Structures and phase transformations of asymmetric bent main-chain liquid crystalline polyesters

Article abstract of DOI:10.1021/ma051389j

A series of new asymmetric bent main-chain liquid crystalline (LC) polyesters (BPE-Cn) were synthesized through the condensation polymerization of A-B type asymmetric α,ω-carboxylic acid-hydroxyl-terminated monomers containing different even numbers of me

Full text of DOI:10.1021/ma051389j

Macromolecules 2005, 38, 8333-8344
8333
Structures and Phase Transformations of Asymmetric Bent Main-Chain
Liquid Crystalline Polyesters
Kwang-Un Jeong,† Brian S. Knapp,† Jason J. Ge,† Shi Jin,‡ Matthew J. Graham,†
Huiming Xiong,† Frank W. Harris,† and Stephen Z. D. Cheng*,†
Maurice Morton Institute and Department of Polymer Science, The University of Akron, Akron, Ohio
44325-3909, and Department of Chemistry, College of Staten Island, The City University of New York,
Staten Island, New York 10314
Received June 28, 2005; Revised Manuscript Received July 29, 2005
ABSTRACT: A series of new asymmetric bent main-chain liquid crystalline (LC) polyesters (BPE-Cn)
were synthesized through the condensation polymerization of A-B type asymmetric R,ω-carboxylic acid-
hydroxyl-terminated monomers containing different even numbers of methylene spacers (BPCA-Cn-PmOH,
where n ) 6, 8, and 10). Differential scanning calorimetry results showed three phase transition processes
in this series of BPE-Cn samples. The phase change with the lowest transition temperature is cooling
rate dependent, while the two higher temperature transitions are cooling rate independent. One-
dimensional (1D) powder wide-angle X-ray diffraction (WAXD) results at different temperatures revealed
that during cooling this series of polymers exhibits a low-ordered LC phase and a highly ordered smectic
crystal phase before developing a crystalline phase at lower temperatures. The phase structures and
symmetry were identified by 2D WAXD and selected area electron diffraction (SAED). On the basis of
2D fiber WAXD patterns, the low ordered LC phase was identified to be an anticlinically tilted SmC
(SmCA) phase, which was constructed by an alternating synclinic SmC phase in each chemical repeating
unit layer along the fiber drawn direction. The highly ordered smectic phase was an anticlinically tilted
SmH (SmHA) phase, which was again constructed by an alternating synclinic SmH phase in neighboring
layers. On the basis of the crystallographic point of view, this smectic crystal phase had an orthorhombic
unit cell. The crystalline structure was determined to be an orthorhombic unit cell (KO) with Pna21
symmetry in these three polyesters. The crystalline structures and symmetry were also confirmed by
SAED experiments. As the number of methylene units increased, the dimensions of the crystal structures
also increased, yet they retained a zigzag conformation. A two-chain packing model of the KO phase with
four chemical repeating units was proposed on the basis of the experimental diffraction patterns and
qualitatively supported by simulated structural diffractions. Additionally, the phase identifications were
also supported by the observation of texture changes in polarized light microscopy. Even though the
zigzag molecular arrangement remains in the low ordered SmCA and the highly ordered SmHA phases
which mimics the bent-core small molecules, chiral characteristics such as helical supramolecular
structures are not observed due to the covalent bonding connections in this series of BPE-Cn polymers.
Introduction
bibenzoic acid, and alkylene glycols exhibited a low
ordered smectic A (SmA) phase, but the mesophase of
the odd-numbered ones was identified to be an anti-
clinically tilted smectic C (SmCA) phase.13-20
Thermotropic liquid crystalline (LC) polymers have
been interesting subjects for both academic and indus-
trial research because of their anisotropic structures and
properties. To achieve the desired physical properties,
LC polymers with various chemical structures and
molecular architecture including both the mesogenic
groups and flexible spacers have been designed.1-12
Commonly observed modifications include tailoring LC
stabilities by selecting different mesogenic groups, dif-
ferent lengths of flexible spacers, and different locations
of the mesogens in the LC polymers.
The well-known odd-even effect of the methylene
units in LC polymers is originally described as affecting
their thermodynamic properties such as the transition
temperatures. However, it has also been found that in
some main-chain LC polymers the odd-even effect of
the flexible methylene units can be observed in their
structures and physical properties. For example, the
even-numbered main-chain polyesters (BB-n, n ) the
number of methylene units) polymerized via the melt
condensation of A-A and B-B type monomers, p,p-
A few years ago, a series of chiral polyesters were
designed and synthesized via the solution condensation
polymerization of nonracemic chiral A-B type mono-
mers, (R)-(-)-4′-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-
propyloxy]-1-nonyloxy}-4-biphenylcarboxylic acid [PET-
(R*)-n, n ) the number of methylene units]. This series
of polymers exhibited an important chiral characteristic,
a double-twisted helical single crystal. It is surprising
that the handedness of the helical sense in this series
of polymers is determined not only by the configura-
tional chirality but also by the number of methylene
units.21-29 For example, having the identical R* chiral
center, the even- or odd-numbered methylene units in
PET(R*)-n switched the helical single crystals from
right-handed for n ) odd to left-handed for n ) even.
This demonstrated a morphological odd-even effect.
The chirality transfer between different length scales
has been determined to be dependent upon the packing
scheme within each of the length scales.26,28
To achieve chiral characteristics from achiral mol-
ecules, low molecular weight bent-core (banana-shaped)
LC molecules with a meta-substituted unit at the center
of the mesogen have been synthesized. Some of the most
† The University of Akron.
