Synthesis of thermoplastic-thermosetting merged polyimides by use of the Diels-Alder reaction

Article abstract of DOI:10.1007/s11164-013-1389-4

Abstract Novel thermoplastic-thermosetting merged polyimides (PIs) have been developed by Diels-Alder (DA) intermolecular reaction of the bisfuran 2,5-bis(furan-2-ylmethylcarbamoyl)terephthalic acid A with a series of bismaleimides, viz., m,m′-dimethyl p,p′-dimaleimidobiphenyl (B ₁ ), m,m′-dimethoxy p,p′-dimaleimidobiphenyl (B ₂ ), 1,1′-bis(4-maleimidophenyl)cyclohexane (B ₃ ), and 2,2′-bis-[4-(4-maleimidephenoxy)phenyl]propane (B ₄ ). The intermediate DA adducts obtained, C _1-4, were aromatized and imidized (i.e. cyclized) through the carboxyl and amide groups to afford thermoplastic-thermosetting merged PIs D _1-4 . Bisfuran A was prepared by condensation of pyromellitic dianhydride with furan-2-ylmethanamine and duly characterized. Synthesized DA adducts and PIs were characterized by use of elemental analysis, spectral features, number average molecular weight, degree of polymerization, and thermal analysis. To facilitate the correct structure assignment and to verify the identities of the DA adducts and PIs, a model compound 4 was prepared in a similar way from phthalic anhydride and furan-2-ylmethanamine. FT-IR spectral features of PIs D _1-4 were compared with those of model compound 4 and found to be identical. Glass fiber-reinforced composites GFRC _1-4 were prepared from this system and characterized by chemical, mechanical, and electrical analysis. All the composites had good mechanical, electrical, and thermal properties and good resistance to organic solvents and mineral acids.

Full text of DOI:10.1007/s11164-013-1389-4

Res Chem Intermed
DOI 10.1007/s11164-013-1389-4
Synthesis of thermoplastic–thermosetting merged
polyimides by use of the Diels–Alder reaction
Yogesh S. Patel Hasmukh S. Patel
•
Received: 23 August 2012 / Accepted: 30 August 2013
Springer Science+Business Media Dordrecht 2013
ꢀ
Abstract Novel thermoplastic–thermosetting merged polyimides (PIs) have
been developed by Diels–Alder (DA) intermolecular reaction of the bisfuran
2
mides, viz., m,m -dimethyl p,p -dimaleimidobiphenyl (B1), m,m -dimethoxy
,5-bis(furan-2-ylmethylcarbamoyl)terephthalic acid A with a series of bismalei-
0
0
0
0
0
0
p,p -dimaleimidobiphenyl (B ), 1,1 -bis(4-maleimidophenyl)cyclohexane (B ), and
2
3
2
,2 -bis-[4-(4-maleimidephenoxy)phenyl]propane (B4). The intermediate DA
adducts obtained, C1–4, were aromatized and imidized (i.e. cyclized) through the
carboxyl and amide groups to afford thermoplastic–thermosetting merged PIs D1–4
.
Bisfuran A was prepared by condensation of pyromellitic dianhydride with furan-2-
ylmethanamine and duly characterized. Synthesized DA adducts and PIs were
characterized by use of elemental analysis, spectral features, number average
molecular weight, degree of polymerization, and thermal analysis. To facilitate the
correct structure assignment and to verify the identities of the DA adducts and PIs, a
model compound 4 was prepared in a similar way from phthalic anhydride and
furan-2-ylmethanamine. FT-IR spectral features of PIs D1–4 were compared with
those of model compound 4 and found to be identical. Glass fiber-reinforced
composites GFRC1–4 were prepared from this system and characterized by chem-
ical, mechanical, and electrical analysis. All the composites had good mechanical,
electrical, and thermal properties and good resistance to organic solvents and
mineral acids.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-013-1389-4)
contains supplementary material, which is available to authorized users.
Y. S. Patel ꢀ H. S. Patel (&)
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India
e-mail: drhspatel786@yahoo.com; hsp13152@rediffmail.com
Y. S. Patel
e-mail: yogeshspatel@ymail.com
123

