Phase separation and coalescence, annihilation of liquid crystal textures during polymerization of main-chain liquid crystalline polyesters

Article abstract of DOI:10.1021/jp984332q

Using a novel thin film polymerization technique, we have observed the phase separation, coalescence, and annihilation of liquid crystal textures during polymerization of wholly aromatic main-chain liquid crystalline polymer poly(p-oxybenzoate/2,6-oxynaphthoate) (P(OBA/ONA)). The polycondensation reaction was conducted and observed in situ on the heating stage of a polarizing microscope. After the melting of monomers, the reaction system started from a homogeneous phase and then changed into a heterogeneous one. The following morphological changes during the entire reaction were observed: generation of anisotropic phase, coalescence of liquid crystal domains, formation of schlieren texture, annihilation of disclinations, and formation of stripe texture. We found all the liquid crystal domains formed in the isotropic melt during the early stage of our polymerization reactions had the disclination strength of +1. The number of defects decreased with increasing reaction time through coalescence and annihilation. At the early stage of polymerization, the dominant factor affecting annihilation rate was the viscosity characteristics at elevated temperatures.

Full text of DOI:10.1021/jp984332q

J. Phys. Chem. B 1999, 103, 4923-4932
4923
Phase Separation and Coalescence, Annihilation of Liquid Crystal Textures during
Polymerization of Main-Chain Liquid Crystalline Polyesters
Si-Xue Cheng† and Tai-Shung Chung*,†,‡
Department of Chemical and EnVironmental Engineering, National UniVersity of Singapore,
10 Kent Ridge Crescent, Singapore 119260, and Institute of Materials Research and Engineering,
10 Kent Ridge Crescent, Singapore 119260
ReceiVed: NoVember 6, 1998; In Final Form: February 17, 1999
Using a novel thin film polymerization technique, we have observed the phase separation, coalescence, and
annihilation of liquid crystal textures during polymerization of wholly aromatic main-chain liquid crystalline
polymer poly(p-oxybenzoate/2,6-oxynaphthoate) (P(OBA/ONA)). The polycondensation reaction was conducted
and observed in situ on the heating stage of a polarizing microscope. After the melting of monomers, the
reaction system started from a homogeneous phase and then changed into a heterogeneous one. The following
morphological changes during the entire reaction were observed: generation of anisotropic phase, coalescence
of liquid crystal domains, formation of schlieren texture, annihilation of disclinations, and formation of stripe
texture. We found all the liquid crystal domains formed in the isotropic melt during the early stage of our
polymerization reactions had the disclination strength of +1. The number of defects decreased with increasing
reaction time through coalescence and annihilation. At the early stage of polymerization, the dominant factor
affecting annihilation rate was the viscosity characteristics at elevated temperatures.
1. Introduction
Main-chain liquid crystalline polymers (MCLCPs) form a
class of materials possessing exceptional balance of properties
such as high mechanical properties, easy flow, excellent
dimensional stability, and chemical resistance. As a result, they
have wide applications and received a great deal of attention
from both academia and industry. Many review articles and
books have described the scientific progress on this subject.1-6
Most MCLCPs are polymerized through condensation
reactions.1-6 During the polymerization, liquid crystal (LC)
phase separates from isotropic melting when degree of polym-
erization reaches a certain value, and the melt rapidly becomes
turbid since the orientation fluctuations of the growing me-
sogenic domains are in the range of the wavelength of visible
light. Further removal of volatile from the melt is the key to
increase the molecular weight. The as-synthesized LC texture
and morphology not only determine processability for subse-
quent process, but also affect the properties of final products.3
Since melt polymerization starts from a homogeneous phase to
a heterogeneous LC phase, different optical textures associated
with stringlike and pointlike defects can be observed. Some
defects remain in the LCPs solidified from the melt polymer-
ization and the others disappear through annihilation during the
reaction. The density and types of defects will strongly affect
the rheological behavior, mechanical properties and optical
properties of polymeric materials. So the quality of LCPs for
end uses is strongly affected by polycondensation conditions,
and reproducing a new LCP variant with the same inherent
properties still remains a big challenge for industrial producers.
However, rare attention has been given by academia to
investigate the LC texture formation and evolution during
Figure 1. The sample for thin film polymerization.
polymerization. Such studies are very limited and most of
them focus only on low molecular weight liquid crystals or
the systems in which polymer molecular weight does not
change.7-10
Geil and co-workers are pioneers to employ thin film polym-
erization technique to investigate the inherent microstructure
of LCPs during the syntheses of a series of aromatic polymers
including poly(p-oxybenzoate)11,12 (POBA), poly(2,6-oxynaph-
thoate)13 (PONA), poly(meta-oxybenzoate/2,6-oxynaphthoate)14
(P(mOBA/ONA)), and poly(p-oxybenzoate/2,6-oxynaphthoate)15
(P(OBA/ONA)). The morphology and crystal structure of the
polymers were studied by optical and transmission electron
microscopes and electron diffraction. Polymers with different
inherent crystal structures were obtained after thin film polym-
erization.
* To whom correspondence should be addressed. E-mail: chencts@
nus.edu.sg.
† Department of Chemical and Environmental Engineering.
‡ Institute of Materials Research and Engineering.
10.1021/jp984332q CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/12/1999

