Polyimides from an asymmetric hydroxyl-containing aliphatic-aromatic diamine synthesized via henry reaction

Article abstract of DOI:10.1002/pola.28720

An aliphatic amino and an aliphatic hydroxyl group have been incorporated via Henry reaction highly efficiently toward the synthesis of a novel asymmetric aliphatic–aromatic diamine 2-amino-1-[4-(5-aminopyridyloxy)phenyl]-1-ethanol (AAP_yP_h

Full text of DOI:10.1002/pola.28720

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Polyimides from an Asymmetric Hydroxyl-Containing Aliphatic-Aromatic
Diamine Synthesized via Henry Reaction
Zhiang Wang, Liying Guo, Songyu Han, Haixia Qi, Yusheng Cheng, Feng Liu
College of Chemistry, Nanchang University, 999 Xuefu Avenue, Honggutan New District, Nanchang 330031, People’s Republic of
China
Correspondence to: H. Qi (E-mail: hxqi@ncu.edu.cn) or F. Liu (E-mail: liuf@ncu.edu.cn)
Received 23 March 2017; accepted 23 June 2017; published online 00 Month 2017
DOI: 10.1002/pola.28720
ABSTRACT: An aliphatic amino and an aliphatic hydroxyl group
have been incorporated via Henry reaction highly efficiently
toward the synthesis of a novel asymmetric aliphatic–aromatic
diamine 2-amino-1-[4-(5-aminopyridyloxy)phenyl]-1-ethanol
0
transparency with decreased cutoff wavelength (k ) ranging
from 328 to 370 nm. Comparative studies reveal that the pen-
dent aliphatic hydroxyls in the polymer chains would lead to
interchain cross-linking via condensation and secondary weak
cross-linking by hydrogen bond depending on different load-
(
AAP
y h y h
P E) in three steps. AAP P E shows good copolymeri-
0
zation reactivity with 4,4 -oxydianiline (ODA) toward different
ing of AAP
hydrophobic properties, DMA tan d and dielectric constant of
2017 Wiley Periodicals, Inc. J. Polym.
y h
P E, which result in a fluctuation of hydrophilic–
0
aromatic dianhydrides, especially 4,4 -oxydiphthalic anhy-
the copolymer films. C
V
dride (ODPA). TGA measurement and mechanical test results
show that all polymers maintain the inherent thermal perfor-
mance and tensile properties, while the glass transition tem-
Sci., Part A: Polym. Chem. 2017, 00, 000–000
peratures (T
from 185.5 to 253.3 8C due to the presence of aliphatic seg-
ments. The introduction of AAP E is found to improve the
solubility of the polymers, and the polymer films’ optical
g
’s) by DMA show moderate decrease ranging
KEYWORDS: asymmetric aliphatic–aromatic diamine; aliphatic
hydroxyl; Henry reaction; cross-linking via condensation;
hydrogen bond
y h
P
INTRODUCTION Polyimides (PIs) are
a
class of high-
diamines have attracted considerable attention in preparing
1
7,18
PIs due to their great industrial significances.
Watanabe
performance polymers with cyclic imide groups in main chain.
Aromatic PIs are well known for their excellent thermal,
mechanical performance, and dielectric properties; therefore,
and Imai explored the approach to synthesize high-
molecular-weight polyimides from aliphatic diamines by
high-pressure and at high temperature (using ethylene glycol
1
–3
widely used in many areas especially in microelectronic.
1
9,20
as solvent), respectively.
Ueda used tetracarboxylic dia-
However, aromatic PIs usually suffer from poor solubility and
strong intermolecular charge-transfer complexes (CTCs) that
results in a strong absorption in UV and visible region and a
nhydrides and trans-1,4-cyclohexyldiamine to prepare high-
molecular-weight semiaromatic PAAs in the presence of ace-
2
1
tic acid. Van Mele prepared aliphatic PIs by one step in m-
cresol and NMP and studied the odd–even effect of the ali-
0
4,5
high dielectric constant (E ), limiting their applications. To
overcome these drawbacks, many structural modification
methods have been taken such as (1) introducing fluorine, sul-
fur, and phosphorus substituent and other bulky groups; (2)
designing asymmetric structure; and (3) incorporating alicy-
22
phatic diamine. Ward synthesized high-molecular-weight
soluble, amorphous, partially aliphatic polyimides by con-
densing aliphatic diamine with ester acid to sidestep the for-
23
mation of gel-like nylon salt.
N-silylation of aliphatic
2
,6–11
clic or aliphatic structures.
All these methods could be
diamine also proved an effective measure to avoid the nylon
salt toward the synthesis of high-molecular-weight PIs from
capable of improving the PIs’ solubility. Recently, the less
polarized fluorinated structure and nonaromatic (alicyclic or
aliphatic) structures have shown considerable efficiency in
2
4
aliphatic diamine. It should be noteworthy that none of the
approaches could apply to all kinds of aliphatic PIs.
0
12–16
reducing CTC and thus dielectric constant (E ).
Owing to the presence of the localized lone pair of electrons
2
Although there have been concerns about their strong nucle-
ophilicity and basicity during polymerization, the aliphatic
on the sp orbit of nitrogen atom, the heterocyclic pyridine
ring offers an advantage over aromatic benzene ring in
Additional Supporting Information may be found in the online version of this article.
2017 Wiley Periodicals, Inc.
V
C
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providing desired properties such as better solubility and
melting point apparatus SGW-X4 (Shanghai Science Appara-
tus, China) using capillary samples. Inherent viscosities (ginh
were measured with an Ubbelohde viscometer with a 0.5 g
1
3,25,26
mechanical properties.
