Incorporation of: N -phenyl in poly(benzimidazole imide)s and improvement in H2O-absorbtion and transparency

Article abstract of DOI:10.1039/d0ra10138a

5-Amine-2-(4-amino-benzene)-1-phenyl-benzimidazole (N-PhPABZ) was successfully synthesized and polymerized with 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA) to obtain a novel N-phenyl-poly(benzimidazole imide) (N-Ph-PBII). The successful incorporation of N-phenyl addressed the issue of high H2O-absorption of traditional PBIIs while retained the superheat resistance property. The resulting N-Ph-PBII possessed a high glass-transition temperature (Tg) up to 425 °C and a low affinity for water of 1.4%. Furthermore, the loose molecular packing and noncoplanar structures led to an increase in optical transparency for the modified PBII.

Full text of DOI:10.1039/d0ra10138a

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PAPER
Incorporation of N-phenyl in poly(benzimidazole
imide)s and improvement in H2O-absorbtion and
transparency†
Cite this: RSC Adv., 2021, 11, 3770
Guangtao Qian, Fengna Dai, Haiquan Chen, Mengxia Wang, Mengjie Hu,
*
Chunhai Chen and Youhai Yu
5-Amine-2-(4-amino-benzene)-1-phenyl-benzimidazole (N-PhPABZ) was successfully synthesized and
polymerized with 3,30,4,40-biphenyl tetracarboxylic dianhydride (BPDA) to obtain a novel N-phenyl-
poly(benzimidazole imide) (N-Ph-PBII). The successful incorporation of N-phenyl addressed the issue of
high H2O-absorption of traditional PBIIs while retained the superheat resistance property. The resulting
N-Ph-PBII possessed a high glass-transition temperature (Tg) up to 425 ꢀC and a low affinity for water of
1.4%. Furthermore, the loose molecular packing and noncoplanar structures led to an increase in optical
transparency for the modified PBII.
Received 1st December 2020
Accepted 6th January 2021
DOI: 10.1039/d0ra10138a
rsc.li/rsc-advances
dielectric properties, and failures of the adhesion to metal,
restricting the application.13,14 Hence, low H2O-absorption PBIIs
1. Introduction
without compromising their superior properties are desirable.
Our previous study showed that, having appropriate N-
substituents, such as N-methyl, in the BI ring can solve the
problem of high WA in PBIIs.14 However, this series of N-methyl-
PBIIs have relatively lower Tg values because of the hindrance of
intermolecular H-bonding formation. Recent researches have
proved that the incorporation of large rigid side-bases can
enhance thermal properties of polymers through their steric
hindrance.15,16 So N-PhPABZ with a bulk N-phenyl-substituent
was designed and synthesized (Scheme 1) in this work, which
may provide PBIIs with low WA and high Tg. N-PhPABZ-BPDA was
successfully prepared (Scheme 2) and its physical properties was
compared with that of the traditional PBII (PABZ-BPDA).
Poly(benzimidazole imide) (PBIIs), combining the superiority of
polyimides (PIs) and polybenzimidazoles (PBIs), are a class of
promising polymers in engineering and science.1–3 Among their
outstanding properties, the high thermal properties with Tg
values normally greater than 400 ꢀC have gained attention.4,5
Previous research has proven that the excellent superheat-
resistant ability of PBIIs can be ascribed to the intermolecular
hydrogen bonds formed between the proton donors (N–H
group) and proton acceptors (–N] or C]O).6,7 Also, the benz-
imidazole (BI)-containing diamines strengthen the charge
transfer (CT) interaction between the molecular chains with
their higher electron-donating properties, facilitating the
increased Tg value of polymers.8,9
As a key additive, heteroaromatic BI ring plays a critical role
for the improving thermal properties of PBIIs. However, the N–H
in the key unit leads to an increased chance for hydrogen-bonded
water molecules formation, resulting polymers with a high
affinity of water.10,11 For instance, the water absorption (WA) can
reach 15–18 wt% for poly(2,20-m-(phenylene)-5,50-bibenzimida-
zole) (PBI) and 6–7 wt% for the PBIIs derived from 5(6)-amino-2-
(4-aminobenzene)-benzimidazole (PABZ) and BPDA.12,13 Among
them, the traditional PBIIs can be used as a candidate material
for exible substrates in various image display devices; however,
their high WA may lead the package cracking, degradation of
2. Experiment
2.1 Materials
2,4-Dinitrouorobenzene, aniline and 4-nitrobenzoyl chloride
were purchased from Shanghai Haohong Biomedical Tech-
nology Co. Ltd. and Aladdin Chemical Co. Ltd., respectively.