Y. S. Patel, H. S. Patel
Keywords Bisfuran ꢀ Bismaleimide ꢀ Diels–Alder reaction ꢀ Thermoplastic–
thermosetting merged polyimide
Introduction
In the last few decades there has been a substantial increase in research in use of the
Diels–Alder (DA) reaction in macromolecular chemistry, particularly in the field of
polyimides (PIs). This reaction has been applied to the furan heterocycle with
growing frequency, because its pronounced dienic character makes it particularly
suitable for use as the diene in DA cycloaddition reactions. Use of bismaleimides as
dienophiles is very useful because the double bond of maleimides is very reactive
toward electron-rich dienes. The first investigation of a PI prepared from a bisfuran
and bismaleimide was reported by Tesoro and Sastri [1]; this led to many
publications on chemical modification of adduct by aromatization. Gaina et al. [2],
Patel et al. [3], Sanyal [4], Peterson et al. [5], and Liu and Chen [6] used many
strategies for synthesis or modification of polymer structures by exploiting the
reactivity of this heterocycle with a variety of dienophiles. A systematic study of
DA polymerization involving furan derivatives and a variety of bismaleimides had
been performed by Gandini [7]. Owing to reported work on these topics and the
long-standing interest of our laboratory in the study of PIs via the DA reaction, a
more systematic investigation has been undertaken. Patel and co-workers have
reported studies on such PIs as poly(epoxy–imide) [8], poly(s-triazine–imide) [9],
poly(ether–imide) [10], poly(ester–amido–imide) [11], poly(amido–imide) [12],
poly(ester–oxysilane–imide) [13], and poly(urethane–imide–ester) [14] with wide
structural variation by DA polymerization of bisfurans and bismaleimides.
PIs are of two types. Thermoplastic PIs are usually synthesized from poly(amic
acids) prepared by reaction of an aromatic dianhydride with an aromatic diamine
followed by cyclodehydration to the PI. Because the final imide polymer is insoluble,
infusible, and intractable, processing of the amic acid solution is frequently used for
manufacture of articles. PIs synthesized from unsaturated monomers, e.g. bismalei-
mides, are known as thermosetting PIs. These PIs are processed in situ. Our main
concern was to synthesize novel PIs with a combination of thermoplastic and
thermosetting groups by introducing bisfuran and bismaleimide segments. The
bisfuran (bisamic acid–pyromellitic part) segment is the thermoplastic portion and the
bismaleimide segment is the thermosetting segment. This would alter the properties of
PIs. Combinations of both types of segment in the PI chain has not received any
attention. Hence, initial work on this topic was recently been reported by us [15]. This
prompted us to extend our work by using other auxiliary bismaleimides.
In this study we synthesized PIs via the DA reaction with subsequent
aromatization and imidization. Scheme 1 summarizes our synthetic approach to
the various phases of this work, viz.:
1
2
3
preparation of bisfuran Afrom pyromellitic dianhydride and furan-2-ylmethanamine;
DA reaction between bisfuran A and bismaleimides B1–4; and
aromatization and imidization (i.e. cyclization) reaction to afford thermoplastic–
thermosetting merged PIs D1–4
.
1
23

Thermoplastic–thermosetting merged PIs via DA reaction
Glass fiber-reinforced composites GFRC1–4 were prepared from this system and
characterized by chemical, mechanical, and electrical analysis. To facilitate correct
structure assignment and to verify the occurrence of the DA adducts and PIs, a
model compound 4 was synthesized from phthalic anhydride and furan-2-
ylmethanamine (Scheme 2). The FT-IR spectral features of PIs D1–4 were compared
Scheme 1 Synthesis of thermoplastic–thermosetting merged PIs
123

Y. S. Patel, H. S. Patel
with those of model compound 4 and found to be identical. The details of these
procedures and the results obtained are discussed below.
Experimental
Materials and measurements
All chemicals and solvents used were of laboratory grade. Pyromellitic dianhydride,
phthalic anhydride, and maleic anhydride were obtained from Fluka Analytical,
Japan, and Merck, Mumbai, India. Furan-2-ylmethanamine was obtained locally and
0
redistilled before use. A variety of diamines were used for the preparation: m,m -
0
0
0
dimethyl p,p -dimaleimidobiphenyl (B ), m,m -dimethoxy p,p -dimaleimidobiphenyl
1
Scheme 2 Synthesis of model compound
1
23

