Pyridinyl-containing benzoxazine: Unusual curing behaviors with epoxy resins

Article abstract of DOI:10.1016/j.polymer.2014.02.039

A pyridinyl-containing benzoxazine (6) was synthesized from the Mannich condensation of 4-phenyl-2,6-bis(4-aminophenyl) pyridine (2), phenol, and paraformaldehyde. For the purpose of properties comparison, a benzoxazine (7), which is structurally similar to (6) except for the pyridinyl group, was prepared. The solvent effect on the synthesis of (6) was discussed, and toluene/ethanol was found to provide (6) with the best purity and yield. The pyridinyl group provides solubility and acts as a catalyst for the ring opening of benzoxazine, as supported by the forward curing in the DSC thermograms. When curing with epoxy resins, a carbonyl absorption at 1670 cm^-1 and 192 ppm was observed in the IR and ¹³C NMR spectra. It is proposed that the formation a cyclic amide structure is responsible for the absorption. A reaction mechanism including nucleophilic addition, Diels-Alder reaction, and rearrangement was proposed. The pyridinyl group acts as a crosslinking site, and results in thermosets with good thermal properties.

Full text of DOI:10.1016/j.polymer.2014.02.039

Polymer 55 (2014) 1666e1673
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Pyridinyl-containing benzoxazine: Unusual curing behaviors with
epoxy resins
,
a
a
a
a
Ching Hsuan Lin a , Yu Sin Shih , Meng Wei Wang , Chun Yu Tseng , Tzu Chun Chen ,
*
Hou Chien Chang a , Tzong Yuan Juang
,
b
*
a Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
b Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A pyridinyl-containing benzoxazine (6) was synthesized from the Mannich condensation of 4-phenyl-
2,6-bis(4-aminophenyl) pyridine (2), phenol, and paraformaldehyde. For the purpose of properties
comparison, a benzoxazine (7), which is structurally similar to (6) except for the pyridinyl group, was
prepared. The solvent effect on the synthesis of (6) was discussed, and toluene/ethanol was found to
provide (6) with the best purity and yield. The pyridinyl group provides solubility and acts as a catalyst
for the ring opening of benzoxazine, as supported by the forward curing in the DSC thermograms. When
curing with epoxy resins, a carbonyl absorption at 1670 cmꢀ1 and 192 ppm was observed in the IR and
13C NMR spectra. It is proposed that the formation a cyclic amide structure is responsible for the ab-
sorption. A reaction mechanism including nucleophilic addition, DielseAlder reaction, and rearrange-
ment was proposed. The pyridinyl group acts as a crosslinking site, and results in thermosets with good
thermal properties.
Received 1 December 2013
Received in revised form
5 February 2014
Accepted 9 February 2014
Available online 21 February 2014
Keywords:
Pyridinyl
Benzoxazine
Curing behaviors
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
phenyl-2,6-bis(4-aminophenyl) pyridine (2), phenol, and para-
formaldehyde. A benzoxazine (7), which is structurally similar to
Benzoxazines are resins that can be polymerized to thermosets
by means of thermally activated ring-opening reactions, and have
been reviewed by many authors [1e5]. Thermosets with low water
absorption, superior electrical properties [6] and low surface en-
ergy [7] can be obtained after curing. There are two types of
difunctional benzoxazines, including bisphenol-based benzox-
azines, and diamine-based benzoxazines. Bisphenol-based ben-
zoxazines are prepared by the condensation of bisphenol, primary
monoamine, and formaldehyde. Diamine-based benzoxazines are
prepared by the condensation of diamine, formaldehyde, and
monophenol. It has been reported that incorporating pyridinyl
groups into polymers can enhance solubility and thermal stability
[8e13]. Therefore, we are interested in developing a pyridine-
containing benzoxazine. Pyridinyl-containing amines are gener-
ally prepared by Chichibabin reaction [14e17] and modified Chi-
chibabin reaction [18]. The diamines, 4-aryl-2,6-bis(4-
aminophenyl) pyridines, can be prepared by a modified Chichiba-
bin reaction [8]. In this work, we report the synthesis of a pyridinyl-
containing benzoxazine (6) from the Mannich condensation of 4-
(6) except for the pyridinyl group, was prepared for properties
comparison. When (6) acts as an epoxy curing agent, unusual
curing behaviors was found. Detailed synthesis of monomers,
curing mechanism, and properties of the resulting thermosets are
provided in this work.
