Synthesis and properties of polyimide foams containing benzimidazole units

Article abstract of DOI:10.1039/c6ra08271h

In this research paper, a series of novel polyimide (PI) foams containing benzimidazole units were prepared derived from polyester ammonium salt (PEAS) precursor powders, which were synthesized by co-polymerization of benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA) with two diamines of 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) and 4,4′-diaminodiphenyl ether (ODA) with various molar ratios. The effects of incorporation of BIA on the morphology, thermal and mechanical properties of co-polyimide (co-PI) foams were explored. The results show that the BIA has a significant influence on foaming degree of PEAS precursor powders. The density of co-polyimide foams increases with increasing the BIA content in the polymer chains. Moreover, the thermal stability of the resultant co-polyimide foams presents a remarkable upward trend with incorporating more BIA units into the polymer chains. As the BIA loading up to 30 mol%, the glass transition temperature of co-polyimide foams increases around 50 °C in comparison with the pristine polyimide foam. Furthermore, the compressive strength of the co-polyimide foams is in the range of 0.30-0.75 MPa, which is superior to their of commercial polyimide foams with the same density. The co-polyimide foams with higher thermal and mechanical properties expand their potential application in many high-tech fields such as aerospace and aviation industries.

Full text of DOI:10.1039/c6ra08271h

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Synthesis and properties of polyimide foams
containing benzimidazole units
Cite this: RSC Adv., 2016, 6, 60094
*
Jianwei Li, Guangcheng Zhang, Yao Yao, Zhanxin Jing, Lisheng Zhou
and Zhonglei Ma
In this research paper, a series of novel polyimide (PI) foams containing benzimidazole units were prepared
derived from polyester ammonium salt (PEAS) precursor powders, which were synthesized by co-
polymerization of benzophenone-3,30,4,40-tetracarboxylic dianhydride (BTDA) with two diamines of 2-
(4-aminophenyl)-5-aminobenzimidazole (BIA) and 4,40-diaminodiphenyl ether (ODA) with various molar
ratios. The effects of incorporation of BIA on the morphology, thermal and mechanical properties of co-
polyimide (co-PI) foams were explored. The results show that the BIA has a significant influence on
foaming degree of PEAS precursor powders. The density of co-polyimide foams increases with
increasing the BIA content in the polymer chains. Moreover, the thermal stability of the resultant co-
polyimide foams presents a remarkable upward trend with incorporating more BIA units into the polymer
chains. As the BIA loading up to 30 mol%, the glass transition temperature of co-polyimide foams
increases around 50 ꢀC in comparison with the pristine polyimide foam. Furthermore, the compressive
strength of the co-polyimide foams is in the range of 0.30–0.75 MPa, which is superior to their of
commercial polyimide foams with the same density. The co-polyimide foams with higher thermal and
mechanical properties expand their potential application in many high-tech fields such as aerospace and
aviation industries.
Received 31st March 2016
Accepted 9th June 2016
DOI: 10.1039/c6ra08271h
www.rsc.org/advances
polymer matrix can enhance the mechanical properties of pol-
yimide foams. Xu et al.5 prepared aromatic polyimide/graphene
1. Introduction
foams through thermal foaming of poly(amic ester)/graphene
oxide (GO) composite precursor powders. The research indi-
cates that GOs act as heterogeneous nucleation sites in the
foaming process, and the homogeneous dispersion of GOs
within polymer matrix prefers to obtain foams with uniform cell
size distributions. And the incorporation of GOs in the polymer
matrix dramatically enhanced the exural strength and
compressive properties of the foam materials. Also, a novel
polyimide/SiO2 hybrid foams were successfully prepared by Qi
et al.6 The results revealed that the SiO2 particles were well-
dispersed in the polyimide matrix. The SiO2 particles
restricted the molecular mobility and enhanced their interfacial
adhesion. The thermal stability, storage modulus, glass transi-
tion temperature, and dielectric constant of the hybrid foams
were improved with the incorporation of SiO2 particles. An
alternative strategy is structural modication on the basis of the
properties of polyimide materials mainly attributed to the
structures of polymer chains. Wang et al.7 prepared the rigid
polyimide foams using cis-5-norbornene-2,3-dicarboxylic anhy-
dride (NA) as end-capping agent. The cross-linked network was
conformed in the thermal foaming process. The cross-linking
points, molecular weight and density of polyimide foams can
be controlled by adding different amount of NA, the mechanical
could be improved from 0.1 MPa to 12.0 MPa by increasing the
Polyimide (PI) foams are a class of high-performance polymer
materials and have been widely used in aerospace and aviation
elds due to their excellent properties such as low weight,
chemical stability, thermal insulating and acoustic absorbing.1,2
However, the conventional polyimide foams are frequently
challenged by weak mechanical properties, which limits its
application in some load-bearing elds signicantly. The
mechanical properties of polyimide foams can be enhanced by
increasing their density, however, lightweight is quite a valuable
parameter for applications in weight-sensitive aerospace,
transportation, or automobile industries. Therefore, it is of
great signicance to prepare lightweight polyimide foams with
higher mechanical properties to satisfy structural material
applications.