4924 J. Phys. Chem. B, Vol. 103, No. 23, 1999
Cheng and Chung
(Figure 2 a-f)
For the LC texture evolution during reaction systems, Yee16
and Ober17 have studied the LC texture evolution during curing
of liquid crystalline epoxy. They found different curing agent
leaded to various LC textures.
Using a novel thin film polymerization technique, we ex-
amined and analyzed in situ polycondensation reactions and
liquid crystal phase evolution for wholly aromatic LCPs, such
as P(OBA/ONA) copolymers and POBA and PONA homopoly-

Main-Chain Liquid Crystalline Polyesters
J. Phys. Chem. B, Vol. 103, No. 23, 1999 4925
Figure 2. Micrographs showing morphological changes of polymerization reaction system. All the micrographs were obtained from the same area
of the same sample. Monomer composition of reaction system was 73/27 ABA/ANA. Reaction temperature was 250 °C.
mers.18 In our experiments, a thin layer of monomers was
sandwiched between two glass slides with a ring spacer and
reactions were carried out on a heating stage of a polarizing
microscope. Through in situ observation of the microscope, we
can use this novel thin film polymerization technique to in situ
investigate morphological changes during the reaction and, thus,

4926 J. Phys. Chem. B, Vol. 103, No. 23, 1999
Cheng and Chung
unfold the evolution of LC structure in polycondensation reac-
tors. In this paper, we intend to extend our previous work to
investigate the whole process of LC texture generation, evolu-
tion, and annihilation of different kinds of defects during the
polymerization reaction, thereby providing important informa-
tion for LCP synthesis.
mixed crystals, and melts to homogeneous phase, and then
changes into a heterogeneous system with the following
sequence of morphological changes: generation of anisotropic
phase, coalescence of LC domains, formation of schlieren
texture, annihilation of disclinations, and formation of stripe
texture.
3.1.1. Generation of Anisotropic Phase: LC Domains. Figure
2 illustrates a set of micrographs showing the typical LC phase
separation from a homogeneous phase and time evolution of
LC texture in the early stage of thin film polymerization. During
the heating, the monomers melted and the whole view area
became isotropic melt phase as shown in Figure 2a. In the early
stage of polycondensation reaction, oligomers formed in the
molten monomer phase. Their molecular weight and chain length
increased with reaction time. When the chain length of the
oligomers reached a certain value, they formed anisotropic phase
(LC phase) and separated from the isotropic melt. Figure 2b
shows the reaction induced phase separation process during the
polymerization. The dark area in the micrograph is the isotropic
phase, while the bright area represents the anisotropic phase.
The first sign of forming anisotropic phase was that many bright
LC domains instantaneously appeared in the view range.
Because of the polydispersity of the chain length, oligomers
were partitioned within the isotropic and anisotropic phase
according to the chain length.19 A fraction of relatively longer
chain length formed anisotropic domains, while others remained
in the isotropic phase.
2. Experimental Section
2.1. Preparation of Monomers: p-Acetoxybenzoic Acid
(ABA) and 2,6-Acetoxynaphthoic Acid (ANA). The monomers
were made by the acetylation of p-hydroxybenzoic acid (HBA)
and 2,6-hydroxynaphthoic acid (HNA) separately, with acetic
anhydride in refluxing toluene in the presence of a catalytic
amount of pyridine. The prepared monomer ABA was then
purified by recrystallization in butyl acetate. Similarly ANA
was purified in methanol. The success of acetylation for both
1
monomers was confirmed by H NMR.
2.2. Thin Film Polymerization. Two monomers, ABA and
ANA, with certain mole ratios were codissolved in acetone to
form a miscible solution. After solvent evaporating, a 0.5 mg
monomer mixture was placed on a glass slide. A drop of acetone
was deposited on the glass slide to dissolve monomers again.
After evaporation of the solvent, a thin layer of reactant mixture
was formed and attached to the glass slide and then sandwiched
between two glass slides with a ring spacer. The monomers
were attached on the bottom slide. The ring spacer was made
of stainless steel with a thickness of 0.5 mm. The whole package
