Effect of multiple H-bonding on the properties of polyimides containing the rigid rod groups

Article abstract of DOI:10.1002/pola.27808

To investigate the influence of hydrogen bonding on the properties of polyimides (PIs) containing rigid rod-like groups, five symmetrical diamines containing benzimidazole, benzoxazole, and hydroxy group were synthesized, and then a series of PIs were prepared. Results showed that hydroxyl-containing poly(benzoxazole imide)s possess higher glass transition temperature (T_g) and dimensional stabilities than their corresponding poly(benzoxazole imide)s. Moreover, the corresponding poly(benzimidazole imide)s presented the best performances, such as the highest T_g, the highest char yield and the highest dimensional stabilities. The influence of hydrogen bonding of benzimidazole on the properties of PIs was stronger than that of hydroxyl groups. Hydroxyl-containing poly(benzoxazole imide)s were formed in crosslinking structures after heat treatment at 400 °C.

Full text of DOI:10.1002/pola.27808

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Effect of Multiple H-Bonding on the Properties of Polyimides Containing
the Rigid Rod Groups
Xiaoye Ma, Chuanqing Kang, Wenhui Chen, Rizhe Jin, Haiquan Guo, Xuepeng Qiu,
Lianxun Gao
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun, 130022, People’s Republic of China
Correspondence to: X. Ma (E-mail: xyma@ciac.jl.cn) or L. Gao (E-mail: lxgao@ciac.jl.cn)
Received 28 May 2015; accepted 24 July 2015; published online 28 August 2015
DOI: 10.1002/pola.27808
ABSTRACT: To investigate the influence of hydrogen bonding
on the properties of polyimides (PIs) containing rigid rod-like
groups, five symmetrical diamines containing benzimidazole,
benzoxazole, and hydroxy group were synthesized, and then a
series of PIs were prepared. Results showed that hydroxyl-
containing poly(benzoxazole imide)s possess higher glass tran-
the highest dimensional stabilities. The influence of hydrogen
bonding of benzimidazole on the properties of PIs was stron-
ger than that of hydroxyl groups. Hydroxyl-containing poly(-
benzoxazole imide)s were formed in crosslinking structures
ꢀ
after heat treatment at 400 C.
V
C
2015 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2016, 54, 570–581
g
sition temperature (T ) and dimensional stabilities than their
corresponding poly(benzoxazole imide)s. Moreover, the corre-
sponding poly(benzimidazole imide)s presented the best per-
KEYWORDS: benzimidazole; benzoxazole; hydrogen bonding;
intra-/intermolecular interaction; polyimides; rigid rod groups
g
formances, such as the highest T , the highest char yield and
INTRODUCTION Aromatic polyimides (PIs), as commercial-
ized aromatic heterocyclic polymers, are widely used in the
semiconductor and electronic packaging industries because
of their outstanding thermal stability, excellent insulation
properties, low dielectric constant, good adhesion to com-
PIs, especially in thin-film formation, through the processing
of soluble poly(amic acid) intermediates. Meanwhile, it can
be predicted that incorporating rigid-rod benzoxazoles and
benzimidazoles into PI main chains can effectively improve
the thermal and mechanical properties of PIs. In recent
years, many research have been carried out to develop new
promising systems of high-performance materials by combin-
1
–4
mon substrates, and superior chemical stability.
The out-
standing properties of PIs are mainly dependent on the
chemical structure, molecular aggregation, and intra-/inter-
molecular interaction. In our previous work, the rigid rod-
like pyridine-containing diamines, 2,5-bis(4-aminophenyl)
pyridine and 2-(4-aminophenyl)25-aminopyridine, were
introduced into the backbone of PI, and the thermal and
8
–24
ing the advantages of PIs, PBOs, and PBIs.
For instance,
0
the asymmetrical diamines 5,4 -diamino-2-phenyl benzimida-
2
2
0
23
zole and 5,4 -diamino-2-phenyl benzoxazole were incor-
porated into PI main chains, and the thermal and mechanical
properties of the resultant PI films were remarkably
enhanced in comparison with those of common PI films.
5
mechanical properties were increased significantly. The PIs
can be enhanced in the original properties by introducing
rigid rod-like nitrogen heterocyclic structure.
Hydrogen bonding (H-bonding) is a kind of intermolecular
interaction, and performs a crucial function to improve the
properties of rigid polymers. There is no H-bonding interac-
tion between molecular chains in the conventional aromatic
PIs. In recent years, the H-bonding interaction of benzimida-
zole in rigid PIs has been researched. The ANHA group of
benzimidazole can form H-bonding with the C@O of the
imide ring in the PIs containing benzimidazole, and improve
Aromatic polybenzoxazoles (PBOs) and polybenzimidazoles
(
PBIs) exhibit excellent mechanical properties and thermal
stability due to the rigidity of polymer backbones, aromatic
heterocyclic chemical structures, and strong intermolecular
6
,7
associations. However, PBOs and PBIs are generally diffi-
cult to process because of their unmeltable behavior and
poor solubility in conventional organic solvents, thus limiting
their potential applications. This problem is circumvented in
2
4,25
the properties of PIs.