Meanwhile, as a closed conju-
)
gated ring with 6 pi electrons that meets Huckel’s rule, the
rigid aromatic pyridine helps to maintain the thermal stabil-
ity when introduced into the polyimide backbone. Herein, we
designed a novel asymmetric aliphatic-aromatic diamine 2-
21
dL
of DMAc solution at 25 8C. Weight-average molecular
weights (M ) and number-average molecular weights (M )
w
n
were obtained via gel permeation chromatography (GPC) on
the basis of polystyrene calibration on a PL-GPC 220 instru-
y h
amino-1-[4-(5-aminopyridyloxy)phenyl]-1-ethanol (AAP P E)
2
1
which bears an aliphatic hydroxyl, an aliphatic amino and an
amino attached to a heteroaromatic pyridine ring. Henry
reaction was used for the incorporation of the aliphatic
amino and aliphatic hydroxyl highly efficiently. To the best of
our knowledge, there has scarcely been report on employing
Henry reaction to introduce aliphatic segments to modify
aromatic diamine for PIs. We envisaged that the asymmetric
aliphatic–aromatic structure would lower the dielectric con-
stant, and simultaneously, improve the optical transparency
and solubility of the resultant PIs. However, how to balance
the reactivity difference between aromatic amino and ali-
phatic amino or how to find an appropriate dianhydride for
effective polymerization needs to be tackled down. Moreover,
it would be very interesting to explore how the aliphatic
hydroxyls introduced by Henry reaction influence the prop-
erties of the polymers as they are pendent to the polymer
chains. It naturally occurred to us that the pendent hydroxyls
would lead to the cross-linked structure in PIs, thus better
strengthening the thermal stability of aliphatic PIs that oth-
ment with DMF as an eluent at a flow rate of 1.0 mL min
.
The film samples were immersed in DMF at room tempera-
ture for 48 h and the soluble fraction (SF) data of the film
samples were measured by dividing the residue weight by
the original weight. Wide-angle X-ray diffraction (WAXD)
measurement were performed at room temperature on a
Bede XRD Di system, using graphite-monochromatized Cu-Ka
radiation (k 5 0.15405 nm). Ultraviolet–visible (UV–vis) spec-
tra of the polymer films were recorded on a Shimadzu UV–
visible spectrophotometer UV-2450. Contact angle measure-
ments were carried out by using Drop Master 300 Contact
AM system (Kyowa Interface Science, Saitama, Japan). Ther-
mogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) were conducted with a Perkin-Elmer TGA-2 in
flowing nitrogen or in air at a heating rate of 10 8C/min.
Dynamic mechanical analyses were conducted with DMA
Q800 V20.22 Build 41 using tensile mode at the frequency
of 1 Hz. An Instron universal tester model 1122 (GB/
T1040.1–2006) was used to study the stress–strain behavior
of the film samples at room temperature with a stretching
rate of 2.5 mm/min, and the data were the average value of
five experiments except for the maximum and the minimum
value. Dielectric spectroscopy measurements of the polymer
2
7,28
erwise would be remarkably sacrificed.
AAP P E was used to condense with different aromatic dia-
In this work,
y
h
nhydrides and we comprehensively studied the influence of
the incorporated AAP P E to the solubility, optical thermal,
y
h
dynamic mechanical thermal, surface, and the dynamic
dielectric properties of the resulted PIs.
7
films with the frequency variation in the range of 1–10 Hz
at room temperature have been performed using a Novocon-
trol Dielectric Spectrometer (GmbH Germany), CONCEPT 40.
Polymer films electrode with diameter of 10 mm and thick-
ness of 30–70 lm having gold plated were placed in a flat
parallel-plate capacitor, and the amplitude of AC applied volt-
age was 1 V.
EXPERIMENTAL
Materials
m-Cresol, methanol, ethanol, dichloromethane, N,N-dimethyl-
formamide (DMF), N,N-dimethylacetamide (DMAc), nitro-
methane, p-hydroxylbenzaldehyde, potassium carbonate,
Monomer Synthesis
triethylamine (TEA), potassium borohydride (KBH ), nick-
4
A nucleophilic substitution reaction between 2-chloro-5-
nitropyridine and p-hydroxybenzaldehyde was carried out to
offer 4-(5-nitropyridyloxy)benzaldehyde, which reacted with
nitromethane via Henry reaction to obtain aliphatic
hydroxyl-functionalized asymmetric aliphatic–aromatic dini-
tro compound. The subsequent reduction of the dinitro
groups was effective enforced by KBH4 with the catalyst of
NiCl ꢀ6H O to generate the corresponding asymmetric ali-
el(II) chloride hexahydrate (NiCl
from Sinopharm Chemical Reagent Inc., China, and 2-chloro-
2
ꢀ6H
2
O) were purchased
5
-nitropyridine was purchased from Nanjing Chemlin Chemi-
0
cal Inc., China. Commercially obtained 4,4 -oxydianiline
(
ODA) was recrystallized from ethanol and dried under vac-
0
uum at 80 8C overnight before use, 4,4 -oxydiphthalic anhy-
dride (ODPA), 4,4 -(hexafluoroisopropylidene) diphthalic
dianhydride (6FDA), 3,3 ,4,4 -biphenyltetracarboxylic dianhy-
dride (BPDA), 3,3 ,4,4 -benzophenonetetracarboxylic dianhy-
dride (BTDA) were commercially obtained and dried under
vacuum at 150 8C overnight prior to use.
0
2
2
0
0
phatic hydroxyl-containing aliphatic–aromatic diamine mono-
mer AAP P E. The synthetic route is shown in Scheme 1.