2. Experimental part
2.1. Materials
Benzaldehyde, and paraformaldehyde were purchased from TCI.
Phenol, ammonium acetate, and hydrazine hydrate 80%were pur-
chased from Showa. Palladium on carbon 10%, p-nitro-
acetophenone were purchased from Acros. Glacial acetic acid and
acetic anhydride were purchased from Scharlau. Boron trifluoride
diethyl etherate was purchased from ALFA. Diglycidyl ether of
bisphenol A (DGEBA) with an epoxy equivalent weight (EEW) of
187 g/eq and dicyclopentadiene epoxy (DCPDE, commercial name
HP-7200) with EEW 269 g/eq were kindly supplied by Chang Chun
Plastics, and Dainippon Ink and Chemicals Corporation, respec-
tively. All solvents were HPLC grade and were purchased from
Tedia.
* Corresponding authors.
E-mail address: linch@nchu.edu.tw (C.H. Lin).
http://dx.doi.org/10.1016/j.polymer.2014.02.039
0032-3861/Ó 2014 Elsevier Ltd. All rights reserved.

C.H. Lin et al. / Polymer 55 (2014) 1666e1673
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Scheme 1. Synthesis of benzoxazines (6e7).
Scheme 2. The proposed reaction mechanism between (6) and epoxy.
2.2. Preparation of diamine (2)
Diamine (2) was prepared by a modified Chichibabin reaction,
followed by reduction (Scheme S1, Supporting Information) [8]. 1H
NMR (ppm, DMSO-d6),
d
¼ 5.42 (4H, NH2), 6.72 (4H, H2), 7.48 (1H,
H11), 7.54 (2H, H10), 7.81 (2H, H6), 7.94 (2H, H9), 8.04 (4H, H3). 13
C
NMR (ppm, DMSO-d6),
d
¼ 112.71(C6), 113.66 (C2), 126.57 (C4),
127.04 (C9), 127.75 (C3), 128.82 (C11), 128.97 (C10), 138.56 (C7),
148.62 (C8), 149.85 (C1), 156.60 (C5). Melting point from DSC ther-
mogram: 200.8 ꢁC. Melting enthalpy 88.6 J/g.
Table 1
Solvent effects on benzoxazine synthesis.
Run Sample Temp Time Solvent
Reaction status
ID
(ꢁC)
(h)
1
2
3
4
5
(6)
(6)
(6)
(6)
(6)
80
80
80
60
80
24
24
24
24
24
1,4-dioxane
xylene
toluene
chloroform
1,4-dioxane/ Homogeneous but low yield
ethanol (2:1)
A lot of gelation
A lot of gelation
A lot of gelation
Homogeneous but low yield
6
7
(6)
(6)
80
80
24
24
xylene/
ethanol (2:1)
toluene/
Homogeneous but low yield
Homogeneous with high yield
2.3. Preparation of diamine (5)
ethanol (2:1)
8
(7)
(7)
(7)
60
80
80
24
24
24
chloroform
toluene/
ethanol (2:1) polymerization
toluene/
Powder precipitation during
polymerization
Powder precipitation during
Diamine (5) was prepared by a three-step procedure according
to the literature (Scheme S2, Supporting Information) [19]. 1H NMR
9
(ppm, DMSO-d6)
d
¼ 5.2 (4H, NH2), 6.7 (4H, H2), 7.4e7.5 (7H,
H3,10,11), 7.5e7.8 (5H, H6, 9,12). Melting point from DSC thermogram:
10
Powder precipitation during
ethanol (1:1) polymerization
229.3 ꢁC. Melting enthalpy 77.5 J/g.

