Partially bio-based poly(amide imide)s by polycondensation of aromatic diacylhydrazides based on lignin-derived phenolic acids and aromatic dianhydrides: Synthesis, characterization, and computational studies

Article abstract of DOI:10.1002/pola.28748

Two new bio-based diacylhydrazide monomers, namely, 4,4′-(propane-1,3-diylbis(oxy))bis(3-methoxybenzohydrazide) and 4,4′-(propane-1,3-diylbis(oxy))bis(3,5-dimethoxybenzohydrazide) were synthesized starting from lignin-derived phenolic acids, namely, vanillic acid and syringic acid. A series of poly(amide imide)s was synthesized by polycondensation of these diacylhydrazide monomers with commercially available aromatic dianhydrides. Poly(amide imide)s showed inherent viscosity in the range 0.44–0.56 dL?g^?1 and exhibited good solubility in organic solvents. Poly(amide imide)s could be cast into transparent, flexible, and tough films from their N,N-dimethylacetamide solutions. Poly(amide imide)s showed 10% weight loss in the temperature range 340–364?°C indicating their good thermal stability. Glass transition temperature (T_g) of poly(amide imides)s were measured by DSC and DMA which were in the range 201–223?°C and 214–248?°C, respectively. The T_g values of poly(amide imide)s were dependent on the number methoxy substituents on aromatic rings of diacylhydrazide monomers. Molecular dynamics simulation studies revealed that chain rigidity is the dominant factor for observed trends in T_g.

Full text of DOI:10.1002/pola.28748

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POLYMER SCIENCE
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ARTICLE
Partially Bio-based Poly(amide imide)s by Polycondensation of Aromatic
Diacylhydrazides Based on Lignin-Derived Phenolic Acids and Aromatic
Dianhydrides: Synthesis, Characterization, and Computational Studies
1
,2
2,3
3
1,2
Sachin S. Kuhire, Pragati Sharma, Suman Chakrabarty, Prakash P. Wadgaonkar
1
Polymers and Advanced Materials Laboratory, Polymer Science and Engineering Division, CSIR-National Chemical Laboratory,
Pune 411008, India
2
Academy of Scientific and Innovative Research, New Delhi 110025, India
3
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, India
Correspondence to: P. P. Wadgaonkar (E-mail: pp.wadgaonkar@ncl.res.in) or S. Chakrabarty (E-mail: s.chakrabarty@ncl.res.in)
Received 14 June 2017; accepted 28 July 2017; published online 00 Month 2017
DOI: 10.1002/pola.28748
ABSTRACT: Two new bio-based diacylhydrazide monomers,
g
their good thermal stability. Glass transition temperature (T )
0
namely, 4,4 -(propane-1,3-diylbis(oxy))bis(3-methoxybenzohy-
of poly(amide imides)s were measured by DSC and DMA
0
drazide) and 4,4 -(propane-1,3-diylbis(oxy))bis(3,5-dimethoxy-
which were in the range 201–223 8C and 214–248 8C, respec-
benzohydrazide) were synthesized starting from lignin-derived
phenolic acids, namely, vanillic acid and syringic acid. A series
of poly(amide imide)s was synthesized by polycondensation of
these diacylhydrazide monomers with commercially available
aromatic dianhydrides. Poly(amide imide)s showed inherent
g
tively. The T values of poly(amide imide)s were dependent on
the number methoxy substituents on aromatic rings of diacyl-
hydrazide monomers. Molecular dynamics simulation studies
revealed that chain rigidity is the dominant factor for observed
trends in T
g
. V
2017 Wiley Periodicals, Inc. J. Polym. Sci., Part
C
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viscosity in the range 0.44–0.56 dL g and exhibited good sol-
ubility in organic solvents. Poly(amide imide)s could be cast
into transparent, flexible, and tough films from their N,N-dime-
thylacetamide solutions. Poly(amide imide)s showed 10%
weight loss in the temperature range 340–364 8C indicating
KEYWORDS: computational studies; lignin; renewable resources;
step-growth polymers; structure-property relations
INTRODUCTION In the recent years, bio-based polymers have
by-product of paper and pulp industries in the world. Depo-
lymerization of lignin yields various phenolic derivatives
such as vanillin, syringaldehyde, vanillic acid, syringic acid,
attracted a great deal of attention due to the environmental
1
–4
issues and depletion of non-renewable resources.
A large
2
4
syringol, guaiacol, ferulic acid, eugenol, and so forth.
number of bio-based polymers are already being manufac-
5
,6
tured in industry. Most of the bio-based polymers are ali-
phatic or cyclo-aliphatic in nature and are derived from
The presence of functional groups in these aromatic chemicals
offers interesting possibilities to use them as precursors for
synthesis of useful monomers. Thus, a range bio-based mono-
mers and polymers have been synthesized from lignin-derived
7
,8
vegetable oils, cellulose, or starch. However, aliphatic and
cyclo-aliphatic polymers have limited applications due to
their poor mechanical properties, low thermal stability, and
1
4,25
26
9
–11
aromatics,
which include (meth)acrylic polymers, epoxy
low glass transition temperatures (T s).