‡ The City University of New York.
* To whom correspondence should be addressed. E-mail:
scheng@uakron.edu.
10.1021/ma051389j CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/30/2005

8334 Jeong et al.
Macromolecules, Vol. 38, No. 20, 2005
Scheme 1
fascinating results from these achiral banana-shaped
molecules is the formation of helical supramolecular
structures in the B7 phase and the spontaneous mac-
roscopic electric polarization in the B2 phase.30-34 These
chiral characteristics were previously thought to only
originate from chiral molecules in the SmC* phase.
Polymers and the subsequent networks incorporating
bent-core mesogenic units have recently been made.35-40
When the bent-core mesogenic units are attached to the
side or end of the fairly flexible backbone systems such
as a carbosilane dendrimer35 or poly(siloxane),36 ferro-
electric polarization was observed in the B2 phase.
We recently reported that a three-dimensional (3D)
helical supramolecular structure formed in a smectic C
(SmC) phase of a series of 4′-[6-(3′-hydroxylphenoxy)-
hexyloxy]-4-biphenylcarboxylic acid compounds with
different methylene spacer units (BPCA-Cn-PmOH, n
) 6-10). This series of compounds was synthesized via
four-step substitution reactions. They have neither
molecular chirality nor a significant bend like the
banana-shaped molecules. On the basis of our experi-
mental observations and computer calculations, we
concluded that the helical supramolecular structures in
these achiral 4-biphenylcarboxylic acid compounds re-
sult from chiral axial conformers of individual head-to-
head dimers with a phenyl group at both ends during
the transition from the SmA f SmC phases.41
The question is: can we design and synthesize a
series of asymmetric bent main-chain LC polymers to
study their chiral characteristics? Several reports have
appeared where the mesogens have symmetric bent-
cores.37-40 The approach we have taken is to use the
condensation polymerization of A-B type asymmetric
BPCA-Cn-PmOH monomers with different methylene
units (n). The corresponding polyesters are abbreviated
BPE-Cn. The bend introduced in BPE-Cn is located at
the end of the mesogen by the meta-linkage. In this
report, the phase structures of this series of BPE-Cn
are identified by wide-angle X-ray diffraction (WAXD)
and selected area electron diffraction (SAED). Their
molecular conformations and packing schemes were
deduced from structural characterization and supported
by computer simulations. The phase morphologies in the
LC and crystalline phases were investigated via polar-
ized light microscopy (PLM). Differential scanning cal-
orimetry (DSC) experiments were utilized to detect the
thermal transitions in this series of BPE-Cn samples.
From these, we achieved an understanding of the
mechanisms involved in these structural and morpho-
logical formations and their effects on determining
chiral characteristics.
polymer underwent 2% and 5% weight loss at 362 and 392
°C, respectively, at a heating rate of 10 °C/min under a dry
nitrogen atmosphere. A more detailed description of the
synthetic and purification processes for the monomers and
their corresponding polyesters can be found in ref 42.
The samples were kept in a vacuum before carrying out
characterization and analysis. For differential scanning calo-
rimetry (DSC) experiments, the sample weight was about 2.0
mg, and the pan weights were kept constant at a precision of
(0.001 mg. To observe the evolution of different phase
structures, film samples with a thickness of 1 mm were
prepared by melting the polymer powders for one-dimensional
(1D) WAXD powder experiments. To prepare oriented fibers
for 2D WAXD experiments, samples were first heated to the
low ordered LC phase. They were uniaxially drawn from the
LC phase to form oriented fibers. The fibers were then
annealed at different temperatures in order to determine
different phase structures using 2D WAXD experiments. The
samples used in PLM observations were melt-pressed between
two glass slides with a typical gap thickness of 10 µm. Thin
film samples prepared for TEM via solution casting from a
0.05% (w/v) chloroform solution on to carbon-coated mica had
a thickness of 50-150 nm. After crystallization, the films were
floated onto the water surface and recovered using the TEM
copper grids.
Equipment and Experiments. The thermal behaviors of
the phase transitions were studied using a Perkin-Elmer
PYRIS Diamond DSC with an Intracooler 2P apparatus. The
temperatures and heat flows were calibrated using material
standards at cooling and heating rates ranging from 2.5 to 40
°C/min. The heating experiments always preceded the cooling
experiments in order to eliminate previous thermal histories,
and the cooling and heating rates were always kept identical.
The transition temperatures were determined by measuring
the onset and peak temperatures from both the cooling and
heating scans at different rates. An onset temperature was
defined by the cross-point of the peak slope and the baseline
in the DSC trace. During cooling, the onset temperature was
Experimental Section
Materials and Sample Preparations. A series of asym-
metric bent main-chain LC polyesters (BPE-Cn) with an even
number of methylene units (n ) 6, 8, and 10) were synthesized
via the condensation polymerization of the A-B type asym-
metric BPCA-Cn-PmOH (where n ) 6, 8, and 10), which were
prepared via four-step substitution reactions as shown in
Scheme 1. Chemical structures of the BPCA-Cn-PmOHs and
their corresponding polyesters (BPE-Cn) were confirmed by
Fourier transform infrared spectroscopy (FT-IR) and solution
proton (1H) and carbon-13 (13C) nuclear magnetic resonance
(NMR). The number-average molecular weights of BPE-Cn (n
) even numbers) were determined by gel permeation chro-
matography (GPC) based on polystyrene standards, and they
ranged between 20 000 and 30 000 g/mol with polydispersities
between 1.8 and 2.4. The thermal gravimetric analysis (TGA)
showed that the BPE-Cn polyesters were thermally stable. The

Macromolecules, Vol. 38, No. 20, 2005
Liquid Crystalline Polyesters 8335
determined on the high-temperature side, and upon heating
the onset temperature was determined on the low-temperature
side.