0
0
y
h
Synthesis of 4-(5-Nitropyridyloxy)Benzaldehyde
Under the atmosphere of nitrogen, to a 100 mL three-necked
round-bottomed flask charged with the 2-chloro-5-
nitropyridine (3.33 g, 21 mmol) and p-hydroxybenzaldehyde
(2.4 g, 20 mmol) dissolved in 40mL of DMF, was added
potassium carbonate (2.77 g, 20 mmol) at room temperature
while stirring for 30 min. The reaction was heated at 45 8C
overnight, poured into mixed solvents of ethanol and water
Measurements
IR spectra were recorded on a Perkin Elmer Fourier Trans-
form Infrared spectrometer. Nuclear magnetic resonance
(
NMR) spectra were measured on a Bruker DRX 400 spec-
1
trometer at 400 MHz for H and 100 MHz for 13C in DMSO-
d6. The melting points were measured with a microscopic
2
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SCHEME 1 The synthesis of 2-amino-1-[4-(5-aminopyridyloxy)-
phenyl]-1-ethanol (AAPyPhE). [Color figure can be viewed at
wileyonlinelibrary.com]
(
volume ratio of 1:2) to get a brown solid, which was recrys-
tallized from petroleum ether and dichloromethane (volume
1
y h
FIGURE 1 The H NMR spectrum of AAP P E.
ratio of 1:4), dried to offer pale yellow solid (4.43 g, yield
1
7
7.3%). m.p.: 114–116 8C. H NMR (DMSO-d , ppm): 9.99 (s,
6
5
1
.20–4.98 (m, 3H), 4.49–4.30 (m, 3H), 1.38 (d, J 5 19.4 Hz,
H), 1.21 (d, J 5 10.0 Hz, 1H).
1
8
H), 9.02 (d, J 5 2.5 Hz, 1H), 8.65 (dd, J 5 9.0, 2.7 Hz, 1H),
.00 (d, J 5 8.4 Hz, 2H), 7.44 (d, J 5 8.4 Hz, 2H), 7.35 (d,
J 5 9.1 Hz, 1H) (Supporting Information, Fig. S1).
13
6
C NMR (DMSO-d , ppm): 155.37 (s), 154.16 (s), 142.24 (s),
1
1
37.51 (s), 132.75 (s), 128.20 (s), 125.74 (s), 119.15 (s),
13.05 (s), 62.94 (s), 40.13 (s).
Synthesis Of b-[4-(5-Nitropyridyloxy)Phenyl]Nitroethanol
The b-nitro alcohol was usually prepared by Henry reac-
2
9
tion. 4-(5-nitropyridyloxy)benzaldehyde (5.2 g, 21.2 mmol)
was added into a 50 mL flask with 20 mL ethanol, then
nitromethane (1.7 mL, 31.8 mmol) and TEA (0.3 mL, 0.1
mol) was added. The reaction was stirred at room tempera-
ture for 12 h, then filtered, recrystallized with ethanol, dried
Polymer Synthesis
For powder preparation, the polymerizations of AAP
P E
y h
with different dianhydrides including BPDA, BTDA, ODPA,
and 6FDA were carried out by one-step approach, as illus-
trated by condensation of AAP
the atmosphere of nitrogen, to a three-necked flask charged
with a Dean-Stark separator, was added AAP E (1 g,
4.0767 mmol) in 30 mL m-cresol and then 6FDA (1.811 g,
4.0767 mmol) at room temperature under magnetic stirring
for 30 min. The reaction were then refluxed for 3 h, cooled
to room temperature, then precipitated by pouring into etha-
nol, washed by hot ethanol three times, and dried to offer
yellow powders.
P E with 6FDA below. Under
y h
to give white crystals (4.38 g, yield 67.4%). m.p.: 131–
1
1
8
6
33 8C. H NMR (DMSO-d , ppm): 9.00 (t, J 5 5.0 Hz, 1H),
P
y h
6
.69–8.50 (m, 1H), 7.68–7.41 (m, 2H), 7.36–7.06 (m, 3H),
.15 (d, J 5 4.8 Hz, 1H), 5.43–5.21 (m, 1H), 4.88 (dd,
J 5 12.4, 2.7 Hz, 1H), 4.60 (dd, J 5 12.3, 10.1 Hz, 1H) (Sup-
porting Information, Fig. S1).
Synthesis of 2-Amino-1-[4-(5-Aminopyridyloxy)Phenyl]-1-
Ethanol (AAP P E)
y
h
The reduction of asymmetric aromatic–aliphatic dinitro com-
pound into the diamine in this work adopted the method as
3
0
reported by John O. Osby. To a 100 mL three-necked
round-bottomed flask containing NiCl ꢀ6H O (1.56 g, 6.56
2
2
mmol) and sonicated KBH (1.08 g, 19.68 mmol) in ice bath,
4
was added b-[4-(5-nitropyridyloxy)phenyl]nitroethanol (2 g,
6
.56 mmol) and KBH (2.46 g, 45.92 mmol) slowly. The reac-
4
tion was maintained in an ice bath overnight, then concen-
trated, purified by column chromatography eluted by mixed
3 2 2
CH OH and CH Cl with volume ratio of 1:50 to offer
yellowish-brown solid (1.31 g, yield 81.6%). m.p.: 116–
119 8C. Elemental analysis (%): calcd.: C, 63.61; H, 6.16; N,
17.13; found: C, 63.17; H, 5.88; N, 17.26.
1
13
The H signals and C signals in NMR spectrum are shown
1
in Figures 1 and 2. H NMR (DMSO-d , ppm): 7.51 (d, J 5 2.0
6
Hz, 1H), 7.24 (d, J 5 7.9 Hz, 2H), 7.03 (dt, J 5 23.4, 11.7 Hz,
1
3
1H), 6.87 (d, J 5 7.7 Hz, 2H), 6.68 (dd, J 5 24.2, 8.2 Hz, 1H),
y h
FIGURE 2 The C NMR spectrum of AAP P E.
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y h
SCHEME 2 The synthesis of the polyimides from AAP P E (for homopolymer powder preparation: n 5 0; for copolymer film prepa-
ration: m/(m 1 n) 5 0, 0.05, 0.10, 0.20, 0.30, 0.50, 0.75, respectively).