On the other
g
2
7
28
29
polymers, polybenzoxazines, cyanate ester resins, poly-
hand, polymers containing aromatic units exhibit higher
3
0
31–34
14,35
thermal stability, higher T , and better mechanical proper-
carbonates, polyesters,
poly(ether ester)s,
and so forth.
polyace-
g
1
2,13
36
37,38
ties.
However, relatively a limited number of examples
tals, polyurethanes,
are available wherein aromatic polymers have been synthe-
1
4–16
Aromatic poly(amide imide)s represent an important class of
high performance polymers due to their excellent thermal
sized from bio-derived chemicals such as lignin,
furan
1
7
18,19
20
21
derivatives, pentadecylphenol,
terpene, eugenol, 4-
1
2 39–41
2
2
and mechanical properties. ,
The excellent properties
aminophenylalanine, and so forth. Of these, lignin is the
2
3
of poly(amide imide)s are derived from their distinctive
chemical structure, which combine both amide and imide
most abundant source of bio-based aromatic chemicals.
Around 70 million tonnes of lignin is produced annually as a
Additional Supporting Information may be found in the online version of this article.
2017 Wiley Periodicals, Inc.
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functional groups in the polymer backbone. Therefore, poly(-
amide imide)s find applications in several fields such as gas
separation, nanofiltration, osmotic power generation, high
temperature adhesives, electronic wire enamels, and so
Inherent viscosity of poly(amide imide)s was measured with
0.5% (w/v) solution of polymer in DMAc using Ubbelhode
suspended level viscometer at 30 6 0.1 8C. Molecular weights
and dispersity values of poly(amide imide)s were deter-
mined on Thermo-Finnigan make gel-permeation chromatog-
raphy (GPC) using N,N-dimethylformamide (DMF) as an
4
2,43
forth.
A spectacular progress has been made in the field
of poly(amide imide)s in terms of chemical structures, mono-
mers, methods/techniques of synthesis, and the application
2
1
eluent at a flow rate of 1 mL min at 25 8C. Sample concen-
21
4
2,44–48
tration was 2 mg mL and narrow dispersity polystyrenes
were used as calibration standards. X-ray diffraction patterns
of poly(amide imide)s were recorded using dried polymer
film on a Rigaku Dmax 2500 X-ray diffractometer at a tilting
areas.
With an objective to use lignin-derived phenolic acids as pre-
cursors for monomers useful for synthesis of high perfor-
mance/temperature polymers, we wish to report herein the
synthesis and characterization of new bio-based diacylhydra-
2
1
rate of 28 min . TGA was performed on Perkin Elmer: STA
2
1
6
000 instrument at heating rate of 10 8C min under nitro-
0
zide monomers, namely, 4,4 -(propane-1,3-diylbis(oxy))bis(3-
gen atmosphere. T was determined on TA instruments DSC
g
0
21
methoxybenzohydrazide) (DVHzC-3) and 4,4 -(propane1,3-
Q-10 with heating rate of 10 8C min under nitrogen atmo-
sphere. Tg of polymer was recorded on second heating
cycles. DMA of poly(amide imide)s as a function of tempera-
ture was determined on films cast from DMAc solutions. The
sample dimensions were 10 3 6 mm (length 3 width) and
thickness was in the range 0.09–0.13 mm. The measure-
ments were carried out on Rheometrics Scientifics (Model
diylbis(oxy))bis(3,5dimethoxybenzohydrazide)
(DSHzC-3).
The key precursors used for the synthesis of monomers
were vanillic acid, syringic acid, and 1,3-dibromopropane. A
series of partially bio-based poly(amide imide)s was synthe-
sized by polycondensation of diacylhydrazides with commer-
cially available aromatic dianhydrides, namely, pyromellitic
0
0
dianhydride (PMDA), 3,3 ,4,4 -biphenyltetracarboxylic dianhy-
Mark IV, UK), using the dynamic mode between 50 and
0
0
dride (BPDA), and 3,3 ,4,4 -oxydiphathalic anhydride (ODPA)
via thermal cyclo-imidization process. Poly(amide imide)s
were characterized by inherent viscosity measurements, sol-
ubility tests, FT-IR, H NMR, and C NMR spectroscopy, ele-
mental analysis, X-ray diffraction, thermogravimetric analysis
21
300 8C, at frequency of 1 Hz and heating rate of 5 8C min
.
Preparations
1
13
General Procedure for Synthesis of a, x-Diester Containing
Oxypropylene Linkage
(
TGA), differential scanning calorimetric (DSC) studies, and
Into a 250-mL two-necked round bottomed flask equipped
with a reflux condenser and an argon inlet were charged,
methyl vanillate or methyl syringate (40 mmol), potassium
carbonate (22.1 g, 160 mmol) and N,N-dimethylformamide
(100 mL). The reaction mixture was heated at 100 8C for 1 h
and then 1, 3-dibromopropane (20 mmol) was added and
heating was continued for 12 h. The reaction mixture was
poured into ice cold water (250 mL). The precipitate was fil-
tered and dissolved in dichloromethane (100 mL). The
dichloromethane solution was washed with water (2 3
dynamic mechanical analysis (DMA). The effect of number of
methoxy substituents on aromatic rings on T of poly(amide
imide)s was investigated. Furthermore, to understand the
molecular origin of the substituent effects on T , molecular
g
g
dynamics (MD) simulation studies were performed.
