopy (POM). A hot-stage
(TH600 type, Linkham Scientific Co.) microscope (Nikon,
model Optiphoto po1XTP-11) with a camera, a programmable
temperature controller, and photomicrographic attachment
was used to take images under cross-polarized light. To obtain
micrographs, an as-cast film sample was placed between two
glass slides.
3. Results and Discussion
We will first compare the thermotropic characteris-
tics, in terms of thermal transitions and mesophase
structure, of PSHQ4-7CNCOOH with that of PSHQ4
to have a better understanding of the roles that the side-
chain mesogenic groups 7CNCOOH play in influencing
the phase transition and mesophase structure of the
main-chain backbone of PSHQ4-7CNCOOH. Such a
comparison has not been reported in the literature31-36
that dealt with the synthesis of combined MCSCLCPs.
Next, we will present the lyotropic characteristics of
PSHQ4-7CNCOOH in terms of POM images of some
representative lyotropic solutions and a phase diagram
over a very wide range of concentrations for three
different solvents.
Thermal Transitions in PSHQ4-7CNCOOH and
PSHQ4 As Determined by DSC. Figure 2 gives DSC
thermograms during the first cooling and the second
heating at a rate of 20 °C/min for as-cast PSHQ4-
7CNCOOH and PSHQ4 specimens. It is of interest to
observe from the left-side panel of Figure 2 that during
the second heating PSHQ4-7CNCOOH exhibits only
two thermal transitions, namely, a glass transition
temperature (Tg) at 85 °C and an isotropization tem-
perature at 172 °C, showing no evidence of crystallinity.
On the other hand, the right-side panel of Figure 2

Macromolecules, Vol. 38, No. 23, 2005
Main-Chain/Side-Chain LC Polymer 9605
Figure 2. DSC thermograms for as-polymerized PSHQ4-
7CNCOOH and PSHQ4 during the first cooling and second
heating cycles at 20 °C/min.
shows that during the second heating PSHQ4 undergoes
a glass transition at 129 °C and isotropization at 259
°C and has a conspicuous endothermic peak at 243 °C
with two small endothermic peaks at 206 and 228 °C.
We found that an isothermal annealing at 130 °C for
24 h did not affect the thermal transition in PSHQ4-
7CNCOOH, whereas an isothermal annealing of PSHQ4
at 220 °C for 24 h steadily increased the area under the
intermediate endothermic peak (i.e., the enthalpy ∆H1
associated with the intermediate endothermic peak) at
ca. 240 °C, signifying that the crystal growth continued
during the isothermal annealing, while the upper endo-
thermic peak at 260 °C remained the same. Thus, we
tentatively conclude that PSHQ4-7CNCOOH is a glassy
TLCP while PSHQ4 is a semicrystalline TLCP. Further,
we found that 7CNCOOH has a clearing temperature
of 150 °C and has a nematic mesophase. It is then clear
that the attachment of the mesogenic 7CNCOOH onto
the flexible methylene unit of the MCLCP, PSHQ4, has
yielded a combined MCSCLCP, the structure of which
has been confirmed by the FTIR spectra given in Figure
1. What is of great interest here is the drastic decrease
(ca. 90 °C) in the TNI of PSHQ4 after it was grafted with
the cyanobiphenyl mesogenic groups having seven me-
thylene units (7CNCOOH), yielding a combined MCS-
CLCP, PSHQ4-7CNCOOH.
Figure 3. WAXD powder patterns at room temperature of
(a) as-polymerized PSHQ4-7CNCOOH specimen annealed at
130 °C for 24 h and (b) an as-cast PSHQ4 specimen annealed
at 220 °C for 2 h.
Figure 4. WAXD fiber patterns at room temperature of (a)
as-polymerized PSHQ4-7CNCOOH specimen annealed at 130
°C for 24 h and (b) an as-cast PSHQ4 specimen annealed at
220 °C for 2 h.