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C.H. Lin et al. / Polymer 55 (2014) 1666e1673
Fig. 1. 1H NMR spectra of (2) and (6) in DMSO-d6.
2.4. Synthesis of benzoxazines (6e7)
one-third of its molecular weight. Benzoxazine/epoxy thermosets
were prepared from an equivalent amount of (6) and epoxy resins
(DGEBA and DCPDE). The mixture was stirred at 140 ꢁC and
transferred to an aluminum mold, and then cured at 160, 180, 200
and 220 ꢁC for 2 h each in an air-circulating oven. Thereafter,
samples were allowed to cool slowly to room temperature to pre-
vent cracking. The sample ID is referred to as the (6)/DGEBA and
(6)/DCPDE thermoset, respectively.
Benzoxazine (6) was prepared by a one-pot procedure (Scheme
1). (2) 3.00 g (9 mmol), phenol 1.6735 g (18 mmol), para-
formaldehyde 1.068 g (36 mmol), and a mixed solvent of toluene/
ethanol 30 mL (2/1, V/V) were introduced into a round-bottom
100 mL glass flask equipped with a nitrogen inlet, a condenser,
and a magnetic stirrer. The reaction mixture was stirred at 80 ꢁC for
24 h. After that, the solution was poured into methanol. The pre-
cipitate was filtered and dried in a vacuum oven. Yellow powder
(80% yield) was obtained. Benzoxazine (7) was similarly prepared
except the starting diamine is (5). Yellow powder (90% yield) was
obtained.
2.6. Characterization
Differential scanning calorimetry (DSC) was performed using a
PerkineElmer DSC 7 in a nitrogen atmosphere at a heating rate of
10 ꢁC/min. Thermal gravimetric analysis (TGA) was performed
with a PerkineElmer Pyris1 at a heating rate of 20 ꢁC/min in a
nitrogen or air atmosphere. Dynamic mechanical analysis (DMA)
was performed with a PerkineElmer Pyris Diamond DMA with a
sample size of 2.0 ꢂ 1.0 ꢂ 0.2 cm. The storage modulus E0 and
2.5. Preparation of benzoxazine/epoxy thermosets
Since the pyridinyl group also reacted with epoxy resins (to be
discussed in Scheme 2), the equivalent weight of benzoxazine (6) is
Table 2
Solubility data of benzoxazines (6e7).a
Sample ID
Solvent
NMP
DMAc
DMF
DMSO
Dioxane
CH2Cl2
CHCl3
THF
Toluene
Xylene
(6)
(7)
þ
ꢃ
þ
ꢃ
þ
ꢃ
þ
ꢃ
þ
ꢃ
þ
ꢃ
þ
ꢃ
þ
ꢃ
ꢃ
ꢃ
e
e
þ, soluble; þh, soluble on heating; ꢃ, partially soluble on heating; e, insoluble on heating.
a
Solubility was tested with a 10 mg sample in 1 mL of solvent.

C.H. Lin et al. / Polymer 55 (2014) 1666e1673
1669
1185
1032
1260
1232
R.T.
945
140 C
160 C
180 C
200 C
220 C
1800
1600
1400
1200
1000
800
Wavenumber (cm-1)
Fig. 2. IR spectra of (6) after accumulative curing at each temperature for 20 min.
tan
d were determined as the sample was subjected to the tem-
perature scan mode at a programmed heating rate of 5 ꢁC/min at
a frequency of 1 Hz. The test was performed using a bending
mode with an amplitude of 5 mm. Thermal mechanical analysis
(TMA) was performed with a PerkineElmer Pyris Diamond TMA
at a heating rate of 5 ꢁC/min. NMR measurements were per-
formed using a Varian Inova 600 NMR in DMSO-d6, and the
chemical shift was calibrated by setting the chemical shift of
DMSO-d6 as 2.49 ppm. The infrared spectra were recorded using a
Nicolet Avatar 320 FTIR spectrophotometer. IR Spectra were ob-
tained from at least 32 scans in the standard wavenumber range
of 400e4000 cmꢀ1 by
a
PerkineElmer RX1 infrared
spectrophotometer.