Thermoplastic–thermosetting merged PIs via DA reaction
0
0
(B ), 1,1 -bis(4-maleimidophenyl)cyclohexane (B ), and 2,2 -bis-[4-(4-maleimide-
phenoxy)phenyl]propane (B ) were obtained locally and used without purification.
2
3
4
Solvents were dried and distilled before use according to standard procedures.
Elemental analysis was performed with a Thermo Finnigan Flash 1101EA (Italy).
1
Infrared spectra were acquired by use of a Nicolet-760 FT-IR spectrophotometer. H
1
3
NMR and C NMR spectra were recorded by use of a 400-MHz Bruker
spectrophotometer. Number average molecular weight (M ) of the PIs was
n
determined by non-aqueous conductometric titration [16–18]. Pyridine was used as
suspending solvent and tetra-n-butylammonium hydroxide in 1,4-dioxane was used as
titrant. A digital conductivity meter from Toshniwal (India) was used for titration.
Thermal behavior was investigated by thermogravimetric analysis by use of a Perkin–
Elmer TGA analyzer at heating rate of 10 K/min in the temperature range 50–700 ꢁC.
Because the PIs were insoluble in common organic solvents, their viscometric
properties were not studied.
Synthesis of 2,5-bis(furan-2-ylmethylcarbamoyl)terephthalicacid A
Bisfuran A was synthesized by condensation of of phthalic anhydride and furan-2-
ylmethanamine [19]. A solution of furan-2-ylmethanamine (0.2 mol) was added
dropwise over 1 h to a stirred solution of pyromellitic dianhydride (0.1 mol) at
0
–5 ꢁC (Scheme 1). The solution obtained was poured into ice water, in which the
reaction product precipitated. The white precipitate was filtered, washed, and
purified by column chromatography. Yield was 65 %; M.wt. 412.35 g; decompo-
sition temp. 250–260 ꢁC (uncorrected); elemental analysis calculated for
1
C H N O : C 58.25, H 3.91, N 6.79 % found: C 57.87, H 3.75, N 6.58 %; H
2
0 16 2 8
NMR (d ppm): 10.9 (s, 2H, –COOH), 8.9 (t, J = 5.2 Hz, 2H, –NH–CO–), 8.1 (s,
1
3
2
H, Ar–H), 7.50–7.93 (m, 6H, Ar–H), 4.32 (d, J = 5.2 Hz, 2H, –CH –); C NMR
2
(
d ppm): 42.24, 117.78, 119.24, 123.45, 126.10, 129.32, 132.67, 135.70, 167.12,
-
72.73; IR (KBr, cm ): 3528 (–COO–H st.), 3254 (–NH–C=O st.), 3123 (C–H st.
1
1
in furan rings), 3038 (–CH st. aromatic ring), 1711 (acid –C=O st.), 1685 (O=C–NH
-
free amide), 1500 (backbone st. aromatic ring), 1465 (COO sym. st.), 1039 (C–O
st.), 730 (furan ring).
Synthesis of bismaleimides B1–4
Bismaleimides (Table 1) were prepared in two steps as reported elsewhere [20–22].
The first step involved addition of one equivalent of diamine to two equivalents of
maleic anhydride to form bisamic acid. In the second step cyclization of the amic
acid end-group was induced by addition of sodium acetate and acetic anhydride.
DA reaction of A with B1–4 (formation of DA adducts C1–4
)
Equimolar amounts of bisfuran A and bismaleimide B1–4 were dissolved in 50 mL
tetrahydrofuran. This solution was heated under reflux for 10 h at 60 ꢁC
(Scheme 1). The reaction mixture was then cooled, poured into a large volume of
dry ether, and the precipitate formed was isolated by filtration, washed, and dried in
123