Mesophase Structures of PSHQ4-7CNCOOH
and PSHQ4 As Determined by WAXD. Figure 3
gives X-ray powder diffraction patterns and the intensi-
ties of X-ray diffraction patterns in smaller angles (2θ
scanning) ranging from 1° to 35° for as-cast specimens
of (a) PSHQ4-7CNCOOH and (b) PSHQ4. These speci-
mens, before being subjected to WAXD measurements,
had the following thermal histories: (a) PSHQ4-
7CNCOOH was annealed at 130 °C for 24 h; (b) PSHQ4
was annealed at 220 °C for 2 h. It is seen in Figure 3a
that the WAXD patterns for PSHQ4-7CNCOOH show
very diffuse lateral spacing with a broad outer halo with
a spacing of 0.45 nm, and the intensity of X-ray
diffraction patterns in smaller angles (2θ scanning) has
a single peak at 20°, which is attributable to the
liquidlike packing of mesogenic groups. In the absence
of sharp diffraction peaks in the WAXD patterns, we
conclude that PSHQ4-7CNCOOH is a glassy LCP
having only a nematic mesophase, confirming the DSC
results presented in Figure 2a. The absence of crystal-
line structure in PSHQ4-7CNCOOH is believed to be
due to the disruption of a well-packed structure in the
presence of side-chain mesogenic groups 7CNCOOH
grafted onto the flexible methylene units of the main-
chain backbone. On the other hand, the WAXD patterns
for PSHQ4 given in Figure 3b show very sharp diffrac-
tion peaks with several weak diffractions and the
intensities of X-ray diffraction patterns in smaller
angles (2θ scanning) have several peaks, indicating that
a highly ordered crystalline structure exists in the
specimen. The values of d spacing determined for
PSHQ4 from Figure 3b are as follows: (1) d ) 0.84 nm
at 2θ ) 10.5°, (2) d ) 0.61 nm at 2θ ) 14.5°, (3) d )
0.54 nm at 2θ ) 16.4°, (4) d ) 0.45 nm at 2θ ) 19.5°,
(5) d ) 0.41 nm at 2θ ) 21.7°, and (6) d ) 0.34 nm at
2θ ) 24.5°. The above observations made for PSHQ4
confirm the DSC results presented in Figure 2.
Figure 4 gives X-ray fiber diffraction patterns and the
intensities of X-ray diffraction patterns in smaller
angles (2θ scanning) ranging from 1° to 35° for melt-
drawn fibers of unannealed (a) PSHQ4-7CNCOOH and
(b) PSHQ4. Fiber specimens were obtained by melt
drawing in the nematic state followed by a rapid
quenching to room temperature. From Figure 4a for

9606 Zhou and Han
Macromolecules, Vol. 38, No. 23, 2005
Figure 5. POM images of (a) as-cast PSHQ4-7CNCOOH
specimen taken at 140 °C and (b) as-cast PSHQ4 specimen
taken at 250 °C.
PSHQ4-7CNCOOH we observe very diffuse meridional
and equatorial diffractions, and the intensity of X-ray
diffraction patterns in smaller angles (2θ scanning) has
a single peak at 20°, suggesting that PSHQ4-7CN-
COOH has virtually perfect amorphous structures in
the solid state. Again, this observation is consistent with
the DSC results presented in Figure 2a and also with
the X-ray powder diffraction patterns presented in
Figure 3a. On the other hand, the X-ray fiber diffraction
patterns given in Figure 4b for PSHQ4 show four-point
spots with a few meridional diffraction patterns, and
the intensities of X-ray diffraction patterns in smaller
angles (2θ scanning) have a large peak at ca. 22° and
two additional small peaks, which are indications of the
presence of reflection planes from the solid crystal
structures. From the four-point spots we obtain the
chain periodicity of 2.18 nm in PSHQ4.
We also observed from 2θ scanning of variable-
temperature X-ray diffraction patterns, although they
are not presented here, that the shape of the main peak
for an unannealed PSHQ4-7CNCOOH specimen dur-
ing the first cooling cycle was almost identical to that
during the second heating cycle and the shape of the
WAXD patterns changed little over the entire temper-
atures tested ranging from 90 to 190 °C, whereas many
reflections appeared at temperatures between 100 and
230 °C for PSHQ4, indicating that it has a very high
rate of crystallization, and the reflection positions in
PSHQ4 in the variable-temperature WAXD patterns
coincided with those in the X-ray powder diffraction
patterns given in Figure 3b.
