Synthesis and liquid crystalline behavior of laterally substituted polyimides with siloxane linkages

Article abstract of DOI:10.1021/ma101751e

Novel siloxane-containing liquid crystalline (LC) polyimides with methyl, chloro, and fluoro substituents on mesogenic units have been developed from siloxane-containing diamines with pyromellitic dianhydride (PMDA) or 3,3',4,4'-tetracarboxybiphenyl dianhydride (BPDA), and their thermotropic LC behavior was examined. Among these, chloro and fluoro substituents are effective for the formation of LC phases, particularly when those are substituted away from the center of the mesogenic unit: the isotropization temperature is not much affected, but the crystal-LC transition temperatures are significantly decreased. On the other hand, the methyl substituent tends to interrupt liquid crystallization as well as crystallization. Thus, the fluoro-substituted polyimide derived from BPDA exhibited the lowest crystalline-LC transition temperature (T_cr-lc = 134 °C) among all polyimides, showing a wide liquid crystal temperature up to 238 °C. From the X-ray diffraction measurement conducted for the oriented mesophases of fibrous polyimides, they were found to form SmA and SmC as high- and low-temperature mesophases, respectively.

Full text of DOI:10.1021/ma101751e

8950 Macromolecules 2010, 43, 8950–8956
DOI: 10.1021/ma101751e
Synthesis and Liquid Crystalline Behavior of Laterally Substituted
Polyimides with Siloxane Linkages
Yu Shoji, Ryohei Ishige, Tomoya Higashihara, Susumu Kawauchi, Junji Watanabe, and
Mitsuru Ueda*
Department of Organic and Polymeric Materials, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1-H-120, O-okayama, Meguro-Ku, Tokyo 152-8552, Japan
Received August 1, 2010; Revised Manuscript Received September 13, 2010
ABSTRACT: Novel siloxane-containing liquid crystalline (LC) polyimides with methyl, chloro, and fluoro
substituents on mesogenic units have been developed from siloxane-containing diamines with pyromellitic
dianhydride (PMDA) or 3,30,4,40-tetracarboxybiphenyl dianhydride (BPDA), and their thermotropic LC
behavior was examined. Among these, chloro and fluoro substituents are effective for the formation of LC
phases, particularly when those are substituted away from the center of the mesogenic unit: the isotropization
temperature is not much affected, but the crystal-LC transition temperatures are significantly decreased.
On the other hand, the methyl substituent tends to interrupt liquid crystallization as well as crystallization.
Thus, the fluoro-substituted polyimide derived from BPDA exhibited the lowest crystalline-LC transition
temperature (Tcr-lc = 134 ꢀC) among all polyimides, showing a wide liquid crystal temperature up to
238 ꢀC. From the X-ray diffraction measurement conducted for the oriented mesophases of fibrous
polyimides, they were found to form SmA and SmC as high- and low-temperature mesophases, respectively.
Introduction
a further decrease in the transition temperatures of polyimides is
required to avoid the destruction of semiconductor chips inside
Insulating polymers with high thermal conductivity have
recently been shown to be attractive materials for effectively
releasing generated heat from densely packaged electronics
devices. Since organic polymeric materials are generally thermal
insulators, the development of low dielectric and thermoconduc-
tive polymers are required at the present time. Liquid crystalline
(LC) polymers are expected to increase thermal conductivity
because the alignment of LC polymers effectively conducts
“phonon” which is the media of thermal conductivity for organic
polymers.1 Furthermore, the direction of phonon conductivity
corresponds to the alignment of polymer main chains, and such
alignment can easily be controlled in common ways, for example,
by stretching, rubbing, magnetic fields, self-alignment, and
so on.1-6
integrated circuits (ICs) during the fabrication process in practical
application. Although methyl-substituted polyimides were synthe-
sized by aiming at twisting the coplanar imide rings and loosening
the packing of the mesogens to decrease their transition tempera-
tures, they unfortunately did not show liquid crystallinity.
In this report, we describe the synthesis of novel laterally
substituted LC polyimides derived from PMDA or BPDA with
diamines containing siloxane spacer units and their thermotropic
LC behavior in detail. The purpose of this work is to determine
which kinds and positions of substituents are effective for
decreasing the crystal-LC transitiontemperatures of polyimides.