Y. S. Patel, H. S. Patel
1
23

Thermoplastic–thermosetting merged PIs via DA reaction
air. The unaromatized and uncyclized products obtained were designated as DA
adducts C1–4. These were purified and characterized by elemental analysis, degree
of polymerization (D ), M , FT-IR, and thermal analysis.
p
n
-
1
C : Yield 45 %; color light brown; empirical wt. 784.72 g; IR (KBr, cm ): 3536
1
(
–OH st. acid), 3250 (–NH st. amide), 3115 (–CH st. furan ring), 3027 (–CH st.
aromatic ring), 2962, 2854 (–CH st. aliphatic), 1777 (–C=O st. asym. imide), 1730
–C=O st. sym. imide), 1711 (–C=O st. acid), 1678 (–C=O st. amide), 1580 (–C=C–
(
-
st. furan ring), 1461 (–COO st. sym.), 1240, 1163, 1061 (furan –CH in-plane
deformation), 1042 (–C–O st.), 913, 872, 731 (furan –CH out-of-plane deforma-
tion/–C=O bending); elemental analysis calculated for C H N O : C 64.28, H
4
2 32 4 12
4
.11, N 7.14 %; found: C 64.20, H 4.04, N 7.09 %; D 7; M 5480 ± 60.
p n
-
C : Yield 40 %; color light brown; empirical wt. 816.72 g; IR (KBr, cm ): 3532
1
2
(
–OH st. acid), 3246 (–NH st. amide), 3112 (–CH st. furan ring), 3033 (–CH st.
aromatic ring), 2950, 2849 (–CH st. aliphatic), 1783 (–C=O st. asym. imide), 1731
–C=O st. sym. imide), 1715 (–C=O st. acid), 1674 (–C=O st. amide), 1583 (–C=C–
(
st. furan ring), 1460 (–COO– st. sym.), 1240, 1161, 1060 (furan –CH in-plane
deformation), 1039 (–C–O st.), 910, 875, 730 (furan –CH out-of-plane deformation/
C=O bending); elemental analysis calculated for C H N O : C 61.77, H 3.95, N
4
2 32 4 14
6
.86 %; found: C 61.70, H 3.91, N 6.80 %; D 7; M 5703 ± 60.
p n
-
C : Yield 50 %; color light brown; empirical wt. 838.81 g; IR (KBr, cm ): 3533
1
3
(
–OH st. acid), 3246 (–NH st. amide), 3117 (–CH st. furan ring), 3034 (–CH st.
aromatic ring), 2956, 2846 (–CH st. aliphatic), 1787 (–C=O st. asym. imide), 1738
–C=O st. sym. imide), 1710 (–C=O st. acid), 1670 (–C=O st. amide), 1585 (–C=C–
(
st. furan ring), 1463 (–COO– st. sym.), 1245, 1167, 1063 (furan –CH in-plane
deformation), 1040 (–C–O st.), 915, 877, 732 (furan –CH out-of-plane deformation/
C=O bending); elemental analysis calculated for C H N O : C 65.87, H 4.57, N
4
6 38 4 12
6
.68 %; found: C 65.82, H 4.53, N 6.60 %; D 8; M 6710 ± 60.
p n
-
C : Yield 40 %; color light brown; empirical wt. 982.94 g; IR (KBr, cm ): 3527
1
4
(
–OH st. acid), 3240 (–NH st. amide), 3122 (–CH st. furan ring), 3028 (–CH st.
aromatic ring), 2957, 2838 (–CH st. aliphatic), 1775 (–C=O st. asym. imide), 1730
–C=O st. sym. imide), 1716 (–C=O st. acid), 1679 (–C=O st. amide), 1587 (–C=C–
(
st. furan ring), 1465 (–COO– st. sym.), 1246, 1163, 1065 (furan –CH in-plane
deformation), 1037 (–C–O st.), 912, 875, 734 (furan –CH out-of-plane deformation/
C=O bending); elemental analysis calculated for C H N O : C 67.21, H 4.31, N
5
5 42 4 14
5
.70 %; found: C 67.15, H 4.25, N 5.63 %; D 8; M 7860 ± 60.
p n
Aromatization and imidization of DA adducts C1–4 (formation of PIs D1–4
)
Aromatization of the polymers was performed by heating 1 g dried DA adduct C1–4
in 1 mL acetic anhydride [1] under reflux for 4 h at 150–160 ꢁC (Scheme 1).
Imidization occurs simultaneously. The resulting solution was cooled and poured
into a large volume of water. The precipitate formed was isolated by filtration,
washed, then dried in air. Thus brown precipitates of PIs D1–4 obtained were
characterized by elemental, IR, and thermal analysis.
-
1
D : Yield 45 %; color brown; empirical wt. 712.66 g; IR (KBr, cm ): 3034
1
(
–CH st. aromatic ring), 2956, 2855 (–CH st. aliphatic), 1782 (–C=O st. asym.
123