EXPERIMENTAL
Materials
Vanillic acid (97%), syringic acid (95%), and 1,3-dibromo-
propane (99%) were received from Sigma Aldrich, St. Louis,
USA. Methyl vanillate and methyl syringate were prepared as
reported in our previous article. PMDA, BPDA, and ODPA
were received from Sigma Aldrich, St. Louis, USA and sub-
limed under reduced pressure before use. Hydrazine hydrate
100 mL), dried over anhydrous sodium sulphate, filtered,
and concentrated under reduced pressure. The crude prod-
uct was purified by column chromatography using pet
ether:ethyl acetate as an eluent to afford white solid. The
product was recrystallized from ethanol:water mixture.
3
7
(
80%), ethanol, methanol, N,N-dimethylacetamide (DMAc),
0
Dimethyl 4,4 -(propane-1,3-diylbis(oxy))bis(3-
and other solvents were received from Thomas Baker Ltd.
Mumbai, India, and were purified as per literature
procedures.
methoxybenzoate)
Yield: 80%; mp 158 8C; H NMR (200 MHz, CDCl , d/ppm):
.35–2.48 (m, 2H), 3.90 (s, 12H), 4.30 (t, 2H), 6.94 (d, 2H),
7.54 (d, 2H), 7.64 (dd, 2H). C NMR (50 MHz, CDCl , d/
1
4
9
3
2
1
3
Measurements
3
Melting points were recorded on Electrothermal MEL-TEMP
apparatus and are uncorrected. FT-IR spectra were recorded
ppm): 28.9, 52.0, 56.0, 65.4, 111.7, 112.3, 122.8, 123.4,
148.9, 152.2, 166.8. Anal. calcd for C H O : C, 62.37; H,
5.98. Found: C, 62.18; H, 5.85.
2
1 24 8
1
13
on a Perkin–Elmer Spectrum GX spectrometer. H and
NMR spectra were recorded on a Bruker-AV 200, 400, or
00 MHz spectrometer using dimethyl sulfoxide-d6 or
C
0
5
Dimethyl4,4 -(propane-1,3-diylbis(oxy))bis(3,5-
dimethoxybenzoate)
chloroform-d as solvent and TMS as an internal standard.
HRMS was carried out on a Thermo Scientific Q-Exacative,
Accela 1250 pump. Elemental analyses were performed with
Flash EA 1112 elemental analyzer.
1
Yield: 76%; mp 120 8C; H NMR (200 MHz, CDCl
,d/ppm):
3
2.12–2.24 (m, 2H), 3.83 (s, 12H), 3.91 (s, 6H), 4.30 (t, 4H),
1
3
7.27 (s, 4H); C NMR (50 MHz, CDCl , d/ppm): 31.0, 52.2,
3
2
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56.1, 70.7, 106.7, 124.9, 141.5, 153.1, 166.8. Anal. calcd for
NMR (125 MHz, DMSO-d , d/ppm): 30.6, 56.1, 69.9, 105.4,
6
C H O : C, 59.48; H, 6.08. Found: C, 58.97; H, 5.85.
119.2, 125.2, 135.4, 140.4, 153.0, 163.8, 164.7. Anal. calcd
for C H N O : C, 57.59; H, 4.05; N, 8.67. Found: C, 56.97;
H, 4.65; N, 8.65.
2
3 28 10
3
1 26 4 12
General Procedure for Synthesis of a, x-Diacylhydrazide
Containing Oxypropylene Linkage
Into a 250-mL two necked round bottom flask equipped
with a magnetic stirrer and a reflux condenser were placed
a, x-diester (28 mmol) and ethanol (50 mL). Hydrazine
hydrate (80%; 28 g) was added dropwise to the reaction
mixture over a period of 15 min and the reaction mixture
was refluxed for 12 h. The solid that separated out was fil-
tered, dried, and recrystallized from ethanol:water mixture.
Synthesis of PAI-2 from DSHzC-3 and BPDA
FT-IR (cm ): 3236 (ANH stretching), 1790 (C@O asymmet-
rical), 1742 (C@O symmetrical), 1680 (C@O amide), 1340
2
1
1
(CANAC stretching),723 (CANAC bending); H NMR (400
MHz, DMSO-d , d/ppm): 2.0–2.07 (m, 2H), 3.85 (s, 12H),
6
4.19 (t, 4H), 7.35 (s, 4H), 8.16 (dd, 2H), 8.44 (d, 2H), 8.52
1
3
(d, 2H), 11.36 (s, 2H); C NMR (100 MHz, DMSO-d
6
, d/ppm)
0.4, 55.9, 69.7, 105.1, 122.9, 124.4, 125.3, 129.2, 130.4,
34.3, 140.1, 144.7, 152.8, 164.5, 164.9. Anal. calcd for
12: C, 61.50; H, 4.18; N, 7.75. Found: C, 61.10; H,
.30; N, 7.56.