For film preparation, polymerizations of the combined dia-
mines of AAP P E and ODA in different ratio with equimolar
BPDA, or BTDA, or ODPA and or 6FDA were conducted by
two-step polycondensation. As an illustration, ODPA
The synthesis of the polyimides was shown in Scheme 2. As
seen in Supporting Information, Figures S2 and S3, all the
y
h
2
1
polymers had the typical absorption bands in IR (cm
)
spectra as at 2924 (saturated CAH stretching), 1785 (C@O
asymmetric stretching of imide), 1727 (C@O asymmetric
stretching of imide), 1393 (CAN stretching), 1254 (CAOAC
stretching), and 719 (imide ring deformation).
(
0.3794 g, 1.223 mmol) dissolved in 2.6 g DMAc in a 25 mL
flask under nitrogen, and then equal molar of diamine mix-
tures composed of AAPyPhE and ODA were added cautiously
to avoid salt solid. The reaction was stirred for 12 h at room
temperature, then poured the viscous liquid onto a clean
glass plate, thermally treated at 80 8C/5 h, 100 8C/1 h,
RESULTS AND DISCUSSION
The Solubility and Thermal Properties of the
150 8C/1 h, 200 8C/1 h, 250 8C/1 h to enforce imidization,
and stripped from glass plate by soaking in hot water to
offer PI film. Seven copolyimide films were prepared from
ODPA, coded as OPI-x (x 5 0, 5, 10, 20, 30, 50, 75) according
to different molar content of AAP P E in the diamine mix-
Homopolymer Powders
The solubility, thermal properties, and GPC data of the
homopolymer powders are summarized in Table 1. All the
powders are confirmed as amorphous structure (see WAXD
diagrams in Supporting Information, Fig. S4), and most of
them are soluble in typical polar solvents such as NMP,
DMAc, DMF, THF, cresol, and chlorinated solvent (Chloro-
form). The general good solubility of the PIs might originate
from asymmetrical aromatic–aliphatic backbone structure,
which disrupted the polymer chain packing and inhibited the
y
h
tures which is 0%, 5%, 10%, 20%, 30%, 50%, and 75%,
respectively. Excepted for OPI-7 which presented as brittle
film, all OPI-x were flexible films with good tensile strength.
For comparison, two copolymer films for each case of BPDA,
BTDA, and 6FDA were prepared, coded as PPI-x (x 5 0, 20),
TPI-x (x 5 0, 20), and FPI-x (x 5 0, 20) according to the
molar content of AAP P E in the diamine mixtures which is
3
1
interchain CTC. 6FDA-AAP
P E and BPDA-AAP P E show
y h y h
y
h
0% and 20%, respectively.
good solubility in all tested solvents at room temperature,
TABLE 1 The Solubility, Thermal Properties, and GPC Data of PI Powders
a
Solubility
Thermal properties
GPC
b
T
d5
T
d10
Char
T
g
M
n
M
w
PIs
NMP DMAc DMF DMSO m-Cresol THF CHCl
3
(8C) (8C)
Yield (%) (8C)
(KDa) (KDa) PDI
ODPA-AAP
y
P
h
E
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1h
1
1h
1
371 414
346 394
335 387
340 393
51.0
49.6
51.2
53.3
221
126
145
125
11.6
6.82
5.83
4.59
15.4
8.69
8.67
6.69
1.33
1.27
1.49
1.46
y h
6FDA-AAP P E
BPDA-AAP
BTDA-AAP
y
P
P
h
h
E
E
1
1
y
6
6
a
b
1
, soluble; 1h, soluble on heating; 6, partially soluble on heating; 2,
Determined by DSC.
insoluble on heating.
4
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FIGURE 3 The TGA curves of the PI powders. [Color figure can
be viewed at wileyonlinelibrary.com]
FIGURE 5 The X-ray diffraction diagrams of OPI-x films. [Color
figure can be viewed at wileyonlinelibrary.com]
which could be due to the fluorinated bulky bistrifluoro-
methylene linkage between two phenyls in 6FDA and the
seemingly rigid yet twisted biphenyl structure in BPDA.
in Figures 3 and 4. The results are summarized in Table 1,
with Td5 ranging from 335 8C to 371 8C, Td10 from 387 8C to
3
However, it is unexpected that the flexible sp -oxygen-linked
414.5 8C, char yield from 49.6% to 53.3%. It is found that
ODPA rather than the planar rigid BTDA shows the poor sol-
ubility when condensed with AAP P E. That may be attrib-
AAP P E-ODPA has the best thermal properties and its T by
y
h
g
y
h
far exceeds those of the other three PIs, which, similarly to the
case of solubility, could be explained by the higher molecular
weight polymer formed by AAP P E condensed with ODPA
uted to the smaller molecular weight of BTDA-AAP P E than
y
h
ODPA-AAP P E. Owing to the high basicity of aliphatic amino
y
h
y
h
in AAP P E, nylon salt was formed during polymerization for
y
h
than the other dianhydrides. The above comparisons between
polyimides from different aromatic dianhydrides reveal the
sufficient condensation reactivity of AAP P E toward ODPA
y h
both dianhydrides, while ODPA offers greater flexibility in
that polymerization would proceed much further to give
higher molecular weight polymer, other dianhydrides includ-
ing 6FDA, BPDA, and BTDA lead to rigid backbone that
would trap the nylon salt and prevent high polymerization
degrees. As indicated by GPC data in Table 1, ODPA-AAP P E
when even adopting traditional imidization approach. There-
fore, ODPA was adopted as the desired dianhydride toward
AAP P E in the following parts in this work.
y
h
y
h
presents larger molecular weight than 6FDA-AAP P E, BPDA-
The Solubility, WAXD, and Optical Properties of the
Copolymer Films
y
h
AAP P E, and BTDA-AAP P E do.