To overcome these problems, a lot of investigation has been
concentrated on the design and preparation of novel polyimide
foams with higher mechanical properties.3–8 There are mainly
two strategies improving the mechanical properties of poly-
imide foams by PEAS precursor powders foaming method. One
strategy involving the introduction of inorganic llers into the
MOE Key Laboratory of Applied Physics and Chemistry in Space, Department of
Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an
710072, China. E-mail: zhangguc@nwpu.edu.cn
60094 | RSC Adv., 2016, 6, 60094–60100
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foam density from 50 kg mꢁ3 to 420 kg mꢁ3. A series of rigid purity). 2-(4-Aminophenyl)-5-aminobenzimidazole (BIA) was
polyimide foams were prepared using poly(amic acid) (PAA) as provided by Changzhou Sunlight Pharmaceutical Co., Ltd,
foaming precursor by Zhan et al.8 The cross-linking points and China (98% purity). Ethanol and tetrahydrofuran (THF) were
density of foams can be adjusted by controlling the content of purchased from Guangzhou Chemical Reagent Co., Ltd, China.
isocyanate and water. The experiment indicates that the glass BTDA, ODA and BIA were dried at 120 ꢀC in the vacuum oven for
transition temperature of obtained polyimide foams increases 12 hours prior to use. The THF was puried by distillation
markedly. Furthermore, the compressive strength also achieve under reduced pressure over sodium prior to use. Other
a big growth up to 1.31 MPa at the density of 117.6 kg mꢁ3
.
commercially available reagent grade chemicals were used
Recently, there are several methods reported in the literature without further purication.
for fabricating novel polyimide materials to improve its
mechanical properties based on molecular design, such as
combining the advantages of polybenzimidazoles (PBI) and
polyimide to synthesize PBI–PI co-polymers to obtain the
desired properties.9–11 For example, the heterocyclic diamine 2-
(4-aminophenyl)-5-aminobenzimidazole (BIA) is an rigid asym-
metric monomer, which has been successfully introduced into
polyimide backbone in order to strengthen the mechanical and
thermal properties of resulting materials in recent years.12–14
Zhang's group have successfully synthesized a variety of poly-
imide materials such as bers and lms containing benzimid-
azole units,15–17 and the results revealed that the mechanical
and thermal properties of resultant materials improved signif-
icantly owing to the incorporation of benzimidazole units into
the polyimide backbone. In order to adapt the specic envi-
ronment, the mechanical and thermal properties of light-weight
polyimide foams are still in need of further improvement. As we
know, the incorporation of benzimidazole moieties into the
polymer chains is one way to enhance the mechanical and
thermal properties of polyimide materials. It may be also an
effective approach to strengthen the mechanical and thermal
properties of polyimide foams. However, the polyimide foams
containing benzimidazole units have been rarely involved.
Therefore, it is worthy to investigate the inuence of benz-
imidazole units on the foaming process and properties of the
resultant co-polyimide foams, which may serve as a general rule
in designing and preparing novel polyimide foams.
