Temperature for curing phthalonitrile-terminated poly(phthalazinone ether nitrile) reduced by a mixed curing agent and its curing behavior

Article abstract of DOI:10.1039/c5ra17234a

A mixture of ammonium molybdate tetrahydrate and urea (AMTU) was developed successfully to catalyze phthalonitrile to afford 2,4,6-tris(2-cyanophenyl)-1,3,5-triazine. Then AMTU was employed as a curing agent to promote the curing reaction of phthalonitril

Full text of DOI:10.1039/c5ra17234a

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Temperature for curing phthalonitrile-terminated
poly(phthalazinone ether nitrile) reduced by
a mixed curing agent and its curing behavior†
abc
Cite this: RSC Adv., 2015, 5, 92055
Zhihuan Weng,abc Jiayi Fu,bc Lishuai Zong,bc Cheng Liu,abc Jinyan Wang
*
and Xigao Jianabc
A mixture of ammonium molybdate tetrahydrate and urea (AMTU) was developed successfully to catalyze
phthalonitrile to afford 2,4,6-tris(2-cyanophenyl)-1,3,5-triazine. Then AMTU was employed as a curing
agent to promote the curing reaction of phthalonitrile-terminated poly(phthalazinone ether nitrile)
(PPEN-Ph), and the effect of the novel curing agent was studied by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). The initial curing temperature and apparent activation
energy Ea (based on the Kissinger equation) were reduced from 268.5 ꢀC and 201.5 kJ molꢁ1 using the
common curing agent ZnCl2, to 223.0 ꢀC and 78.4 kJ molꢁ1 using AMTU, respectively, under identical
curing conditions. Moreover, the resulting thermosetting resin over AMTU showed excellent thermal
stability; the Td5% and Td10% were 487 ꢀC and 540 ꢀC, respectively, and the char residue was up to 75% at
800 ꢀC. These results indicated that AMTU could reduce the curing temperature for PPEN-Ph effectively,
and it may be a good candidate as a curing agent for phthalonitrile resins.
Received 26th August 2015
Accepted 12th October 2015
DOI: 10.1039/c5ra17234a
www.rsc.org/advances
additives.12 Selvakumar et al. reported that the use of ionic
liquid as solvent and microwave irradiation signicantly
increased the rate and yield of the reaction.13 One of the
1. Introduction
Three-dimensional polymers containing aryl-s-triazine moieties
have superior thermal stability, excellent mechanical proper-
ties, unique optical and electrical properties; they have found
many applications in advanced technologies, such as aerospace
and marine.1–5 These polymers are synthesized via the thermal
curing reaction of phthalonitrile-terminated oligomers at high
temperature. Usually, there are two common curing agents for
this reaction: Lewis acids such as ZnCl2;6,7 and aromatic
diamines such as 1,3-bis(3-aminophenoxy)benzene (m-APB), bis
[4(4-aminophenoxy)phenyl]sulfone (p-BAPS), and bis(4-
aminophenyl)-sulfone (BAS).8–11 However, relatively high
curing temperature and long curing time are the major prob-
lems concerning the current state of research and application.
Thus, several methods have been developed to improve this
situation.12–18 Amir et al. successfully synthesized some new
amino- and imide-containing phthalonitrile compounds with
a 1 : 1 molar ratio of amino group to phthalonitrile unit, and
they were thermally polymerized even in the absence of curing
powerful approaches is to design high-performance curing
agent, it is also benet to enhance the properties of phthalo-
nitrile resins and extend their application, however, there are
a few papers on this area to date.19–22 4-Aminophenoxy phtha-
lonitrile has been synthesized and utilized to catalyze the curing
of resorcinol-based phthalonitrile monomer, this strategy was
limited by the design of monomer structure.19 Graphene oxide
(GO) possessing hydroxyl and epoxy groups was prepared and
examined as a curative agent for amino novolac phthalonitrile,
however, the dispersion of GO in monomers should be
improved at higher GO loading.20
The mixture of metal salt, urea, ammonium chloride and
ammonium molybdate have been employed as catalyst to
synthesize peruoroalkyl phthalocyanine from phthalonitrile
successfully.23,24 Our research has demonstrated that the
mixture of ammonium molybdate tetrahydrate and urea
(AMTU) also could effectively catalyze phthalonitrile to afford
2,4,6-tris(2-cyanophenyl)-1,3,5-triazine (TCPT) with NMP as
solvent at 200 ꢀC under air atmosphere (Scheme 1). In this
article, we attempted to employ AMTU as a curing agent of
phthalonitrile-terminated oligomers to reduce the curing
temperature.