3. Results and discussion
3.1. Synthesis of benzoxazine (6)
Benzoxazine (6) was prepared from the Mannich condensation
of (2), paraformaldehyde, and phenol (Scheme 1).
Table 1 lists the solvent effect on the synthesis of (6). Using
homo-solvents, such as 1,4-dioxane (Run 1), xylene (Run 2), and
toluene (Run 3) led to insoluble gelation. Homogeneous solution
can be achieved using chloroform (Run 4) and mixed solvents
such as 1,4-dioxane/ethanol (Run 5), xylene/ethanol (Run 6), and
Toluene/ethanol (Run 7). Among these solvents, toluene/ethanol
Fig. 4. IR spectra of (a) (6)/DGEBA and (b) (6)/DCPDE after accumulative curing at each
temperature for 20 min.
(2/1, V/V) (Run 7) gave the best purity and yield. In our previous
work [20], we proposed that the solvation effect of ethanol en-
hances the solubility of benzoxazine. This strategy also works
effectively in this study. Fig. 1 shows the NMR spectra of (2) and
(6) prepared by Run 7. The amino peaks of (2) disappeared
completely, and new peaks a and b for oxazine were observed at
4.6 and 5.5 ppm. However, signals of impurity at 9.2e9.8 ppm
and 4.0e4.5 were observed in (6), probably due to partially ring-
opened structure of (6). Recently, Endo et al. reported the prep-
aration of 1,3-benzoxazine bearing 4-pyridyl group, and found
viscous substance. They speculated that the viscous product was
a resole-type byproduct, of which formation was promoted by
highly basic 4-aminopyridine [21]. Therefore, the impurity in
Fig. 1 might also be related with the basicity of pyridinyl group. In
the IR spectrum, absorptions at 945 cmꢀ1 for oxazine, and ab-
sorptions at 1032 cmꢀ1 (symmetric stretch) and 1232 cmꢀ1
(asymmetric stretch) for AreOeC were observed, supporting the
structure of (6).
Fig. 3. DSC thermograms of benzoxazines (6) and (7).

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C.H. Lin et al. / Polymer 55 (2014) 1666e1673
13
Fig. 5. C NMR spectrum of (6)/DGEBA after curing at (a) 140 and (b)180 ꢁC for 20 min.
3.2. Synthesis of benzoxazine (7)
in chloroform (Run 8) and toluene/ethanol (Runs 9e10) results in
powder precipitate (not gelation) after 1 h due to the poor solubility
of (7), which limits the purity of (7). Fig. S1 shows the 1H NMR
spectra of (5) and (7). The amino peaks of (5) at 5.2 ppm dis-
appeared completely, while new peaks a and b for oxazine were
observed at 4.7 and 5.5 ppm. Table 2 lists the solubility of (6) and
(7). While pyridinyl-containing (6) shows good solubility in several
Diamine (5), a precursor for benzoxazine (7) synthesis, was
prepared by a three step procedure [19]. Benzoxazine (7) was
prepared from the Mannich condensation of (5), para-
formaldehyde, and bisphenol A (Scheme 1 and Table 1 Run 8e10).
In contrast to the good solubility of (6), preparing benzoxazine (7)
Fig. 6. DSC thermograms of (6) and (6)/DGEBA.

C.H. Lin et al. / Polymer 55 (2014) 1666e1673
1671
10
Compared with (7), a forward curing temperature was observed for
(6). The result demonstrates the catalytic effect of pyridinyl moiety.