Other chemicals used in this study were purchased from Sino-
pharm Chemical Reagent Beijing Co. Ltd. Among them, BPDA
were dried overnight at 80 ꢀC under vacuum, and N,N-dime-
thylacetamide (DMAc) were puried by vacuum distillation,
prior to use. All the other commercially available reagents were
used without further purication.
Center for Advanced Low-Dimension Materials, State Key Laboratory for Modication
of Chemical Fibers and Polymer Materials, College of Material Science and
Engineering, Donghua University, Shanghai 201620, P. R. China. E-mail:
yuyouhai@dhu.edu.cn
2.2 Characterization
1
1H NMR and H–1H COSY data were recorded on a Bruker 600
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra10138a
AVANCE III spectrometer, using dimethyl sulfoxide-d6 as
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Scheme 1 Synthesis of N-PhPABZ.
a solvent. Elemental analyses were determined in by Elmentar
WA of the polymer lms was measured by dipping in distilled
Vario EL-III. The inherent viscosities (hinhs) were measured with water. In detail, large lm specimens (>0.1 g) were weighed
an Ubbelohde viscometer at 25 ꢁ 0.1 ꢀC with a concentration of before being immersed in water at 25 ꢀC for 24 h and then
0.5 g Lꢂ1 in DMAc. Attenuated total reectance Fourier trans- blotted carefully clean with tissue paper and weighed. The water
form infrared (ATR-FTIR) spectra were measured at a Nicolet absorption ratio was calculated from the follow equation:
6700 infrared spectrometer in the range of 4000–700 cmꢂ1. UV-
WA ¼ [(Wwet ꢂ Wdry)/Wdry] ꢃ 100%.
vis spectra were recorded on a Shimadzu UV-3600 spectropho-
tometer in a transmittance mode with the wavelength range of
300–700 cmꢂ1. Wide-angle X-ray diffraction (WAXD) was
Among them Wdry is the initial weight of the samples, Wwet is
collected on a Rigaku Denki D/MAX-2500 diffractometer at
the sample weight aer treatment and WA is the mean value of
room temperature. The thermogravimetric analysis was
three parallel samples.
assessed usinꢂg1Discovery TGA 550 with a constant heating rate
ꢀ
of 10 C min under nitrogen. Dynamic mechanical analysis
(DMA) was performed on a TA Instrument DMA Q800 in
2.3 Synthesis of monomers
a tension mode with a heating rate of 5 ꢀC minꢂ1 and
2.3.1 2,4-Dinitrodiphenylamine (1). Aniline (5.0 g, 53.7
mmol) was added into a stirred mixture of 2,4-dinitro-
uorobenzene (9.5 g, 51.1 mmol) in methanol (100.0 mL)
a frequency of 1 Hz. Coefficient of thermal expansion (CTE) was
examined using a TA Instrument TMA Q400 in the range of 50–
300 ꢀC at a heating rate of 5 ꢀC minꢂ1 with a xed load of 0.5 g
mmꢂ1 (lm thickness) in nitrogen. The tensile properties of
polymer lms were evaluated on a Shimadzu AG-I universal
testing apparatus at speed of 5.0 mm minꢂ1, and tensile
strength (s), tensile modulus (E) and elongation at break (3)
were demonstrated as the average of ve strips. The net charge
on nitrogen atoms and energy minimized structures on the
molecules was achieved through the B3LYP (Becke, three-
parameter, Lee–Yang–Parr) exchange–correlation functional
for the 6-311G(d,p) basis set basis set in Gaussian 09W.
ꢀ
maintained at 0 C. The mixture was stirred at room tempera-
ture for 18 h (TLC monitoring) and then ltrated. The residue
was dried in vacuum at 80 ꢀC for 24 h to afford the title
compound (11.3 g, yield: 85.3%) which was used for the next
1
ꢀ
reaction without purication. Melting point: 160 C. H NMR
(DMSO-d6, ppm): d ¼ 10.16 (s, 1H), 8.90 (d, 1H, J ¼ 2.7 Hz), 8.23
(dd, 1H, J ¼ 9.6 Hz, 2.8 Hz), 7.57–7.49 (m, 2H), 7.44–7.32 (m,
3H), 7.10 (d, 1H, J ¼ 9.5 Hz). Anal. calcd for C12H9N3O4: C, 55.60;
H, 3.50; N, 16.21. Found: C, 55.21; H, 3.82; N, 16.22.