Previous reports indicated that the
incorporation of hydroxyl groups can also form H-bonding
Additional Supporting Information may be found in the online version of this article.
2015 Wiley Periodicals, Inc.
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ꢀ
and enhance the properties of high-performance fibers,
a thick paste. The mixture was slowly heated to 110 C to
2
6,27
among others, like M5 fibers,
but the H-bonding of
inhibit excessive foaming and then continuously stirred for
2 h at 110 C. The temperature was slowly increased to
00 C and then maintained for 6 h. The reaction mixture
was then slowly cooled to 100 C and poured into ice-cold
water with rapid stirring. The precipitate was collected by
filtration and soaked overnight in a sodium bicarbonate solu-
tion. The precipitate was collected, thoroughly washed with
water, and dried under vacuum at 80 C. The crude product,
-DA, was further purified by sublimation. TOF-MS: m/z 5
ꢀ
hydroxyl group in rigid PIs has not been reported.
ꢀ
2
In this study, five fully symmetrical diamines containing
benzimidazole, benzoxazole, and hydroxyl group were syn-
thesized, and then a series of PIs were prepared by the reac-
tion of these diamines with commercial aromatic
dianhydrides. The influences on the properties of PIs by the
introduction of rigid rod-like nitrogen heterocyclic structure
and the formation of intra-/intermolecular H-bonding were
discussed.
ꢀ
ꢀ
1
3
1
42.1. H NMR (400 MHz, DMSO-d6) d (ppm): 8.06(s, 1H,
Ar-H), 7.86–7.88 (m, 5H, Ar-H), 6.72, 6.70 (d, 4H, Ar-H), 5.99
s, 4H, NH2). C NMR (100 MHz, DMSO-d6) d (ppm): 164.3,
1
3
(
EXPERIMENTAL
152.8, 147.6, 139.9, 129.2, 113.9, 113.1, 107.4, 93.6.
Materials
Diamine (1-OHDA) was synthesized in a procedure similar to
that of 1-DA, but using 4,6-diaminoresorcinol dihydrochlor-
ide and 4-aminosalicylic acid as starting materials. TOF-MS:
4,6-Diaminoresorcinol dihydrochloride (Zhejiang Jiangnan
Pharmaceutical Factory), 4-aminobenzoic acid (Adamas Rea-
gent Co., Ltd.), 4-aminosalicylic acid (Adamas Reagent Co.,
1
m/z 5 374.1. H NMR (400 MHz, DMSO-d6) d(ppm): 11.07
0
Ltd.), 3,3 -diaminobenzidine (Shanghai Zhuorui Chemical Co.,
(s, 2H, AOH), 8.15(s, 1H, Ar-H), 7.91 (s, 1H, Ar-H), 7.66, 7.64
0
Ltd.), and 3,3 -dimethoxybenzidine dihydrochloride (Shang-
(
d, 2H, Ar-H), 6.29–6.31 (m, 2H, Ar-H), 6.201, 6.197 (d, 2H,
hai Zhuorui Chemical Co., Ltd.) were purchased and directly
13
Ar-H), 6.09 (s, 4H, NH2). C NMR (100 MHz, DMSO-d6) d
0
0
used. 3,3 ,4,4 -Biphenyl tetracarboxylic dianhydride (BPDA),
(
9
ppm): 164.5, 159.9, 154.9, 146.0, 138.0, 128.6, 107.6, 105.9,
9.6, 98.2, 94.1.
0
0
0
4
,4 -oxydiphthalic dianhydride (ODPA), and 3,3 ,4,4 -benzo-
phenone tetracarboxylic dianhydride (BTDA) were obtained
by sublimation under vacuum. 1,4-Bis(3,4-dicarboxyphenox-
y)benzene dianhydride (HQDPA, prepared in our laboratory)
was purified by dehydration of acetic anhydride. N,N-dime-
thylacetamide (DMAc) and N-methyl-2-pyrrolidinone (NMP)
were distilled under reduced pressure before use. Other
common reagents were purchased from commercial sources
and used as received.
0
0
In a 500 mL three-necked flask, 3,3 -dihydroxy-4,4 -diamino-
biphenyl (5.0 g, 0.023 mol) and 4-aminobenzoic acid
(6.342 g, 0.046 mol) were dissolved in 80 g of PPA under a
nitrogen atmosphere to produce a thick paste. The mixture
ꢀ
was slowly heated to 200 C and then maintained for 6 h.