Very recently, Takeichi et al. reported the synthesis of pyridinyl-
containing benzoxazine monomer (B-3py) from bisphenol A,
paraformaldehyde, and 3-aminopyridine [27]. They also found a
forward curing temperature and attributed it to the catalytic effect
on the basicity of the pyridinyl group. They proposed that the
zwitterionic intermediate of ring-opened benzoxazine is stabilized
by the basic pyridinyl group, thus accelerating the ring-opening
polymerization at lower temperatures. Although the curing
occurred, as supported by IR and DSC data, the preparation of void-
free thermoset of (6) is not successful for unknown reasons.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
10
10
10
10
10
10
10
(6)/DGEBA
(6)/DCPDE
3.4. Curing of (6)/epoxy
50
100
150
200
250
Temperature( C)
Fig. 4a shows the IR spectra of the (6)/DGEBA curing system. The
epoxy absorption at 915 disappeared completely after curing at
160 ꢁC. The characteristic absorptions at 945, 1032, and 1232 cmꢀ1
disappeared gradually with the progress of curing. Unlike the
curing of neat (6) (Fig. 2), the characteristic peaks of oxazine did not
disappear completely in the (6)/DCPDE system after curing at
220 ꢁC for 20 min, suggesting a slower curing rate. This result in-
dicates that the catalytic effect of pyridinyl group on the curing of
benzoxazine is not obvious, probably due to dilution effect when
compared polymerization behaviors of benzoxazine/epoxy mixture
with that of benzoxazine. Unexpectedly, a special carbonyl ab-
sorption at 1670 cmꢀ1 was observed in Fig. 4a. A similar phenom-
enon was observed in the (6)/DCPDE system (Fig. 4b).
Xue and Ishida et al. studied the reactions between poly(4-vinyl
pyridine) and epoxy resins [28e30]. They reported that the re-
actions include nucleophilic addition, DielseAlder reaction, and
rearrangement. The reactions led to pyridone or cyclic amide
structure, as observed in the IR spectra at 1650 and a shoulder at
1670 cmꢀ1, respectively [28e30]. The order of reactivity of the
pyridine derivate and epoxy is related with the structure of the
pyridine derivate, where the order of reactivity is pyridine >> 2-
methylpyridine > 2,6-dimethylpyridine [29]. The reactivity de-
creases rapidly with substituents in the ortho position of the pyr-
idinyl group due to the steric hindrance of nucleophilic addition. In
the case of the 2,6-dimethylpyridine, both positions adjacent to the
nitrogen are occupied by the methyl group, and almost no reaction
was observed in their reports. In this case, the two phenyl groups
that are ortho to the nitrogen are expected to hinder the reaction
between pyridinyl and epoxy resin. However, an absorption of
Fig. 7. DMA thermograms of the (6)/DGEBA and (6)/DCPDE thermosets.
solvents, (7) shows poor solubility in organic solvents. This result
shows the advantage of the pyridinyl group in solubility. The poor
solubility of (7) makes it difficult to process, which will be dis-
cussed below.
3.3. Curing of benzoxazine (6)
Fig. 2 shows the IR spectra of (6) after accumulative curing at
each temperature for 20 min. The absorptions at 945, 1032, and
1232 cmꢀ1 decreased gradually with the progress of curing, and
disappeared completely after curing at 200 ꢁC for 20 min. The
Mannich-type CeNeC absorption at 1185 cmꢀ1 and AreOH ab-
sorption at 1260 cmꢀ1 appeared at a later stage of curing. This result
suggests that N,O-acetal-type structure was formed at the initial
stage of curing, and rearranged to Mannich-type structure, a ther-
modynamically more stable structure, at elevated temperatures
[22e25]. This curing behavior was found in the catalyzed ring-
opening of benzoxazine [22]. Since no extra catalyst was added in
this system, a self-catalyzed characteristic was found. The unpaired
electrons of the pyridinyl group are thought to be related with the
self-catalyzed characteristic. Gu et al. also found a self-catalyzed
phenomenon in the curing of a benzimidazole-containing ben-
zoxazine [26]. Fig. 3 shows the DSC thermograms of (6) and (7).