3
1
C
4
0
4
,4 -(Propane-1,3-diylbis(oxy))bis(3-
methoxybenzohydrazide) (DVHzC-3)
37 30 4
H N O
1
Yield: 90%; mp 208 8C; H NMR (200 MHz, DMSO-d , d/
6
ppm): 2.16–2.22 (m, 2H), 3.79 (s, 6H), 4.16 (t, 4H), 4.42 (s,
4
H), 7.04 (d, 2H), 7.41 (d, 2H), 7.43 (dd, 2H), 9.63 (s, 2H);
Synthesis of PAI-3 from DSHzC-3 and ODPA
FT-IR (cm ): 3240 (ANH stretching), 1789 (C@O asymmet-
1
3
21
C NMR (50 MHz,DMSO-d , d/ppm): 28.6, 55.6, 64.9, 110.5,
6
1
12.2, 120.1, 125.7, 148.4, 150.2, 165.6; HRMS (ESI, m/z):
ric), 1742 (C@O symmetric), 1680 (C@O amide), 1346
(CANAC stretching) 722(CANAC bending); H NMR (200
1
1
[
M 1 H] calcd for C H N O , 405.1769; found, 405.1768.
1
9 25 4 6
Anal. calcd for C H N O : C, 56.43; H, 5.98; N, 13.85.
MHz, DMSO-d
6
, d/ppm): 1.92–2.05 (m, 2H), 3.83 (s, 12H),
1
9 24 4 6
Found: C, 56.41; H, 5.42; N, 13.97.
4.17 (t, 4H), 7.32 (s, 4H), 7.73 (dd, 2H), 7.79 (d, 2H), 8.10
1
3
(
d, 2H), 11.31 (s, 2H); C NMR (50 MHz, DMSO-d , d/ppm)
6
0
4
,4 -(Propane-1,3-diylbis(oxy))bis(3,5-
3
1
0.6, 56.1, 69.9, 105.3, 114.7, 125.2, 125.4, 126.6, 132.4,
40.3, 153.0, 161.1, 164.7, 164.8. Anal. calcd for
dimethoxybenzohydrazide) (DSHzC-3)
1
Yield: 97%; mp 222 8C; H NMR (200 MHz, DMSO-d , d/
6
C H N O : C, 60.16; H, 4.09; N, 7.59. Found: C, 59.82; H,
3
7 30 4 13
ppm): 1.87–2.0 (m, 2H), 3.76 (s, 12H), 4.09 (t, 4H), 4.47 (s,
3.84; N, 7.37.
1
3
4H), 7.15 (s, 4H), 9.71 (br. s, 2H); C NMR (50 MHz,DMSO-
d , d/ppm): 30.6, 55.9, 69.8, 104.4, 128.2, 138.9, 152.7,
Synthesis of PAI-4 from DVHzC-3 and PMDA
FT-IR (cm ): 3240 (ANH stretching), 1790 (C@O asymmet-
6
1
21
1
4
5
65.4; HRMS (ESI, m/z): [M 1 H] calcd for C H N O ,
2
1 29 4 8
65.1980; found, 465.1975. Anal. calcd for C21
H
28
N
4
O
8
: C,
ric), 1741 (C@O symmetric), 1677 (C@O amide), 1339
1
4.30; H, 6.08; N, 12.06. Found: C, 54.24; H, 5.74; N, 12.36.
(CANAC stretching) 727(CANAC bending); H NMR (400
MHz, DMSO-d , d/ppm): 2.25–2.31 (m, 2H), 3.87 (s, 6H),
6
Synthesis of Poly(amide imide)s Containing Oxypropylene
Linkages
4
2
.27 (t, 4H), 6.56 (d, 2H), 7.22 (d, 2H), 7.59 (s, 2H), 7.65 (d,
H), 8.53 (s, 2H), 11.41 (s, 2H); C NMR (100 MHz, DMSO-
13
General Procedure for Synthesis of Poly(amide imide)s
from DVHzC-3/DSHzC-3 and Aromatic Dianhydrides
Into a 50-mL three-necked round bottom flask equipped
with a guard tube, a nitrogen inlet, and a magnetic stirring
bar were charged DVHzC-3/DSHzC-3 (1.20 mmol) and DMAc
d6, d/ppm): 28.5, 55.7, 65.1, 111.0, 112.3, 119.2, 121.7,
22.7, 135.4, 148.7, 151.8, 163.8, 163.9, 164.8. Anal. calcd
for C H N O : C, 59.39; H, 3.78; N, 9.55. Found: C, 58.84;
1
2
9 22 4 10
H, 4.14; N, 9.13.
(
10 mL). After complete dissolution of diacylhydrazide, aro-
Synthesis of PAI-5 from DVHzC-3 and BPDA
FT-IR (cm ): 3239 (ANH stretching), 1794 (C@O asymmet-
ric), 1741 (C@O symmetric), 1678 (C@O amide), 1345
2
1
matic dianhydride (1.20 mmol) was added in one portion to
the stirred solution of diacylhydrazide. The reaction was
allowed to proceed for an additional 12 h under nitrogen
atmosphere. At the end of the reaction time, the viscous
solution of poly(hydrazide acid) was obtained. The solution
of poly(hydrazide acid) was cast onto a glass plate and the
solvent was evaporated by heating at 80 8C with continuous
nitrogen flow for 1 h. The semi-dried film was heated at
1
(CANAC stretching) 720 (CANAC bending); H NMR (400
MHz, DMSO-d , d/ppm): 2.20–2.30 (m, 2H), 3.85 (s, 6H),
6
4.25 (t, 4H), 7.20 (d, 2H), 7.56 (s, 2H), 7.63 (d, 2H), 7.70 (d,
2H), 7.78 (s, 2H), 8.90 (d, 2H), 11.21 (s, 2H); C NMR (100
1
3
MHz, DMSO-d , d/ppm): 28.5, 55.7, 65.1, 111.0, 112.3, 114.6,
6
121.6, 122.9, 125.1, 125.5, 126.6, 132.4, 148.7, 151.7, 161.1,
230 8C/0.75 mm Hg for 12 h under reduced pressure to
164.8, 164.9. Anal. calcd for C H N O : C, 63.44; H, 3.96;
3
5 26 4 10
effect imidization.