was placed on a heating stage (Linkam THMS-600) of a
microscope and heated to a proposed temperature with a heating
rate of 90 °C/min. The sample was held at that specific
temperature during the whole reaction process. When heating
stage reached the proposed temperature, the reaction time began
to be recorded. During the heating, monomers sublimated to
the top slide from the bottom one because of the temperature
difference between these two slides. After reaching proposed
reaction temperature, all monomers were attached on the top
slide except the loss, which was less than 5%. Nothing was
remained on the bottom slide. The temperature of the top slide
was calibrated by testing the melting points of the pure
monomers as well as by measuring with thermocouple. The
temperature difference between the heating stage set by the
programmer and the top slide was 20 ( 2 °C in our experimental
temperature range. The polymerization reaction was carried out
on the top slide and all the temperatures mentioned refer to the
temperatures of the top slide. The reaction process was observed
in situ by a polarizing light microscope (Olympus BX50) with
crossed polarizers between which a red plate having the
retardation of 530 nm was inserted or not inserted. The optical
images were recorded by a digital video cassette recorder (DHR-
1000NP). The data of the micrographs were analyzed by
imaging software (Image-Pro Plus 3.0). The sample for thin
film polymerization is shown in Figure 1.
3.1.2. Coalescence of LC Domains: Formation of Schlieren
Texture. After the appearance of anisotropic phase, the size of
LC domains quickly increased, and correspondingly the number
of domains decreased because of domain growth and coales-
cence of adjacent LC domains.
Figures 2d-i illustrate a detailed coalescence process. Ac-
cording to previous study,6 we know that black brushes
originating from the points are regions where the director is
either parallel or perpendicular to the plane of polarization of
incident light, therefore the incident light is extinguished by
the crossed polarizers. When rotating the crossed polarizers, the
position of the points remains unchanged but the brushes rotate
continuously showing that the orientation of the director changes
continuously about the disclinations. If the sense of rotation is
the same as that of polarizers, the disclination is a positive one.
On the contrary, if the sense of rotation is opposite to that of
polarizers, the disclination is a negative one. The strength S of
a disclination is determined by the number N of the dark brushes
around the single disclination: |S| ) N/4. We found in our
experiment all LC domains formed in the isotropic melt had
the disclination strength of +1. In Figure 2e, the two domains
with strength S ) +1 are indicated by red arrows. When
coalescence happened, a negative disclination of strength S )
-1 as indicated by green arrow formed at the contact point of
these two domains as soon as they contacted each other. The
process followed was the annihilation of the two defects with
opposite signs as shown in Figures 2e-i. After the formation
of the disclination of S ) -1 during coalescence, this discli-
nation (indicated by green arrow) and one of adjacent discli-
nations of S ) +1 (indicated by red arrow) immediately moved
toward each other and then disappeared together. Figure 3 shows
the dependence of distance D between the two disclinations on
reaction time t (for the case of Figure 2e-i). The distance
decreased with reaction time almost linearly and two disclina-
tions disappeared together after 0.8 s of the coalescence. Thus,
a large domain with only one declination of S ) +1 was
formed.14
2.3. Characterization. FTIR Characterization. The mono-
mers and polymers were characterized by FTIR (Perkin-Elmer
FTIR Spectrometer Spectrum 2000.). Polymer sample (in KBr
pellet) was prepared by scraping materials from the slide.
3. Results and Discussion
3.1. Phase Separation and Time Evolution of Liquid
Crystal Texture during Polymerization Reaction. Using the
reaction system with the monomer composition of 73/27 (mole
ratio) ABA/ANA at a reaction temperature of 250 °C as an
example, we investigated the polycondensation reaction system.
During whole polymerization reaction, the system starts from