Mesophase Structure of PSHQ4-7CNCOOH and
PSHQ4 As Determined by POM. In obtaining POM
images of PSHQ4-7CNCOOH and PSHQ4, the speci-
mens were first heated to a temperature ca. 20 °C above
the clearing temperature of the respective polymers to
achieve an isotropic state, at which the melt looked dark
under a polarizing microscope, and then were cooled at
a rate of 2 °C/min down to the measurement tempera-
ture (140 °C for PSHQ4-7CNCOOH and 250 °C for
PSHQ4). A nitrogen atmosphere was provided to mini-
mize oxidative degradation of the specimens. Figure 5
gives POM images of PSHQ4-7CNCOOH taken at 140
°C and PSHQ4 taken at 250 °C. It is seen in Figure 5
Figure 6. Cross-polarized optical micrographs of 22 wt %
solution of PSHQ4-7CNCOOH in o-dichlorobenzene at various
temperatures in the nematic region, as indicated on each
image.
that both PSHQ4-7CNCOOH and PSHQ4 exhibit
Schlieren texture in the nematic state. It should be
remembered that according to the DSC thermograms
given in Figure 2, nematic texture is expected at
temperatures of ca. 88-172 °C for PSHQ4-7CNCOOH
and at 242-260 °C for PSHQ4.
Domain Textures and Phase Behavior of Lyo-
tropic Solutions of PSHQ4-7CNCOOH. Figure 6
gives POM images obtained at temperatures ranging
from 0 to 23 °C of 22 wt % lyotropic solution of PSHQ4-
7CNCOOH dissolved in o-dichlorobenzene. Notice in
Figure 6 that the completely dark image at 23 °C
represents the isotropic state of the lyotropic solutions.
We have also taken POM images at various tempera-
tures for 25, 27, 30, 40, 50, 60, and 70 wt % lyotropic
solutions in o-dichlorobenzene, 33 and 41 wt % lyotropic
solutions in m-dibromobenzene, and 50 and 60 wt %
lyotropic solutions in tetrachloroethane. For the reason
of space limitation we do not present those POM images
here. It should be mentioned that we were not able to
prepare lyotropic solutions of PSHQ4 because the
polymer precipitated out via crystallization from a
homogeneous solution of PSHQ4 as the solvent evapo-
rated, a phenomenon commonly observed from super-
saturated solutions of crystallizable organic or inorganic
materials (e.g., sugar; sodium chloride salt).
Having taken POM images of the lyotropic solutions
of PSHQ4-7CNCOOH in o-dibromobenzene, m-dichlo-
robenzene, and tetrachloroethane, we have constructed
a phase diagram. Figure 7 gives a phase diagram thus
constructed, in which the shaded areas represent bi-
phasic region. It can be seen in Figure 7 that the
temperature at which the nematic phase persists in a
lyotropic solution increases with increasing solution
concentration.

Macromolecules, Vol. 38, No. 23, 2005
Main-Chain/Side-Chain LC Polymer 9607
Figure 7. Phase diagram determined by polarizing optical
microscopy for lyotropic solutions of PSHQ4-7CNCOOH in
three different solvents: (O) o-dichlorobenzene, (4) m-dibro-
mobenzene, and (0) tetrachloroethane, in which the shaded
areas represent the biphasic region.
Origin in the Lyotropic Characteristics of
PSHQ4-7CNCOOH. Earlier, Piao et al.39 synthesized
a series of combined MCSCLCPs with the chemical
structure
Figure 8. DSC thermograms for (a) PHQ4-7CNCOOH and
(b) PHQ4.
SCLCPs. In contrast, in the present study we compared
the thermal transition temperatures and mesophase
structure of the MCLCP, PSHQ4, with those of the
MCSCLCP, PSHQ4-7CNCOOH. By doing so, we were
able to assess the roles the mesogenic side chain,
7CNCOOH, played in controlling the thermal transition
temperatures and mesophase structure of PSHQ4-
7CNCOOH.