2.3.2 4-Nitro-1-N-phenylbenzene-1,2-diamine
(2).
To
a solution of compound 1 (10.0 g, 38.6 mmol) in the solvent
Scheme 2 Reaction schemes of N-PhPABZ-BPDA.
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system ethanol (80.0 mL)-water (40 mL) was added sodium
bicarbonate (3.9 g, 46.3 mmol) and sodium sulde (3.6 g, 46.3
ꢀ
mmol). The reaction was allowed to warm to 80 C and stirred
overnight. Aer this time, the reaction was cooled with an ice-
water bath and then poured into water. The precipitate which
appeared was collected using suction ltration and dried to
yield compound 2 as a brown colored powder (7.2 g, yield:
1
ꢀ
81.8%). Melting point: 123 C. H NMR (DMSO-d6, ppm): d ¼
7.79 (s, 1H), 7.58 (d, 1H, J ¼ 2.7 Hz), 7.46 (dd, 1H, J ¼ 8.9 Hz, 2.7
Hz), 7.34 (t, 2H, J ¼ 7.8 Hz), 7.16 (d, 2H, J ¼ 7.9 Hz), 7.07 (d, 1H, J
¼ 8.9 Hz), 7.01 (t, 1H, J ¼ 7.3 Hz), 5.44 (s, 2H). Anal. calcd for
C
12H11N3O2: C, 62.87; H, 4.84; N, 13.91. Found: C, 63.11; H,
4.81; N, 14.12.
2.3.3 5-Nitro-2-(4-nitrobenzene)-1-phenyl-benzimidazole
(3). To compound 2 (5.0 g, 21.8 mmol) and triethylamine (2.6 g,
26.2 mmol) in tetrahydrofuran (50.0 mL) at 0 C was added 4-
Fig. 1 1H NMR spectra of synthesized monomers (compound 3 and
N-PhPABZ).
ꢀ
nitrobenzoyl chloride (4.9 g, 26.2 mmol) in equal portions over
0.5 h. The reaction was allowed to stir at room temperature for
16 h. Water (100.0 mL) was added and the reaction mixture was
ltered. Aer drying, the residue was dissolved in the acetic acid
at 80 ꢀC overnight to slowly evaporate the solvent. Subsequently,
the polyamic acid (PAA) lm was heated at 200 ꢀC, 300 ꢀC, and
400 ꢀC each for 1 h to complete the imidization process. Finally,
the glass disc was cooled to room temperature and soaked in
deionized water to get the desired PBII lm.
ꢀ
(200.0 mL) and the solution was heated under argon at 120 C
for 3 h. The mixture was then diluted with water and ltered
thru ltration fabric (ethanol rinse). The residue puried by
recrystallization from DMSO yielded compound 3 (7.0 g, yield:
89.3%) as a yellow powder. Melting point: 195 ꢀC. 1H NMR
(DMSO-d6, ppm): ¼ 7.50 (dd, 1H, J ¼ 8.6 Hz, 2.1 Hz), 7.10 (d, 1H,
J ¼ 1.6 Hz), 6.56 (d, 1H, J ¼ 8.6 Hz), 6.15 (s, 2H), 5.23 (d, 1H, J ¼
4.4 Hz), 2.79 (d, 3H, J ¼ 4.7 Hz). Anal. calcd for C19H12N4O4: C,
63.33; H, 3.36; N, 15.55. Found: C, 63.35; H, 3.65; N, 15.29.
2.3.4 5-Amine-2-(4-aminobenzene)-1-phenyl-benzimidazol
(N-PhPABZ). Compound 3 (5.0 g, 13.8 mmol) and palladium
catalyst (10% Pd/C, 0.5 g) was dissolved in methanol (50.0 mL).
Aer stirred for a further 1 h whilst being reuxed, the mixture
was hydrogenated by the dropwise addition of 7.0 mL of
hydrazine hydrate. The reaction was reuxed for 4.5 hours and
then cooled to room temperature. The catalyst was removed by
ltration, and then the ltrate was poured into water (100.0
mL). The precipitate was ltered off and puried in water/EtOH
to afford the desired compound (a white powder, 3.8 g, yield:
2.5 Other monomers and PABZ-BPDA
The synthesis processes of PABZ and PABZ-BPDA had been
illustrated in our previous study.14 In this work, the physical
properties of the traditional PBII (PABZ-BPDA) were used as
references.
