Crosslinked polyamide based on main-chain type polybenzoxazines derived from a primary amine-functionalized benzoxazine monomer

Article abstract of DOI:10.1002/pola.24867

Two types of main-chain type polybenzoxazines with amide and benzoxazine groups as repeating units in the main chain, termed as poly(amide-benzoxazine), have been synthesized. They have been prepared by polycondensation reaction of primary amine-bifunctio

Full text of DOI:10.1002/pola.24867

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ARTICLE
Crosslinked Polyamide Based on Main-Chain Type Polybenzoxazines
Derived from a Primary Amine-Functionalized Benzoxazine Monomer
Tarek Agag,1 Carlos R. Arza,2 Frans H. J. Maurer,2 Hatsuo Ishida1
*
1Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202
2Department of Polymers and Materials Chemistry, Lund University, SE-22100 Lund, Sweden
Correspondence to: H. Ishida (E-mail: hxi3@cwru.edu)
Received 1 March 2011; accepted 20 June 2011; published online 3 August 2011
DOI: 10.1002/pola.24867
ABSTRACT: Two types of main-chain type polybenzoxazines
with amide and benzoxazine groups as repeating units in the
main chain, termed as poly(amide-benzoxazine), have been
synthesized. They have been prepared by polycondensation
reaction of primary amine-bifunctional benzoxazine with adi-
poyl and isophthaloyl dichloride using dimethylacetamide as
solvent. Additionally, a model reaction is designed from the
reaction of 3,30-(4,40-methylenebis(4,1-phenylene))bis(3,4-dihy-
dro-2H-benzo[e][1,3]oxazin-6-amine) with benzoyl chloride. The
structures of model compound and polyamides are confirmed
by Fourier transform infrared (FTIR) and proton nuclear mag-
netic resonance (1H NMR) spectroscopies. Differential scanning
calorimetry and FTIR are also used to study crosslinking behav-
ior of both the model compound and polymers. Thermal prop-
erties of the crosslinked polymers are also studied by
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thermogravimetric analysis.
2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 49: 4335–4342, 2011
KEYWORDS: crosslinked polyamide; main chain benzoxazine
polymer; polymerization behavior; primary amine-functional
benzoxazine
INTRODUCTION Polybenzoxazine is a novel phenolic-type ther-
moset, which has attracted much interest in recent years due to
its outstanding properties. Despite having a few good properties
and wide industrial use of traditional phenolic resins, the short-
comings associated with these materials are well known. For
example, volatiles such as water or formaldehyde are released
during polymerization process due to condensation reaction.
Thus, these volatiles lead to poor mechanical properties due to
the formation of microvoids, not to mention about the health
concern the reaction byproducts cause. Another shortcoming is
the use of base or acid catalyst for polymerization, which results
in corrosion of the processing equipment. In this manner, prop-
erties belonging to polybenzoxazines, such as near-zero volu-
metric shrinkage and no byproduct release on polymerization,
low water absorption, excellent electrical thermal properties,
and good chemical resistance, low flammability,1–7 and very
good molecular design flexibility, make polybenzoxazines attrac-
tive for numerous applications. Polybenzoxazines are obtained
by thermally activated cationic ring-opening polymerization of
the corresponding benzoxazines without any added initiator1
and without generating any byproducts. Benzoxazine resins can
also be polymerized with initiators. Depending on the type of
initiators used, the resultant polymer takes either the phenolic
or arylether type.8,9 However, thermal treatment of the aryl
ether–type polybenzoxazine leads to the phenolic type.9 The
benzoxazine ring-containing monomers are easily prepared
from a phenolic derivative, a primary amine, and formalde-
hyde.1,10,11 For some applications, brittleness, necessity of mod-
erately high temperature for the ring-opening polymerization,
and difficulty of processing into thin film from the typical mono-
mers need to be overcome. Such needs lead to the recent
approaches in polybenzoxazine research. One of the approaches
consists of designing main-chain type polymers incorporating
benzoxazine moieties as a repeating unit either as pendant
groups or in the main chain.12–26 In this manner, this class of
benzoxazine polymers can be processed into tough films,
thereby behaving as thermoplastics. In addition, the product can
be fully or partially crosslinked by thermal treatment, adding
typical advantages of thermosets to the final product.