The molecular designs of the substituents include methyl, fluoro,
and chloro groups, expected to reduce the molecular interaction
of the mesogenic units.
There are two requirements for thermal conductive materials
used in practical applications. One is solution processability, and
the other is low transition temperatures of LC polymers to de-
crease the processing temperature. Aromatic polyimides should
be applicable as thermal conductive materials because the pre-
cursory polyimides, which are poly(amic acid)s, have the advan-
tage of solution processability, although most polyimides are
generally insoluble. However, only a few LC polyimides have
so far been reported, such as wholly aromatic and thermotropic
LC polyimides with methylene or oxyethylene units.7-11 In a
preceding work, we reported the synthesis of thermotropic LC
polyimides from diamines containing siloxane spacer units and
pyromellitic dianhydride (PMDA) or 3,30,4,40-tetracarboxy-
biphenyl dianhydride (BPDA).12,13 The crystal-LC transition
temperatures ranged from 222 to 275 ꢀC in the heating process,
depending on the number of methylene or siloxane units. These
LC transition temperatures were lower than those of corresponding
polyimides with alkylene or oxyethylene spacer units. However,
Results and Discussion
An LC polyimide backbone with octamethyltetrasiloxane
spacer units was selected, which showed the lowest crystal-LC
transition temperature (222 ꢀC) in a previous work.13 Methyl-,
fluoro-, or chloro-substituted diamine monomers containing
octamethyltetrasiloxane units (3b-d, 3f) were newly synthesized
under standard conditions for Williamson’s ether synthesis,
hydrosilylation, and reduction, as shown in Scheme 1.
Figure 1 representatively shows the 1H and 13C NMR spectra
for diamine 3d in CDCl3. The signal assignments in the 1H NMR
spectrum of 3d are consistent with the proposed structures. The
signals at 6.53 (b) and 6.76 ppm (a, c) correspond to aromatic
protons. The resonances for methylene protons (g, f, e, and d)
were clearly observed at 0.57-0.63, 1.47-1.58, 1.76-1.85, and
3.93 ppm, respectively. Furthermore, the signals of protons
adjacent to the methylsilyl groups appeared at 0.05 and 0.08
ppm. The target structure of 3d was also confirmed from the 13C
NMR spectrum, showing the expected 12 signals.
*Corresponding author. E-mail: mueda@polymer.titech.ac.jp.
pubs.acs.org/Macromolecules
Published on Web 10/14/2010
r 2010 American Chemical Society

Article
Polyimides 6 were synthesized by using a two-step polycon-
Macromolecules, Vol. 43, No. 21, 2010 8951
curves of all polyimides 6 are shown in Figure 2. The crystal
melting temperatures in polyimides 6 were 237 (6a), 197 (6b), 237
(6c), 242 (6d), 222 (6e), 193 (6f), 163 (6g), 186 (6h), and 210 ꢀC (6j)
(Table 2, Figure 3), while polyimide 6i showed an amorphous
nature. Polyimides 6a and 6c derived from PMDA showed
more than two exothermic peaks in the cooling process. In
addition, birefringence and fluidity were observed in the tem-
perature range between these peaks by polarizing optical micros-
copy (POM). Therefore, the phase between the transition tem-
peratures corresponds to the liquid crystalline phase. On the
other hand, the methyl- or chloro-substituted polyimides, 6b and
6d, did not show the liquid crystallinity according to DSC trace
and POM. With respect to the melting points of the polyimides
derived from PMDA, there was no noticeable change among
polyimides 6a, 6c, and 6d. Only methyl-substituted polyimide 6b
had the lowest melting point among them, probably because the
molecular interaction was reduced by a bulky substituent. The
melting point of chloro-substituted polyimide 6d was almost the
same as nonsubstituted polyimide 6a and fluoro-substituted 6c,
despite the similar bulkiness of the chlorine substituent of 6d to
the methyl one. We speculate that the dipole moment of the
chloro group might enhance the molecular interaction of meso-
genic units.