(6)/DGEBA
(6)/DCPDE
100
(6)/DGEBA (N2)
(6)/DGEBA (air)
(6)/DCPDE (N2)
(6)/DCPDE (air)
80
60
40
20
0
100
200
0
100
200
300
400
500
600
700
800
Temperature (oC)
Temperature(0C)
Fig. 8. TMA curves of the (6)/DGEBA and (6)/DCPDE thermosets.
Fig. 9. TGA thermograms of (6)/epoxy in N2 and air.

1672
C.H. Lin et al. / Polymer 55 (2014) 1666e1673
Table 3
Thermal properties of the (6)/DGEBA and (6)/DCPDE thermosets.
Curing system
E0 (GPa)a
Tan
d
(ꢁC) (DMA)a
Tg (ꢁC) (TMA)b
CTE (ppm/ꢁC)c
Td5% (ꢁC)d
Char yielde
N2
N2
air
air
(6)/DGEBA
(6)/DCPDE
2.19
2.20
209
217
175
185
51
48
396
410
400
414
38
27
35
25
a
Measured by DMA at a heating rate of 5 ꢁC/min; storage modulus (E0) are recorded at 50 ꢁC; Tg was determined from the peak temperature of the tan
Measured by TMA at a heating rate of 5 ꢁC/min.
d curve.
b
c
Coefficient of thermal expansion is recorded from 50 ꢁC to 150 ꢁC.
d
e
Temperature corresponding to 5% weight loss by thermogravimetry at a heating rate of 20 ꢁC/min in nitrogen and air.
Residual weight % at 800 ꢁC in nitrogen and air.
1670 cmꢀ1 indicates that the reaction occurred, and the cyclic
values are relatively high when compared with the B-a/epoxy
thermoset, which possesses 5% degradation temperature at 302e
309 ꢁC, dependent on the curing catalyst [32]. Takeichi et al. re-
ported that pyridinyl-containing polybenzoxazine (PB-3py) has the
same thermal stability as PB-a [27]. This result in Fig. 9 demon-
strates that thermosets based on diamine-based benzoxazine
display better thermal stability than thermosets based on
bisphenol-based benzoxazine, which are consistent with those
reported in our previous work [33].
amide is the product of this system. Fig. 5 shows the 13C NMR
spectrum of (6)/DGEBA after curing at 140 ꢁC and 180 ꢁC (the same
thermal treatment as those in Fig. 4). The spectrum is difficult to
assign for each peak. However, a carbonyl signal at 192 ppm was
observed after thermal treatment at 180 ꢁC. The carbonyl signal,
which was not seen after 140 ꢁC treatment, supports the formation
of cyclic amide. Fig. 6 shows DSC thermograms of (6)/epoxy. A
forward exothermic peak was observed for (6)/epoxy when
compared with that of (6). The forward exotherm is attributed to
the nucleophilic addition of pyridinyl group to epoxy. According to
Xue and Ishida [28e30], IR (Fig. 4), 13C NMR (Fig. 5), and DSC data
(Fig. 6), a reaction mechanism between (6) and epoxy is proposed
in Scheme 2. Step I is the nucleophilic addition of the pyridinyl
group on epoxy, forming an oxygen ion. Step II is the nucleophilic
4. Conclusions
We have successfully prepared a pyridinyl-containing benzox-
azine (6) from the Mannich condensation of (2), phenol, and
paraformaldehyde using toluene/ethanol as the reaction solvent.