Although many benzoxazine resins have been reported,7 no
successful attempt has been reported until recently27 on the
synthesis of primary amine-functional benzoxazines despite
its great potential to be the raw materials for many benzoxa-
zine monomers and polymers.
Polyamides, as a major class of polymers, find applications in
fibers and film technology due to their inherent flame
retardance, thermal and chemical resistances, and good me-
chanical properties. Having realized this versatile applicabil-
ity of polyamides, it appears worthwhile to study the effect
*On leave from Tanta University, Egypt.
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of incorporating benzoxazine into polyamide structure. How-
ever, the common weakness of polyamide is its sensitivity of
the properties to moisture content as well as poor creep re-
sistance, as is usually the case for many thermoplastic poly-
mers. On the other hand, the problem usually raised for
thermosetting polymers is their brittleness. By combining
the thermoplastic nature of polyamide and thermosetting
nature of the crosslinked polybenzoxazine, we created ther-
moplastic/thermoset crossover molecules that offer processi-
bility, ductility, and film formability advantages of thermo-
plastics while adding to them are the improved thermal,
chemical, and creep resistances as well as dimensional stabil-
ity of thermosetting resins. Reduced moisture sensitivity was
an added advantage. Thus, developing the current cross-
linked polyamide has various advantages, in particular to
balance various mechanical and physical properties that was
lacking in traditional pure thermoplastics and thermosetting
resins.
Preparation of 3,30-(4,40-Methylenebis(4,1-
phenylene))bis(3,4-dihydro-2H-benzo[e][1,3]oxazin-6-
amine)
Primary amine-bifunctional benzoxazine (3,30-(4,40-methyle-
nebis(4,1-phenylene))bis(3,4-dihydro-2H-benzo[e][1,3]oxazin-6-
amine), P-ddm-NH2) was prepared following the method
developed in our laboratory, which is published elsewhere.27
1H NMR (DMSO-d6), ppm: d ¼ 3.71 (s, CH2), 4.44 (s, Ar-
CH2AN, oxazine), 4.57 (s, NH2), 5.22 (s, NACH2AO, oxazine),
6.28–7.05 (14H Ar). 13C NMR (DMSO-d6), ppm: d ¼ 49.07
(Ar-CAN, oxazine), 78.45 (OACAN, oxazine). FTIR m (cmꢁ1
)
¼ 3351 (NAH stretching), 1502 (stretching of trisubstituted
benzene ring), 1222 (asymmetric stretching of CAOAC),
1182 (asymmetric stretching of CANAC), 950 (out-of-plane
CAH).
Preparation of N,N0-(3,30-(4,40-Methylenebis(4,1-
phenylene))bis(3,4-dihydro-2H-benzo[e][1,3]oxazine-6,3-
diyl))dibenzamide, Model Compound
In this study, a primary amine-bifunctional benzoxazine,
which was recently developed in our laboratory,27 was used
for the synthesis of aliphatic and aromatic benzoxazine-con-
taining polyamides, which are collectively abbreviated as
poly(amide-benzoxazine). In addition, a model amide-con-
taining bifunctional benzoxazine monomer was also studied.
The preparation, characterization, and properties of model
compound and main-chain type polybenzoxazines as well as
their thermosets will be discussed.
P-ddm-NH2 (0.4 g, 0.861 mmol) and triethylamine (2 mL)
were dissolved in 13 mL of DMAc, cooled in an ice bath, and
stirred for 15 min. Then, a solution of benzoyl chloride
(0.254 g, 1.808 mmol) in 15 mL chloroform was added drop-
wise for 15 min. After addition of acid chloride, the mixture
was stirred for 1 h in the ice bath. After diluting with chloro-
form, the crude product was washed three times with 0.5 N
NaOH aqueous solution and additional three times with
water. The solution was dried over sodium sulfate anhy-
drous, followed by precipitation in hexanes. Removal of sol-
vent by evaporation afforded a white powder (Yield 95%).