3. Results and discussion
3.1 Molecular structure
As shown in Scheme 1, the N-Ph was successfully introduced in
N-PhPABZ as a key unit through the use of aniline with the
above reaction sequence. The new BI-diamine was fully char-
acterized by means of elemental analysis, FTIR spectrum, 1H
1
1
NMR (Fig. 1) and H–1H COSY (Fig. 2) spectra. In the H NMR
spectra of N-PhPABZ, no signal (ca. 11.84 ppm) of the –NH–
1
ꢀ
92.8%). Melting point: 270 C. H NMR (DMSO-d6, ppm): d ¼
7.50 (dt, 3H, J ¼ 27.1 Hz, 7.3 Hz), 7.31 (d, 2H, J ¼ 7.4 Hz), 7.12 (d,
2H, J ¼ 8.6 Hz), 6.81 (dd, 2H, J ¼ 11.0 Hz, 5.1 Hz), 6.52 (dd, 1H, J
¼ 8.5 Hz, 1.9 Hz), 6.43 (d, 2H, J ¼ 8.6 Hz), 5.44 (s, 2H), 4.79 (s,
2H). Anal. calcd for C19H16N4: C, 75.98; H, 5.37; N, 18.65. Found:
C, 75.60; H, 5.65; N, 18.88.
2.4 Preparation of the N-PhPABZ-BPDA lm
N-PhPABZ-BPDA was prepared through a typical two-stage
thermal imidization (Scheme 2). In detail, a sealed bottle
charged with anhydrous DMAc (9.5 mL), N-PhPABZ (1.2015 g,
4.0 mmol) was mixed in. Aer the diamine was completely
dissolved, BPDA (1.1768 g, 4.0 mmol) was added slowly at a rate
sufficient to maintain the reaction solution clear (addition time
ca. 2.0 h). The mixture was continuously stirred under room
temperature for 24 h. The above solution was then cast on
a dust-free glass disc, and the glass plate was placed in an oven Fig. 2 1H–1H COSY spectrum of N-PhPABZ.
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nucleophilicity and reactivity than the phenyl linked –NH2. It
indicated that the employ of N-Ph did not affect the nucleo-
philic advantage for such diamines.
To get clear geometric optimization conformation, the
energy minimized structure of N-PhPABZ was calculated by the
DFT [B3LYP/6-311G(d,p)] method, using the Gaussian 09W
soware (Fig. 3). Obviously, the net charge of the amine N
atoms on BI ring was signicantly higher than that on phenyl
ring, because of the higher electron-donating property of BI
group. Since the nucleophilicity of diamines increased by
introducing strong electron-donating BI, it could be inferred
that N-PhPABZ would possess higher nucleophilicity than full-
phenyl diamines. Furthermore, the stable geometry proved
the resultant diamine possessed more noncoplanar conforma-
tion in comparison with PABZ. As depicted in FOHURE 3, the
dihedral angle between phenyl ring and imide ring exhibited
25.2ꢀ. It was indicative of the BI-diamine molecular structure
was signicantly affected by the N-substitution group, and its
steric hindrance resulted in the distorted backbone.
Fig. 3 Energy minimized structure of N-PhPABZ and net charges on
the nitrogen atoms of amino groups calculated by the DFT [B3LYP/6-
311G(d,p)] calculations.
proton in the BI group was observed, and three sets of peaks
(7.5–7.3 ppm) were attributed to aromatic protons, proving the
placement of phenyl on BI nitrogen atom. To further assign the
1
aromatic protons of N-Ph in the diamine, H–1H COSY spectra
had been given and the data accorded with the expected
structure. In addition, the high eld peak at 5.44 ppm (H-14)
and 4.79 ppm (H-13) would correspond to the amine protons
linked to phenyl ring and BI unit. The electron-donating effect
of BI on amine H-13 caused an obviously 0.65 ppm high-eld
shi in comparison with H-14 attached to phenyl ring, that is,
the terminal amine attached to BI ring had higher
3.2 Polymer characteristics
ATR-FTIR was utilized to investigate the molecule structures of
the PI lm as shown in Fig. 4A. The typical characteristic imide
peak at around 1783, 1719, 1367 cmꢂ1 were assigned to imide
carbonyl asymmetric stretching, imide carbonyl symmetric
stretching and cyclic C–N stretching, respectively, and the peaks
corresponding to the C]O in –CONH– (1660 cmꢂ1, gray band
Fig. 4 ATR-FTIR spectra (A), WAXD diffractogram (B), TGA thermogram (C) and DMA curve (D) of N-PhPABZ-BPDA.