Then, the reaction mixture was slowly cooled to 100 C and
poured into ice-cold water with rapid stirring. The precipi-
tate was collected by filtration and soaked overnight in a
sodium bicarbonate solution. The precipitate was collected,
thoroughly washed with water, and dried under vacuum at
C. The crude product, 3-DA, was further purified by
recrystallization from DMAc/H O. TOF-MS: m/z 5 418.1.
NMR (400 MHz, DMSO-d6) d (ppm): 8.02 (s, 2H, Ar-H), 7.88,
.86 (d, 4H, Ar-H), 7.70 (s, 4H, Ar-H), 6.71, 6.68 (d, 4H, Ar-
H), 5.99 (s, 4H, NH2). C NMR (100 MHz, DMSO-d6) d
ppm): 164.2, 152.6, 150.7, 141.6, 136.4, 129.0, 123.7, 118.8,
13.6, 112.6, 108.6.
ꢀ
Characterization
1
13
H and C nuclear magnetic resonance (NMR) spectra were
recorded on a Bruker 400 spectrometer at 400 MHz and
with tetramethylsilane as an internal standard. Inherent vis-
cosities were measured with 0.5 g/dL concentration of pol-
y(amic acid)s (PAAs) in DMAc using an Ubbelohde
ꢀ
80
1
H
2
7
ꢀ
viscometer at 30 C. Thermogravimetric analyses (TGAs)
13
ꢀ
were performed at a heating rate of 10 C/min under nitro-
(
1
gen with a Perkin Elmer Pyris Diamond TG/DTA. Dynamic
mechanical analysis (DMA) was carried out with a TA instru-
ꢀ
ment DMA Q800 at a heating rate of 5 C/min and a load
Diamine (2-DA) was synthesized in a procedure similar to
that of 3-DA, but with 3,3 -diaminobenzidine and 4-
0
frequency of 1 Hz in film tension geometry in nitrogen
atmosphere, and glass transition temperature (T
g
) was
aminobenzoic acid as starting materials. TOF-MS: m/z 5
416.2. H NMR (400 MHz, DMSO-d6) d (ppm):12.46 (s, 2H,
imidazole, ANH), 7.87, 7.85(d, 4H, Ar-H), 7.70(s, 2H, Ar-H),
1
regarded as the peak temperature of the tan d curve. Small-
angle and wide-angle X-ray diffraction (XRD) patterns were
obtained using a Bruker D8 ADVANCE XRD with Cu-K radia-
tion at a wavelength (k) 1.54 Å. The dimensional stability of
the films was investigated using a TMA Q400 thermal
mechanical analyzer, film samples sized 2 mm 3 4 cm, and
with an initial tension of 0.05 N.
7.54 (s, 2H, Ar-H), 7.44, 7.42 (d, 2H, Ar-H), 6.68, 6.66 (d, 4H,
1
3
Ar-H), 5.60 (s, 4H, NH2). C NMR (100 MHz, DMSO-d6) d
(ppm): 153.4, 150.9, 135.5, 128.1, 121.3, 117.6, 113.9.
Diamine (3-OHDA) was synthesized in a procedure similar to
0
0
that of 3-DA, but with 3,3 -dihydroxy-4,4 -diaminobiphenyl
Synthesis of Diamines
In a 1000 mL three-necked flask, 4,6-diaminoresorcinol dihy-
drochloride (12.78 g, 0.06 mol) and 4-aminobenzoic acid
and 4-aminosalicylic acid as starting materials. TOF-MS: m/z 5
450.1. H NMR (400 MHz, DMSO-d6) d (ppm): 11.13 (s, 2H,
AOH), 8.10 (s, 2H, Ar-H), 7.75 (s, 4H, Ar-H), 7.68, 7.65 (d, 2H,
1
(
16.46 g, 0.12 mol) were dissolved in 120 g of poly(phos-
Ar-H), 6.31, 6.29 (d, 2H, Ar-H), 6.19 (d, 2H, Ar-H), 6.11 (s, 4H,
NH2). C NMR (100 MHz, DMSO-d6) d (ppm): 164.1, 159.9,
1
3
phoric acid) (PPA) under a nitrogen atmosphere to produce
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(
1-DA), 11 (1-OHDA), 11 (3-DA), 7 (2-DA), and 13 (3-OHDA)
peaks, all of which corresponding to the carbon atoms of the
1
13
target product. The H and C NMR spectra indicate a con-
sistent structure of the products with the target.
The polymerization of diamine and dianhydride involves the
nucleophilic substitution reaction of amino group. The pro-
1
tons peak of amino group appear in low field in the H-NMR
spectra, the electron cloud density is lower, and the reactiv-
2
9
SCHEME 1 Synthesis of diamines.
ity of diamine is also lower. The results showed that the
polymerization activities of diamines possibly decreased in
the order of the diamines containing benzimidazole
1
54.7, 149.1, 139.5, 136.5, 128.4, 124.2, 118.0, 108.8, 107.3,
9.3, 97.8.