EXPERIMENTAL
Materials
Adipoyl chloride, p-aminophenol, benzoyl chloride, 4-chloro-
benzene, isophthaloyl chloride, sodium borohydride, tri-
fluoroacetic anhydride, and paraformaldehyde were used as
received from Sigma-Aldrich. 4,40-Diaminodiphenyleneme-
thane (DDM) was purchased from Johnson Matthey Catalog.
Dimethylacetamide (DMAc), chloroform, ethyl acetate, hex-
ane, methanol, triethylamine, xylenes, sodium hydroxide, and
sodium sulfate were obtained from Fisher.
1H NMR (DMSO-d6), ppm: d ¼ 3.70 (s, CH2), 4.60 (s, Ar-
CH2AN, oxazine), 5.37 (s, NACH2AO, oxazine), 6.70–7.94
(24H, Ar) and 10.08 (s, NH). 13C NMR (DMSO-d6), ppm: d ¼
49.13 (Ar-CAN, oxazine), 78.95 (OACAN, oxazine), 131.32
(CANH) and 164.58 (Ar-(C¼¼O)ANH). FTIR (KBr), cmꢁ1
:
3311 (NAH stretching), 1646 (carbonyl stretching), 1498
(stretching of trisubstituted benzene ring), 1226 (asymmet-
ric stretching of CAOAC), 1186 (asymmetric stretching of
CANAC), 946 (out-of-plane CAH).
Characterization
A Varian Oxford AS600 at a proton frequency of 600 MHz
and a corresponding carbon frequency of 150.9 MHz was
Preparation of Main-Chain Type Aliphatic
Poly(amide-benzoxazine)
1
used to acquire H and 13C NMR spectra in deuterated dime-
thylsulfoxide (DMSO). Tetramethylsilane was used as an in-
ternal standard. Thermal stability was examined with a TA
Instruments Model 2920 differential scanning calorimeter
(DSC) using a heating rate of 10 ꢀC/min under a nitrogen
atmosphere. Fourier transform infrared (FTIR) spectra were
acquired on a Bomem Michelson MB100 equipped with a
deuterated triglycine sulfide detector and a dry air purging
unit at a resolution of 4 cmꢁ1 using KBr plates. Dynamic me-
chanical analysis (DMA) was done on a TA Instruments
Model Q800 DMA applying controlled strain tension mode
In a three-necked, round-bottomed flask with a rubber sep-
tum and stirring magnet, a solution of P-ddm-NH2 (1 g,
2.152 mmol) in DMAc (13 mL) and triethylamine (1 mL)
was added. The flask was cooled to ꢁ5 ꢀC using an ice/
ethanol bath, and a solution of adipoyl chloride (0.394 g,
2.152 mmol) in DMAc (7 mL) was dropwise syringed to
the reaction mixture for 15 min. The reaction mixture was
then allowed to warm to room temperature and stirred for
24 h under nitrogen atmosphere. Then, the crude product
was filtrated to remove the formed salt, followed by pre-
cipitation into ethyl acetate (ꢂ100 mL). The solid was fil-
tered, washed with ethyl acetate, and dried overnight in
vacuum oven at room temperature to give a white solid
(yield 85%).
ꢀ
with amplitude of 10 lm and a ramp rate of 3 C/min. Ther-
mogravimetric analysis (TGA) was performed on a TA Instru-
ments Model Q500 TGA with a heating rate of 10 ꢀC/min
under a nitrogen atmosphere.
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1H NMR (DMSO-d6), ppm: d ¼ 1.58 (COACH2ACH2), 2.26
(CH2ACH2ACH2), 3.68 (Ar-CH2-Ar), 4.53(Ar-CH2AN, oxa-
zine), 5.31 (OACH2AN, oxazine), 6.5–7.5 (Aromatics) and
9.68 (NH). FTIR (KBr) (cmꢁ1): 3272 (NAH stretching), 1660
(carbonyl stretching), 1500 (stretching of trisubstituted ben-
zene ring), 1226 (asymmetric stretching of CAOAC), 1187
(asymmetric stretching of CANAC), 935 (out-of-plane CAH).