y
h
y h
Figures 5 and 6 show the wide-angle X-ray diffraction
The thermal properties of PI powders evaluated by TGA and
DSC both at heating rate of 10 8C/min under N are shown
(
WAXD) spectra of all PI films of OPI-x, TPI-x, BPA-x, and
2
FIGURE 4 The DSC curves of the PI powders. [Color figure can
FIGURE 6 The X-ray diffraction diagrams of FPI-x, PPI-x, and TPI-
be viewed at wileyonlinelibrary.com]
x films. [Color figure can be viewed at wileyonlinelibrary.com]
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TABLE 2 The Viscosity, Solubility, WAXD, and Optical Properties of the PI Films
b
Solubility
WAXD
Optical properties
a
c
d
˚
PIs
g
inh (dL/g) DMSO THF CHCl
3
m-Cresol DMAc NMP DMF 2h (8)
d-Spacing (A)
T500 (%)
0
k (nm)
OPI-0
OPI-5
1.17
1.06
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1h
1h
1h
1h
1
18.9
18.4
18
4.7
4.8
4.9
5.0
5.1
5.1
5.4
5.6
4.3
4.7
5.0
4.9
72.1
83.3
75.6
76.2
80.1
75.9
80.3
74.9
73.7
61.7
63.5
55.6
378
365
380
369
374
381
382
370
404
445
439
435
2
2
1
OPI-10 0.95
OPI-20 0.92
OPI-30 0.97
OPI-50 0.89
2
2
1
1h
1h
1h
6
2
1
17.8
17.5
17.3
16.5
15.8
20.5
19
2
1
1h
12
1h
2
1
1
FPI-0
1.14
0.86
1.23
0.85
1.15
0.79
1
6
FPI-20
PPI-0
1h
6
1
1h
2
1h
1
PPI-20
TPI-0
1h
2
1h
2
1h
2
2
17.7
17.8
TPI-20
1
1h
1h
1h
a
c
The inherent viscosity of PAA.
Transmittance at 500 nm.
UV–vis cutoff wavelength.
b
d
1
, soluble at room temperature; 1h, soluble after heating; 6, partially
soluble on heating; 2, insoluble.
FPI-x series, and all the films present an amorphous state.
The average interchain spacing distance (d-spacing) values
were calculated from scattering angles (2h) based on Bragg’s
equation nk 5 2dsinh (n 5 1, k 5 1.54 Å), as summarized in
Table 2. It is shown that except for the films of TPI-x series,
the scattering angles of the films generally decrease with the
increasing molar content of AAP P E for all series, and the
d-spacing values present gradual increase which contributes
to the enhanced solubility due to the more ready solvation
at enlarged polymer interchain distance. The enhancement in
solubility could be accounted for by the increased flexibility
of polymer backbone due to the presence of the aliphatic
The Surface Properties and Water Absorption of the
Copolymer Films
Water contact angle (h) measurements were carried out for
OPI-x and FPI-x films, as shown in Supporting Information,
1
/2
Figure S6. According to eq 1, 1 cosh 5 2 (c
[2b(c /c )2], where both c and b are constant at room tem-
perature, c could be calculated from the determined h
and the results together with the water absorp-
tions are summarized in Table 3. It is shown that the values
of water contact angle (h) and surface tension (c ) of the
films display a fluctuation with the variation of molar con-
tent of AAP E. For OPI-x series, with the molar content of
AAP E increasing from 0 (OPI-0) to 10 (OPI-10), the h
decreases from 77.98 to 52.98, and c undergoes a corre-
/c
)
exp
s
1
1
s
1
y
h
s
3
2,33
value,
s
P
y h
ethylene structure in AAP
y
P
h
E that inhibits chain packing
y h
P
and interchain charge transfer as well.
s
sponding increase from 36.7 to 54.1 mN/m, indicating that
As seen in Table 2 and Figure 7, k0 values of OPI-x range
from 365 to 381 nm and OPI-5 exhibits the best optical
transparency with 83.3% of T500 and 365 nm of k , which
0
exceed those of OPI-0 (72.1% and 378 nm), indicating that
the addition of AAP P E helps to improve the optical proper-
y
h
ties of the polymers. The enhancement in optical properties
could be attributed to the effects of the aliphatic segments
and the asymmetric alternating pyridine and phenyl struc-
ture of AAP P E moieties on breaking the conjugation and
y
h
weakening the CTC. The variation of AAP P E loading
y
h
amount shows fluctuated effects on the optical properties
properly due to the film thickness difference and the differ-
ent presence of the aliphatic hydroxyls pendent to the poly-
mer chains which will be investigated in details in the
following parts of this paper. The introduction of 20% molar
content of AAP P E show different effects on the optical
y
h
properties of FPI-x, PPI-x, and TPI-x (Supporting Information,
Fig. S5), and that may be because of the different structural
characteristic (such as fluorinated or rigid structure) of the
dianhydrides and different polymerization degree as well.
FIGURE 7 The UV–vis spectra of the OPI-x films. [Color figure
can be viewed at wileyonlinelibrary.com]
6
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TABLE 3 The Surface Water Contact Properties and Water Absorption of the PI Films
PIs
Entries
OPI-0
OPI-5
OPI-10
OPI-20
OPI-30
OPI-50
FPI-0
FPI-20
a
h (8)
77.9
67.4
52.9
73.9
57.1
60.7
87.4
65.3
b
c
s
(mN/m)
36.7
42.4
54.1
38.7
50.1
47.1
32.7
43.6
W
0
0.0363
0.0367
1.14
0.0370
0.0375
1.23
0.0346
0.0352
1.65
0.0343
0.0347
1.16
0.0355
0.0361
1.56
0.0340
0.0345
1.47
0.0371
0.0374
0.92
0.0353
0.0358
1.33
W
c
WA
a
c
Equilibrium contact angle was measured at ambient temperature and
double distilled water as solvent for a time period of 120 s depending
on the stability of the drop, and the data were the average value of 10
experiments.