Y. S. Patel, H. S. Patel
imide), 1735 (–C=O st. sym. imide), 1500 (backbone st. aromatic ring), 1370
(
C 70.78 H 3.39 N 7.86 %; found: C 70.71 H 3.32 N 7.80 %.
imide, imide ring vibration, axial); elemental analysis calculated for C H N O :
42 24 4 8
-
D : Yield 45 %; color brown; empirical wt. 744.66 g; IR (KBr, cm ): 3040
1
2
(–CH st. aromatic ring), 2960, 2853 (–CH st. aliphatic), 1780 (–C=O st. asym.
imide), 1732 (–C=O st. sym. imide), 1502 (backbone st. aromatic ring), 1373
(
C 67.74, H 3.25, N 7.52 %; found: C 67.68, H 3.18, N 7.46 %.
imide, imide ring vibration, axial); elemental analysis calculated for C H N O :
42 24 4 10
-
D : Yield 40 %; color brown; empirical wt. 766.75 g; IR (KBr, cm ): 3041
1
3
(–CH st. aromatic ring), 2964, 2858 (–CH st. aliphatic), 1781 (–C=O st. asym.
imide), 1733 (–C=O st. sym. imide), 1504 (backbone st. aromatic ring), 1376
(
C 72.06, H 3.94, N 7.31 %; found: C 72.00, H 3.89, N 7.26 %.
imide, imide ring vibration, axial); elemental analysis calculated for C H N O :
46 30 4 8
-
D : Yield 40 %; color brown; empirical wt. 910.88 g; IR (KBr, cm ): 3046
1
4
(–CH st. aromatic ring), 2963, 2850 (–CH st. aliphatic), 1785 (–C=O st. asym.
imide), 1731 (–C=O st. sym. imide), 1503 (backbone st. aromatic ring), 1375
(imide, imide ring vibration, axial); elemental analysis calculated for C H N O :
55 34 4 10
C 72.52, H 3.76, N 6.15 %; found: C 72.45, H 3.69, N 6.10 %.
Bulk polymerization
A mixture of A (0.01 mol), B1–4 (0.01 mol), and 2 ml acetic anhydride was heated
at approximately 150 ꢁC for 10 h with vigorous agitation. The resulting brown solid
products were cooled and poured into a large volume of water. The precipitates
formed were isolated by filtration, washed, then dried in air. The results obtained by
bulk polymerization were analogous with those for the synthesized PIs.
Synthesis of model compound
0
A suspension of 2-(furan-2-ylmethylcarbamoyl)benzoic acid (0.01 mol) and 4,4 -
diaminodiphenylmethane bismaleimide (0.01 mol) in THF was heated under reflux
for 10 h at 60 ꢁC. The resulting unaromatized and unimidized product was
designated as DA adducts 3. It was heated under reflux at 150–160 ꢁC for 4 h with
2
ml acetic anhydride to afford aromatization and imidization (Scheme 2). The
resulting solution was cooled and poured into a large volume of water, washed,
filtered, then dried in air. The brown precipitate of 4 thus obtained was characterized
by elemental and IR analysis. Yield was 65 %; M.wt. 567.55 g; elemental analysis
calculated for C H N O : C 71.95, H 3.73, N 7.40 %; found: C 71.86, H 3.67, N
3
4 21 3 6
-
1
1
7
.31 %; IR (KBr, cm ): 3028, 2972, 2862, 1771, 1730, 1513, and 1379; H NMR
(
Ar–H), 8.41 (d, J = 8.8, 2H, maleimide) (Supplementary Fig. 1).
d ppm): 2.49 (s, 2H, Ph–CH –Ph), 4.40 (s, 2H, N–CH –Ph), 7.28–8.06 (m, 15H,
2
2
Preparation of glass fiber-reinforced composites GFRC1–4
To prepare glass fiber-reinforced composites GFRC1–4, suspensions of A and B1–4
in THF were prepared on a weight basis and stirred well for 10 min. These resin
1
23