Preparation of Main-Chain Type Aromatic
Poly(amide-benzoxazine)
This polymer was prepared similarly as the aliphatic poly-
mer from P-ddm-NH2 (1 g, 2.152 mmol) in DMAc (13 mL),
triethylamine (1 mL), and isophthaloyl chloride (0.437 g,
2.152 mmol) in DMAc (7 mL). The solid was filtered,ꢀwashed
with ethyl acetate, and dried in vacuum oven at 50 C over-
night to give a white solid (yield 85%).
1H NMR (DMSO-d6), ppm: d ¼ 3.69 (CH2), 4.59 (Ar-CAO, ox-
azine), 5.36 (OACAN, oxazine), 6.69–8.46 (Aromatics) and
10.26 (NH). FTIR (KBr), cmꢁ1: 3253 (NAH stretching), 1660
(carbonyl stretching), 1502 (stretching of trisubstituted ben-
zene ring), 1228 (asymmetric stretching of CAOAC), 1191
(asymmetric stretching of CANAC), 931 (out-of-plane CAH).
FIGURE 1 1H NMR spectra of P-ddm-NH2 and the amide-con-
taining model compound.
firmed by 1H NMR, 13C NMR, and FTIR spectroscopies. 1H
NMR spectra shown in Figure 1 for the model compound ex-
hibit a slight increase in the chemical shifts of the methylene
proton resonances of the oxazine ring from that of P-ddm-
NH2 benzoxazine structure from 4.44 and 5.22 ppm to 4.60
and 5.37 ppm, respectively. In addition, the appearance of
the NH-proton due to the amide bond at 10.08 ppm and the
disappearance of the resonance at 4.57 ppm due to the pri-
mary amine of P-ddm-NH2 were observed, indicating suc-
cessful formation of the amide groups.
Crosslinking of Poly(amide-benzoxazine)s
A solution of polymers in DMAc (30% solid content) has been
prepared. The solution has been cast over dichlorodimethylsi-
lane-treated glass plates. The solvent films have been dried at
ꢀ
70 C for 24 h under vacuum. Thereafter, the film as fixed on
glass plate has been thermally treated using the ꢀfollowing
heating schedule: 100, 120, 150, 180, 200, and 240 C for 2 h
each, unless otherwise indicated, to afford film of reddish
brown color and thickness of about 100 lm. The highest poly-
meriꢀzation temperature for the aliphatic sample was at
ꢀ
220 C for 2 h, instead of 240 C for 2 h of the aromatic poly-
Figure 2 shows FTIR spectra of model compound and P-ddm-
NH2. The characteristic peaks due to the NAH and C¼¼O
stretching modes of amide linkage are observed at 3311 and
mer. Films obtained under these polymerization conditions
were used for the dynamic mechanical analysis. However, for
the DSC study, the highest temperature for both linear and ar-
omatic polymers was set at 250 ꢀC for 1 h.
RESULTS AND DISCUSSION
Preparation of Model Compound
In order to study the reactivity of primary amine-bifunc-
tional benzoxazine (P-ddm-NH2) and the properties of am-
ide-containing benzoxazine, a model compound was synthe-
sized from the reaction of P-ddm-NH2 with benzoyl chloride,
as shown in Scheme 1. The chemical structure was con-
SCHEME 1 Preparation of an amide-containing model com-
pound. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
FIGURE 2 IR spectra of P-ddm-NH2 and the amide-containing
model compound. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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SCHEME 2 Preparation of aliphatic and aromatic main-chain type poly(amide-benzoxazine)s. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
1,646 cmꢁ1, respectively. In addition, characteristic absorp-
tions assigned to benzoxazine structure due to the CAOAC
and CANAC stretching, and the benzene ring mode of ben-
zoxazine at 1,226, 1,186, and 946 cmꢁ1, respectively, were
also observed. Both NMR and IR spectra suggest the forma-
tion of model amide-containing benzoxazine in high yield.