(2-DA) > the diamines containing benzoxazole (1-DA and
9
3-DA) > the diamines containing hydroxyl group and benzox-
Synthesis of PIs
azole (1-OHDA and 3-OHDA).
PIs were prepared using a conventional two-stage solution
polymerization and imidization process. The polymerization
of 3-BPPI is described here as a typical example: BPDA
Solution polymerization of diamines and various dianhy-
drides yielded viscous solution of PAAs. Thermal imidization
(
0.4413 g, 1.5 mmol), 3-DA (0.6272 g, 1.5 mmol), and 15 mL
DMAc were placed in a 100 mL three-necked flask equipped
with a mechanical stirrer and a nitrogen inlet. The mixture
was stirred at room temperature for 8 h to form a viscous
solution of PAA with a solid content of 7 wt %.
A film was cast from the PAA solution onto a flat glass plate
ꢀ
and dried at 50 C for 12 h. Then, a programmed procedure
ꢀ
in a ventilated oven at 80 and 140 C for 1 h, respectively,
was used to remove residual solvents. The plate was placed
into a vacuum oven and treated at 200, 250, 300, and
ꢀ
3
50 C for 1 h, respectively, to obtain a fully imidized film.
The polymer films were easily stripped off from the glass
plate by soaking in water.
RESULTS AND DISCUSSION
Synthesis
Five aromatic diamines containing benzimidazole, benzoxa-
zole, and hydroxyl groups were synthesized according to a
2
8
published method by the reaction of 4-aminobenzoic acid or
-aminosalicylic acid and monomer B in PPA with P O , as
4
2
5
shown in Scheme 1. PPA is widely used for catalyzing intra-
molecular cyclization reactions because of its high dehydra-
1
13
tion and low oxidizability. TOF-MS and H and C NMR
spectroscopy were used to identify the structure of the target
diamines. TOF-MS results showed the molecular weights of
342.1 (1-DA), 374.1 (1-OHDA), 418.1 (3-DA), 416.2 (2-DA),
and 450.1 (3-OHDA), which were close to the theoretical val-
1
ues of the target products. The H-NMR spectra are shown in
Figure 1. The peaks at 11.07 (1-OHDA) and 11.13 ppm (3-
OHDA) are assigned to the protons of hydroxyl groups. The
peak at 12.46 ppm (2-DA) is assigned to the protons of the
imidazole ring ANHA group. The aromatic protons in the
skeleton of 1-DA, 1-OHDA, 2-DA, 3-DA, and 3-OHDA molecules
are located within the range of 6.69–8.05, 6.19–8.14, 6.66–
7.87, 6.68–8.01, and 6.18–8.09 ppm, respectively, and the
attribution of each proton is labeled in Figure 1. The protons
of amino groups in the skeleton of 1-DA, 1-OHDA, 2-DA, 3-DA,
and 3-OHDA molecules are located at 5.99, 6.09, 5.60, 5.99,
1
3
1
and 6.11 ppm, respectively. C NMR spectra exhibited 9
FIGURE 1 H NMR spectra of diamines.
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SCHEME 2 Synthesis of PIs.
via a programmed heating procedure followed, as shown in
Scheme 2. PAAs solution showed high inherent viscosities of
tion viscosity of 1-OHDA and BPDA. Consequently, the result-
ing film of 1-BPOHPI was brittle.
0.5 to 2.3 dL/g, as listed in Table 2. These samples produced
flexible films, except 1-BPOHPI, by casting from the PAA sol-
utions and thermal treatment. Considering that the diamine
FT-IR Spectroscopic Studies
FT-IR spectroscopy is frequently used in the detection of H-
bonding interactions, since IR is sensitive to the frequency
shift of the proton donor and acceptor when H-bonding is
1-OHDA possessed intramolecular H-bonding in comparison
with 1-DA, and the molecule is fully rigid as shown Figure 4.
Crystal precipitates appeared with the increase in polymeriza-
3
0–34
formed.