N, 8.46. Found: C, 62.52; H, 3.78; N, 8.46.
Synthesis of PAI-1 from DSHzC-3 and PMDA
FT-IR (cm ): 3237 (ANH stretching), 1795 (C@O asymmet-
Synthesis of PAI-6 from DVHzC-3 and ODPA
FT-IR (cm ): 3237 (ANH stretching), 1794 (C@O asymmet-
2
1
21
rical), 1743 (C@O symmetrical), 1680 (C@O amide), 1338
ric), 1739 (C@O symmetric), 1678 (C@O amide), 1340
(CANAC stretching), 734 (CANAC bending); H NMR (400
1
1
(
CANAC stretching), 730 (CANAC bending); H NMR (500
MHz, DMSO-d
6
, d/ppm): 1.99–2.04 (m, 2H), 3.85 (s, 12H),
MHz, DMSO-d , d/ppm): 2.27 (br. s, 2H), 3.85 (s, 6H), 4.25 (t,
4H), 7.19 (dd, 2H), 7.56 (dd, 2H), 7.60 (d, 2H), 7.66 (d, 2H),
6
1
3
4.19 (t, 4H), 7.36 (s, 4H), 8.55 (s, 2H), 11.51 (s, 2H);
C
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SCHEME 1 Synthesis of diacylhydrazides starting from vanillic/syringic acid.
1
3
7
.78 (d, 2H), 8.09 (d, 2H), 11.21 (s, 2H); C NMR (100 MHz,
at 4.09 d ppm and methoxy protons attached to aromatic
ring appeared as a singlet at 3.76 d ppm. The remaining
methylene protons showed a multiplet in the range 1.87–2.0
6
DMSO-d , d/ppm) 28.5, 55.7, 65.1, 111.0, 112.4, 114.7,
21.6, 122.8, 126.6, 132.4, 148.6, 151.7, 161.1, 164.8, 164.9.
Anal. calcd for C H N O : C, 61.95; H, 3.86; N, 8.26.
1
1
3
3
5 26 4 11
d ppm. C NMR and HRMS spectral data of diacylhydrazides
Found: C, 61.10; H, 4.26; N, 7.84.
were in good agreement with the proposed structures.
RESULTS AND DISCUSSION
Synthesis of Poly(amide imide)s
Monomer Synthesis
A series of new partially bio-based poly(amide imide)s was
synthesized by polycondensation of diacylhydrazide mono-
mers, namely, DVHzC-3 and DSHzC-3 with commercially avail-
able aromatic dianhydrides, namely, PMDA, BPDA, and ODPA
by two-step polycondensation reaction (Scheme 2). In the first
step, poly(hydrazide acid) was synthesized by addition of stoi-
chiometric quantity of aromatic dianhydride to the solution of
diacylhydrazide in DMAc at room temperature.
Scheme 1 depicts the route followed for synthesis of new
bio-based aromatic diacylhydrazide monomers, namely,
DVHzC-3 and DSHzC-3. The starting materials, namely, vanil-
lic acid, syringic acid, and 1,3-dibromopropane are all bio-
5
0,51
based.
into respective methyl esters and were further reacted with
,3-dibromopropane to afford corresponding a, x-diesters. a,
Vanillic acid and syringic acid were converted
1
x-Diesters were reacted with excess hydrazine hydrate to
obtain aromatic diacylhydrazide monomers. The overall
yields of diacyl hydrazide were in the range 65–73% and
synthetic procedure is convenient and straight forward. The
diacylhydrazides and intermediates involved in their prepa-
The polycondensation reactions proceeded in a homogenous
solution throughout the course of reaction. In the second
step, cyclization of poly(hydrazide acid) was carried out by
thermal cyclo-imidization method. The poly(hydrazide acid)
solution was poured into a glass petri dish and solvent was
evaporated by heating at 80 8C with continuous nitrogen
flow. The semi-dried poly(hydrazide acid) film was heated at
1
13
ration were characterized by FT-IR, H NMR, C NMR spec-
troscopy, HRMS, and elemental analyses.
FT-IR spectrum of DSHzC-3 is shown in Supporting Informa-
tion Figure S1. FT-IR spectrum showed absorption bands at
2
1
21
3
353 cm and 1642 cm corresponding to ANHA/ANH2
1
and AC@O group, respectively. A representative H NMR
spectrum of DSHzC-3 is reproduced in Figure 1.
The singlets appeared at 9.71 d ppm and 4.47 d ppm are
attributed to ANHA and ANH2 protons, respectively. The
aromatic protons exhibited a singlet at 7.15 d ppm. The
methylene protons attached to oxygen atom showed a triplet
1
0
FIGURE 1 H NMR Spectrum of 4,4 -(propane-1,3diylbis(oxy))-
bis(3,5dimethoxybenzohydrazide) in DMSO-d
can be viewed at wileyonlinelibrary.com]
6
.