The molecular structure of the polymers was confirmed by
1H NMR and FTIR spectroscopies. Figure 3 shows the 1H
NMR spectra of both aliphatic and aromatic poly(amide-ben-
zoxazine)s along with the primary amine-bifunctional benzox-
azine (P-ddm-NH2). The spectrum of aliphatic poly(amide-
benzoxazine) exhibits a slight increase in the chemical shifts
of the methylene groups of the oxazine ring from 4.44 and
5.22 ppm to 4.53 and 5.31 ppm, respectively, besides the
appearance of the characteristic resonance at 9.68 ppm repre-
senting NAH proton from the amide linkage. Similarly, in case
of the aromatic poly(amide-benzoxazine), the chemical shifts
of benzoxazine structure are observed at 4.59 and 5.36 ppm,
and a new resonance of the amide NAH is shown at 10.26
ppm. The NMR data of the polymers are in accordance with
that of model compound. Common to both polymers is the
ACH2A of ddm unit at 3.70 ppm. The multiplets in the range
of 7.0–7.1 ppm are due to the aromatic protons of the ddm
unit. The multiplets of the aromatic protons of P-ddm-NH2 in
the range of 6.3–6.5 ppm shifted to the range of 6.3–6.9 ppm
on main-chain polymer formation. Multiple singlets in the
Preparation of Main-Chain Type
Poly(amide-benzoxazine)
Similar conditions as for the model compound were applied
for the synthesis of both aliphatic and aromatic poly(amide-
benzoxazine)s as shown in Scheme 2. Main-chain type poly-
benzoxazines were synthesized by solution polycondensation
of P-ddm-NH2 with both adipoyl and isophthaloyl dichloride
in DMAc as a solvent at room temperature, using triethyl-
amine as a base catalyst. Isolated yields of polymers were
about 85%, and the products were soluble in aprotic organic
solvents, such as dimethylformamide, N-methylpyrrollidone,
DMAc, and DMSO, at room temperature with ꢂ20% solid
content.
FIGURE 3 1H NMR spectra of P-ddm-NH2 and aliphatic and aro-
matic poly(amide-benzoxazine)s. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 4 IR spectra of P-ddm-NH2 and amide-containing poly-
mers. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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would result in the disappearance of the first exothermic
event as shown in the secꢀond scan after the treatment of the
model compound at 175 C for 10 s. On heat treatment, the
softening event around 110 ꢀC is clearly observed, further
supporting the destruction of the partial order. The exotherm
with the maximum centered at 244 ꢀC is attributed to the
ring-opening polymerization of benzoxazine. This is the typi-
cal temperature where ring-opening reaction is observed
without the presence of initiator, catalyst, or any special
structural effect, such as the influence of the orientation. The
heat treatment at 175 ꢀC did not influence at all the main
exothermic behavior, including the exotherm peak tempera-
ture and heat of polymerization. As no clear melting behav-
ior is observed for the nonheat-treated model compound, de-
spite its rather high purity, it is possible that the packing
structure of this model compound resembles metamorphic
structure of polyamide, whose ordered structure is between
amorphous and crystalline structures. Although aliphatic
polyamide, such as nylon 6, forms metamorphous structure
very easily30 and only with special conditions the amorphous
structure can be observed, the relaxation of model com-
pound may be slow due to the steric hindrance in packing
by the oxazine ring that is not coplanar to the benzene
ring.31
FIGURE 5 DSC of thermograms of P-ddm-NH2, model com-
pound, and heat-treated model compound at 175 ꢀC for 10 s.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
range of 7.3–8.5 ppm of both aliphatic and aromatic polymers
are due to the aromatic protons of the isophthalamide unit
introduced on aromatic main-chain formation.
The structure of model compound was studied after the heat
treatment. FTIR analysis shown in Figure 6 before and after
the thermal treatment indicates no changes in the structure.
This ꢀsupports the claim that the first exotherm centered at
163 C is not due to any configurational changes of the ben-
zoxazine structure.