The strength of intermolecular interaction was
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21
The FTIR spectra in the range of 1650–1750 cm
that
belonged to Vas(C@O) of imide were carefully investigated to
evaluate the evolution of H-bonding interactions. The curve-
resolved FTIR spectra are displayed in Figure 3 for PIs, and
3
6
four peaks are separately identified. Konieczny and Wunder
2
1
reported that band IV (center: around 1690 cm ) is due to
the asymmetric stretching of C@O. Bands III (center:
2
1
21
1
1
703 cm ), band II (center: 1714 cm ), and band I (center:
2
1
726 cm ) correspond to hydrogen-bonded C@O, van der
Waals forces associated C@O, and “free” C@O stretching of
the cyclic imides for poly(benzimidazole imide) (2-HPI),
2
5
respectively. Given the absence of H-bonding, band III is not
present in poly(benzoxazole imide)s (1-HPI and 3-HPI). In the
hydroxyl-containing poly(benzoxazole imide)s (1-HOHPI and
3-HOHPI), hydrogen-bonded C@O stretching band III also
does not exist. For quantitative evaluation, the percentage of
FIGURE 2 FT-IR spectra of the PIs.
the C@O stretching bands was calculated and analyzed results
3
7–39
are summarized in Table 1.
Waals forces associated C@O stretching band II were 51%,
1%, 52%, and 62% for 1-HPI, 1-HOHPI, 3-HPI, and 3-HOHPI,
The percentages of van der
6
investigated in terms of H-bonding by analyzing the fre-
quency difference of C@O between difference diamines
derived PIs. Figure 2 shows the FT-IR spectra of PIs pre-
pared from BPDA. The peak positions of the symmetrical
and asymmetrical C@O stretching vibration, which are char-
acteristic of the imide ring, appeared in the vicinity of 1700
respectively. The values of hydroxyl-containing poly(benzoxa-
zole imide)s (1-HOHPI and 3-HOHPI) were higher than those
of poly(benzoxazole imide)s (1-HPI and 3-HPI). This result
suggests that the hydroxyl-containing poly(benzoxazole
imide)s have higher intermolecular force than their corre-
sponding poly(benzoxazole imide)s.
2
1
and 1770 cm for all samples.
Thermal Properties
Given that the C@O group was expected to form H-bonding
with ANHA and AOH groups due to the presence of the
lone electron pair at the oxygen atom, the C@O band would
be weaker than that whose electron was not transferred.
Moreover, red shift would occur upon the formation of H-
bonding. Compared with the C@O wave numbers of 1710.8
The different intermolecular and intramolecular forces of PIs
result in significant differences in their properties. The
dynamic mechanical properties of PI films are investigated by
DMA, and T
PIs exhibited good thermal properties and all T
exceeded 300 C because of the rigid rod structure of the
nitrogen heterocyclic ring. The properties of PIs prepared from
diamines (1-OHDA, 1-DA) and diamines (2-DA, 3-DA, and 3-
was defined as the peak of tan d (Table 2). The
except 3-HPI
g
g
2
1
ꢀ
and 1772.5 cm for 3-BPPI, the C@O wave numbers of 2-
2
1
BPPI shifted to lower frequencies at 1705 and 1770 cm
,
whereas the CAO wave numbers of 3-BPOHPI shifted to
21
higher frequencies at 1716.6 and 1776.4 cm , suggesting
the H-bonding structural difference between 2-BPPI and 3-
BPOHPI, as shown Figure 4. The imide C@O is a intermolec-
ular H-bonding with the imidazole ANHA group for poly(-
benzimidazole imide) 2-BPPI, accordingly the C@O vibration
OHDA) are comparable. The T values of the PIs are displayed
g
in Figure 5. The T
values of imidazole-containing diamine 2-
g
ꢀ
DA derived PIs were the highest, reaching 421 C for 2-BPPI,
and this result can be regarded as the effect of strong intermo-
lecular forces that the benzimidazole unit imparted.
22,24
The Tg
2
4,25
occurred red shift than poly(benzoxazole imide) 3-BPPI.
values of 3-OHDA derived PIs were higher than that of 3-DA
For the containing hydroxyl poly(benzoxazole imide) 3-
BPOHPI, the hydroxyl group and oxazole nitrogen atom form
intramolecular H-bonding to a large conjugated system,
resulting in the enhanced inductive effect of imide ring nitro-
gen atom for the C@O group. Thus, the C@O vibration of 3-
BPOHPI appeared in higher frequencies than those of 3-
BPPI. The peak positions of the symmetrical and asymmetri-
cal C@O stretching vibration appeared in the wave numbers
derived PIs, and the T values of 1-OHDA derived PIs were
g
higher than that of 1-DA derived PIs, which are due to the
intermolecular and intramolecular H-bonding interaction of
AOH groups. Another characteristic that was taken into
account was the T difference (DT ), which is listed in Table 3.
g
g
The T differences, DT 1, DT 2, and DT 3, between 1-OHDA
g
g
g
g
and 1-DA, between 2-DA and 3-DA, and between 3-OHDA and
ꢀ
ꢀ
3
3
-DA derived PIs ranged within 6–44 C, 62–83 C, and 22–
21
of 1712.7 and 1774.4 cm
for 1-BPPI and 1716.6 and
ꢀ
9 C, respectively, with the same dianhydride, and the DT 2
g
21
1775.0 cm
for 1-BPOHPI, and the C@O stretching vibra-
were highest. All characteristics showed that the PIs containing
imidazole formed stronger intermolecular forces than the PIs
containing hydroxyl and benzoxazole.