densation procedure, as shown in Scheme 2. Poly(amic acid)s
(PAAs), which are the precursors of polyimides, were prepared
from each diamine 3 and tetracarboxydianhydride derivatives
(PMDA (4a) or BPDA (4b)) at room temperature in N-methyl-
pyrrolidone (NMP). The resulting PAAs had high inherent
viscosities in the range 0.62-2.04 dL/g (Table 1). Then they were
cast and gradually heated up to 200 ꢀC for 2 h on glass substrates
under nitrogen to obtain polyimide films. In the representative
FT-IR spectrum of prepared polyimide 6g, the peaks for the amic
acid groups disappeared and the characteristic imide peaks at
1770 (CdO asymmetric stretching), 1713 (CdO symmetric
stretching), and 1389 cm-1 (C-N stretching) appeared, indicat-
ing complete imidization. The expected polyimide structures were
also confirmed by elemental analysis.
All polyimides 6 possess high thermal stability over 440 ꢀC
(Td5%) according to thermogravimetric analysis (TGA) under
nitrogen (Table 1). The differential scanning calorimetry (DSC)
Scheme 1. Synthetic Routes of Monomers, Diamines 3
Next, we investigated the effect of substituents at the R1
position for polyimides 6e-h, derived from BPDA. Polyimdes
6e, 6g, and 6h also showed liquid crystallinity by DSC and POM,
while polyimide 6f did not. Figure 4 shows the optical micro-
scopic texture observed for the mesophase of polyimide 6h at
225 ꢀC. Compared with the transition temperatures of polyimides
6e, 6g, and 6h, fluoro-substituted polyimide 6g had the lowest
crystal-LC transition temperature (134 ꢀC in the cooling
process) among them, although the LC-isotropic transition
temperatures were almost the same as the others. It probably
means that the fluoro group effectively reduced the molecular
interaction between the mesogens and prevented the highly
ordered structure of the polyimides; in other words, it decreased
the crystal melting temperature, leading to the expansion of the
LC temperature range. To elucidate the effect of the substitution
position, we synthesized polyimides 6i and 6j, which have a
methyl- or fluoro-substituted group at the R2 position and
expected that the transition temperature of polyimide 6i and 6j
would further be decreased compared with polyimide 6g, which
has a fluoro-substituted group at the R1 position, by twisting the
imide planes of the mesogenic units. However, polyimides 6i and
6j did not show LC properties, and their isotropic transition
Figure 1. (A) 1H and (B) 13C NMR spectra of the diamine 3d.

8952 Macromolecules, Vol. 43, No. 21, 2010
Scheme 2. Synthetic Routes of LC Polyimides 6
Shoji et al.
Table 1. Polymerization Results and Thermal Stabilities
substituent is the most efficient for producing a smectic C phase
when it is substituted away from the center of the mesogen
molecule and has reduced the occurrence of higher-ordered
phases.14 In our results, the fluoro group at the R1 position was
also the most effective substituent for decreasing the crystal-LC
transition temperatures; in other words, the fluoro substituent
properly destabilized the crystalline phase. Consequently, the
fluoro or chloro substitutions at the R1 position of polyimides 6
from BPDA reduce the crystal-LC transition temperatures
almost without changing their LC-isotropic transition tempera-
tures, while the substitutions at the R2 position reduce the
isotropic transition temperatures for destabilizing the crystalline
or LC phase. There is no clear explanation for the reason why the
methyl-substituted polyimide 6i derived from BPDA exception-
ally shows an amorphous nature.
polymer Ar
R1
R2
η
inh [dL/g]a Td1% [ꢀC]b Td5% [ꢀC]b
6a
6b
6c
6d
6e
6f
6g
6h
6i
4a
4a CH3
4a
4a Cl
4b
4b CH3
4b
4b Cl
H
H
H
H
H
H
H
H
H
0.66
0.67
0.62
0.86
1.43
2.04
0.83
0.75
0.78
1.60
415
421
412
414
430
415
426
420
408
392
455
443
443
444
457
443
443
446
452
440
F
From optical microscopic observation of the fanlike texture,
all the liquid crystals formed are smectic phase. X-ray diffraction
observation also identifies the smectic phase, showing an outer
broad hallow and inner sharp reflections. For polyimides 6c, 6e,
6g, and 6h, forming two mesophases, the higher temperature
mesophase is the SmA and the lower temperature mesophase is
the SmC. The layer spacings of the SmA and SmC phases are
H
F
4b
4b
H
H
CH3
F
6j
a Inherent viscosities were measured at 30 ꢀC in NMP at a PAA 5
concentration of 0.5 g/dL. b Decomposition temperature. Td1%: 1%
weight loss temperature, Td5%: 5% weight loss temperature.