Main-Chain Liquid Crystalline Polyesters
J. Phys. Chem. B, Vol. 103, No. 23, 1999 4927
nihilation of the loops were two methods to release the excess
deformation energy.
A. Annihilation between Two Opposite Disclinations. During
the thin film polymerization reaction, we observed that schlieren
texture evolved to inversion walls and, at the same time,
annihilation occurred between disclinations. The micrographs
in Figure 4 were taken from the same area of same sample when
the range of reaction time was 441.0-449.4 s. They show the
annihilation process of a pair of disclinations with opposite
strength S ) +1 and -1. According to Figure 4, we can see
the distance between the pair of disclinations gradually de-
creased. At last, two disclinations disappeared together. During
this process, the molecules within the area rearranged their
orientation and yielded a locally perfect orientation after the
annihilation.
Figure 5 shows the distance between the two disclinations
decreases in an approximately linear manner with the reaction
time because the slope of the straight line in logarithmic scale
is 1. (Here to is the time that two disclinations joined and
disappeared.) For the nonreacting systems, the annihilation
kinetics follows a power law.7-10 Our experiment data indicates
the annihilation kinetics for reacting system also follows the
power law. Although the viscosity increases with the progress
of polymerization reaction because of the increase in molecular
weight, we have not observed its effect on annihilation. This is
due to the fact that the change in viscosity was not obvious
because the annihilation happened within a relatively short time
interval.
Figure 3. Time dependence of annihilation process after coalescence.
Monomer composition of reaction system was 73/27 ABA/ANA.
Reaction temperature was 250 °C. (For the case of Figure 2e-i).
The growth of the LC phase occurred when reaction time
was in the range from 202 s to 392 s for this sample. As a re-
sult, the total view area became anisotropic LC phase and the
sum of the strength of all disclinations in the sample tended
to be zero. The typical schlieren texture was formed at this
moment.
3.1.3. Time EVolution of Schlieren Texture: Annihilation of
Disclinations. As we know, the dynamical properties of LCP
are quite different from that of LC, but the static properties of
LCP are similar to that of LC.6 In the continuum theory of LC,
the distortion energy density is given by:
At the beginning of polycondensation reaction, the annihila-
tion occurred very quickly, indicating the molecular weight at
this stage was relatively low and thus viscosity of the system
was not very high. With an increase in the reaction time, the
annihilation rate slowed because the increasing viscosity
restricted the motion of disclinations for further annihilation.
When the energy needed for the reorganization of molecular
orientation was higher than that released by the annihilation of
defects, the annihilation process was completely retarded. The
annihilation between two pairs of defects during different
reaction time intervals in the same sample is compared in Figure
6. In this Figure, the annihilation rate (V ) dD/dt) for the same
pair of defects remained approximately the same during the
whole annihilation process. For the annihilation occurred during
the time interval of 441.0-449.4 s, as already shown in Figure
5, the annihilation rate was about 25 µm/s. While for another
annihilation occurred during the time interval of 530-716 s,
the annihilation rate was much slower, about 2 µm/s. In our
reaction system at 250 °C, no further movement of defects could
be observed after reaction time reached 1000 s.
Fd ) 1/2K1 (divn)2 + 1/2 K2(n ‚ curln)2 + 1/2 K3(n x curln)2
where n is the variable director, K1, K2, and K3 are elastic
constants for splay, twist, and bend, respectively.20 According
to this formula, the larger the elastic constants are, the higher
the distortion energy is. Normally, the elastic constants decrease
with increasing temperature, and increase with increasing length
of molecules. According to Odijk’s deduction, the splay, twist,
and bend elastic constants for the polymer chains which are
not completely rigid can be expressed as follows:4
K1 ≈ 3K2
(if the chain length L is less than the persistence length q)
K1
L
(if the chain length L exceeds the persistence length q)
K2 ≈ φ1/3(q/d)1/3(kT/d)
K3 ≈ φ(q/d)(kT/d)
However, annihilation between the defects was a complicated
process and not fully understood yet. We found that the
annihilation process was affected by many factors. For example,
other defects surrounding the particular pair of defects more or
less affect the annihilation. In a polymerization system, a more
complicated factor is added to the annihilation process due to
the elastic constant change because of the increase in molecular
weight. However, the annihilation rates for different pairs of
disclinations decrease with reaction time is a universal trend.
where the term (kT/d) redimensionalizes the constant to give
the units of force, φ ) 1 for thermotropic polymers, and q/d is
the persistence ratio. Thus the elastic constants increase with
increasing persistence length. If the molecular length exceeds
the persistence length, the values of K2 and K3 will tend to be
constant, while K1 will increase monotonically with L.
Since the defects are caused by the deformation of molecular
chains, they involve very high distortion energy in the case of
rigid or semirigid polymers.7-10,20 Disclinations with opposite
signs are trendy to attract each other in order to release the
energy. This interaction leads to the annihilation and the decrease
in the number of defects. In our polymerization systems, the
annihilation between two opposite sign disclinations and an-
B. Shrinkage of Loop. Figure 7 shows the typical annihilation
of loops for the polymerization at 250 °C. The images were
taken at reaction time ranging from 480-546 s. With an increase
in the reaction time, the loop gradually shrank, at the same time
changed its shape into circular one, and finally disappeared.
Time dependence of the loop (indicated by the arrow) area A
and perimeter P is shown in Figure 8. (to is the time that the