aState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116024, China. E-mail: wangjinyan@dlut.edu.cn
bLiaoning High Performance Resin Engineering Research Center, Dalian University of
Technology, Dalian 116024, China
Our laboratory has synthesized a series of organosoluble
poly(phthalazinone ether nitrile) (PPEN), these polymers
showed high mechanical strengths and good thermal proper-
ties. Moreover, they can be easily cast into tough and creasable
cDepartment of Polymer Science & Materials, Dalian University of Technology, Dalian
116024, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra17234a
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value was taken at the inection point, and Tp and T 0 were
p
recorded at the exothermal peak of DSC curves.
2.3. Synthesis of TCPT
TCPT can be prepared from phthalonitrile over ZnCl2, BAS or
AMTU, a procedure with ZnCl2 was given as a typical example. A
100 mL three-necked round-bottom ask equipped with
mechanical stirrer under reux was charged with a mixture of
phthalonitrile (1.28 g, 10 mmol) and ZnCl2 (0.064 g, 0.47 mmol)
at a mass ratio of 20 : 1. Then, N-methyl pyrrolidone (NMP)
(5 mL) was added. The reaction mixture was stirred at 200 ꢀC for
8 h under air atmosphere. The reaction solution was diluted
with NMP and analyzed by HPLC and HPLC-MS. In the case of
AMTU, the mass ratio of ammonium molybdate tetrahydrate
and urea was 1 : 1. The reaction mixture was stirred at 150 ꢀC for
Scheme 1 Synthesis of TCPT from phthalonitrile over catalyst.
lms, their applications such as proton exchange membrane
and high-temperature coating have been successfully
studied.25,26 In order to improve the thermal stability of PPEN
and extend its application, two phthalonitrile (Ph) moieties
were introduced to the chain terminal of the polymers to afford
PPEN-Ph, and the curing reaction can be proceeded between the
terminal cyano groups over curing agent.27–29 With increasing
molecular weight, the glass transition temperature (Tg) of PPEN-
Ph will be risen and the number of terminal cyano groups will
be declined. Thus, the collision rate of cyano groups will be
reduced and the curing temperature will be increased,
furthermore, the curing peaks in differential scanning calo-
rimetry (DSC) will not be obvious. According to preliminary
studies, PPEN-Ph with molecular weight of 800 was employed in
this work, to test its curing behavior over various curing agents.
ꢀ
1 h rstly, and then hold at 200 C for another 7 h under air
atmosphere.
2.4. Synthesis of PPEN-Ph
The preparation of PPEN-Ph24,25 was carried out under nitrogen
atmosphere. DHPZ (23.80 g, 0.10 mmol), ground anhydrous
K2CO3 (19.35 g, 0.14 mmol), DMSO (30 mL), and toluene (50
mL) were placed in a 250 mL three-necked round-bottomed
ask equipped with mechanical stirring and reux as well as
a waꢀter segregator. The mixture was stirred and reuxed at 140–
150 C until the water was completely removed via azeotropic
distillation with toluene. Aer eliminated the excess toluene via
2. Experimental
2.1. Materials
ꢀ
elevating the temperature to 165 C, the system was cooled to
DHPZ was purchased from Dalian Polymer New Material Co.,
Ltd and recrystallized with DMAc. Anhydrous K2CO3 was
ground and dried in vacuum at 110 ꢀC for 24 h prior to use. All
solvents and other reagents were purchased from commercial
sources and used as received.
room temperature.