[Color figure
SCHEME 2 Synthesis of partially bio-based aromatic poly(am-
ide imide)s.
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TABLE 1 Inherent Viscosity, Molecular Weight and Thermomechanical Properties of Poly(amide imide)
b
GPC
ꢀ
ꢀ
c
d
e
0
f
Poly(amide
imide)
g
inh
Mn 3 Mw 3
T
10
T
g
T
g
E (Pa)
21 a
)
3
3
9
Diacylhydrazide
Dianhydride
(dL g
10
10
Dispersity (8C) (8C) (8C) 3 10
PAI-1
PAI-2
PAI-3
PAI-4
PAI-5
PAI-6
0.53
38.4
40.8
36.6
25.5
37.0
29.4
67.8
72.6
63.0
53.5
66.6
56.0
1.7
1.7
1.7
2.0
1.8
1.9
355 ND 248 3.5
356 223 246 2.2
352 209 238 3.0
340 218 226 1.6
360 212 224 0.4
364 201 214 1.1
0.56
0.47
0.44
0.51
0.45
a
c
g
inh was measured with 0.5% (w/v) solution of poly(amide imide) in
Temperature at which 10% weight loss was observed.
d
e
f
DMAc at 30 6 0.1 8C.
T
g
determined by DSC.
g
T determined by DMA.
Storage modulus measured at 50 8C. Not detected.
b
Measured by GPC in DMF, polystyrenes were used as the calibration
standard.
ND
2
30 8C under reduced pressure for 12 h to form poly(amide
linkage. The absorption band of the five membered imide car-
4
5
21
21
imide).
bonyl appeared at 1795 cm and 1743 cm corresponding
to asymmetric and symmetric stretching vibration, respec-
The results of polymerization reactions are summarized in
Table 1. Inherent viscosity values of poly(amide imide)s were
in the range 0.44–0.56 dL g . Poly(amide imide)s were solu-
ble in DMF and their molecular weights were measured by
GPC using polystyrenes as calibration standards. Number aver-
21
tively. A band at 1680 cm appeared due to carbonyl group
21
21
of amide. The absorption bands at 1338 cm and 730 cm
were assigned to stretching and bending vibrations, respec-
2
1
4
5
tively, of CANAC linkage of imide ring.
1
ꢀ
H NMR spectrum of PAI-1 is reproduced in Figure 3. A mul-
age molecular weights (Mn ) and dispersity values were in the
range 25,500–40,800 and 1.7–2.0, respectively. Inherent vis-
cosity values and GPC molecular weight data indicated the for-
mation of reasonably high molecular weight polymers.
tiplet in the range 1.99–2.04 d ppm was observed due to
ꢀ
Poly(amide imide)s being semi-rigid in nature, the Mn values
reported with polystyrene standards are expected to be over-
estimated. Similar observations have been reported in the lit-
erature for measurements of molecular weight by GPC of
5
2,53
polymers which adopt a rod-like conformation in solution.
Poly(amide imide)s could be cast into transparent, flexible,
and tough film from their DMAc solution.
Structural Characterization of Poly(amide imide)s
The chemical structures of poly(amide imide)s were con-
1
13
firmed by FT-IR, H and C NMR spectroscopy, and elemen-
tal analyses.
FT-IR spectrum of PAI-1 is reproduced in Figure 2. The absorp-
2
1
tion band at 3237 cm
corresponds to ANHA of amide
FIGURE 2 FT-IR spectrum of PAI-1.
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1
FIGURE 3 H NMR spectrum of PAI-1 in DMSO-d
6
. [Color fig-
ure can be viewed at wileyonlinelibrary.com]
FIGURE 5 TG curves of poly(amide imide)s. [Color figure can
be viewed at wileyonlinelibrary.com]
methylene protons b to oxygen atom. Methoxy protons
exhibited a singlet at 3.85 d ppm while methylene protons
attached to oxygen atom appeared as a triplet at 4.19 d ppm.
Aromatic protons ortho to amide group displayed a singlet
at 7.36 d ppm. The aromatic protons of PMDA moiety
appeared as a singlet at 8.55 d ppm. Amide protons (ANHA)
exhibited a broad singlet at 11.51 d ppm. C NMR spectrum
of PAI-1 along with assignments of carbon atoms is repro-
duced in Supporting Information Figure S2.
Thermal Properties of Poly(amide imide)s
The thermal properties of poly(amide imide)s were evalu-
2
1
ated by TGA and DSC at a heating rate of 10 8C min under
nitrogen atmosphere. TG and DSC curves are shown in Fig-
ures 5 and 6, respectively, and the data are summarized in
Table 1. The 10% weight loss temperatures of poly(amide
imide)s under nitrogen atmosphere were in the range 340–
1
3
364 8C, indicating their acceptable thermal stability. However,
the thermal stability (T10 values) of present poly(amide
imide)s are inferior compared with the conventional fully
Solubility of Poly(amide imide)s
The solubility of poly(amide imide)s was investigated at 3%
4
2,44
aromatic poly(amide imide)s
. The lowering of thermal
(
w/v) in various organic solvents. Poly(amide imide)s exhib-
stability could be attributed to the presence of thermally
labile oxypropylene linkages and methoxy groups. The
weight residues of poly(amide imide)s at 800 8C were in the
range 24–34%, which could be attributed to the formation
of carbonaceous materials due to anaerobic heating of the
ited good solubility in organic solvents such as DMF, DMAc,
DMSO, NMP, pyridine, and m-cresol and were insoluble in
chloroform and dichloromethane. The improvement in solu-
bility characteristic of poly(amide imide)s could be attrib-
uted to the combined effect of flexible oxypropylene linkages
and polar methoxy substituents
polymers. T values of poly(amide imide)s were in the range
g
2
01–223 8C except for PAI-1 which did not show T in DSC
g
X-ray Diffraction Studies of Poly(amide imide)s
analysis.