4928 J. Phys. Chem. B, Vol. 103, No. 23, 1999
Cheng and Chung
Figure 4. Micrographs showing the annihilation process of a pair of disclinations with opposite signs. All the micrographs were obtained from the
same area of the same sample. Monomer composition of reaction system was 73/27 ABA/ANA. Reaction temperature was 250 °C.
Figure 5. Time dependence of the distance D between a pair of
disclinations. The disclinations are shown in Figure 4. t is reaction time,
and to is the time that two disclinations joined and disappeared.
Monomer composition of reaction system was 73/27 ABA/ANA.
Reaction temperature was 250 °C.
Figure 6. Annihilation of two pairs of disclinations during different
reaction time intervals. (a) 441.0-449.4 s and (b) 530-716 s. Monomer
composition of reaction system was 73/27 ABA/ANA. Reaction
loop disappeared.) We can find that the decrease in area and
perimeter of the loop follows a power law within the middle
stage of the shrinkage.
temperature was 250 °C.

Main-Chain Liquid Crystalline Polyesters
J. Phys. Chem. B, Vol. 103, No. 23, 1999 4929
Figure 7. Micrographs showing the shrinkage process of loops. All the micrographs were obtained from the same area of the same sample.
Monomer composition of reaction system was 73/27 ABA/ANA. Reaction temperature was 250 °C.
In the reaction systems we investigated, for the pairs of
disclinations with opposite signs, in some cases they annihilated
and then completely disappeared, while in other cases the
distance between two opposite disclinations decreased but
defects still existed in the system. For the loops, in some cases
they shrank, were annihilated, and then disappeared, while in
other cases, they shrank into a small area or a spot and stayed
in the system. At the beginning stage of schlieren texture, the
density of defects in the reaction system was considerably high.
After annihilation, the density dramatically decreased.
3.1.4. Formation of Stripe Texture. LCPs exhibit a variety
of textures of different length scales originated from flow and
deformation. A frequently observed texture called “stripe
texture” or “banded texture” often emerges in the samples in
which relaxation or shrinkage of molecular chains occurs. The
Figure 8. Time dependence of area A and perimeter P of the loop.
The loop is shown in Figure 7. t is reaction time, and to is the time that
the loop disappeared. Monomer composition of reaction system was
73/27 ABA/ANA. Reaction temperature was 250 °C.
conditions inducing this texture include: subjecting to shear
and/or elongation flow,21 evaporating the solvent from a
lyotropic LC,22 and quenching a thermotropic LC from high