To help understand the reason(s) why PAZOn and
PEFBPn are semicrystalline MCSCLCPs, while PSHQ4-
7CNCOOH is a glassy MCSCLCP giving rise to lyotropic
characteristics, in the present study we synthesized two
additional LCPs with the following chemical structures:
with n ) 3, 4, 5, 6, or 10. According to them, (i) all
PAZOn synthesized were found to be semicrystalline
polymers having melting temperatures (Tm) ranging
from 166 to 188 °C depending upon the length (n) of
methylene units in the side-chain mesogenic groups and
a clearing temperature higher than the thermal degra-
dation temperature, ca. 320 °C, and (ii) PAZOn with n
e 6 had nematic mesophase, while PAZO10 had layered
mesophase. Being highly crystalline, PAZOn would not
exhibit lyotropic characteristics.
Ge et al.40 synthesized a series of combined MCS-
CLCPs with the chemical structure
having n ) 6-12. According to them, all PEFBPn
synthesized were found to be semicrystalline polymers
and exhibited complex morphologies (different crystal-
line phases) during heating. They carried out detailed
structural characterizations of PEFBPn using WAXD,
POM, and TEM.47 Again, being highly crystalline,
PEFBPn would not exhibit lyotropic characteristics.
Referring to PAZOn and PEFBPn, we have no way
of knowing the roles the mesogenic side chains, which
were grafted onto the backbone of the main chain,
played in controlling the thermal transition tempera-
tures and mesophase structures of the respective MC-
Notice that PHQ4 and PHQ4-7CNCOOH do not have
a phenylsulfonyl pendent group, while PSHQ4 PSHQ4-
7CNCOOH have a phenylsulfonyl pendent group in the
main-chain backbone.
Figure 8 gives DSC thermograms during the first
heating at a rate of 20 °C/min for (a) as-cast PHQ4-
7CNCOOH and (b) as-cast PHQ4 specimens. It is seen
in Figure 8 that PHQ4-7CNCOOH has a clearing
temperature well above its thermal degradation tem-
perature (ca. 340 °C) and PHQ4 has a Tg at 130 °C, a
Tm at 245 °C, and a clearing temperature exceeding the
highest experimental temperature (320 °C) employed.

9608 Zhou and Han
Macromolecules, Vol. 38, No. 23, 2005
Thermogravimetric analysis (TGA) indicates that both
PHQ4-7NCOOH and PHQ4 begin to undergo thermal
degradation at 340-345 °C, as given in the Supporting
Information. Comparison of Figure 8 with Figure 2
reveals the dramatic effect of the presence of phenyl-
sulfonyl pendent group on the thermal transition tem-
peratures of PSHQ4-7CNCOOH and PSHQ4.
why the grafting of bulky 7CNCOOH onto the flexible
methylene unit of PSHQ4 has decreased its TNI so much
(ca. 90 °C) from 260 to 172 °C. Equally significant in
the study of Kim and Han53 is that the grafting of bulky
tert-butyl or phenylsulfonyl pendent groups onto a
segmented LCP yielded nematic mesophase, while the
grafting of less bulky ethoxy pendent groups onto the
same segmented LCP yielded smectic mesophase. Their
study clearly demonstrates that attachment of bulky
pendent groups onto the backbone of a segmented
MCLCP would hinder the formation of highly ordered
packing of the MCLCPs.
Using POM, we found that an as-cast PHQ4-7CN-
COOH has a smectic mesophase at 260 °C. A POM
image of as-cast PHQ4-7CNCOOH specimen is given
in the Supporting Information. We can then conclude
that the attachment of phenylsulfonyl pendent group
onto the backbone of PHQ4-7CNCOOH has trans-
formed the smectic mesophase of PHQ4-7CNCOOH
into a nematic mesophase in PSHQ4-7CNCOOH. On
the other hand, we were not able to find a suitable
solvent that could dissolve PHQ4, which could then
enable us to obtain a POM image of PHQ4 at a
temperature higher than 245 °C, an endothermic peak
appearing in Figure 8b. To demonstrate that the endo-
thermic peak appearing at 245 °C in Figure 8b is the
melting temperature of PHQ4 and it has a mesophase
at temperatures above 245 °C, we conducted the fol-
lowing experiment. Namely, a polymerized PHQ4 speci-
men was first heated to 260 °C and then quenched
rapidly into ice water. Subsequently, we took WAXD
powder patterns of the quenched specimen at room
temperature. The rationale behind this experiment was
based on the premise that the quenched PHQ4 specimen
would have retained the structure it had at 260 °C
before being quenched into ice water. The WAXD
powder patterns for the quenched PHQ4 specimen are
given in the Supporting Information, showing very
sharp diffraction peaks. Also, we took X-ray diffraction
patterns in smaller angles (2θ scanning), and they are
given beneath the WAXD powder patterns in the
Supporting Information. Thus, we can conclude that
PHQ4 has a highly ordered crystalline structure at 260
°C, and thus indeed the clearing temperature of PHQ4
exceeds the highest experimental temperature (320 °C)
employed.