Phase structures and transformations were identified using
1D WAXD powder experiments. These were conducted in the
transmission mode of a Rigaku 12 kW rotating-anode X-ray
(Cu KR radiation) generator coupled to a diffractometer. The
diffraction peak positions and widths were calibrated with
silicon crystals of known crystal size in the high 2θ angle
region (>15°) and silver behenate in the low 2θ angle region.
A hot stage was coupled to the diffractometer in order to study
the LC structural evolutions with temperature changes during
cooling and heating. The temperature of this hot stage was
calibrated to be within an accuracy of (1 °C. Samples were
scanned between 1.5° and 35° at a scanning rate of 4°/min.
The oriented fiber 2D WAXD patterns were obtained using a
Rigaku X-ray imaging system with an 18 kW rotating anode
X-ray generator. A hot stage was also used to obtain diffraction
patterns from the crystal and LC structures at different
temperatures. The patterns were recorded on an image plate
(Rigaku, R-AXIS-IV). A 30 min exposure time was required
for a high-quality pattern. In both 1D and 2D WAXD experi-
ments, background scattering was subtracted from the sample
scans.
TEM (FEI Tacnai 12) experiments were carried out to
examine crystal morphology on the nanometer scale using an
accelerating voltage of 120 kV. SAED patterns of the sample
having different zones were also obtained using a tilting stage
to determine the crystal unit cell symmetry and dimensions.
The camera length was set at 2.1 m, and calibration of the
SAED spacing smaller than 0.384 nm was carried out using
evaporated thallous chloride, which has a largest first-order
spacing diffraction of 0.384 nm. Spacing values larger than
0.384 nm were calibrated by doubling the d spacing values of
the first-order diffractions. Morphological observation of the
phases on the micrometer scale at different temperatures was
conducted on an Olympus (BH-2) PLM coupled with a Mettler
heating stage (FP-90).
Figure 1. Sets of DSC cooling and subsequent heating
thermal diagrams for BPE-Cn (n ) 6, 8, and 10) samples at a
rate of 2.5 °C/min (a) and for BPE-C8 at cooling and heating
rates ranging from 2.5 to 40 °C/min (b).
the two high-temperature transitions measured during
cooling are 3.36 kJ/mol at 188 °C and 6.47 kJ/mol at
180 °C. These two heats of transitions and the onset
transition temperatures at the high-temperature side
do not exhibit cooling rate dependence, although these
two transition peak widths become broadened with
increased cooling rate. This indicates that these two
high-temperature transitions take place close to ther-
modynamic equilibrium, and they may be associated
with the transitions between an isotropic melt (I) and
a LC phase and between two LC phases.43-46 In con-
trast, the heat of transition (2.17 kJ/mol) at the lowest
transition temperature (148 °C, measured at 2.5 °C/min)
is cooling rate dependent. Furthermore, if we compare
the cooling diagram with the heating diagram in Figure
1b, it is clear that the lowest transition process pos-
sesses a large undercooling; i.e., the melting endotherm
is over 30 °C higher than the exotherm during cooling,
commonly indicating a crystallization process.
Although DSC experiments are sensitive to heat
absorption and release events, this technique cannot
provide direct information about structural changes and
molecular interactions. Therefore, 1D WAXD experi-
ments at different temperatures are utilized to identify
the corresponding structural evolutions.
Figure 2a shows a set of 1D WAXD powder patterns
during cooling at different temperatures (at a rate of
2.5 °C/min) for BPE-C8. In this figure, structures with
two different length scales can be identified. One is on
the nanometer scale in the low 2θ angle region between
1.5° and 9°, and another on the subnanometer scale can
be identified between 9° and 35°. The 1D WAXD power
patterns in Figure 2a show three phase transitions, and
they agree with the observations in the DSC cooling
Crystallographic simulation was performed using Cerius2
(Version 4.6) simulation software from Accelrys utilizing the
COMPASS force field. The lowest energy conformation of the
single polymer chain was chosen as the starting conformation.
Basic unit cell parameters determined by crystallographic
experimental data from 2D WAXD and SAED experiments
were used in order to build the crystal unit cell. The positions
of atoms in this unit cell were judged by comparing their
calculated diffraction patterns with those of experiments.
Results and Discussion
Thermodynamic Transitions and Their Corre-
sponding Structural Evolutions. Figure 1a shows a
set of DSC cooling and subsequent heating thermal
diagrams for asymmetric bent main-chain BPE-Cn (n
) 6, 8, and 10) at a rate of 2.5 °C/min. Three transition
processes are observed during cooling. In the case of
BPE-C6, a broad high-temperature exothermic process
can be deconvoluted into two exothermic peaks at 200
and 196 °C, which can also be verified by 1D WAXD
results at these temperatures. For BPE-C8, the three
transition processes are observed at 188, 180, and 148
°C. In the case of BPE-C10, three transition processes
take place at 162, 157, and 94 °C, respectively. It is
evident that the BPE-Cn samples containing the longer
methylene spacers exhibit lower transition tempera-
tures, which is often observed in main-chain LC poly-
mers. These three transition processes are also detected
during heating at the same rate, but the transitions
overlap.
