Published on the web September 14, 2013
1545
Highly Thermally Resistant and Flexible Polyimide Aerogels Containing Rigid-rod Biphenyl,
Benzimidazole, and Triphenylpyridine Moieties: Synthesis and Characterization
Dengxiong Shen, Jingang Liu,* Haixia Yang, and Shiyong Yang*
Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, P. R. China
(Received August 19, 2013; CL-130758; E-mail: liujg@iccas.ac.cn)
O
O
O
O
O
O
Polyimide (PI) aerogels containing rigid-rod biphenyl,
benzimidazole, and triphenylpyridine moieties have been
developed. The flexible PI aerogels exhibited excellent thermal
stability with glass-transition temperatures (Tg) higher than
350 °C. Initial porous structures in the PI aerogels could be
maintained even after isothermal aging at 300 °C for 24 h in
nitrogen. Furthermore, the weight losses of the aerogels after
isothermal aging at 300 and 400 °C for 24 h in nitrogen were
below 5 and 10 wt %, respectively.
N
H2N
N
H2N
n
m
+
or
NH2
N
H
N
H
NH2
BPDA
O
4-APBI
3-APBI
NMP
room temperature
nirogen, 24h
O
O
H
O
H
O
O
O
O
O
O
H
N
N
N
N
N
N
H
HO
OH
O
N
HO
N
H
OH
H
O
O
O
n
PAA
TAPP:
(1) TAPP
NH2
(2) Ac2O/pyridine
(3) scCO2
Organic aerogels have attracted increasing attention in
recent years due to their intrinsic low density, low thermal
conductivity, high thermal insulation, and low dielectric
constants compared to their inorganic counterparts.1 Various
organic aerogels such as polyurethane, polyurea, polystyrene,
aramide, and polydicyclopentadiene have been widely inves-
tigated in literature and have found applications in high-tech
fields.2-6 However, common organic aerogels usually suffer
from low thermal and dimensional stability at elevated temper-
atures; therefore, they cannot meet the severe demands of high-
temperature applications such as thermal protective coatings or
interlayer thermal insulation systems for microelectronic device
fabrications.7 In order to prevent thermal deformation of the
aerogels in the above applications, high-temperature resistant
organic aerogels are highly desired.
Polyimides (PIs) represent an important class of high-
temperature resistant polymers and have been widely used in the
electronic, microelectronic, and optoelectronic industry owing to
their excellent combined thermal, mechanical, and dielectric
properties.8 Thus, it can be anticipated that PI aerogels might
exhibit good comprehensive properties. Some pioneering work
on PI aerogels has been reported very recently.9-13 Several PI
aerogels based on aromatic dianhydrides, 3,3¤,4,4¤-biphenyl-
tetracarboxylic dianhydride (BPDA), 3,3¤,4,4¤-benzophenon-
tetracarboxylic dianhydride (BTDA), aromatic diamines, para-
phenylenediamine (PPD), 4,4¤-oxydianline (ODA), bisaniline-
p-xylidene (BAX), 2,2¤-dimethylbenzidine (DMBZ), and
multifunctional end-cappers, 1,3,5-tris(4-aminophenyl)benzene
(TAPB), 1,3,5-tris(4-aminophenoxy)benzene (TAB), octa(ami-
nophenyl)silsesquioxane (OAPS), have been developed. Exper-
imental results obtained in the above-mentioned reports suggest
that PI aerogels derived from flexible dianhydride BTDA,
flexible diamine ODA, and TAB crosslinker usually exhibited
glass-transition temperatures (Tg) below 280 °C, while their
volume shrinkages were as low as 20%.9 Conversely, those
derived from rigid dianhydride BPDA, rigid diamine PPD or
DMBZ, and TAB showed high Tg values up to 346 °C; however,
their volume shrinkages were as high as 48%.9 Thus, it is a
challenge to achieve a good balance between the thermal and
N
H2N
NH2
PIA-1 (BPDA/4-APBI/TAPP)
PIA-2 (BPDA/3-APBI/TAPP)
Scheme 1. Synthesis of PIA aerogels.
dimensional stability of the PI aerogels at elevated temper-
atures.
In this communication, we report a series of novel flexible
PI aerogels with excellent thermal stability at elevated temper-
atures. The molecular design, synthesis, and effects of their
molecular structures on the properties were investigated.
The aerogels were synthesized based on a well-established
route for PI aerogel preparation, as shown in Scheme 1. For this
purpose, a triphenylpyridine-containing triamine compound,
2,4,6-tris(4-aminophenyl)pyridine (TAPP) was synthesized first,
according to the pathway shown in Scheme S1. According to the
pathway, TAPP was obtained in good yields via the catalytic
reduction of the nitro compound, 4-(4-nitrophenyl)-2,6-bis(4-
aminophenyl)pyridine (TNPP) with hydrazine monohydrate and
palladium catalysts in refluxing ethanol. TNPP was prepared via
a modified Chichibabin reaction with 4-nitroacetophene and
4-nitrobenzaldehyde in the presence of ammonium acetate in
glacial acid.14 The structure of TAPP was confirmed using
Fourier transform infrared (FTIR) spectroscopy, nuclear mag-
netic resonance (1H NMR) spectroscopy, and elemental analysis.
The obtained pale-brown crystalline TAPP showed high purity
and was used as the end-capper for aerogel synthesis. Then, two
PI aerogels, PIA-1 (BPDA/4-APBI/TAPP) and PIA-2 (BPDA/
3-APBI/TAPP) were prepared by the polycondensation of rigid
BPDA dianhydride, rigid benzimidazole-containing diamines,
and TAPP, respectively, followed by drying in supercritical
carbon dioxide (scCO2). The detailed synthesis procedure is
shown in the Supporting Information.15 Both APBI and TAPP
monomers contain rigid components (benzimidazole or triphen-
ylpyridine segments), which are very beneficial for improving
the thermal resistance of the derived PI aerogels.
Chem. Lett. 2013, 42, 1545-1547
© 2013 The Chemical Society of Japan