Thermoplastic–thermosetting merged PIs via DA reaction
1
Fig. 1 H NMR of bisfuran A
suspensions were used to vanish PI-compatible glass cloth. The suspensions were
applied with a brush and the solvent was left to evaporate for 10–20 min at room
temperature. The ten dried prepregs so prepared were stacked one over another and
placed between stainless steel plates and compressed under approximately
60–70 psi pressure at 150 ꢁC for 10 h in air circulating oven. The composites
were cooled to room temperature before the pressure was released. The composites
were then machined to desire final dimension for mechanical and chemical tests.
Chemical resistance test
The chemical resistance test for all the composite samples was performed according
3
to ASTMD 543–67. The composite sample dimensions were 25 9 25 9 3 mm .
The samples were immersed in 25 % v/v H SO , 25 % v/v HCl, 25 % w/v NaOH,
2
4
ethanol, acetone, DMF, and THF for 7 days at room temperature. After 7 days the
samples were removed from the reagents and analyzed for the percentage change in
their thickness and weight.
Mechanical and electrical tests
All mechanical tests were performed using three specimens.
•
The compressive strength was measured on a Universal Instron testing machine
model no. A-74-37, at room temperature according to ASTM D 695.
123

Y. S. Patel, H. S. Patel
•
•
•
The notched Izod impact strength of the composites was measured on a Zwick
model no. 8900 impact machine at room temperature according to ASTM D256.
Rockwell hardness was measured at room temperature by use of a model RAS
(Saro Engineering, India) Rockwell hardness tester according to ASTM D 785.
Measurement of flexural strength was performed with a Universal Instron
Testing Machine, model no. A-74-37, at room temperature according to ASTM
method D790.
Electrical strength measurements were conducted on a high voltage tester
machine oil test set.
Results and discussion
Monomer synthesis
To the best of our knowledge, product A has not been reported previously.
Characterization of the reaction product provided the first unambiguous proof of the
successful synthesis of 2,5-bis(furan-2-ylmethylcarbamoyl)terephthalic acid (A).
The FT-IR spectrum of A showed the most relevant peaks of the furan ring and the
1
3
,2,4,5-tetra-substituted benzene ring, and typical absorption of carboxylic acid at
-
1
-1
1
528 and 1711 cm and of the O=C–NH group at 3254 and 1680 cm [23]. In H
NMR spectroscopy, signals in the range 7.5–8.1 ppm were ascribed to the protons
of the aromatic rings. The singlet at 10.9 ppm was ascribed to the protons of
carboxyl –OH group and a triplet at 8.9 ppm was attributed to the –NH proton of the
1
3
amide group (Fig. 1), which was further confirmed by C NMR. Monomer A lost
weight during thermal degradation. TGA data for A indicated that degradation
occurred into two steps. The first stage was from 180 to 300 ꢁC and might be
attributed to decarboxylation of A. The weight loss of 20.91 % is consistent with the
theoretical value, 21.34 %. The second major stage at approximately 300–700 ꢁC
was attributed to decomposition and/or pyrolysis of the monomer. Three to four
percent of charred residue remained at 700 ꢁC. The expected structure was thus
clearly verified by this spectroscopic and thermal analysis which indicated,
moreover, the absence of any detectable impurity, particularly of the two reagents
used to prepare A. This was also confirmed by elemental analysis.
Polymer synthesis
The thermoplastic–thermosetting merged PI system was prepared by DA intermo-
lecular reaction followed by aromatization of the DA adduct and simultaneous
imidization via cyclization. Results from elemental analysis of the DA adducts
indicated that the reactions led to good yields in cycloaddition of the furan rings,
and were consistent with their predicted structures (Scheme 1). D for polymers
p
C1–4 was estimated by non-aqueous titration and was in the range 8–9. The IR
spectra of the adducts revealed important changes compared with the initial
monomers; these clearly confirmed the DA reaction had occurred with good yields
1
23

