Synthesis of thermotropic liquid crystalline polyimides with siloxane linkages

Article abstract of DOI:10.1246/cl.2009.716

Thermotropic liquid crystalline (LC) semi-aliphatic polyimides have been developed from 3,3',4,4'-tetracarboxybiphenyl dianhydride (BPDA) and diamines containing siloxane spacer units. These polyimides exhibited high thermal stability and lower transition

Full text of DOI:10.1246/cl.2009.716

716
Chemistry Letters Vol.38, No.7 (2009)
Synthesis of Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages
Yu Shoji, Tomoya Higashihara, Junji Watanabe, and Mitsuru Uedaꢀ
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
(Received April 17, 2009; CL-090380; E-mail: mueda@polymer.titech.ac.jp)
Thermotropic liquid crystalline (LC) semi-aliphatic poly-
In virtual electronics applications, further decrease of fabri-
imides have been developed from 3,30,4,40-tetracarboxybiphenyl
dianhydride (BPDA) and diamines containing siloxane spacer
units. These polyimides exhibited high thermal stability and
lower transition temperature compared to the corresponding
polyimides containing alkyl or oxyethylene spacer units.
cation temperatures are required to avoid oxidation of copper
wires inside ICs. It should be mentioned that there has been no
example of LC fabrication temperature less than 243 ꢁC among
polyimide-based thermotropic liquid crystalline materials de-
rived from commercially available BPDA. As siloxane units
are well known to be highly flexible, the introduction of siloxane
linkages to a polyimide main-chain should decrease LC transi-
tion temperatures. In addition, siloxane linkages are also respon-
sible for thermal stability. In this communication, we describe
the first synthesis of new polyimides derived from BPDA and di-
amines containing siloxane spacer units and their thermotropic
LC behaviors.
Diamines 3 containing siloxane linkages were prepared
from p-nitrophenol in three steps as shown in Scheme 1. Reac-
tion of p-nitrophenol with 4-bromo-1-butene in the presence
of potassium carbonate in acetonitrile yielded 4-(3-butenyloxy)-
nitrobenzene, (1). Hydrosilylation of compound 1 with 1,1,3,3-
tetramethyldisiloxane or 1,1,3,3,5,5,7,7-octamethyltetrasiloxane
in the presence of Karstedt’s catalyst gave compounds 2, which
were hydrogenated to monomer diamines 3 (see Supporting
Information12).
The development of electronics is currently accelerated
toward integration, miniaturization, and high functionalization.
Under the current situation, both conductors and insulating
materials in ICs have been packaged as dense as possible, and
it is highly desirable to develop new polymeric materials which
can release generated heat effectively, while maintaining electri-
cally insulating functions. Organic materials are used in many
semiconductors as electric insulators; however they are also
thermal insulators. Therefore, the development of low dielectric
and thermoconductive polymers is a challenging topic at the
present time. Thermal conductive media depends on materials;
free electrons for metals whereas lattice vibrations, defined as
‘‘phonon,’’1,2 for ceramics and organic materials. To increase
phonon conductivity, highly oriented polymers, that is, LC poly-
mers are powerful candidates. Among them, there are establish-
ed studies on segmented LC polymers which possess rigid rod as
well as flexible spacer units.3,4
Aromatic polyimides are high-performance polymeric
materials that have been widely used in the aerospace, electron-
ics, and microelectronic industries because of their outstanding
thermal and chemical stabilities, mechanical properties, electri-
cal properties, and radiation resistance.5,6 Thermoconductive
polymers for insulators are required to have relatively low phase
transition temperature and low elastic modulus. Therefore, poly-
imides with flexible chains are good candidates. Very few LC
polyimides with sequences of methylene or ethylene oxide units
have been reported. Watanabe and co-workers reported pioneer-
ing researches on thermotropic LC polyimides from 4,400-terphen-
yltetracarboxylic dianhydride and aliphatic diamines with meth-
ylene spacer units (8–12).7–9 The LC temperatures ranged from
144 to 249 ꢁC depending on the number of methylene units. In
this case, the specially designed dianhydride was used. Recently,
LC polyimides 6 shown in Scheme 2 were prepared by using
a two-step polycondensation procedure. BPDA (4) reacted with
diamines 3 at room temperature in N-methyl pyrrolidone (NMP)
to produce poly(amic acid)s (PAAs) 5 with high inherent viscos-
ities in the range of 1.1–1.4 dL gꢂ1. Then, PI films were obtained