0 0
WA(water absorption) (%) 5 (W 2 W )/W 3 100%; where W is the
weight of the polymer sample after immersed in water for 96 h at room
temperature and W is the weight of the polymer sample after being
dried in vacuum at 100 8C for 8 h.
0
b
Surface energy, obtained by Owen equation and contact angle.
the introduction of hydroxyl groups by adding AAP P E
strengths at break of 89.35–170.73 MPa, elongation at break
of 8.33–10.82%, and tensile modulus of 1.35–2.28 GPa. As
compared with OPI-0, the addition of AAP P E with any
y h
y
h
effectively increases the hrydrophilicity of the PI films. How-
ever, with the molar content of AAP P E continuously
y
h
increasing to 20% (OPI-20), the film surface properties
reverse sharply to a remarkable increase in hydrophobicity
molar content generally leads to an improvement in mechan-
ical properties of OPI-x in terms of tensile strength, modulus,
and elongation at break, inferring that asymmetric aromatic–
with a maximum h value (73.98) and a minimum c value
s
(
38.7 mN/m). This reversal at OPI-4 is presumed to result
aliphatic segments in AAP
mechanical properties of the films. The improvement in
mechanical properties by adding AAP E to flexible dianhy-
dride ODPA-ODA PI film is believed to be associated with the
interchain cross-linking and the presence of pyridine ring as
well, which combiningly increase the polymer backbone
rigidity and toughen the films. Similar to the effects of the
surface properties as discussed above, the variation of molar
y h
P E helps to strengthen the
from the synergistic effects of the increased presence of ali-
phatic ethylene structure and the interchain cross-linking via
condensation of the hydroxyls, which largely eliminated the
wetting effect of the hydroxyls and turning the film surface
to hydrophobic. However, as the molar content of AAP P E
continues to increase, the presence of free hydroxyl groups
would increase because the cross-linked structure acted as a
rigid matrix and hindered the condensation for pendent
hydroxyl groups at a remote distance. So OPI-30 was more
hydrophilic than OPI-20 as indicated by smaller h value
P
y h
y
h
content of AAP P E shows the fluctuated effects on the
y h
mechanical properties of OPI-x series, with the maximum
value of mechanical data appearing at OPI-10. Similarly, the
introduction of AAP P E with molar content of 20% was
(
57.18 vs 73.98) and larger c value (50.1 vs 38.7 mN/m).
y h
s
found to increase the tensile properties of 6FDA-based FPI-
0 film. Whereas, the incorporation of AAP P E with 20%
When the molar content of AAP P E further increases, the
y
h
2
increasing number of exposed hydroxyl groups would be
capable of forming interchain hydrogen bond that turns the
polymer OPI-50 into secondary weak cross-linking structure
and leads OPI-50 to be more hydrophobic than OPI-30, as
y h
molar content to rigid dianhydride BPDA- and BTDA-based
PI films leads to a decrease in tensile properties. Because
ODPA and 6FDA are dianhydrides of flexible structure, yet
BPDA and BTDA are rigid, we tend to believe that the cross-
linking structure caused by hydroxyls condensation in AAPy-
PhE was moderate and so helpful to increase the mechanical
properties for OPI and FPI series, but may aggravate the
rigidity of polymer backbones for PPI and TPI series into
fragile structures, thus reducing their tensile properties.
s
evidenced by the comparison of h and c value.
Owing to the presence of two trifluoromethyl groups, 6FDA
could be used to increase the hydrophobicity nature of the
traditionally less hydrophobic polyimide composed of
repeated polar and hydrophilic imide rings. As examined in
this work, FPI-0 derived from 6FDA and ODA shows superior
hydrophobicity to ODPA equivalent OPI-0. Similar to OPI-x
series, the incorporation of the aliphatic hydroxyl-containing
The DMA tan d curves for OPI-x series is shown in Figure 8,
and the DMA tan d curves for the other series are shown in
Supporting Information, Figure S8 and the TGA curves for all
the films shown in Supporting Information, Figure S9. As
summarized in Table 4, the increasing addition of AAP P E
y h
20% molar content of AAP P E leads to an increase of
hydrophilicity of 6FDA-based PI films’ surface.
y
h
Mechanical and Thermal Properties and Cross-Linking
Degree of the Copolymer Films
The mechanical properties of all PI films were measured,
and the tensile stress–strain curves are provided in Support-
ing Information, Figure S7. As summarized in Table 4, except
for the fragile OPI-75, ODPA-based OPI-x series show tensile
gradually decreases the glass transition temperatures (Tg)
and thermal decomposition temperatures (Td5 and Td10) of
the films for all tested series, which is a general rule for
aliphatic-containing polyimide and may be good for improv-
ing the processing performances of the intended intractable
PIs. The addition of AAP P E generally increases the DMA
y
h
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TABLE 4 The Thermal and Mechanical Properties of the PI Films
a
Thermal properties
Mechanical properties
b
PIs
T
g
(8C)
T
d5 (8C)
Td10 (8C)
Char yield (%)
Tan d (DMA)
TS (MPa)
EB (%)
TM (GPa)
OPI-0
OPI-5
OPI-10
OPI-20
OPI-30
OPI-50
FPI-0
253
248
246
239
233
219
295
267
267
252
283
282
478
498
462
438
407
376
503
394
498
414
473
425
534
546
538
524
506
451
537
509
554
532
544
526
54.2
56.5
55.2
54.6
56.6
52.8
53.7
52.2
59
1.15
1.35
1.12
1.21
1.28
1.15
1.43
1.12
0.60
0.23
1.09
0.36
89.3
9.26
8.33
10.05
9.47
10.82
8.43
7.93
8.97
11.93
9.06
11.8
7.94
1.35
1.70
2.28
1.35
1.29
1.49
0.97
1.30
1.05
0.66
1.49
1.07
109.2
170.7
108.2
106.2
91.1
68.3
FPI-20
PPI-0
105.9
80.1
PPI-20
TPI-0
57.9
56.6
57.1
30.5
116.0
74.7
TPI-20
a
b
21
TS, tensile strength; EB, elongation at break; TM, tensile modulus.