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Table 1 Properties of the PBII films
s
E
a
Tg (DMA) (ꢀC)
Sample no.
hinh (PAA) (dL gꢂ1
)
T5db (N2) (ꢀC)
(MPa)
(GPa)
3 (%)
CTEc (ppm Kꢂ1
)
WA (%)
PABZ-BPDA
N-PhPABZ-BPDA
1.16d
0.81
423d
425
549d
534
218d
130
6.7d
4.7
5.9d
3.7
11d
27
5.9d
1.4
a
Glass transition temperature was recorded by DMA. b 5% weight loss temperature was recorded by TGA. c Coefficient of thermal expansion along
d
the X–Y direction, measured in the range of 100–200 ꢀC with TMA. Data from ref. 14.
domain) was hardly observed, clarifying the imidization process CT interaction and the strengthened physical interaction
was fully completed.17,18 Moreover, the characteristic absorption between the molecular chains also increased the difficulty in
of BI ring at 1303 cmꢂ1 (imidazole ring breathing peak) molecular-chain motion when these PBIIs experienced a high
conrmed the successful incorporation of the characteristic temperature.6–9 In this work, N-PhPABZ, as a modied BI-
group.19,20 Additionally, the characteristic absorption peaks of diamine, inherited the superiority of higher nucleophilicity,
imide C]O in the N-PhPABZ agreed well with the values of free evidenced by the 1H NMR and calculation results. Furthermore,
carbonyl, that was no hydrogen bonding occurred in the the rigid N-Ph group limited the rotational freedom around
prepared PBII.14 The results implied the N-Ph on the imidazole backbone of PIs. As expected, The N-PhPABZ-BPDA lm dis-
nitrogen atoms limited the formation of intermolecular H- played a high Tg.
bonding and increased the difficulty in the hydrogen-bonded
water molecules formation.
It was believed that the CTE values were susceptible to the
alteration in molecular-chain orientation, and the interchain
The chain aggregation conducted by X-ray diffraction interaction and chain stiffness/linearity played an key role in
measurement was illustrated in Fig. 4B. The N-PhPABZ-BPDA the occurrence of in-plane orientation.25,26 The PABZ-BPDA had
lm exhibited a broad diffraction peak centered at 2q ¼ 22.5ꢀ, a high level of coplanarity and much H-bonding, which was
typical of an amorphous material. The loose molecular packing benet to the spontaneous orientation during thermal imid-
was attributed to the increased interchain distances resulted ization, therefore, such series of PIs exhibited higher dimen-
from the existence of the bulky N-phenyl in the backbone. In sional stability and lower CTE. Indeed, with a bulky pendant
contrast, the reported PBIIs usually had highly ordered molec- group and distorted conformations, N-PhPABZ-BPDA had rela-
ular arrangements and close chain packing with their H- tively higher CTE compared with the PABZ-BPDA. However, the
bonding, which could provide materials with deep color CTE value of 27 ppm Kꢂ1 was close to that of the commercially
because of the enhanced charge-transfer (CT) interactions.4,7,21 available Kapton H (Dupont) used as a representative heat-
In this work, with the incorporation of N-substituent, the resisting high polymer.27
density and packing coefficient decreased, and the optical
As the result (Table 1 and Fig. S2†), N-PhPABZ-BPDA had
transparency of the PBII might be improved.
a tensile strength of 130.0 MPa, a modulus of 4.7 GPa and an
3.2.1 Physical properties of N-PhPABZ-BPDA. The thermal elongation at break of 3.7%, affirming its toughness and exi-
properties of N-PhPABZ-BPDA were evaluated by TGA (Fig. 4C), bility. It was believed that PI with higher crystallinity and
DMA (Fig. 4D) and TMA (Fig. S1†), and the analysis data was
presented in Table 1. The 5% weight loss temperature was
above 500 ꢀC, which is similar to that of PABZ-BPDA, testifying
N-PhPABZ-BPDA retained the advantages of traditional PBII in
higher thermal stability. In general, the more conjugated rigid
chain led to a higher thermal stability for PIs.22,23 This was
because the resulting p-electron system facilitated rapid heat
transfer between molecular bonds, which prevented the rupture
of polymer main chains when they experience a high tempera-
ture. The rigidity of imidazole diamine had not decreased aer
the introduction of rigid N-Ph, thus, N-PhPABZ-BPDA possessed
a high thermal decomposition temperature.