Figure 1b shows two sets of DSC cooling and subse-
quent heating thermal diagrams for BPE-C8 at different
rates ranging from 2.5 to 40 °C/min, as an example for
the three BPE-Cn samples. The heats of transition for

8336 Jeong et al.
Macromolecules, Vol. 38, No. 20, 2005
a slight narrowing of this scattering halo with a sudden
shift of 2θ angle to 19.6° (d ) 0.453 nm) takes place,
and they correspond to an exothermic process at the
same temperature in Figure 1b. Finally, sharp reflec-
tions develop in the high 2θ angle region below 148 °C,
and the reflection peak in the low 2θ angle region is
also enhanced as shown in Figure 2a.
The structural changes of BPE-C8 in Figure 2a can
be characterized by the d spacing changes and changes
in the width at half-height (WAHH) of the high 2θ angle
scattering halo with respect to temperature above the
crystallization temperature of 148 °C. The correlation
lengths of this halo can be calculated using the WAHH
data based on the Scherrer equation. The results are
shown in Figure 2b. The vertical dashed lines are
indications of the thermal transitions based on the DSC
cooling results. The changes in these structural param-
eters correspond well to the thermal transitions ob-
served in DSC. The 1D WAXD powder patterns during
heating at 2.5 °C/min (Figure 2c) are less obvious to
observe the structure changes compared with those
during cooling since the crystal melting takes place at
a higher temperature (∼180 °C). However, they agree
with the thermal transitions observed in the DSC
heating results (Figure 1b).
Identification of Crystal Structures. To determine
the structural symmetry and lattice parameters, 2D
WAXD experiments on oriented BPE-Cn samples are
conducted. A 2D WAXD pattern for BPE-C8, from a
fiber drawn in the LC phase at 185 °C and annealed at
145 °C for 1 day, is shown in Figure 3a. This pattern
exhibits diffractions not only on the equator and merid-
ian but also in the quadrants. As indicated in this figure,
the a* and b* axes are assigned to the equator, and the
c* axis is along the fiber drawn (FD) direction. On the
equator of this fiber pattern, only one strong diffraction
spot appears at 2θ ) 21.1° (d ) 0.42 nm). When the
diffraction spots in the quadrant are projected onto the
equator, the locations of these four extinct diffraction
spots on the equator can be identified at 2θ ) 10.5° (d
) 0.84 nm), 14.9° (d ) 0.60 nm), 18.2° (d ) 0.49 nm),
and 25.9° (d ) 0.34 nm). They can be assigned to be
the (010), (100), (110), and (120), respectively, based on
the triangulation method of building a 2D a*b* lattice
of the unit cell.49 The strong diffraction spot observed
at 2θ ) 21.1° is assigned to be the (020). On the basis
of these diffraction spots on the equator, the γ angle is
calculated to be 90°. On the meridian of this fiber
pattern, there are four diffraction spots at 2θ ) 3.90°
(d ) 2.27 nm), 7.80° (d ) 1.13 nm), 11.7° (d ) 0.75 nm),
and 15.6° (d ) 0.57 nm), respectively, and they should
be assigned as the (002) and its higher order (004), (006),
and (008) diffractions. This assignment is based on two
facts. First, the bend at the end of the mesogen of BPE-
C8 is an asymmetric chemical repeat unit; therefore,
two repeat units of each BPE-C8 chain must be involved
to construct the minimum structural repeat unit of this
unit cell. Second, the diffraction spots (hkl) in the
quadrants of the fiber pattern with l ) odd numbers
can be observed in between the neighboring (00l) (l )
even numbers) diffractions. Careful structural analysis
gives an orthorhombic unit cell with dimensions of a )
0.60 nm, b ) 0.84 nm, c ) 4.53 nm, and R ) â ) γ )
90° via the refinement of the reciprocal lattice using a
procedure developed in our laboratory, and this phase
is abbreviated to be the KO phase. Table 1 lists the
experimentally observed and the calculated 2θ values
Figure 2. Set of 1D WAXD powder patterns of BPE-C8 during
cooling at a rate of 2.5 °C/min at different temperatures (a).
The relationship of the d spacings of the center scattering halos
and correlation lengths of these scattering halos of BPE-C8
for temperatures above the crystallization temperature (b). The
correlation lengths were calculated on the basis of the Scherrer
equation. The vertical dashed lines are indications of the
thermal transitions based on the DSC cooling rate. Set of 1D
WAXD powder patterns of BPE-C8 during heating at a rate
of 2.5 °C/min at different temperatures (c).