The objective of this work is to fabricate light-weight
aromatic co-polyimide foams with higher mechanical and
thermal properties. The co-polyimide foams containing benz-
imidazole units were prepared via thermal foaming method
derived from PEAS precursor powders, which was synthesized
by co-polymerization of benzophenone-3,30,4,40-tetracarboxylic
dianhydride (BTDA) with two diamines of 2-(4-aminophenyl)-5-
aminobenzimidazole (BIA) and 4,40-diaminodiphenyl ether
(ODA) with various molar ratios. In comparison, pristine poly-
imide (BTDA/ODA) foam also has been prepared. The effects of
the incorporation of BIA moieties on foaming process of PEAS
precursor powders and the physical properties of the resultant
co-polyimide foams are systematically investigated.
2.2 Preparation of polyimide foams
PI-1 was prepared by the following procedures.18,19 In a 250 mL
round-bottom ask, BTDA (32.22 g, 0.1 mol) was rst dispersed
in a mixture solvent of ethanol (100 mL) and THF (20 mL) at
room temperature under the protection of N2 atmosphere. This
solution was treated at 70 ꢀC for 2 hours with magnetic stirring.
A trace (3 drops) of 2-methylimidazole was added to accelerate
the esterication reaction of BTDA. Aer the color of the solu-
tion changed from white to clear, the mixed monomers of ODA
(0.09 mol, 18.02 g) and BIA (0.01 mol, 2.25 g) was added. The
resulting mixture was stirred for 2 hours to yield a homoge-
neous precursor solution. The PEAS solution had a viscosity of
12.0 Pa s at room temperature and the solid content of the PEAS
precursor solution was ca. 35 wt%. Secondly, the PEAS
precursor solution was heated to remove the extra solvent by
rotating distillation. The resultant resin was crushed into ne
powders and separated into particles with diameters in the
range of 75–100 mm by a group of sieves. Eventually, the PEAS
precursor powders were treated for 3–5 hours at 80 ꢀC until the
weight remained unchanging and the residual solvent was
maintained at 14 wt%.
The thermal foaming process was performed in a graphite
mold. The mold was preheated at 80 C for 1 h in oven. Aer
ꢀ
being loaded with the precursor powders, the graphite mold
was rapidly transferred into the heated oven. The foaming and
imidization processes of the PEAS precursor powders were
performed by the ꢀfollowing schedule: 130 ꢀC for 60 min, 200 ꢀC
for 120 min, 280 C for 120 min. Aer this, the graphite mold
was cooled down to room temperature.20 The resultant co-
polyimide foam was denoted as PI-1 and cut into the designed
size dimensions for testing.
The same procedures used for PI-1 were repeated via
adjusting the molar ratio of ODA/BIA (10 : 0, 8 : 2, 7 : 3 and
6 : 4) to fabricate other polyimide foams (Table 1). The resultant
Table 1 The properties of resultant PI and co-PI foams
Cell diameter
(mm)
Density
(kg mꢁ3
)
Molar ratio
(ODA/BIA)
Viscositya
(Pa s)
PI
2. Experimental
2.1 Materials
The benzophenone-3,30,4,40-tetracarboxylic dianhydride (BTDA)
was purchased from Shanghai Aladdin Industrial Co., Ltd,
China (99% purity). 4,40-Diaminodiphenyl ether (ODA) was ob-
tained from Sinopharm Chemical Reagent Co., Ltd, China (99%
PI-0
PI-1
PI-2
PI-3
615 ꢂ 87
440 ꢂ 75
339 ꢂ 68
220 ꢂ 84
27
35
42
54
10 : 0
9 : 1
8 : 2
7 : 3
12.0
13.4
13.7
15.3
a
The viscosity of the PEAS precursor solution, measured using digital
rotational viscometer at room temperature.
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polyimide foams were denoted as PI-0, PI-2, PI-3, PI-4 respec- Compressive properties of the foams were measured using
tively. The overall process for the formation of co-polyimide a CMT 8502 multifunctional experimental machine at room
foams is schematically described in Scheme 1.
temperature according to Chinese national standard GB/T8813.
The compressive measurement were performed using speci-
mens with a dimension of 50 mm ꢃ 50 mm ꢃ 25 mm (length,
2.3 Characterization
width and thickness) at a compressive rate of 3.0 mm minꢁ1
,
The viscosity of the PEAS precursor solution was tested at room
temperature using a digital rotational viscometer (CAP2000+L,
Brookeld, USA) at a speed of 30 rpm. The chemical structures
of polyimide foams were characterized on a Nicolet iS10 Fourier
transform infrared spectrometer (FT-IR). The FT-IR measure-
ments were performed in the 400–4000 cmꢁ1 range by averaging
32 scans. Scanning electron microscope (SEM) images were
and the specimens were compressed to a total deection of 80%
of the full-part thickness.