Thermoplastic–thermosetting merged PIs via DA reaction
Fig. 2 FT-IR of DA adduct C
1
Fig. 3 FT-IR of PI D
1
with all the bismaleimides. The IR spectra of DA adducts C1–4 had prominent
characteristic bands of furan, amide, and carboxylic acid groups (Fig. 2). The bands
-
near 3125 and 1585 cm were attributed to furan. The bands at approximately
1
-
1
3
approximately 3530 and 1710 cm were attributed to –OH and –C=O groups,
225 and 1675 cm
were attributed to amide groups and the bands at
-
1
123

Y. S. Patel, H. S. Patel
respectively, of the carboxylic acid. Comparison of the IR spectra of non-
aromatized and-non cyclized C1–4 with aromatized and imidized (cyclized) D1–4
revealed discernible differences. The bands of the carboxylic acid and amide groups
in the spectra of C1–4 were almost absent from the spectra of D1–4, indicating that
imidization had occurred very smoothly, and the disappearance of the bands of the
furan ring confirmed aromatization reaction had been completed simultaneously.
Fig. 4 FT-IR of model compound 4
Fig. 5 Thermogram of the DA adducts (C1–4
)
1
23

Thermoplastic–thermosetting merged PIs via DA reaction
For the sake of convenience, the IR spectra of PIs D1–4 (Fig. 3) were compared with
that of the model compound (Fig. 4). IR spectral features of PIs D1–4 were identical
with those of the model compound. These features confirmed the predicted structure
of aromatized–imidized PIs D1–4
.
The thermal stability of C1–4 (Fig. 5) and D1–4 (Fig. 6) were evaluated from their
TG curves. C1–4 underwent two stages of mass loss. The first, major, stage, in the
temperature range 180–280 ꢁC, corresponded to decarboxylation of the polymer.
The weight loss observed for C1–4 was consistent with the theoretical value. The
second stage of mass loss, above 280 ꢁC, was because of polymer pyrolysis.
Approximately 4–6 % charred residue of C was left at 700 ꢁC. Very rapid weight
1
–4
loss was observed between 400 and 600 ꢁC.
Decomposition of D started at 350 ꢁC, depending upon the nature of PIs. For
1
–4
almost all the polymer samples weight loss was 50 % in the range 500–600 ꢁC.
Complete weight loss (approx. 95–97 %) was observed at approximately 700 ꢁC.
Aromatized samples D
were more thermally stable that their prepolymers
–4
1
because of the decarboxylation of the former. Thus degradation of D1–4 started at
slightly higher temperatures than that of the non-aromatized samples C1–4
Comparison of the thermal stability of all PIs revealed the following order of
.
stability: B [ B [ B [ B . Thermal data reported for most commercial PIs were
4
1
2
3
determined in N . This study was conducted in air, so it is worth mentioning that
these PIs were quite thermally stable compared with commercial PIs.
2
Fig. 6 Thermogram of the PIs (D1–4
)
123