diagrams (Figure 1b). At temperatures above 188 °C,
BPE-C8 is in the I phase. Only an amorphous halo at
2θ ) 18.6° (d spacing ) 0.477 nm) is observed. This
corresponds to a liquidlike, short-range order in the
lateral molecular packing. Its correlation length is
estimated to be about 1.2 nm by the Scherrer equa-
tion.43-48 When the temperature reached 188 °C, a low-
angle diffraction at 2θ ) 4.05° (d ) 2.18 nm) starts to
develop, and this usually indicates the formation of the
layer structure of a smectic LC phase. The temperature
where the diffraction at 2θ ) 4.05° appeared also
corresponds to a sudden narrowing of the high 2θ angle
scattering halo with a shift of the 2θ angle from 18.6°
to 19.3° (d spacing from 0.477 to 0.460 nm). At 180 °C,

Macromolecules, Vol. 38, No. 20, 2005
Liquid Crystalline Polyesters 8337
Table 1. Experimental and Calculated Crystallographic
Parameters of the KO Crystalline Phase of BPE-C8
2θ (deg)
d-spacing (nm)
expta calcdb
hkl
expta
calcdb
intc
020
011
101
111
002
012
022
023
113
123
004
014
114
124
015
115
125
006
016
116
21.1
10.7
15.0
18.3
21.1
10.7
15.0
18.3
3.90
11.2
21.4
21.9
19.2
26.6
7.80
13.1
19.9
27.1
14.3
20.7
27.7
11.7
15.8
21.7
0.42
0.83
0.59
0.48
2.27
0.79
0.42
0.40
0.46
0.34
1.13
0.68
0.45
0.33
0.62
0.43
0.32
0.75
0.56
0.41
0.42
0.83
0.59
0.48
2.27
0.79
0.41
0.41
0.46
0.34
1.13
0.68
0.45
0.33
0.62
0.43
0.32
0.76
0.56
0.41
vs
vw
m
w
vs
m
m
s
vs
w
m
w
vs
s
w
vs
m
m
w
s
3.90
11.2
21.4
21.9
19.2
26.6
7.80
13.1
19.9
27.1
14.4
20.7
27.7
11.7
15.8
21.7
a The accuracy of the experimental data is (0.005 nm. b The
calculated data listed are based on the KO orthorhombic unit cell
with a ) 0.60 nm, b ) 0.84 nm, c ) 4.53 nm, and R ) â ) γ )
90.0°. c The intensities are semiquantitatively estimated via a
microdesitometer and classified as very strong (vs), strong (s),
medium (m), weak (w), and very weak (vw).
the strongest (114) diffraction spot among the set of (11l)
diffraction spots. The measured tilting angle is 28°,
which is identical to the calculated value. Therefore, the
bending angle at the center of the structural repeating
unit (the center of two chemical repeating units) is 124°.
Since the only experimentally observed diffraction on
the equator is the (020), the two zigzag chains must lie
more or less on the (0k0) plane.
This structural determination and schematic two-
chain unit cell model for the KO phase in BPE-C8 can
be supported by SAED experiments conducted on BPE-
C8 single crystals in TEM. Figure 4a shows a stack of
lamellar crystals grown at a cooling rate of 0.1 °C/min
from the I phase and annealed at 145 °C for 12 h under
a dry nitrogen atmosphere. The lamellae appear to be
edge-on. The [010] zone SAED pattern is obtained from
this lamellar stack of the KO phase when the sample is
not tilted, as shown in Figure 4b. In this SAED pattern,
it is evident that no diffractions can be observed on the
a* axis due to the reason that all the (hk0) diffractions
are extinct (as shown in the 2D WAXD fiber pattern in
Figure 3a), except for the (020) diffraction. However, the
(020) diffraction cannot be seen in this SAED pattern
since the b* axis is perpendicular to the a*c* plane. The
observation of the (101) ED spot confirms that the (00l)
(l ) odd numbers) are extinct in the 2D WAXD fiber
pattern, and the diffraction spots on the meridian must
be assigned as the (002), (004), (006), and (008). The
(100) ED spot is also extinct in this ED pattern, as
observed in the case of the 2D WAXD fiber pattern. To
obtain an ED pattern from the [001] zone, which
provides the packing symmetry in the unit cell, the
BPE-C8 sample in the I phase was quenched to room
temperature and annealed at 145 °C for 12 h (the inset
of Figure 4c). A SAED pattern from the [001] zone can
be observed and is shown in Figure 4c. Only the (020)
and (040) ED spots are observed as seen in the X-ray
diffractions on the equator of the 2D WAXD fiber
pattern (Figure 3a). The symmetry group of the KO
phase was determined to be Pna21 on the basis of the
SAED results.
Figure 3. 2D WAXD fiber pattern of the KO crystalline phase
of BPE-C8 after the sample was annealed at 145 °C for 1 day
(a). The fiber draw direction and c* axis is along the meridian
direction while the a* and b* axes are along the equatorial
direction. A schematic unit cell model with two chains for the
KO crystalline phase of BPE-C8 (b).
and the d spacings based on this unit cell lattice. With
two chains (four chemical repeating units) in this
orthorhombic unit cell, its calculated crystallographic
density is 1.23 g/cm3. The experimentally observed
density is 1.21 g/cm3, which fits well with the calculated
data.
A schematic two-chain unit cell model for this KO
crystalline phase of BPE-C8 is shown in Figure 3b. The
d spacing of the (002) diffraction is 2.27 nm, while the
calculated length of the chemical repeating unit is 2.57
nm. This repeat unit should thus be tilted (28° with
respect to the c axis (the FD direction) toward the a axis.
The molecular tilting angle can also be measured from

8338 Jeong et al.
Macromolecules, Vol. 38, No. 20, 2005
Figure 4. TEM morphology (a) and SAED pattern of the [010] zone (b) without tilting for the edge-on lamellar crystals of BPE-
C8. SAED pattern of the [001] zone for the KO phase of BPE-C8 (c). The inset of (c) is the TEM morphology of the [001] zone
SAED pattern.