3. Results and discussion
3.1 FT-IR analysis
obtained using a VEGA 3 electron microscope (TESCAN, Czech The chemical structures of the resultant polyimide and co-
Republic). The calculation of cell diameter on the SEM photo- polyimide foams with various ODA/BIA molar ratios were
graphs was conducted using soware Image-Pro Plus. DSC analyzed by FT-IR and the obtained results are shown in Fig. 1.
(Mettler Toledo DSC1) was used to study the glass transition The peaks at 1778 cmꢁ1 and 1720 cmꢁ1 are correspond to the
temperature of the polyimide foams, the samplesꢁw1ere heated C]O asymmetrical and symmetrical stretching of aromatic
from 25 C to 370 C at a ramp rate of 10 C min under the imide ring respectively. The peaks at 1380 cmꢁ1 and 738 cmꢁ1
nitrogen atmosphere. Thermal gravimetric analysis (TGA) was represent the C–N stretching vibration and bending of imide
performed on TGAQ 600 (TA Company) at a ow rate of 60 cm3 ring.21 The asymmetric and symmetric stretching of N–H bands
minꢁ1 under the nitrogen atmosphere. Sample tests were pꢁer1- in the benzimidazole could be observed in the regions of 3300–
ꢀ
ꢀ
ꢀ
3400 cmꢁ1 in three co-polyimide foams. The characteristic
ꢀ
ꢀ
ꢀ
formed from 25 C to 800 C at a heating rate of 10 C min
.
Scheme 1 Synthesis of the co-PI (BTDA/ODA/BIA).
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of BIA into the polymer backbone. And the average cell diameter
and foaming degree decreased with increasing the BIA content.
However, the PEAS precursor powders could not be foamed partly
or totally as the content of BIA exceeding 50 mol% at the same
foaming temperature. These results indicate that the BIA content
is a key factor controlling the density and foaming degree of PEAS
precursor powders, and the high content of BIA is suitable for
fabricating polyimide foams with high density.
3.3 The foaming process of PEAS precursor powders
DSC technology was employed to trace the thermal foaming
behavior of PEAS precursor powders under the heating condi-
tions. Fig. 3 presented the DSC curves of PEAS precursor
powders with different contents of BIA and the corresponding
status of precursor powders at various temperatures. The
foaming process can be divided into three steps as follows.23
The rst step was used to evaporate extra THF and ethanol
(serving as foaming agents) within the solid PEAS precursor
powders, which is critical in determining the foaming degree of
PEAS precursor powders. Because excessive or insufficient
residual solvent within PEAS precursor powders are not suitable
for fabricating polyimide foams with uniform cell size distri-
butions. The second step is foaming process, the PEAS
precursor powders melted and inated to microspheres under
the given temperature during this process. The cell structures
could be formed with a further growth of microspheres owing to
the collision.24 Following this, the imidization process endow
polyimide foams with high exible and mechanical properties.
The polyimide foams with uniform morphology can be ob-
tained by controlling these parameters that governs the
powders ination kinematics. In principle, the foaming process
of PEAS precursor powders were mainly determined by the
following four factors: chemical structure of PEAS, size of
precursor particles, foaming temperature and content of
residual solvent. In this study, the foaming temperature and the
content of residual solvent were same, therefore, the
Fig. 1 The FT-IR spectra of the PI and co-PI foams.
peaks suggest the formation of imide ring and benzimidazole
moieties have been successfully incorporated into the back-
bones of co-polyimide foams.22
3.2 The morphology of PI and co-PI foams
The scanning electron microscopy (SEM) was used to investigate
the cell morphology of the resultant foams. The results are shown
in Fig. 2. The average cell diameter has been statistically calcu-
lated from SEM images as displayed in Table 1. Drastic changes
in the morphology can be observed from SEM aer introduction
Fig. 2 SEM photographs of the PI and co-PI foams.
Fig. 3 DSC spectra of the PEAS precursor powders.
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