˚
listed in Table 2. The SmA layer spacings (32.7-33.3 A) of 6a and
˚
6c are relatively smaller than those (35.4-35.9 A) of 6e, 6g, and
6h, reflecting the difference in length in the central unit of the
mesogenic group.
temperatures decreased. This is probably because the methyl or
fluoro substitution at the R2 position twisted the imide plane and
led to a significant decrease in the molecular interaction of the
mesogenic units, destabilizing the crystalline phase or mesophase.
To support this result, molecular simulation of the mesogenic
units of polyimides 6e, 6g, and 6j was carried out by density
functional theory. The equilibrium structures of those polyimides
were optimized and verified by frequency analysis at B3LYP/6-3/
G(d) (Figure 5). The dihedral angle of the phenyl and imide
groups for polyimide 6j was 58.1ꢀ, which was wider than 42.7ꢀ for
polyimide 6e and 39.9ꢀ for polyimide 6g. These results are similar
to a previous report, wherein Coates suggested that a lateral
The detailed structural change in the SmA-SmC transition
was investigated by wide-angle X-ray diffraction (WAXD) mea-
surement in polyimide 6g, which forms enantiotropic mesophases
in the widest temperature range. To obtain a uniaxial orientation
of the mesophases, the fiber was spun from isotropic melt at
280 ꢀC. Figure 6a shows a typical WAXD pattern observed in a
low-temperature liquid-crystal phase (180 ꢀC in the heating
process). Here, the fiber axis is set in the vertical direction. In a
small-angle region of the fiber pattern (see enlarged view of small-
angle region of Figure 6a), sharp layer reflections split on the left

Article
Macromolecules, Vol. 43, No. 21, 2010 8953
Figure 2. DSC heating and cooling curves of polyimides 6 by the measurement performed at a scanning rate of 10 ꢀC/min.
˚
and right side of the meridian are observed. The spacing is 31.1 A.
On the other hand, an outer broad reflection with a spacing of
5.5 A can be found on the equator. This diffraction profile gives
that this high-temperature phase is the SmA phase followed by
SmC-SmA transition. In Figure 7, the tilt angle of the layer
corresponding to the split angle μ of the layer reflections is plotted
against temperature. Of interest is that the tilt angle is somewhat
independent of the SmC temperature and becomes zero discon-
tinuously at the SmC-SmA transition, showing that the transi-
tion is of the first order. From a comparison of the spacing
between the SmC and SmA phases, we can estimate a tilt angle
of 30ꢀ under the assumption that the repeating unit length is
maintained throughout the transition. This value is nearly equal
to the observed one of 29ꢀ.
˚
just the luminescence of the SmC structure with the polymer
chains lying parallel to the fiber axis and the layers normal tilted
from the fiber axis.15 Figure 6b shows the fiber pattern obtained
for a high-temperature phase at 235 ꢀC. This pattern has sharp
˚
reflections of the smectic layer on the meridian at 35.4 A in the
small-angle region and broad outer reflections whose intensity
˚
peak is on the equator at 6.6 A. The orthogonal relationship
between the layer reflection and outer broad reflection indicates

8954 Macromolecules, Vol. 43, No. 21, 2010
Table 2. Thermal Transition Temperatures of Polyimides 6
Shoji et al.