Table 2. Crystal System, Axial System, and Density of the KO Phase of BPE-Cn at Room Temperature
axial system
density
calcd
n
crystal system
a (nm)
b (nm)
c (nm)
R (deg)
â (deg)
γ (deg)
expt
6
orthorhombic
orthorhombic
orthorhombic
0.61
0.61
0.62
0.83
0.84
0.85
4.02
4.53
4.75
90.0
90.0
90.0
90.0
90.0
90.0
90.0
90.0
90.0
1.25
1.21
1.16
1.27
1.23
1.18
8
10
Confirmation of this structure determination and the
detailed positions of atoms in the KO two-chain (four
chemical repeating units) unit cell of BPE-C8 with a
symmetry group of Pna21 can be provided via computer
calculations. These calculations can generate a detailed
chain-packing scheme. To examine whether this pro-
posed packing scheme fits to the real structure, the
calculated diffraction patterns are generated and com-
pared with the experimentally obtained diffraction
patterns. First, the global equilibrium conformation of
the monomer BPCA-C8-PmOH was constructed at 0 K
using the COMPASS force field. The energy-minimized
monomers were used to construct the BPE-C8 polyester.
The lowest energy conformation of the single polymer
chain was chosen as the starting conformation. To build
the KO structure with two polymer chains using Pna21
symmetry, basic unit cell parameters determined by
experimental data from 2D WAXD and SAED experi-
ments were used. Figure 5a shows the two-chain pack-
ing scheme suggested by the computer calculations from
the side (the [100] zone, Figure 5b) and top (the [001]
zone, Figure 5c) views. The calculated 2D WAXD fiber
patterns and SAED pattern with the [001] zone based
on this scheme are shown in parts d and e of Figure 5,
respectively. Both (Figures 5d and 5e) qualitatively
match with the experimental observations (Figures 3a
and 4c, respectively).
For the other two polymers BPE-Cn (n ) 6 and 10),
their 2D WAXD fiber patterns are shown in parts a and
b of Figure 6, respectively. It is evident that these two
patterns are similar to that of the BPE-C8 as shown in
Figure 3a. Detailed structural determinations of these
two crystal structures lead to the identical orthorhombic
lattice and symmetry of the BPE-C8 crystal structures.
Their lattice, unit cell dimensions, and density data are
listed in Table 2. As shown in this table, with an
increasing the number of methylene units, the unit cell
dimensions increase, while the densities decrease.
Identification of Liquid Crystal Structures. A 2D
WAXD for an oriented BPE-C8 fiber sample, drawn in
the LC phase and annealed at 175 °C for 10 h under a
dry nitrogen atmosphere, is shown in Figure 7a. Com-
pared with the 2D WAXD fiber pattern of the KO phase
in Figure 3a, this figure shows fewer and diffused
diffraction spots on the meridian and in the quadrants,
but the overall diffraction pattern does not exhibit any
substantial changes. At first, Figure 7a appears to show
a highly ordered smectic crystal phase based on our
previous publications.43,44 Therefore, chain conforma-
tions and packing scheme in this highly ordered smectic
LC phase should not be very different from those in the
KO phase.
When we start to determine the structural lattice,
difficulty immediately arises due to the large zigzag

Macromolecules, Vol. 38, No. 20, 2005
Liquid Crystalline Polyesters 8339
Figure 5. Computer simulated BPE-C8 chain packing scheme in unit cell with the best match to the experimental 2D WAXD
and SAED patterns (a) with views from the side ([100]-zone) (b) and the top ([001]-zone) views (c). Computer calculated 2D WAXD
pattern (d) and SAED pattern of the [001] zone (e), which agree well with those of the experimental 2D WAXD (Figure 3a) and
SAED (Figure 4c) diffraction patterns.
chain conformation and its packing scheme. It is evident
that in this structure the chemical repeating units form
layers perpendicular to the FD direction, and the
neighboring layers are anticlinically tilted by (31.5°
with respect to FD direction (as described in Figure 7b).
Within each individual layer, the repeat units are
synclinically oriented. If we view each chemical repeat
unit layer as the structural block, a monoclinic lattice
can be determined with a ) 1.05 nm, b ) 0.50 nm, c )
2.21 nm, R ) 90.0°, â ) 58.5°, and γ ) 90.0°. Along the
long axis of the chemical repeating unit, a 2D rectan-
gular lattice can be observed.
From the crystallographic point of view, however, one
smectic crystal repeating unit must include two neigh-
boring chemical repeating unit layers along the chain
direction because of the symmetry requirement of the

8340 Jeong et al.
Macromolecules, Vol. 38, No. 20, 2005
Figure 6. 2D WAXD fiber pattern of the KO crystalline phase
of BPE-C6 (a) and BPE-C10 (b) after the sample was annealed
for 1 day at 150 and 85 °C, respectively.
Table 3. Experimental and Calculated Crystallographic
Parameters of the Highly Ordered Smectic Crystal Phase
of BPE-C8
2θ (deg)
d-spacing (nm)
expta calcdb
hkl
expta
calcdb
intc
002
023
004
114
115
116
4.00
4.00
20.4
8.00
18.9
19.9
21.0
2.21
0.43
1.10
0.47
0.45
0.42
2.21
0.43
1.11
0.47
0.45
0.42
vs
vs
w
s
vs
m
20.4
8.01
18.9
19.9
21.0
a The accuracy of the experimental data is (0.005 nm. b The
calculated data listed are based on the orthorhombic unit cell with
a ) 0.63 nm, b ) 0.91 nm, c ) 4.42 nm, and R ) â ) γ ) 90.0°.
c The intensities are semiquantitatively estimated via a microde-
sitometer and classified as very strong (vs), strong (s), medium
(m), weak (w), and very weak (vw).
structural repeat unit, as illustrated in Figure 7b.