4930 J. Phys. Chem. B, Vol. 103, No. 23, 1999
Cheng and Chung
Figure 10. The FTIR spectra of (a) monomers and (b) polymers.
Monomer composition was 73/27 ABA/ANA. Reaction temperature
was 250 °C.
+1 and the right one is -1 because the macromolecular chain
orientation is perpendicular to the stripes. The stripe texture
developed fully within more than 10 min. The width of the
stripes is about 5 µm.
However, the stripe texture may not appear in some other
composition and reaction temperatures if the crystallization
occurs first and interrupts the formation of stripe texture.18
3.2. FTIR Characterization. During thin film polymeriza-
tion, ester exchange reactions between acetoxy groups and
carboxyl groups occur with elimination of acetic acid.
This polycondensation reaction follows second-order kinet-
ics.24 The detail kinetics study will be reported in our following
paper.
Figure 10 shows the FTIR spectra of monomers and thin film
polymerization product. Figure 10a is the spectrum of a 73/27
ABA/ANA monomer mixture. The band at 1685 cm-1 is νCdO
of the -COOH group and the band at 1759 cm-1 is νCdO of
the CH3COO- group. After a 2 h reaction, as shown in Figure
10b, the bands at 1685 and 1759 cm-1 almost completely
disappear which indicates that nearly all of acetoxy and carboxyl
groups are consumed, with a substantial number of ester groups
formed in 1735 cm-1. On the basis of these spectra, we believe
copolymers are obtained.
Figure 9. Micrographs showing the formation of stripe structure. All
the micrographs were obtained from the same area of the same sample.
Monomer composition of reaction system was 73/27 ABA/ANA.
Reaction temperature was 250 °C.
3.3. Effect of Reaction Temperature on Annihilation
during the Polymerization. To investigate the effect of the
reaction temperature on annihilation, we conducted thin film
polymerizations at different reaction temperatures from 230 to
310 °C for 73/27 ABA/ANA system. The annihilation processes
at different areas of the same sample as well as different samples
with same reaction conditions were recorded and the annihilation
rates V were calculated. The results are summarized in Figure
11, where t1 is the time for the appearance of the anisotropic
phase. The dashed line represents that annihilation occurred
before formation of schlieren texture (before total view area
became LC phase), while the solid line represents that annihila-
tion occurred after formation of schlieren texture (after total
view area became LC phase). The length of the line indicates
the range of annihilation rates.
temperature.23 As a consequence of stress relaxation after shear
or volume deficiency induced by solvent evaporating or
quenching, stripe texture appears while local order does not
decrease significantly.21-23
In our reaction system, with the increase of molecular weight,
the volume deficiency caused the appearance of stripe texture
since adhesion to the substrate prevented uniform shrinkage in
three dimensions. Figure 9 shows formation of stripe texture at
the reaction time ranging from 30-55 min. Figure 9a is the
typical schlieren texture with two disclinations of opposite signs.
The dark line connecting the defects is quite diffuse, indicating
the defects are isolated. After the stripe texture appeared (Figure
9b), the situation changed, the defects are connected by an
inversion wall (the sharp dark line). From the stripes in Figure
9c we can deduce that the strength of the left disclination is
Figure 11 suggests that the annihilation rates for different
pairs of disclinations vary in a wide range even during the same