The structure of the poly(amide-benzoxazine)s was further
confirmed by FTIR. Figure 4 shows the spectra of the poly-
mers. The band assignments were as per Bellamy28 and
Dunkers and Ishida.29 The characteristic band due to the
NAH stretching mode of the amide group was observed at
3,272 cmꢁ1 for polymers, whereas the characteristic absorp-
tion at 3,351 cmꢁ1 due to the NAH stretching mode of the
primary amine has disappeared. In addition, characteristic
band attributed to the Amide I band was also observed at
1,660 cmꢁ1 in both polymers. Furthermore, the benzene ring
mode of the benzoxazine group in the main chain was
observed at 935 and 931 cmꢁ1 for the aliphatic and aro-
matic poly(amide-benzoxazine)s, respectively. Thus, both
NMR and IR spectra indicate the formation of poly(amide-
benzoxazine)s.
DSC of Poly(amide-benzoxazine) as a Function of Heat
Treatment
The thermally activated crosslinking behavior of the main-
chain type poly(amide-benzoxazine)s through ring-opening
Study of Polymerization Behavior by DSC and FTIR
The effect of the presence of aromatic amide linkage on the
ring opening of benzoxazine in model compound was studied
by DSC. Figure 5 shows DSC thermographs of P-ddm-NH2
and model compound. As has been reported previously, the
polymerization behavior of P-ddm-NH2 consists of multither-
mal events, suggesting the catalytic interaction of the pri-
mary amine group with the benzoxazine structure [28]. The
DSC thermogram of model compound shows an exotherm
with the sharp maximum at 163 ꢀC, immediately after the
initial endothermic peak at 154 ꢀC. Such a phenomenon,
later confirmed by 1H NMR and FTIR (not shown), suggests
the enthalpic relaxation caused by partially ordered rear-
rangement of the molecules probably due to a favorable mo-
lecular structure at such a temperature assisted by the
hydrogen bonding among amide groups. This rearrangement
FIGURE 6 FTIR spectra of model compound before and after
the heat treatment at 175 ꢀC for 10 s. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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FIGURE 7 DSC of the aliphatic poly(amide-benzoxazine) after
various thermal treatment. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 9 FTIR spectra of the aliphatic poly(amide-benzoxa-
zine) after various thermal treatment.
bility for ring opening polymerization to take place easier
than the whole aromatic structure.
reaction was also studied by DSC and FTIR. Figures 7 and 8
show the DSC thermograms of the main-chain type poly(amide-
benzꢀoxazine)s after thermal treatments at 100, 150, 200, and
250 C for 1 h at each temperature.
In both poly(amide-benzoxazines)s, the exothermic peak
decreased after each treatment temperature. The exotherm
ꢀ
practically disappeared at 250 C, thereby showing the com-
pletion of the ring-opening reaction. The completeness of the
crosslinking via ring-opening polymerization of benzoxazine
was also confirmed by FTIR. Figure 9 shows the FTIR spec-
tra of the aliphatic poly(amide-benzoxazine) as an example.
The characteristic bands are seen due to the benzoxazine
structure at 1,500 cmꢁ1 (the trisubstituted benzene ring
mode), 1,226 cmꢁ1 (the CAOAC antisymmetric stretching
mode of oxazine), 1,187 cmꢁ1 (the CANAC antisymmetric
stretching mode of oxazine), and 935 cmꢁ1 (the benzene
ring mode of benzoxazine). By the end of thermal treatment
It was observed that the onset of exothermic peak, which is
attributed to the ring-opening polymerization of benzoxazine
in the main chain, took place at about 181 C with the maxi-
mum at 236 ꢀC for the aliphatic poly(amide-bemzoxazine).
However, the onset of the exothermic peak of aromatic poly-
(amide-benzoxazine) started at slightly higher temperature
of 204 ꢀC with maximum at 245 ꢀC. This can be attributed
to the flexiblizing effect of the aliphatic chain in aliphatic
poly(amide-benzoxazine), which assisted the segmental mo-
ꢀ
FIGURE 10 Dynamic mechanical spectra of the crosslinked
poly(amide-benzoxazine)s. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 8 DSC of the aromatic poly(amide-benzoxazine) after
various thermal treatment.