Similar to the KO crystal phase case of BPE-C8, the a*
and b* axes can still be assigned to the equator and the
c* axis is along the FD direction (Figure 7a). On the
equator of this fiber pattern, the (020) diffraction spot
is not seen possibly due to disordered arrangements of
the zigzag chains. When the diffraction spots in the
quadrant are projected onto the equator, extinct dif-
fraction spots on the equator can be found at 2θ ) 17.1°
(d ) 0.518 nm) and 19.5° (d ) 0.455 nm), which can be
assigned to be the (110) and (020), respectively. On the
meridian of this fiber pattern, a pair of strong diffraction
spots are at 2θ ) 4.00° (d ) 2.21 nm) with a higher order
weak diffraction at 2θ ) 8.01° (d ) 1.10 nm). Again,
they are assigned to the (002) and (004), respectively,
indicating that the structural repeat unit consists of two
chemical repeat units of BPE-C8 in this highly ordered
smectic crystal phase. The two strongest pairs of scat-
tering spots in Figure 7a are at 2θ ) 20.5° (d ) 0.43
nm), which is attributed to the lateral packing of the
molecules, and possess a correlation length of ∼10 nm
as estimated by the Scherrer equation. Careful struc-
tural analysis gives rise to an orthorhombic unit cell
with dimensions of a ) 0.63 nm, b ) 0.91 nm, c ) 4.42
nm, and R ) â ) γ ) 90° via the refinement. Therefore,
this highly ordered smectic crystal phase should be an
anticlinically tilted SmH (SmHA) phase. Table 3 lists
the experimentally observed and calculated 2θ angles
and d spacings based on this unit cell lattice. With two
chains (four chemical repeating units) in this orthor-
hombic unit cell, its calculated crystallographic density
is 1.20 g/cm3.
Figure 7. 2D WAXD fiber pattern of the highly ordered
smectic crystal phase of BPE-C8 after the sample was an-
nealed at 175 °C for 10 h (a). The fiber draw direction and c*
axis are along the meridian direction while the a* and b* axes
are along the equatorial direction. A schematic chain-packing
model for the highly ordered smectic crystal phase of BPE-C8
(b).
point of view, yet the dimensions increase with an
increasing number of methylene units. All of the struc-
tural parameters, based on experimentally observed and
calculated data, for these three polyesters in the highly
ordered smectic crystal phase are listed in Table 4.
The structure of the LC phase observed between 180
and 188 °C can also be identified using the 2D WAXD
fiber pattern from BPE-C8. The fiber samples were
obtained by annealing the fiber at 185 °C for 2 h under
a dry nitrogen atmosphere, and the resultant WAXD
pattern is shown in Figure 9a. In the high 2θ angle
region, two pairs of scattering halos at 2θ ) 19.3° (d )
0.46 nm) in the quadrants ((20° away of the equator)
represent a liquidlike lateral short-range ordering within
Parts a and b of Figure 8 are two 2D WAXD fiber
patterns for BPE-C6 and BPE-C10 in the highly ordered
smectic crystal phases. The structures and lattices are
identical to that of BPE-C8 from the crystallographic

Macromolecules, Vol. 38, No. 20, 2005
Table 4. Crystal System, Axial System, and Density of BPE-Cn in the Highly Ordered Smectic Crystalline Phase
axial system density
Liquid Crystalline Polyesters 8341
n
crystal system
a (nm)
b (nm)
c (nm)
R (deg)
â (deg)
γ (deg)
calcd
6
orthorhombic
orthorhombic
orthorhombic
0.61
0.63
0.64
0.90
0.91
0.93
4.02
4.42
4.70
90.0
90.0
90.0
90.0
90.0
90.0
90.0
90.0
90.0
1.25
1.20
1.15
8
10
the layer. Their correlation lengths are ∼2.5 nm, as
estimated by the Scherrer equation. This illustrates that
the mesogens possess (20° chain tilting with respect
to the layer normal (the FD direction), as shown in
Figure 9b. A pair of low 2θ angle diffraction spots on
the meridian appear at 2θ ) 4.05° (d ) 2.18 nm),
indicating that the layer normal is along the FD
direction. It is predicted that the chain orientation in
the drawn polymer sample in this low ordered LC phase
does not significantly change during the transition from
the highly ordered smectic crystal phase. Within each
chemical repeating unit layer, the chain orientation is
synclinic. However, two neighboring layers construct
one smectic layer structure (the diffraction on the
meridian), and the alternated neighboring layers con-
struct an anticlinically tilted SmC (SmCA) phase,13-20,48
as illustrated in Figure 9b. The bending angle of the
BPE-C8 zigzag conformation in this SmCA phase is
∼140°. On the basis of the tilting angle ((20°) and the
measured layer thickness (2.18 nm), the length of a
chain in one layer of this SmCA phase is calculated to
be 2.32 nm, which is shorter than the calculated length
of the chain (2.56 nm). This difference may be due to
the fact that the alkoxyl chains of BPE-C8 in this SmCA
phase contain gauche and trans conformations instead
of the assumed all-trans conformation in the calcula-
tion.11,50-54 The 2D WAXD fiber patterns of the other
two BPE-Cn samples (n ) 6 and 10) are shown in Figure
10a,b, and they are also identified as SmCA phases. The
layer diffractions of these SmCA phases for BPE-C6 and
BPE-C10 are at 2θ ) 4.39° (d ) 2.01 nm) and 2θ ) 3.70°
(d ) 2.39 nm), respectively.
Figure 8. 2D WAXD fiber pattern of the highly ordered
smectic crystal phase of BPE-C6 (a) and BPE-C10 (b) after
the sample was annealed for 10 h at 175 and 145 °C,
respectively.