X-ray diffractograms of poly(amide imide)s are shown in Fig-
ure 4. Poly(amide imide)s exhibited two broad halos in the
range 2h 5 20–308 and 10–158. The broad halos in XRD pat-
terns indicated their amorphous nature, which could be
mainly because of the presence of methoxy substituents
which disrupt close packing of polymer chains.
DMA of Poly(amide imide)s
The thermomechanical properties of poly(amide imide)s
were determined by DMA. DMA curves of poly(amide
imide)s are displayed in Figure 7 and the data are included
in Table 1. The storage modulus of oxypropylene containing
FIGURE 4 X-Ray diffractograms of poly(amide imide)s. [Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 6 DSC curves of poly(amide imide)s. [Color figure can be viewed at wileyonlinelibrary.com]
poly(amide imide)s based on DSHzC-3 and DVHzC-3 were in
similar trends in T values have been reported in the litera-
ture for polyimides and poly(amide imide)s.
g
9
9
9
48
the range 2.2 3 10 to 3.5 3 10 and 0.4 3 10 to 1.6 3
9
10
Pa, respectively, indicating their high mechanical
The effects of methoxy substituents in diacylhydrazide
strength. Results showed that poly(amide imide)s based on
DSHzC-3 exhibited higher storage modulus than correspond-
ing poly(amide imide)s based on DVHzC-3 which could be
attributed to the higher chain rigidity imposed by additional
methoxy substituents.
monomers on T values of corresponding poly(amide imide)s
g
were evaluated. Poly(amide imide)s derived from tetrame-
thoxy substituted diacylhydrazide, namely, DSHzC-3, showed
higher Tg values than corresponding poly(amide imide)s
based on dimethoxy substituted diacylhydrazide, namely,
DVHzC-3. MD simulation studies were performed on selected
poly(amide imide)s to analyze the molecular origin of the
observed substituent effects on Tg.
All the poly(amide imide)s showed two transitions above
room temperature. The transition appearing at higher tem-
perature, that is, the a transition, corresponds to T
transition that appeared in the temperature range 122–
55 8C corresponds to the sub-glass transition or b transi-
g
. The
MD Simulation Studies of Representative Poly(amide
imide)s (PAI-1 and PAI-4)
1
5
4
g
tion. The T values of poly(amide imide)s were in the
Two representative poly(amide imide)s, namely, PAI-1 and
PAI-4, which differ in the number of methoxy substituents
on aromatic rings were selected for MD simulation studies.
All atomistic models of the repeating units of poly(amide
imide)s are shown in Supporting Information Figure S5. The
details of polymer melt preparation, equilibration, and simu-
lation procedure are given in Supporting Information. The
range 214–248 8C and these values were slightly higher com-
pared with corresponding T values measured by DSC. The
g
difference in T values by DMA and DSC may be due to dif-
g
ferent principles of measurements; DMA measures the
mechanical response while DSC records the changes in heat
flow of polymer.
5
5
It was noticed that T values of poly(amide imide)s based on
simulation studies were carried out using Gromacs-5.0.5
g
5
6
both diacylhydrazides followed the order: ODPA < BP-
package and OPLA-AA force field (see Supporting Informa-
tion for simulation details). Tg values of PAI-1 and PAI-4
were calculated from temperature dependence of the density
DA < PMDA. The lowest T observed for ODPA-based poly(-
g
amide imide)s could be explained on the basis of
introduction of additional ether linkages while the highest Tg
values of PMDA-based poly(amide imide)s are attributed to
rigidity imparted by the rigid dianhydride monomer. The
g
as shown in Figure 8. This approach of T calculation has
5
7
been used previously for polyimides by Lyulin et al. The
calculated Tg values were higher compared with the
FIGURE 7 Dynamic mechanical curves of poly(amide imide)s. [Color figure can be viewed at wileyonlinelibrary.com]
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g
FIGURE 8 T of poly(amide imide)s measured by MD simulation. (A) Poly(amide imide) based on DVHzC-3 (PAI-4). (B) Poly(amide
imide) based on DSHzC-3 (PAI-1). [Color figure can be viewed at wileyonlinelibrary.com]
corresponding experimental values obtained from DMA stud-
lower density. The radial distribution function (RDF) between
2
1
ies due to the higher cooling rates applied (10 K ns ) in
simulation studies. However, the shift in the experimental T
the centers of mass of all the chains [Supporting Information
Figure S7(A)] showed the first peak at a larger distance for
PAI-1 depicting higher inter-chain spacing. Similarly, RDF
between the centers of mass of the phenyl rings and all other
atoms within single chains also showed peak at a larger dis-
tance for PAI-1 [Supporting Information Figure S7(B)]. This
points out that the chains expand and span larger volume in
the case of PAI-1. Thus, more inter- and intra- chain spacing in
PAI-1 leads to higher free volume and lesser density. Represen-
tative snapshots showing inter-chain spacing for PAI-1 and
PAI-4 are shown in Supporting Information Figure S8. Similar
counterintuitive observations of increased Tg accompanied
with decreased density have been reported by Nair and cow-
g
value has been well reproduced. The calculated T of PAI-1
g
is 18 K higher than PAI-4 as compared with an increase in
22 K observed in DMA studies. It is generally accepted that
T of an amorphous polymer is determined by a relaxation
g
5
8
or segmental motion of the chains,
which in turn is
5
9
60
affected by the free volume and/or rigidity of the chains.