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benzoxazine) exhibits higher thermal stability than the poly-
mer containing aliphatic segment. For aliphatic poly(amide-
benzoxazine), the initial decomposition temperatures of 5
and 10% weighꢀt losses (T5 and T10) and the char yield at
ꢀ
ꢀ
800 C are 259 C and 312 C and 41%, respectively. For the
aromatic poly(amide-benzoxazine), the initial decomposition
temperatures of 5 and 10% weight losses (T5 and T10) and
the char yield at 800 ꢀC are 347 ꢀC and 403 ꢀC and 57%,
respectively. The uncrosslinked aromatic amide reported by
Behniafar and Khosrani-borna showed excellent thermal sta-
ꢀ
bility in the temperature range of 300–500 C. However, the
weight quickly reduced in the 600–700 ꢀC range, showing
ꢀ
the char yield at 700 C less than 35%. Yet, another uncros-
slinked aromatic polyamide based on sulfone group studied
by Zulfiqar and Sarwar42 reported the char yield at 600 ꢀC
to be about 40%. Our crosslinked poly(amide-benzoxazine)s,
even the aliphatic material, showed much higher char yield
at 800 ꢀC, indicating significant improvement in char forma-
tion by crosslinking through the oxazine reactions.
FIGURE 11 TGA thermograms of the crosslinked poly(amide-
benzoxazine)s after thermal treatment at 250 ꢀC for 30 min.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
CONCLUSIONS
A novel approach has been developed to synthesize thermo-
setting polyamides by incorporating benzoxazine structure as
a repeating unit in the main chain. For better understanding of
the polymerization reaction, a model compound was synthe-
sized from the primary amine-bifunctional benzoxazine with
benzoyl chloride in high purity. The main-chain type polymers
have also been prepared by polycondensation reaction of the
primary amine-bifunctional benzoxazine with adipoyl and iso-
phthaloyl dichloride in dimethylacetamide as solvent. The
structures of model compound and polyamides are confirmed
by FTIR and 1H NMR. The crosslinking of aliphatic poly(am-
ide-bezoxazine) took place at slightly lower temperature than
the aromatic polymer due to the flexiblizing effect of the ali-
phatic segment in the main chain. The polymers after cross-
linking showed good thermal stability appointing these mate-
rials as a good candidate for high performance applications.
ꢀ
at 250 C, the characteristic bands attributed to the benzoxa-
zine structure disappeared, supporting the conclusion
obtained by the DSC analysis.
Thermal Properties of Crosslinked
Poly(amide-benzoxazine)s
The results of dynamic mechanical analysis of the thermoset
obtained from the heat treatment of the poly(amide-benzoxa-
zine) at 250 ꢀC are shown in Figure 10 for the storage (E0)
modulus, loss (E00) modulus, and tan d for the two polymers.
The Tg was observed from the maxima of E00 at 236 and 169 C
ꢀ
for aromatic and aliphatic crosslinked poly(amide-benzoxa-
zine)s, respectively. In addition, a very weak transition was
observed under 100 ꢀC from the maxima of E00 for both ali-
phatic and aromatic polymers. As described elsewhere,3,22 b-
transition has been observed in crosslinked polybenzoxazine
resulting from a monomeric benzoxazine based on bisphe-
nol-A and aniline, abbreviated as poly(BA-a), around 90 ꢀC,
whereas the corresponding Tg was 170 ꢀC. It is therefore
possible that for crosslinked aromatic poly(amide-benzoxa-
zine), this transition is due to the b-transition, taking into
consideration that even though Tg of the current polymer is
higher than poly(BA-a), the presence of an aliphatic segment
in the repeating unit makes possible a b-transition at such a
value. Crosslinked aromatic polyamides32–40 have been stud-
ied in the past. However, the majority of the studies relate to
the reverse osmosis membrane development and either the
degradation as a result of crosslinking and permeation prop-
erties were of interest. Very little studies have been pub-
lished on the thermal property improvement of crosslinked
polyamide, in particular aliphatic polyamide.
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