Figure 9. 2D WAXD fiber pattern of the low-ordered SmCA
LC phase of BPE-C8 after the sample was annealed at 185 °C
for 2 h (a). The fiber draw direction is along the meridian. A
schematic chain-packing model for the SmCA LC phase of BPE-
C8 (b).
One issue still remains: Figures 9 and 10 do not
warrant the SmCA chain packing, since this 2D WAXD
pattern can also be constructed by a mixture of two
(conventional) SmCS domains with opposite chain tilting
directions. However, the argument can be made that
the chain conformation in the KO phase and SmHA
phase is zigzag type, and the c axes of the unit cells are
identified to include two chemical repeat units. If the
SmCS phase were formed, the transition between the
SmCS and SmHA phases would require a large extra
volume in order to provide space for the significant
change between a zigzag to a linear chain conformation.
Both the free volume creation and conformation changes
would take an enormous amount of energy. Therefore,
the most probable chain conformation in the SmC phase
must retain the zigzag to form the SmCA phase.
Liquid Crystalline Textures. The phase identifica-
tions of BPE-Cn polyesters based on diffraction methods
can also be supported by the observation of texture
changes in PLM at the transitions. Figure 11a-c shows

8342 Jeong et al.
Macromolecules, Vol. 38, No. 20, 2005
Figure 10. 2D WAXD fiber pattern of the low-ordered SmCA
LC phase of BPE-C6 (a) and BPE-C10 (b) after the sample
was annealed for 2 h at 198 and 160 °C, respectively.
changes in the BPE-C8 morphological textures upon
cooling. At the cooling rate of 0.1 °C/min, the transition
from the I melt to the SmCA phase occurs at 188 °C. At
this point a baˆtonnet-like texture having cauliflower-
shaped domains against the I melt can be observed
(Figure 11a).55,56 When the temperature reaches 180 °C,
a fan-shaped texture with spherulitic domains appears
as shown in Figure 11b, which is often observed in
smectic phases.55 At a lower temperature deep into the
highly ordered smectic crystal phase, concentric rings
start to develop. By further decreasing the temperature
below the exothermic process at 148 °C, a pronounced
ring striation, a nonequidistant arc pattern, can be
observed as shown in Figure 11c. The rings show three
different colors: green, yellow, and brown, which may
correspond to the three orientations of the orthorhombic
structure.55 The rotation would occur along the normal
direction of the ring striations. A detailed quantitative
investigation is currently underway.
Figure 12a shows a cluster of oriented baˆtonnet-like
textures induced by mechanical shearing of the sample
in the SmCA phase, similar to the observations previ-
ously reported.56 The chain orientation is parallel to the
shear direction. When this sample is annealed more
than 10 h in the SmCA phase under a dry nitrogen
atmosphere, bigger elongated baˆtonnet-like texture
domains appear. In the SmCA phase of this series of
BPE-Cn, no helical supramolecular texture can be
observed even though the bending angle of BPE-Cn
polymer chain is ∼140°, which is similar to that of
banana-shaped LC small compounds.30-34 This is be-
cause the BPE-Cn are polymers, so the chemical repeat-
ing units are connected by covalent bonds. The strong
chemical bonding allows the retention of the zigzag
conformation but does not permit continuous twisting
of the chemical repeat unit from one to another along
the same twisting direction as in the case of their
corresponding monomers.41 Additionally, in BPCA-Cn-
PmOH, the chemical repeat units are connected in a
head-to-head sequence,41 while in the BPE-Cn polymers,
it is a covalent head-to-tail connection. On the other
hand, the ring striation, observed in PLM, shows three
different colors which may be an indication of limited
crystal twisting on the micrometer length scale.
Figure 11. Morphological observations of BPE-C8 under PLM
at different temperatures at a cooling rate of 0.1 °C/min: the
SmCA phase at 185 °C (a), the highly ordered smectic crystal
phase at 160 °C (b), and the KO phase at 30 °C (c).
On the basis of 2D WAXD and SAED results, these
three phases are identified to be the low-ordered SmCA
phase, the highly ordered SmHA phase, and the crystal-
line orthorhombic KO phases. A detailed chain-packing
scheme for the orthorhombic crystalline phase (KO) has
been proposed on the basis of the experimental diffrac-
tion patterns and qualitatively supported using simu-
lated structural diffractions. This structural model
consists of two crystallographic units in each unit cell
(which contains four chemical repeating units), and its
symmetry group is Pna21. Of particular interest is that
in all of these ordered phases the BPE-Cn chains retain
the large zigzag conformation which becomes the basic
building block of these ordered structures. This leads
to a difficulty in determining the LC phases. In each
chemical repeat unit layer, the highly and low ordered
smectic crystals should be syclinically packed, but
because of the crystallographic rule of structure sym-
Conclusion
A series of new asymmetric bent main-chain LC
polyesters (BPE-Cn) synthesized through the condensa-
tion polymerization of the A-B type asymmetric BPCA-
Cn-PmOH monomers containing different numbers of
methylene spacers (n ) 6, 8, and 10) exhibit three
different phases as observed via DSC measurements.

Macromolecules, Vol. 38, No. 20, 2005
Liquid Crystalline Polyesters 8343
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Acknowledgment. This work was supported by
the National Science Foundation (DMR-0203994 and
0516602). We also acknowledge Perkin-Elmer Co. for
their support in providing a Diamond DSC instrument
for our laboratory.
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