2
Tested by DMA heated at 5 8C min in N .
tan d value of ODA-ODPA films as compared by the cases of
OPI-x (x 5 5–50) to OPI-0. OPI-5 has the biggest tan d value
of 1.35 and this may be attributed to the introduction of
flexible aliphatic ethylene segments. However, the tan d val-
ues of PIs from OPI-10 to OPI-50 present a complex fluctua-
tion. Initially, the intermolecular cross-linking structure by
condensation of hydroxyl groups leads to a sharp decrease
of tan d of OPI-10. When the molar content of AAP P E fur-
Cross-linking correlates with the viscoelastic properties of
3
4
the polymers. Lu and Larock calculated the cross-linking
0
0
density based on the equation E 5 3v RT, where E is the
e
storage modulus at T 1 40 8C, R is the gas constant, T is the
g
absolute temperature at T 1 40 8C, and v is the cross-
g
e
3
5
linking density. Meanwhile, they introduced the soluble
fraction (SF) in DMF to estimate the cross-linking degree of
the soybean oil-based polymers containing different content
of hydroxyls which react with functional cyanate groups to
build cross-linking. According to their methods, we obtained
the ve and SF values for the OPI-x samples from OPI-0 to
OPI-50 based on the DMA storage modulus in Figure 9 and
the SF detection method described in Measurements, and the
results are summarized in Table 5.
y
h
ther increases, the condensation of hydroxyls becomes diffi-
cult and the increasing presence of flexible aliphatic ethylene
segments results in the increase of tan d for OPI-20 and OPI-
30. However, when the presence of free hydroxyl groups
continues to increase with the increasing addition of AAPy-
PhE, the interchain hydrogen bond formation becomes avail-
able and that makes the polymer rigid again, as indicated by
the decrease of the tan d value of OPI-50.
Seen in Table 5, the change of SF values are inversely pro-
portional to that of the cross-linking density ve for OPI-x
series. It shows that OPI-0 has smaller v than the rest of
e
FIGURE 8 The DMA curves of the OPI-x films. [Color figure can
FIGURE 9 The storage modulus of the OPI-x films. [Color fig-
be viewed at wileyonlinelibrary.com]
ure can be viewed at wileyonlinelibrary.com]
8
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TABLE 5 The v
e
and SF Values of OPI-x Films
0
3
Code
E (Pa)
v
e
(mol/m )
SF (%)
OPI-0
565578
40.1
31.69
26.02
16.52
18.65
34.25
28.37
OPI-5
1678727.3
2073718
1735257.6
1484370.9
718846.9
119.9
148.7
126.1
109.1
54.2
OPI-10
OPI-20
OPI-30
OPI-50
OPI-x films, indicating the introduction of AAP P E leads to
y
h
cross-linking structure. It is also found that the cross-linking
density changes in a fluctuated manner with the increasing
loading of AAP P E for OPI-x films.
y h
Dielectric Properties of the Copolymer Films
0
FIGURE 11 The dielectric constant (E ) versus frequency for OPI-
To elucidate the influence of AAP P E to the dielectric prop-
y
h
x films. [Color figure can be viewed at wileyonlinelibrary.com]
0
erties of PIs, the dielectric constant (E ) and dielectric loss
0
0
(
E ) of OPI-x series with different content of AAP P E were
y
h
studied as the function of frequency and temperature.
becomes more obvious at higher frequency, as shown by Fig-
ure 11.
0
Dielectric Constant (E )
Figure 10 shows the dielectric constant (E ) versus frequency
0
0
As shown in Table 6, OPI-5 has the lower E than OPI-0, as
and temperature three-dimensional diagram of OPI-20. It can
the incorporation of aliphatic structure reduces the electron
0
be seen that E generally increases with increasing tempera-
polarization and inhibits the CTC. However, as the molar con-
ture and there exist some relaxations that are similar to the
0
tent of AAP P E increase, the E value of OPI-10 presented
y
h
3
6
0
dielectric loss later presented in this article. E variation
with frequency is determined by the ability of the dipoles in
polymer to move fast enough for their orientation to keep
pace with the increased oscillations in an alternating electric
an increase, which may be due to the increasing presence of
polar hydroxyl groups that has offset the dielectric constant-
reducing effect of the alicyclic ethylene segments. With the
molar content of AAP P E continues to increase to 20%
y
h
3
7–39
0
field.
With the frequency increasing, the orientational
(OPI-20), the E reverses to decrease. It is understood that
during the thermal treatment of the film preparation process,
the increasing number of hydroxyls would condense to cre-
ate a cross-linked structure of the polymer. This reversal at
OPI-20 could be the synergistic effect of the aliphatic ethyl-
ene structure and the cross-linking structure. When the
molar content of AAP P E increase to 30%, the rigid cross-
polarization of the dipoles would gradually lose pace with
the alternation of the applied electric field, and so the E
value of the PI films would decrease, and this tendency
0
y
h
linking polymer chains would restrict the free motion of the
hydroxyls and so make further condensation difficult, leading
to some exposed hydroxyls. Hence, the increased presence of
0
polar hydroxyls contributes to an increase in the E of OPI-
TABLE 6 Dielectric Constant of OPI-x Films with Varied Fre-
quency at 25 8C
0
Dielectric constant (E )
PIs
1 Hz
1 kHz
1 MHz
10 MHz
OPI-0
4.61
4.02
5.15
4.76
5.55
5.13
4.48
3.76
5.02
4.67
5.39
4.64
4.32
3.62
4.84
4.52
5.11
4.33
4.24
3.57
4.74
4.45
4.96
4.20
OPI-5
OPI-10
OPI-20
OPI-30
OPI-50
0
FIGURE 10 The dielectric constant (E ) versus frequency and
temperature for OPI-20. [Color figure can be viewed at wileyon-
linelibrary.com]
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3
0. Nevertheless, when the molar content of AAP P E contin-
y h
ues to increase, the increasing number of exposed hydroxyls
may form hydrogen bond and lead to secondary weak cross-
0
linking structure, leading to a decrease of the E for OPI-50.