When cured with epoxy resins, the pyridinyl group provides solu-
bility, as supported. An unexpected carbonyl absorption was
observed at 1670 cmꢀ1 in the IR spectrum. In addition, a carbonyl
signal at 192 ppm was observed in the 13C NMR spectrum. Referring
to IR, DSC, and 13C NMR data and the conclusion of Xue and Ishida
[28e30], a reaction mechanism including nucleophilic addition,
DielseAlder reaction, rearrangement, and ring-opening of ben-
zoxazine was proposed in the (6)/epoxy curing system. DMA data
show Tg is as high as 209 and 217 ꢁC for the (6)/DGEBA and (6)/
DCPDE thermoset, respectively. The Tg value of 209 ꢁC is relatively
high for a thermosets based on DGEBA, showing a moderate-to-
high Tg characteristic of (6) as an epoxy curing agent. A possible
reason is the difunctional benzoxazine (6) is acting as a trifunc-
tional curing agent, since pyridinyl also acts as a curing site with
epoxy resins. Although the reactions between pyridine and epoxy
have been reported [28e30], to the best of our knowledge, no
literature has reported the reactions between epoxy and pyridinyl-
containing benzoxazine with two bulky substitutions ortho to
pyridinyl.
attack of the oxygen ion on the a-position of the pyridine, forming a
conjugated diene after the aromatic structure of pyridine was
destroyed. A DielseAlder reaction is then carried out in step III for
the conjugated diene. Finally, a cyclic amide was formed after the
rearrangement in step IV. The rearrangement starts at tempera-
tures higher than 160 ꢁC, as supported by the appearance of
carbonyl absorption in Fig. 4. Step V is the ring-opening of oxazine
leading to crosslinking structure.
3.5. Thermal properties of benzoxazine/epoxy thermosets
Fig. 7 shows the DMA thermograms of the (6)/epoxy thermo-
sets. Only one tan
geneous copolymer was obtained. The Tg obtained from the peak
temperature of tan
is 209 and 217 ꢁC for the (6)/DGEBA and (6)/
d peak was observed, indicating that a homo-
d
DCPDE thermoset, respectively. The value is relative high for
benzoxazine/DGEBA-based thermosets. For example, the Tg of B-a/
DGEBA is 154e175 ꢁC, depending on the ratio of B-a and DGEBA
[31]. Note that B-a is a benzoxazine based on bisphenol A/formal-
dehyde/aniline. This result demonstrates a high Tg characteristic of
(6) as an epoxy curing agent. A possible reason is the difunctional
benzoxazine (6) is acting as a trifunctional curing agent, since
pyridinyl also acts as a curing site with epoxy resins, as shown in
Scheme 2. Takeichi et al. reported that the Tg of pyridinyl-
containing polybenzoxazine (PB-3py) is 40 ꢁC higher than bisphe-
nol A/formaldehyde/aniline-based polybenzoxazines (PB-a) due to
the strong hydrogen bonding between the pyridinyl moiety and
phenolic group [27]. However, no (7)/epoxy data are available for
the comparison of properties in this work due to the poor solubility
of (7) in epoxy and in organic solvents. TMA measurement (Fig. 8)
shows the Tg value of (6)/DGEBA and (6)/DCPDE thermoset is 175
and 185 ꢁC, respectively. Coefficient of thermal expansions (CTE) is
51 and 48 ppm/oC, respectively.
Acknowledgments
The authors thank the National Science Council of the Republic
of China for financial support.
Appendix A. Supplementary data
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.polymer.2014.02.039.
References
[1] Ghosh NN, Kiskan B, Yagci Y. Prog Polym Sci 2007;32(11):1344e91.
[2] Ishida H, Agag T. Handbook of benzoxazine resins. Elsevier; 2011.
[3] Yagci Y, Kiskan B, Ghosh NN. J Polym Sci Part Polym Chem 2009;47(21):5565e
76.
[4] Nair CPR. Prog Polym Sci 2004;29(5):401e98.