ΔH [kJ mol-1 b
Tm [ꢀC]a Tcr-lc [ꢀC]b Tlc-lc [ꢀC]b Tlc-i [ꢀC]b lc-cr lc-lc iso-lc dLC1 [A] dLC2 [A] tilt angle f [deg]
]
˚
˚
polymer Ar
R1
R2
6a
6b
6c
6d
6e
6f
6g
6h
6i
4a
4a CH3
4a
4a Cl
4b
4b CH3
4b
4b Cl
H
H
H
H
H
H
H
H
H
237
197
237
242
222
192
163
186
e
214
d
216
d
203
d
134
178
e
c
d
221
d
216
d
211
198
e
231
d
249
d
254
d
238
230
e
16.2
10.9
20.8
2.7
1.5
1.7
32.7
33.3
35.9
F
1.3
4.3
H
32.6
24
F
8.6
13.5
4.5
5.4
1.4
1.7
31.1
32.2
35.4
35.4
29
36
4b
4b
H
H
CH3
F
6j
210
d
d
d
a Melting points of polyimides 6 on the heating process. b Transition temperatures were determined as those at peak tops by DSC during a cooling scan
at 10 ꢀC/min. Tcr-lc: crystal-LC transition temperature; Tlc-lc: LC-LC transition temperature; Tlc-i: LC-isotropic transition temperature. c No
detection of LC-LC transition temperature. d There are no LC phases. e Amorphous nature. f These tilt angles were measured from the splitting angle of
inner layer reflections in oriented X-ray patterns taken for the fibrous SmC phases (refer to Figure 6a).
Figure 3. Comparison of mesomorphic properties for polyimides 6.
˚
˚
Although the SmA-SmC transformation is thus obvious, the
present SmC and SmA phases possess some unusual structural
features. First, the SmC phase shows a few of broad reflections
along unusual directions in addition to the ordinal equatorial one
with a spacing of 5.5 A, that is, 6.4 and 5.0 A ones along the
meridional and off-meridional directions (see Figure 6a). This
unusual pattern may be attributable to the formation of a two-
dimensional pseudolattice; the strong segregation takes place

Article
Macromolecules, Vol. 43, No. 21, 2010 8955
between the alkyl spacer part and dimethylsiloxane part,16 giving
a rise to short-range positional order along the layer plane
direction. A second unusual feature is observed also on the broad
outer reflection profile of the SmA phase. Irrespective of the high
orientation of the layer, as can be found from the sharp inner
reflection concentrated on the meridian, the outer reflection is
widely spread along the azimuthal direction. From the intensity
profile of the outer broad reflection along the azimuthal direc-
tion, in fact, the orientational order parameter ÆP2æ of the
mesogenic part is evaluated as 0.4. This low orientational order
reminds us of the “de Vries” type of SmA phase, in which the
mesogens are significantly tilted to the layer normal, but their
average axis is perpendicular to the layer. The average tilt angle is
estimated as 37ꢀ from the layer normal. A third feature is the first-
order SmA-SmC transition. Usually, the SmC-SmA phase
transition is second-order, and the tiltangle changes continuously
following the equation15 (TC - T)1/2. Inthiscase, however, the tilt
angle changes discontinuously. The clear endothermic peakof the
SmC-SmA transition observed on DSC thermograms on both
the heating and cooling scan also supports that the transition is
first-order. The first-transition behavior of the SmC-SmA
transition has been predicted by Saunders et al. using a general-
ized theoretical Landau-de Gennes model.17-19 As pointed out
by Saunders, the fact that all known de Vries material does not
form the nematic phase implies that the formation of the SmC
phase is driven by an increase in the layering rather than
orientation as the temperature decreases. In fact, our synthesized
polyimide has a dimethylsiloxane group in the spacer which will
be strongly segregated from the alkyl chains and aromatic
mesogens, and this strong segregation is considered as a driving
force to form the LC phase. Detailed structural analyses are now
being carried out.
Figure 6. Wide-angle X-ray diffraction patterns of (a) SmC and (b)
SmA phases observed in the fiber samples of 6g. To show the smectic
layer reflection, patterns in small-angle region are enlarged in the lower
photographs. Fiber axis is set in a vertical direction.
Figure 4. Polarized optical microscopic texture observed for SmC of
polyimide 6h at 225 ꢀC (the scale bar indicates 25 μm).
Figure 5. Optimized geometries of the mesogenic units of polyimides 6e, 6g, and 6j.