0
0
Dielectric Loss (E )
0
0
The dielectric loss (E ) is one of the important indexes to
evaluate the quality of the insulating material. It is decided
by the energy loss caused by the molecular dipoles orienta-
tional polarization and the intrinsic friction of electron trans-
3
7
00
0
fer. The frequency and temperature dependences of the E
for OPI-20 is shown in Figure 12. Basically similar to E
change tendency, the E increased with increasing tempera-
ture, and three relaxation processes designated as c, b, and
0
0
0
0
a, respectively, are observed for the E variation with
increasing temperature. We have reported these relaxations
3
0
00
in our previous work. Generally, the E decreases with the
increasing frequency over the frequency range tested, but, as
shown in Supporting Information, Figure S10, there
appeared some unreported relaxations on which we will con-
duct further investigation in future.
0
0
FIGURE 12 The dielectric loss (E ) versus frequency and tem-
perature for OPI-20. [Color figure can be viewed at wileyonline-
library.com]
FIGURE 13 The proposed copolymer interchain structures with the variation of the loading of aliphatic hydroxyl-containing AAPy-
E. [Color figure can be viewed at wileyonlinelibrary.com]
P
h
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CONCLUSIONS
8 H. C. Yu, S. V. Kumar, J. H. Lee, S. Y. Oh, C. M. Chung, Mac-
romol. Res. 2015, 23, 566.
In this work, a novel asymmetric hydroxyl-containing ali-
phatic–aromatic diamine AAP P E was facilely synthesized by
9 D. J. Liaw, C. C. Huang, W. H. Chen, Macromol. Chem. Phys.
006, 207, 434.
y
h
2
way of Henry reaction in three steps. Preliminary screening on
different aromatic dianhydrides reveals the desirable conden-
1
0 A. S. Mathews, L. L. Kim, C. S. Ha, J. Polym. Sci. Part A:
Polym. Chem. 2006, 44, 5254.
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y
h
11 K. Goto, T. Akiike, Y. Inoue, M. Matsubara, Macromol.
Symp. 2003, 199, 321.
parative investigations show that the incorporation of
AAP P E improves the solubility and optical transparence by
y
h
12 X. D. Ji, Z. K. Wang, J. L. Yan, Z. Wang, Polymer 2015, 74,
inhibiting CTC and reducing the chain packing due to the
asymmetric structure and aliphatic ethylene segments, and
meanwhile retains the inherent thermal and mechanical prop-
erties of the polymers. The molar content of the incorporated
AAP P E was found to have fluctuated influences on the film
38.
13 Y. Guan, D. M. Wang, G. L. Song, G. D. Dang, C. H. Chen, H.
W. Zhou, X. G. Zhao, Polymer 2014, 55, 3634.
1
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8 J. A. Kreuz, B. S. Hsiao, C. A. Renner, D. L. Goff, Macromo-
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9 T. Namikoshi, K. Odahara, A. Wakino, M. Murat, S.
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0 K. Itoya, Y. Kumagai, M. Kakimoto, Y. Imai, Macromolecules
y
h
1
s
surface water contact properties such as h and c , and the simi-
lar effects of the variation of molar content of AAP P E were
observed on the DMA tan d and dielectric constant E .
y
0
h
1
1
We proposed that with the increase of molar content of
AAP P E, the increasing hydroxyl groups would lead the
y
h
1
polymer chain structure to four different stages as outlined
in Figure 13. It is postulated that when the molar content of
1
AAP P E reaches certain value, the pendent alicyclic hydroxyl
y
h
would condense to form interchain cross-linked network. On
increasing addition of AAP P E, the cross-linked polymer
2
y
h
1994, 27, 4101.
would bear some exposed hydroxyl groups because the rigid
polymer chain restricted the free motion of the pendent
hydroxyls. Further increase of AAP P E leads to increasing
21 T. Ogura, M. Ueda, Macromolecules 2007, 40, 3527.
2
2 C. Koninga, A. Delmottea, P. Larnoa, B. V. Mele, Polymer
1998, 39, 3697.
y
h
presence of pendent hydroxyls that would tend to form sec-
ondary interchain weak cross-linking by hydrogen bond.
23 A. E. Eichstadt, T. C. Ward, M. D. Bagwell, I. V. Farr, D. Dunson,
J. E. Mcgrath, J. Polym. Sci. Part B: Polym. Phys. 2002, 40, 1503.
2
4 Y. Watanabe, Y. Sakai, Y. Shibasaki, S. Ando, M. Ueda, Mac-
romolecules 2002, 35, 2277.
5 Y. Guan, C. B. Wang, D. M. Wang, G. D. Dang, C. H. Chen,
H. W. Zhou, X. G. Zhao, Polymer 2015, 62, 1.
6 C. B. Wang, X. G. Zhao, D. B. Tian, D. M. Wang, C. H. Chen,
H. W. Zhou, Des. Monomers Polym. 2017, 20, 97.
7 B. Jarza˛bek, E. Schab-Balcerzak, T. Chamenko, D. S eR ka, J.
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ACKNOWLEDGMENTS
2
The financial supports from the NSFC (Grant Nos. 51263014 and
2
21271099) and Jiangxi Provincial Education Department (Grant
No. GJJ13113) are greatly appreciated. The authors are thankful
to Prof Haoqing Hou from Jiangxi Normal University, China for
kindly providing DMA measurements of all these samples.
2
2
8 Y. T. Chern, H. C. Shiue, Macromolecules 1997, 30, 4646.
2
9 Noboru Ono, The Nitro Group in Organic Synthesis; Wiley-
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