tion of the hydroxyl-containing poly(benzoxazole imide) is
also moved to a higher wave number. Meanwhile, we sur-
2
1
mise that the small absorption at 1850 cm for 3-BPOHPI
and 1-BPOHPI caused by C@C-C@O groups formed by intra-
molecular H-bonding (Fig. 4) in the hydroxyl-containing pol-
The thermal stability of PIs were analyzed using TGA, and
the results are summarized in Table 2. All derived PIs exhib-
ited good thermal stability. The 5% and 10% weight-loss
3
5
y(benzoxazole imide).
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21
FIGURE 3 Curve-resolved FTIR spectra in the range of 1650–1750 cm for PIs; Band I belongs to “free” C@O, Band II belongs to
van der Waals forces associated C@O and Band III belongs to H-bonding associated C@O. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
21
TABLE 1 Curve-Fitting Result of the FTIR Spectra in the Range of 1650–1750 cm for PIs
1-HPI 1-HOHPI 2-HPI
3-HPI
3-HOHPI
Freq
Area
(%)
Freq
(cm
Area
(%)
Freq
Area
(%)
Freq
Area
(%)
Freq
(cm
Area
(%)
21
21
21
21
21
PIs
(cm
)
)
(cm
)
(cm
)
)
Band I
1726
1711
37
51
1729
1714
27
61
1726
1714
1703
1690
23
34
33
10
1725
1709
36
52
1727
1714
27
62
Band II
Band III
Band IV
1691
12
1695
12
1690
12
1696
11
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TABLE 2 Inherent Viscosity (c¸ inh) and Properties of the PIs
Char
CTE
ꢀ
a
ꢀ
b
ꢀ
c
d
ꢀ
PI
-BPPI
Diamines
Dianhydrides
g (dL/g)
T
g
( C)
T
d
5 ( C)
T
d
10 ( C)
Yield (%)
(ppm/ C)
1
1
1
1
1
1
1
1
2
2
2
2
3
3
3
3
3
3
3
3
1-DA
BPDA
BTDA
ODPA
HQDPA
BPDA
BTDA
ODPA
HQDPA
BPDA
BTDA
ODPA
HQDPA
BPDA
BTDA
ODPA
HQDPA
BPDA
BTDA
ODPA
HQDPA
2.33
1.13
1.34
1.70
1.02
0.77
0.89
0.96
1.03
0.81
0.80
1.11
2.26
0.99
1.58
1.30
374
361
346
302
–
589
553
562
555
608
576
587
572
64
58
61
57
26.4
6.1
-BTPI
1-DA
-OPI
1-DA
7.2
-HPI
1-DA
10.2
-BPOHPI
-BTOHPI
-OOHPI
-HOHPI
-BPPI
1-OHDA
1-OHDA
1-OHDA
1-OHDA
2-DA
367
358
346
421
397
387
361
338
335
316
296
372
357
355
326
533
553
524
485
512
521
493
579
544
548
535
547
538
521
499
561
574
545
574
557
571
536
599
572
577
557
581
569
558
521
59
59
56
71
72
70
69
67
64
64
62
62
64
60
58
4.0
3.3
8.8
20.35
10.1
9.0
-BTPI
2-DA
-OPI
2-DA
-HPI
2-DA
37.6
4.9
-BPPI
3-DA
-BTPI
3-DA
18.3
24.9
42.3
3.5
-OPI
3-DA
-HPI
3-DA
e
-BPOHPI
-BTOHPI
-OOHPI
-HOHPI
3-OHDA
3-OHDA
3-OHDA
3-OHDA
0.71
e
0.55
17.0
23.2
38.3
e
0.61
e
0.78
a
b
c
d
e
ꢀ
Peak temperature of tan d by DMA in nitrogen.
Temperature at which a 5% weight loss was recorded by TGA.
Temperature at which a 10% weight loss was recorded by TGA.
Residual weight retention when heated to 800 C in nitrogen.
Inherent viscosities were measured in NMP.
– The 1-BPOHPI film is brittle.
FIGURE 4 H-bonding in poly(benzimidazole imide) and poly(benzoxazole imide) (Refs. 24 and 34).
576
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FIGURE 5 T
g
values of PIs. [Color figure can be viewed in the
FIGURE 6 Char yield of PIs.
online issue, which is available at wileyonlinelibrary.com.]