Y. S. Patel, H. S. Patel
Composite characterization
Glass fiber-reinforced dark brown composites were prepared and characterized.
Results from testing of the chemical resistance of the PI composites to organic
solvents, acids, and alkalis (Table 2) revealed that weight loss was negligible in
ethanol, acetone, THF, and HCl. Weight loss of approximately 1.40–1.56 % was
observed in 25 % H SO . Concentrated alkali resulted in weight changes of
2
4
approximately 1.70–2.04 %; in DMF approximately 1.88–2.06 % weight change
was observed. The results indicated that the composites had remarkable resistance
to organic solvents, acids, and alkali.
Mechanical tests were performed on the specimens and the results obtained
Table 3) were compared with reported data [15]. The flexural strength of a material
(
is defined as its ability to resist deformation under load. The flexural strength of the
composites was between 272 and 287 Mpa. Compressive properties describe the
behavior of a material when it is subjected to a compressive load. The compressive
strength of the composites was between 220 and 239 Mpa. The notched Izod impact
test is a single-point test that measures a material’s resistance to impact from a
swinging pendulum. Izod impact is defined as the kinetic energy needed to initiate
fracture and continue the fracture until the specimen is broken. The impact strength
of the composites was between 232 and 256 Mpa. Rockwell hardness is a measure
of the indentation resistance of a material. Testing is performed by first forcing a
steel ball indenter into the surface of a material by use of a specified minor load. The
load is then increased to a specified major load, then reduced to the original minor
load. Rockwell hardness is a measure of the net increase in depth of the indenter.
Rockwell hardness was between 106 and 110 Mpa. The results of mechanical
testing (Table 3) revealed that all the composites have good mechanical properties
compared with our previously reported data. This may be attributed to increased
rigidity because of the aromatic backbone in the bismaleimides. The overall trend of
the mechanical properties of the prepared PI glass fiber-reinforced composites
decreases as follows: GFRC [ GFRC [ GFRC [ GFRC . The dielectric
4
1
2
3
a
Table 2 Chemical resistance of glass fiber-reinforced composites
b
Composite
GFRC
1
% change
GFRC
2
% change
GFRC
3
% change
4
GFRC % change
Thickness
Wt.
Thickness
Wt.
Thickness
Wt.
Thickness
Wt.
1
.1
1.0
5 % NaOH 1.3
1.46
1.23
1.90
1.11
1.12
1.96
1.41
1.2
1.1
1.4
1.0
1.2
1.4
1.4
1.53
1.27
1.94
1.13
1.17
1.99
1.57
1.2
1.2
1.4
1.2
1.2
1.5
1.4
1.56
1.29
2.04
1.19
1.28
2.06
1.64
1.1
1.0
1.2
0.8
0.9
1.3
1.1
1.40
1.19
1.70
0.87
1.10
1.88
1.21
2
2
5 % HCl
Ethanol
Acetone
DMF
THF
1.0
1.0
1.3
1.3
a
Condition: reinforcement—E type glass cloth, plain weave, 10 mm, 10 layers, resin content 40 ± 2 %,
molar ratio: A:B1–5 = 1:1 (mol/mol). Curing temperature = 150 ± 10 ꢁC, curing
time = 10 h, curing pressure = 60–70 psi. Composite size: 25 9 25 9 3 mm
weight
%
3
1
23

Thermoplastic–thermosetting merged PIs via DA reaction
Table 3 Mechanical and electrical properties of glass fiber-reinforced composites
Composite Flexural strength Compressive Notched impact Rockwell
strength
MPa)
Electrical strength
strength (MPa) hardness (MPa) in air (kV/mm)
(MPa)
(
GFRC
GFRC
GFRC
GFRC
1
2
3
4
281
275
272
287
231
224
220
239
250
238
232
256
109
107
106
110
17.2
18.6
19.8
21.9
strength of an insulating material is defined as the maximum voltage required to
cause dielectric breakdown. The electric strength of all the composites was in the
range of 17.2–21.9 kV/mm. This was high compared with previous data.
Conclusion
Novel PIs with a merged segment system and both thermoplastic and thermosetting
properties have been successfully prepared by DA reaction of 2,5-bis(furan-2-
ylmethylcarbamoyl)terephthalic acid with different bismaleimides, followed by
aromatization and imidization. The in-situ-produced PIs adhere well to glass fiber.
All the glass fiber-reinforced composites had good mechanical, electrical, and
thermal properties and good resistance to organic solvents and mineral acid.
Acknowledgments One of the authors, Y.S. Patel, is grateful to the University Grants Commission
(UGC) for sanctioning a Teacher Fellowship under the scheme Faculty Improvement Programme for this
research work.
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