by thermal imidization of PAAs cast on glass substrates under
nitrogen, followed by immersion in warm water. Successful
thermal conversion was confirmed by the FT-IR spectra. The
characteristic absorptions which originated from the imide
moiety were observed at around 1770, 1712, and 1389 cmꢂ1
.
Self-standing and opaque yellow films with low elasticity were
obtained.
Thermal properties such as 5% wight loss temperature and
transition temperature were measured by TGA and DSC under
K2CO3
+
HO
NO2
O
NO2
Br
O
acetenitrile
1
Eastmond and co-workers used
a common dianhydride,
3,30,4,40-tetracarboxybiphenyl dianhydride (BPDA), and report-
ed the segmented polyimides based on BPDA and ꢀ,!-bis(4-
aminophenoxy)oxyethylene units. The resulting polyimides ex-
hibited liquid crystallinity (smectic A) at temperatures decreas-
ing from 352, 301, 256, to 243 ꢁC with increasing spacer units (3,
4, 5, and 6), respectively.10 These temperatures are relatively
lower than those of the polyimides based on BPDA and diamines
with methylene spacer units (>350 ꢁC).11 Therefore, more flex-
ible oxymethylene units seem to be favorable rather than meth-
ylene units to lower transition temperatures.
NO2
+
Si
Si
H
O
H
n
n = 1, n = 3
Karstedt cat.
toluene
Si
Si
O2N
O
O
O
O
NO2
NH2
n
n = 1 : 2a
n = 3 : 2b
2
H2 / EtOAc
Pd / C
Si
Si
H2N
O
O
n
n = 1 : 3a
n = 3 : 3b
3
Scheme 1. Synthesis route of monomers, diamines 3.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.7 (2009)
717
O
O
(a)
Tc2
Si
Si
Tc1
+
O
O
cooling
heating
H2N
O
O
O
NH2
n
4
3
O
O
n = 1 : 3a
n = 3 : 3b
NMP, r.t.
O
O
H
H
Si
Si
O
N
C
C
N
O
O
Tm2
Tm1
n
HOOC
COOH
Tm3
n = 1 : 5a
n = 3 : 5b
5
6
80
120
160
200
240
280
320
360
Temperature/°C
∆
(b)
Tc3
O
N
O
N
O
Si
Si
Tc2
Tc1
O
O
O
cooling
heating
n
O
n = 1 : 6a
n = 3 : 6b
Scheme 2. Synthesis route of LC polyimides 6.
Tm2
Tm1
Table 1. Thermal transition temperatures of polyimidesa
100
150
200
250
Temperature/°C
Tx/ꢁCb
6a
6b
ꢀHx/J gꢂ1 b
6a
6b
Figure 1. DSC traces for polyimides 6: (a) 6a and (b) 6b.
Tm1
Tm2
Tm3
Tc1
Tc2
Tc3
253
264
335
325
236
—
222
268
—
254
216
203
ꢀHm1
ꢀHm2
ꢀHm3
ꢀHc1
ꢀHc2
ꢀHc3
15.3
2.8
10.1
10.1
15.3
—
31.9
2.3
—
2.0
4.5
24.4
(a)
(b)
aAll determined by DSC at a second heating or cooling rate of
10 ꢁC minꢂ1. 5a: ꢁinh ¼ 1:07 [dL/g], 5b: ꢁinh ¼ 1:43 [dL/g].
Inherent viscosities were measured at 30 ꢁC in NMP at a PAA
Figure 2. Polarized optical micrographs of polyimides 6: (a)
polyimide 6a at 320 ꢁC and (b) polyimide 6b at 240 ꢁC (ꢃ400).
b
5 concentration of 0.5 g dLꢂ1. Transition temperature. Tm:
peak top temperature of endotherm peak, Tc: peak top tem-
perature of exotherm peak, ꢀHm: endotherm enthalpy,
ꢀHc: exothermo enthalpy.
transition temperature compared to polyimides containing alkyl
or oxyethylene spacer units. We expect that these polyimides
will exhibit relatively high thermal conductivity, and can be used
as insulators in electronics fields.
nitrogen, respectively (Table 1). Both polyimides 6 showed high
thermal stability with 5% weight loss temperature over 440 ꢁC.
The DSC traces of polyimides 6 are shown in Figure 1. Three en-
dotherm peaks on heating and two exotherm peaks on cooling
were observed in polyimide 6a, while two endotherm peaks on
heating and three exotherm peaks on cooling in polyimides 6b.
These transition phases in these polyimides correspond to crys-
tal, liquid crystal, and isotropic phases, and thermal transitions
are summarized in Table 1. All polyimides 6 have high thermal
stability and lower transition temperatures compared to poly-
imides containing alkylene or oxyethylene spacer units having
the same numbers of atoms. Moreover, the transition tempera-
ture decreased with increasing siloxane sequence length because
of increasing flexibility of the main chains.
Liquid crystallinity of polyimide 6 was supported by optical
microscopy. In the case of polyimide 6b, the fluidity and bire-
fringence were observed above the first melting point (Tm1),
and LC–isotropic transition was observed at the second endo-
thermic temperature (Tm2). The optical micrographs of each
polyimide annealed at 320 or 240 ꢁC for 10 min are shown in
Figure 2. The clear fan-shaped textures were assigned to smectic
LC. The detailed characterization of the crystal and LC phases
by X-ray diffraction and measurement of thermal conductivity
are in progress.
References and Notes
1
2
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12 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
In conclusion, we have developed new LC semialiphatic
polyimides containing siloxane spacer units. They showed lower
Published on the web (Advance View) June 13, 2009; doi:10.1246/cl.2009.716