The availability of more free volume and less chain rigidity
leads to faster relaxation resulting in lower Tg and vice
versa. However, prior studies indicate that these two factors
might work independently and either of them can be a con-
6
1,62
trolling factor in affecting Tg.
The calculated free volume,
6
3
64
densities along with other physical properties of PAI-1 and
PAI-4 are shown in Supporting Information Table S1. Inter-
orkers for polyimide-HFPE-30 and Gu and coworkers for
0
polyimide based on BPDA and 4,4 oxydianiline. In both the
estingly, despite having higher T , PAI-1 showed lower den-
studies, density of the higher T polymer was reported to be
g
g
sity and higher free volume as compared with PAI-4. The
density of PAI-1 and PAI-4 would depend on the intra- as
well as inter-chain spacing/packing. The higher radius of
gyration (Supporting Information Table S1) of PAI-1 indi-
cated a slightly expanded conformation that leads to the
lesser than its counterpart. These reports attributed the
observed increase in T with substitution and isomerization to
g
the increased rigidity of polymer backbone. Both these studies
have demonstrated higher rotational barrier associated with
groups involving substitution or isomerization.
FIGURE 9 (A) Torsional angle distributions of PAI-4 and PAI-1. (B) Rotational energy barrier from PMF for PAI-4 and PAI-1. [Color
figure can be viewed at wileyonlinelibrary.com]
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dibromopropane. A series of new partially bio-based poly(am-
ide imide)s was prepared from diacylhydrazides and commer-
cially available aromatic dianhydrides. Poly(amide imide)s
exhibited amorphous nature and good solubility in polar sol-
vents. Poly(amide imide)s could be cast into transparent, flexi-
ble, and tough films from their DMAc solutions. Poly(amide
ꢀ
imide)s showed inherent viscosity and Mn in the range 0.44–
2
1
0
.56 dL g
imide)s showed 10% weight loss in the temperature range
40–364 8C indicating their acceptable thermal stability. Ther-
and 25,500–40,800, respectively. Poly(amide
3
momechanical properties of poly(amide imide)s were studied
by DMA which indicated that poly(amide imide)s based on
DSHzC-3 exhibited higher storage modulus than correspond-
ing poly(amide imide)s based on DVHzC-3. Both DMA and DSC
FIGURE 10 Comparison of the decay of dihedral autocorrela-
tion function (C(t)) for PAI-4 and PAI-1. [Color figure can be
viewed at wileyonlinelibrary.com]
showed higher T for poly(amide imide)s based on DSHzC-3
g
compared with the corresponding poly(amide imide)s based
on DVHzC-3. MD simulation studies on representative poly(-
amide imide)s indicated that chain rigidity, imposed by addi-
tional methoxy substituents on phenyl ring, as the molecular
Along the similar lines, effect of the chain rigidity, in particular
the rotational barrier between the phenyl ring and the oxypro-
pylene linkage on respective T values was explored. Torsional
g
g
origin for the observed increase in T of poly(amide imide)s
based on DSHzC-3. As a final remark, vanillic acid, and syringic
acid represent valuable precursors for the synthesis of difunc-
tional monomers useful in the preparation of high perfor-
mance/temperature thermoplastic polymers.
angle distribution of a [see Supporting Information Figure
S5(A) for a structural representation] is shown in Figure 9(A).
The dramatic increase in the peak intensities in PAI-1 indi-
cated that the rotation around phenyl ring is more hindered
due to the extra methoxy substituents. To quantify the
increase in the rotational barrier with substitution, potential
of mean force (PMF), G(a) was calculated from the torsional
distributions using the Boltzmann inversion relation:
ACKNOWLEDGMENTS
The authors thank CSIR-4PI supercomputing facility for the
computational resources. This work was supported by a fellow-
ship from the University Grants Commission (UGC), New Delhi,
India (to S.S.K.), Senior Research Fellowship from CSIR, New
Delhi (to P.S.), and the Ramanujan Fellowship from the Depart-
ment of Science and Technology (DST), India (SR/S2/RJN-84/
GðaÞ52kBTln PðaÞ
The difference between the minima and maxima of PMF
[
Figure 9(B)] corresponds to the free energy barrier for tor-
21
sional rotation. PAI-1 exhibited 1.6 kcal mol
higher rota-
2012) (to S.C.).
tional barrier than PAI-4. Thus, in PAI-1 rotation around
phenyl ring is hindered, which restricts the segmental
motion and thereby increases the relaxation time. To analyze
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