Main-Chain Liquid Crystalline Polyesters
J. Phys. Chem. B, Vol. 103, No. 23, 1999 4931
Figure 12. Comparison of typical annihilation processes when the
fraction of LC phase is 70% at different reaction temperatures. t is
reaction time, and to is the time that two disclinations joined and
disappeared. Monomer composition of reaction systems was 73/27
ABA/ANA.
Figure 11. Time dependence of annihilation rate for reaction systems
at different temperatures. t is reaction time, and t1 is the time of
appearance of anisotropic phase. (a) 310 °C: t1 ) 37 s, (b) 290 °C: t1
) 90 s, (c) 270 °C: t1 ) 140 s, (d) 250 °C: t1 ) 202 s and (e) 230 °C:
t1 ) 510 s. Monomer composition of reaction systems was 73/27
ABA/ANA.
reaction time interval and at the same reaction temperature. For
example, the highest and lowest annihilation rates we have
observed were 133 and 60 µm/s, respectively, when the reaction
temperature was 310 °C and t-t1 was around 15 s. However,
Figure 11 clearly exhibits that, for most cases, annihilation rate
decreases with increasing reaction time as we have mentioned
before. In addition, the higher the polymerization temperature
we used, the faster the change of annihilation rate is. This is
due to the fact that viscosity plays a critical role to determine
the speed of the annihilation since a higher reaction temperature
causes a higher reaction rate and a faster increase in viscosity.
The speed of the annihilation monotonically decreases with
the increasing reaction time if polymerization reactions take
place at temperatures ranging from 250 to 310 °C. However,
the situation changed when the reaction temperature was low:
230 °C. Before the total view area became an anisotropic phase,
the annihilation rate was around 15-45 µm/s when t - t1 ) 51
s and 15-27 µm/s when t - t1 ) 160 s. After the total view
area became an anisotropic phase, the annihilation rate increased
to 15-50 µm/s when t - t1 ) 388 s. It is a challenge to explain
the increase in annihilation rate. The possible explanation for
this interesting phenomenon is the change in elastic constants
played a more important effect than the change in viscosity.
Since the reaction at 230 °C was slow, the system viscosity did
not change significantly from t -t1 ) 51 to 388 s. But the elastic
constants increased with an increase in molecular length, thus
the distortion energy of the system increased and resulted in a
relatively high annihilation rate.
The annihilation after coalescence of LC domains when the
fraction of LC phase was 70% (before the total view area
became an anisotropic phase) at different reaction temperatures
is compared in Figure 12. Figure 12 suggests that annihilation
rate increases with the increasing reaction temperature. This
phenomenon also shows the reaction rate and viscosity are
important factors affecting the annihilation since a higher
temperature resulted in lower viscosity.
3.4. Annihilation in Reaction Systems with Different
Monomer Composition. We also studied the annihilation
process during homopolymerizations of pure ABA and pure
ANA at 250 °C. Results are illustrated in Figure 13. The
relationship between the annihilation rate and the reaction time
for systems with different monomer composition is similar. Both
annihilation rate and its range decrease with an increase in t -
t1. Although the molecular structures will affect the values of
Figure 13. Time dependence of annihilation rate for reaction systems
with different monomer composition. t is reaction time, and t1 is the
time of appearance of anisotropic phase. (a) ABA: t1 ) 246 s, (b)
73/27 ABA/ANA: t1 ) 202 s and (c) ANA: t1 ) 328 s. Reaction
temperatures were 250 °C.
elastic constants, and thus affect the annihilation, we have not
observed obvious difference in annihilation rate among these
three systems. This indicates that these three systems might have
similar values of elastic constants at early stage of polymeri-
zation.
4. Conclusions
We have observed phase separation, coalescence, and an-
nihilation of liquid crystal textures during polymerization of
liquid crystalline polyesters by a novel thin film polymerization
technique. The polycondensation system continuously changed
from random monomer melt to highly orientated polymers
through the following morphological changes: generation of
anisotropic phase, coalescence of LC domains, formation of
schlieren texture, annihilation of disclinations, and formation
of stripe texture. All the LC domains formed in the isotropic
melt during the early stage of polymerization for these wholly
aromatic liquid crystalline polyesters had the disclination
strength of +1. The number of defects decreased with increasing
reaction time through coalescence and annihilation. In most
cases, the higher reaction temperature we used, the faster the
annihilation rate was. At the early stage of polymerization, the
dominant factor affecting annihilation rate was the low viscosity
characteristics at elevated temperatures. For reaction systems

4932 J. Phys. Chem. B, Vol. 103, No. 23, 1999
Cheng and Chung
(7) Wang, W. Liq. Cryst. 1995, 19, 251.
(8) Liu, C.; Muthukumar, M. J. Chem. Phys. 1997, 106, 7822.
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with different monomer composition, their annihilation patterns
were similar.
Acknowledgment. The authors would like to express their
gratitude to National University of Singapore (NUS) for the
financial support with Research Fund RP 950696. Thanks are
also due to Prof. P. H. Geil at University of Illinois for providing
us many useful and knowledgeable comments, Dr. S. Mullick
for providing the monomers, and Dr. W. Wang, Mr. S. L. Liu,
Ms. W. H. Lin, and Mdm. L. K. Leong for their kind help and
useful comments. Special thanks are due to the Institute of
Materials Research and Engineering (IMRE) of Singapore for
the equipment.
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