What is significant here is that the grafting of
mesogenic side group 7CNCOOH onto the MCLCP,
PHQ4, has transformed the nematic PHQ4 into smectic
PHQ4-7CNCOOH. Under such a circumstance we
cannot expect that PHQ4-7CNCOOH would be able to
form lyotropic solutions. Indeed, we have found that this
is the case. Therefore, we conclude that the presence of
the bulky phenylsulfonyl pendent group in the main-
chain backbone of PSHQ4-7CNCOOH also has played
a decisive role in obtaining a glassy, nematic-forming
LCP, enabling us to prepare lyotropic solutions from the
thermotropic PSHQ4-7CNCOOH.
At this juncture it is worth mentioning a previous
study by Kim and Han,53 who investigated the effect of
bulkiness of pendent groups on the mesophase structure
and the TNI of segmented MCLCPs having aromatic
triads with 10 methylene units as flexible spacer and
varying sizes of pendent groups. They found that the
bulkier a pendent group, the lower the TNI of the
MCLCPs; namely (i) the MCLCP having ethoxy pendent
groups undergoes smectic-to-nematic transition at 148
°C and N-I transition at 239 °C, (ii) the MCLCP having
tert-butyl pendent groups undergoes only N-I transition
at 193 °C, and (iii) the MCLCP having phenylsulfonyl
pendent groups undergoes only N-I transition at 179
°C. The above experimental observations now explain
4. Concluding Remarks
In this study, we have found that the grafting of
mesogenic cyanobiphenyl groups having seven methyl-
ene units onto the flexible methylene units of a semi-
crystalline MCLCP, PSHQ4, decreased its clearing
temperature as much as ca. 90 °C, and the resulting
combined main-chain/side-chain LCP has become a
glassy, nematic-forming mesophase at temperatures
between the glass transition temperature and the
clearing temperature. We attribute this observation to
the fact that the grating of mesogenic cyanobiphenyl
groups having seven methylene units (7CNCOOH) onto
the flexible methylene units of a semicrystalline PSHQ4
hinders its structural regularity, preventing the forma-
tion of a well-packed structure. As a result, we have
been able to successfully prepare lyotropic solutions of
PSHQ4-7CNCOOH, which was the primary objective
of the present study. This will then enable us to
investigate the rheological behavior of the lyotropic
solutions of PSHQ4-7CNCOOH. The results of such an
investigation will be reported in the future.
It is important to point out that the successful
preparation of lyotropic solutions of PSHQ4-7CNCOOH
presented in this paper was made possible by the fact
that PSHQ4-7CNCOOH is a glassy, nematic-forming
LCP having a relatively low TNI. Such a low value of
TNI (172 °C) for the glassy, nematic-forming PSHQ4-
7CNCOOH is attributed to the presence of bulky phe-
nylsulfonyl pendent group in the main-chain backbone.
Acknowledgment. This study was supported in
part by the Petroleum Research Funds (#PRF35694-
AC7) of the American Chemical Society, for which we
are very grateful. We gladly acknowledge the effort of
Dr. Kyung Min Lee, who participated in our early effort
for the synthesis of PSHQ4-7CNCOOH.
Supporting Information Available: TGA traces for
PHQ4 and PHQ4-7CNCOOH, POM image of PHQ4-7CN-
COOH, and WAXD powder patterns together with X-ray
diffraction patterns in smaller angles (2θ scanning) of an as-
polymerized PHQ4 specimen. This material is available free
of charge via the Internet at http://pubs.acs.org.
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