In Fig. 4D, the peak temperature of the tan delta curve was
regarded as Tg value,24 and the value of N-PhPABZ-BPDA was
almost the same as that of PABZ-BPDA, which was up to 425 ꢀC.
In previous studies, the outstanding Tg of PBIIs could be
explained from two aspects. First, the intermolecular H-
bonding (C]N/N–H and C]O/N–H) derived from imid-
azole unit restricted chain-segment motion. Second, the higher
nucleophilic of BI-contained diamines resulted in an increased
Fig. 5 UV-vis spectra and images of the PBII films.
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Table 2 Optical properties of the PBII films
substituted PABZ. The N-substituent hindered the hydrogen-
bonded water molecules formation and the N-Ph-PBII showed
lower WA. Furthermore, the merged N-Ph distorted the back-
bone and increased the difficulty of chain motion in the
prepared PIs. As a result, the N-PhPABZ-BPDA lm exhibited an
extremely high Tg and signicantly improved optical trans-
parency. Although the dimensional stability and mechanical
properties of the N-Ph-PBII were lower than traditional PBIIs,
but they could still meet the demand for practical application.
This study provided new insights to prepare the modied PBIIs
with desirable properties.
a
Sample no.
Thickness (mm)
lcut-off (nm)
T500b (%)
PABZ-BPDA
N-PhPABZ-BPDA
20
21
436
405
31
63
a
Cutoff wavelength, the wavelength with a transmittance of lower than
1%. b Transmittance at 500 nm.
intermolecular interactions possessed higher mechanical
properties.28 The obviously decreasing mechanical performance
of the N-Ph-PBII had been observed in the Table 1. Based on the
above analysis, the higher mechanical properties of PABZ-BPDA
resulted from its semi-crystalline state, and the lower properties
of PABZ-BPDA corresponded to its amorphous state. Besides,
the presence of N-substituent bulky not only hindered hydrogen
bond formation but also loosened the chain packing, resulting
the reduced physical interaction and the decreasing trend of Tg.
Nevertheless, the tensile strength exceeded 100 MPa, the initial
modulus was over 2 GPa, which was able to meet the require-
ments in commercial use.29,30
Polymers containing imidazole rings usually had a high
affinity for water. The primary cause was the hydrophilic
imidazole N–H group which could form hydrogen bonds with
water. So the feasible substituents on the imidazole nitrogen
atom could hinder the hydrogen-bonded water molecules
formation, partially eliminating the issue of high H2O-
absorption for such polymers. Obviously, N-PhPABZ-BPDA
illustrated a low H2O-absorption down to 1.4%, and the value
decreased 76% compared to that of PABZ-BPDA (Table 1). It
veried that the introduction of N-Ph was one of the useful
technologies for healing the inherent problem of high WA for
the PBII series.
The cut-off wavelength values and transmittances at 500 nm
of the N-Ph-PBII lm were listed in Fig. 5 and Table 2, which was
investigated by UV-vis spectroscopy. The color intensity of PIs
was generally related to the charge transfer complex (CTC)
formation theory.31,32 According to the theory, the intra- and
inter-molecular CTC formation was easily affected by electron-
accepting property of dianhydride, electron-donating property
of diamine and stacking state of chain-segments. Polyimides
with bulky electron withdrawing units, exible groups or non-
coplanar structures possessed lighter color compared with PIs
had a rigid chain. PABZ, as one of traditional BI-diamines, had
a strong electron-donating ability and a stiff framework, and
thus PABZ-BPDA presented a deep color. N-PhPABZ also showed
strong nucleophilicity, however, its N-Ph moiety had the ability
to distort molecules and increased the spacing between back-
bones for the polymers contained it. As a result, the N-PhPABZ-
PBII had a superior optical transparency than that derived from
PABZ.
Conflicts of interest
There are no conicts to declare.
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