ꢀ
the benzobisoxazole unit. Given enhanced molecular chain
rigidity and intermolecular force because of the intra-/inter-
molecular H-bonding of hydroxyl groups as shown Figure 4,
the movement and rotation of molecular chains are limited,
leading to the lower CTE value of 1-OHDA prepared PIs than
that of 1-DA prepared PIs.
temperatures of the polymers were recorded as 485–589 C
ꢀ
and 521–608 C, respectively. In addition, the Char yield of
ꢀ
the PIs in nitrogen atmosphere at 800 C were 56%–72%, as
summarized in Figure 6. The Char yield of 2-DA prepared
PIs were the highest within 69%–72%, the finding are due
to the following: (1) the strong H-bonding of the imidazole
group, which functions as physical crosslinking and increases
the contact between molecular chains, and (2) the higher
proportion of nitrogen in the molecule. The Char yield of
hydroxyl-containing poly(benzoxazole imide)s were slightly
lower than their corresponding poly(benzoxazole imide)s
because of the hydroxyl degradation. In general, the different
thermal stabilities of the polymers were related to their com-
positions and structures, the existence of highly rigid benzox-
azole and benzimidazole leads to the increase in the thermal
stability of PIs.
In comparing PIs prepared from 2-DA, 3-DA, and 3-OHDA,
we can see that the CTE values of 3-OHDA derived hydroxyl-
containing poly (benzoxazole imide)s were lower than 3-DA
derived poly(benzoxazole imide)s. However, the CTE values
of 2-DA derived poly(benzimidazole imide)s were the lowest.
The strong H-bonding of imidazole was speculated to per-
form a physical crosslinking function, and the intermolecular
H-bonding interaction of imidazole was stronger than the
hydroxyl group. Among all PIs, low CTE is observed due to
their rigid rod-like characteristics imparted by benzoxazole
and benzimidazole.
Dimensional Stability
The rigidity of the PI backbone would be considerably
enhanced after the employment of benzoxazole and benzim-
idazole, as can been seen from the outstanding thermal
resistance. Dimensional stability was expected to be
improved. The thermal expansion coefficient (CTE) values
were directly measured by the TMA instrument, and CTE
values are shown in Figure 7. The PIs prepared from dia-
mines 1-DA and 1-OHDA presented low CTE values of below
ꢀ
1
0 ppm/ C because of the fully rigid diamines containing
g
TABLE 3 DT of Polyimides
DT
g
1
DT
g
2
g
DT 3
BPDA
BTDA
ODPA
HQDPA
–
83
62
71
65
34
22
39
30
6
12
44
T
g g g g
difference (DT 1, DT 2, and DT 3) between 1-OHDA and 1-DA,
FIGURE 7 CTE of PIs. [Color figure can be viewed in the online
between 2-DA and 3-DA and between 3-OHDA and 3-DA derived PIs
with the same dianhydride.
issue, which is available at wileyonlinelibrary.com.]
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FIGURE 8 XRD patterns of PIs. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
2
1
Aggregation State
The morphological structures of PI films were examined by
XRD, as shown in Figure 8. The XRD curves show ordered
the scattering peaks appeared at around q 5 1.2 nm . The
calculated d spacing was 52 Å, which corresponded to the
long period of the 2-OPI film detected by the XRD test. These
results indicate that the periodical lamellar structure formed
in poly(benzimidazole imide) films.
ꢀ
ꢀ
structures around 2h 5 15 (d 5 5.9 Å), 18 (d 5 4.9 Å), and
ꢀ
2
5 (d 5 3.6 Å) for 1-DA [Fig. 8(a)], 1-OHDA [Fig. 8(b)], 3-DA
[
Fig. 8(c)], and 3-OHDA [Fig. 8(d)] derived poly(benzoxazole
For poly(benzoxazole imide)s and hydroxyl-containing poly(-
benzoxazole imide)s, given the rigid rod-like structure of
benzoxazole and relatively weak intermolecular H-bonding
imide)s, which indicate crystalline morphology. The crystal-
line structure should be related to rigidity and planar struc-
ture of benzoxazole, which enables the effective packing of
molecular chains, particularly for 1-DA and 1-OHDA contain-
ing benzobisoxazole units.
The 2-DA derived poly(benzimidazole imide) films were
essentially amorphous, as can be deduced from the broad
patterns in the diagrams of Figure 8(e). However, signs of
crystallization were observed, as demonstrated by the pres-
ence of discrete diffraction peaks in the diagrams of Figure
8
(e) (2-HPI) and Figure 8(f) (2-OPI). In addition, diffraction
ꢀ
ꢀ
peaks (2h) were located at 18 and 1.67 for 2-HPI and
-OPI, respectively, and d spacing could be calculated as 4.9
2
Å for 2-HPI and 52 Å for 2-OPI. Considering that the d spac-
ing obtained from XRD reflections corresponds to interchain
4
0–42
Packing,
the result indicated that the long period, the
average distance between the ordered regions, was shorter
for 2-HPI because of its greater flexibility than 2-OPI.