8956 Macromolecules, Vol. 43, No. 21, 2010
Shoji et al.
scanning calorimetry (DSC). WAXD measurements were per-
formed at ambient temperature by using a Rigaku-Denki
RINT-2500 X-ray generator with monochromic Cu KR radia-
tion (40 kV, 50 mA) from graphite crystal of monochromator
and flat-plate type of imaging plate. Density functional theory
calculations were carried out by using Gaussian09 program on
TSUBAME supercomputer.
Materials. PMDA and BPDA were purified by sublimation
prior to use. NMP, acetonitrile, and toluene were purified by
distillation. Other reagents and solvents were purchased from
TCI, Japan. The syntheses of diamine monomers 3a and 3e as
well as polyimides 6a, 6e, and 6i were described in the previous
report.7
General Synthesis for Diamines and Polymers. The substituted
diamine monomers 3 containing siloxane linkages were synthe-
sized according to the previous report7 under the standard
conditions for Williamson’s ether synthesis, hydrosilylation,
and reduction. Polyimides were synthesized from PMDA or
BPDA with diamines 3 by the solution polymerization in NMP
followed by thermal imidization. Detailed synthesis and char-
acterization of individual diamines 3 and polyimides 6 are
described in the Supporting Information.
Figure 7. Tilt angle of the layer estimated from the split angle μ of the
layer reflection in WAXD fiber pattern is plotted against temperature.
Conclusion
We have synthesized a new series of laterally substituted LC
semialiphatic polyimides containing siloxane spacer units which
showed high thermal stability (Td5% > ∼450 ꢀC). Among the
methyl, chloro, and fluoro substituents, the fluoro substituent
was the most effective in stabilizing the liquid crystal phase: it
reduces the crystal melting temperature substantially, but not the
isotropization temperature of LC. The chloro substituent was
found to be effective as well, but the methyl substituent, irrespec-
tive of being the same size as the chloro one, substantially
destabilizes the LC phase. We have also been concerned with
the position of the substituent. The R1 position, apart from the
central part of the mesogen, was more effective than the R2
position. The liquid crystals formed are the SmA and SmC
phases, as found from the oriented X-ray patterns taken for the
fibrous samples. The structural features of these phases and the
SmA-SmC transition behavior were observed especially for
the fluoro substituent polyimide based on BPDA, 6g showing
the widest LC temperature region. Of interest is that an anom-
alous diffraction profile is observed on the outer broad reflection
both in the SmA and SmC phases, although inner layer reflec-
tions are observed ordinarily in the meridional direction. In the
SmC phases, some other broad reflections appear along the
meridional direction in addition to the ordinary equatorial one
and in the SmA phase; the broad reflection is significantly spread
along the azimuthal direction irrespective of the high orientation
of the layer. Further, these features are considered to be caused by
the segregation of the siloxane group from the other moieties in
the polymer chain, which will be examined in more detail in the
near future.
Acknowledgment. The financial support from Mitsubishi
Chemical Corporation is gratefully acknowledged.
Supporting Information Available: Text giving experimental
details about the synthesis and characterization of 1b-1d, 1f,
2b-2d, 2f, 3b-3d, 3f, 6b-6d, 6f-6h, and 6j and Figure S1
showing the X-ray patterns of oriented polyimides 6a, 6c, 6e,
and 6h. This material is available free of charge via the Internet
at http://pubs.acs.org.
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Experimental Section
Measurement. FT-IR spectra were measured on a Horiba FT-
720 spectrometer. 1H and 13C NMR spectra were recorded with
a Bruker DPX300S spectrometer. Inherent viscosities were
measured at 30 ꢀC in N-methylpyrrolidinone (NMP) at a
polymer concentration of 0.5 g/dL. The transition characteris-
tics were surveyed with a polarizing microscope (Olympus
BX51), together with the use of a LINKAM LTS-350 hot stage
equipped with a temperature controller by setting a polyimide
film between crossed polarizers. Thermal analysis was per-
formed on a Seiko EXSTAR 6000 TG/DTA 6300 thermal
analyzer at a heating rate of 10 ꢀC/min for thermogravimetry
(TG) and a Perkin-Elmer DSC7 calorimeter connected to a
cooling system at a heating rate of 10 ꢀC/min for differential
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