[5] Takeichi T, Agag T. High Perform Polym 2006;18(5):777e97.
[6] Ning X, Ishida H. J Polym Sci Part Polym Chem 1994;32(6):1121e9.
Fig. 9 shows the TGA thermograms of the (6)/DGEBA and (6)/
DCPDE thermosets, with the results listed in Table 3. The 5%
degradation temperature for (6)/DGEBA and (6)/DCPDE is 396 and
410 ꢁC in nitrogen, and 400 and 414 ꢁC in air, respectively. The

C.H. Lin et al. / Polymer 55 (2014) 1666e1673
1673
[7] Wang C-F, Su Y-C, Kuo S-W, Huang C-F, Sheen Y-C, Chang F-C. Angew Chem
Int Ed 2006;45(14):2248e51.
[20] Lin CH, Chang SL, Shen TY, Shih YS, Lin HT, Wang CF. Polym Chem 2012;3(4):
935e45.
[8] Tamami B, Yeganeh H. Polymer 2001;42(2):415e20.
[9] Wang K-L, Liou W-T, Liaw D-J, Huang S-T. Polymer 2008;49(6):1538e46.
[10] Tamami B, Yeganeh H. J Polym Sci Part Polym Chem 2001;39(21):3826e31.
[11] Liaw D-J, Wang K-L, Chang F-C, Lee K-R, Lai J-Y. J Polym Sci Part Polym Chem
2007;45(12):2367e74.
[12] Wang X, Li Y, Gong C, Zhang S, Ma T. J Appl Polym Sci 2007;104(1):212e9.
[13] Liaw D-J, Wang K-L, Chang F-C. Macromolecules 2007;40(10):3568e74.
[14] Chichibabin A, Zhur Zeide O. Russ Fiz Khim Obshch (J Russ Phys Chem Soc)
1914;46:1212.
[21] Kawaguchi AW, Sudo A, Endo T. J Polym Sci Part Polym Chem 2014;52(3):410e6.
[22] Sudo A, Kudoh R, Nakayama H, Arima K, Endo T. Macromolecules
2008;41(23):9030e4.
[23] Sudo A, Hirayama S, Endo T. J Polym Sci Part Polym Chem 2010;48(2):479e84.
[24] Wang YX, Ishida H. Polymer 1999;40(16):4563e70.
[25] Endo T, Sudo A. J Polym Sci Part Polym Chem 2009;47(19):4847e58.
[26] Yang P, Gu Y. J Polym Sci Part Polym Chem 2012;50(7):1261e71.
[27] Shibayama Y, Kawauchi T, Takeichi T. High Perform Polym; 2013. http://
dx.doi.org/10.1177/0954008313495245.
[15] Frank RL, Seven RP. J Am Chem Soc 1949;71(8):2629e35.
[16] Li JJ. Chichibabin pyridine synthesis. Name reactions. Springer; 2009. pp. 107e
9.
[17] Katritzky A, Rees C. Comprehensive heterocyclic chemistry, vol. 2. Oxford:
Pergamon; 1984. p. 395.
[28] Xue G, Ishida H, Koenig JL. Die Makromol Chem Rapid Commun 1986;7(1):
37e41.
[29] Xue G, Ishida H, Koenig JL. Polymer 1986;27(7):1134e7.
[30] Xue G, Ishida H, Koenig JL. Die Angew Makromol Chem 1986;142(1):17e27.
[31] Ishida H, Allen DJ. Polymer 1996;37(20):4487e95.
[18] Weiss M. J Am Chem Soc 1952;74(1):200e2.
[19] Spiliopoulos IK, Mikroyannidis JA. Macromolecules 1998;31(4):1236e45.
[32] Kimura H, Matsumoto A, Ohtsuka K. J Appl Polym Sci 2008;109(2):1248e56.
[33] Lin CH, Chang SL, Hsieh CW, Lee HH. Polymer 2008;49(5):1220e9.