The morphological structures of poly(benzimidazole imide)
films were further measured using the SAXS measurements
2
1
as presented in Figure 9. A peak at around q 5 0.34 nm
,
FIGURE 9 SAXS patterns of poly(benzimidazole imide) films.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
which corresponded to a periodic distance of 18.5 nm, was
observed for 2-HPI films. In 2-BPPI, 2-BTPI, and 2-OPI films,
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ꢀ
FIGURE 10 XRD patterns (a) and FTIR spectra (b) of the poly(benzimidazole imide) films after heat treatment at 400 C/2 h. [Color
figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 11 XRD patterns (a) and FTIR spectra (b) of hydroxyl-containing poly(benzoxazole imide) films after heat treatment at T
1
ꢀ
ꢀ
(
400 C/4 h) and T
2
(400 C/8 h). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
ꢀ
interactions, the molecular chain can move, rearrange, and
consequently form stable crystalline structures below 350 C
overcome at higher temperature (400 C). Moreover, inter-
molecular forces were weakened and the motion of molecule
chain becomes free. Then the macromolecular packing
becomes more orderly and the crystalline structure appears.
ꢀ
(
imidization temperature). For poly(benzimidazole imide)s,
the motion and rotation of molecular chain are difficult
because of strong intermolecular forces of the imidazole
group, so the formation of a crystalline structure is impeded,
the periodical ordered structure and weak crystallization
1
-BPOHPI and 3-BPOHPI films were denoted as 1-BPOHPI-T
ꢀ
and 3-BPOHPI-T, after heat treatment at 400 C. The crystal-
linity increased than before heat treatment, as seen in the
XRD patterns in Figure 11(a). The IR spectra [Fig. 11(b)]
shows that the vibration peaks at 1850 cm
ened or disappeared after heat treatment at 400 C, indicat-
ing the destruction of intramolecular H-bonding.
ꢀ
appeared below 350 C (imidization temperature).
2
1
Effects of Heat Treatment
were weak-
ꢀ
The morphological structures of PI films are known to be
4
3
strongly influenced by annealing temperatures. Poly (benz-
imidazole imide) films were denoted as 2-BPPI-T, 2-BTPI-T,
ꢀ
2
-OPI-T, and 2-HPI-T after heat treatment at 400 C for 2 h.
According to the XRD patterns in Figure 10(a), signs of crys-
ꢀ
tallization were observed around 2h 5 17 (d 5 5.2 Å) and
ꢀ
2
h 5 23 (d 5 3.9 Å), and crystallinity increased than before
heat treatment. According to the FTIR spectra in Figure
1
1
1
3
0(b), the carbonyl stretching vibration (1770.6 and
21
705 cm ) of 2-BPPI was increased to (1772.5 and
2
1
706.9 cm ) of 2-BPPI-T, respectively. The vibration peak at
21
356 cm was enhanced, it is attributed to the characteris-
1
tic peak of imidazole ANHA. These results manifest that the
part of H-bonding between the imidazole and carbonyl was
FIGURE 12 H NMR spectrum of soluble FOHPI after heat treat-
ꢀ
ment at 350 C/6 h.
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sional stabilities. These results showed that the H-bonding
effect of benzimidazole is stronger than that of hydroxyl
groups. (4) Poly(benzoxazole imide)s show crystalline struc-
ture, whereas poly(benzimidazole imide)s show amorphous
ꢀ
state or weak ordered structure, after imidization at 350 C.
Intermolecular H-bonding interactions were overcome for
ꢀ
poly(benzimidazole imide)s after heat treatment at 400 C,
the molecular chains were rearranged and the crystalline
structures were formed. (5) Hydroxyl-containing poly (ben-
zoxazole imide)s were formed in crosslinking structures after
ꢀ
heat treatment at 400 C. The results of this study contribute
in designing a material with specific properties according to
the structure–property relationship.
ACKNOWLEDGMENTS
The financial support from the National Nature Science Foun-
dation of China (21104085) and the National High Technology
Research and Development Program of China (863 Program no.
FIGURE 13 FTIR spectra of soluble FOHPI film after heat treat-
ꢀ
ꢀ
a b
ment at T (350 C/6 h) and T (400 C/4 h).
2
012AA03A211) and Supported by Major State Basic Research
To further investigate the effects of heat treatment on
hydroxyl-containing poly(benzoxazole imide), the soluble
hydroxyl-containing poly(benzoxazole imide) FOHPI(For
complete experimental details of the FOHPI refer to the Sup-
porting Information) shown in Figure 12 was prepared. The
IR spectra as seen Figure 13 display that the vibration peak
Development Program (973 Program no. 2014CB643603) is
gratefully acknowledged.
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