Then 2,6-diuorobenzonitrile (DFBN, 6.96 g, 0.05 mol) was
added into the mixture, and the reaction was hold at 110 ꢀC for
1 h, 160 ꢀC for 3 h, and 190 ꢀC for 2 h, respectively. Aer that, the
system was cooled down to 80 ꢀC, and 4-nitrophthalonitrile
(NPh, 19.04 g, 0.11 mol) was added. Then reaction temperature
was maintained at 80 ꢀC for 12 h. The nal solution was poured
2.2. Measurement and characterization
High performance liquid chromatogram (HPLC) was performed into hot water. The crude product was washed three times with
on an HP1100 liquid chromatography. Electrospray ionization- hot water and dried. The crude product was rened with CHCl3
mass spectrometry (ESI-MS) was performed on a HP1100LC/ and Soxhlet extracted with methanol for 24 h. A yellow product
MSD mass spectrometer equipped with an Agilent Extend C18 was recovered aer vacuum drying at 150 ꢀC for 24 h. Yield: 39.6
column in a liquid chromatography mode. Samples were g (95.7%). FT-IR (KBr, cmꢁ1): 3080 (]C–H), 2233 (C^N), 1675
determined by selected ion monitoring with ESI negative ioni- (C]O), 1593, 1508 (C]C), 1487, 1461, 1323, 1249 (Ph–O), 1168,
1
zation mode (CID ¼ 50 V). Infrared spectra were recorded on 1020, 977, 857, 783, 695. H NMR (400 MHz, DMSO-d6, ppm):
a Thermo Nicolet Nexus 470 Fourier transform infrared spec- 8.64 (br, 2H), 8.49 (br, 2H), 7.92 (br, 4H), 7.85 (br, 6H), 7.77 (br,
1
trometer, using potassium bromide pellets of solids. H NMR 1H), 7.70 (br, 2H), 7.41 (br, 2H), 7.34 (br, 4H).
(400 MHz) spectra were obtained on a Bruker Avance II 400M
nuclear magnetic resonance spectrometer operating at 25 ꢀC
using CDCl3 as solvent and listed in parts per million downeld
2.5. General procedure for thermal curing
from tetramethylsilane (TMS). Thermogravimetric analysis Thermosetting resins were prepared by the direct thermal
(TGA) curves were measured on a Mettler TGA/SDTA851 thermal curing of PPEN-Ph in the presence of curing agents, a typical
gravimetric analysis instrumentꢀ in owing nitrogen (50 experiment was carried out as following. A mixture of PPEN-Ph
mL minꢁ1) at a heating rate of 20 C minꢁ1 from 50 to 800 ꢀC. and curing agent with a mass ratio of 20 : 1 was blended,
Glass transition temperature (Tg) and curing temperature of ground, and pressed into a sheet. This sheet was then cured
maximum peaks (Tp and T0p) were determined with a Mettler successively at 250 ꢀC for 2 h, 280 ꢀC for 2 h, and 300 ꢀC for 8 h
DSC822 differential scanning calorimeter (DSC) under nitrogen under air atmosphere, to afford a black solid. In the case of
ow at a heating rate of 10 ꢀC minꢁ1 from 100 to 400 ꢀC. The Tg AMTU, the mixture was heated at 150 ꢀC for 1 h prior to curing.
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3. Results and discussion
3.1. Synthesis of TCPT over AMTU
To check the feasibility of employing AMTU as curing agent for
phthalonitrile-terminated oligomers, herein, trimerization of
phthalonitrile was chosen as the model reaction to compare the
activity of formation of 1,3,5-triazine over various catalysts. The
distribution of products and corresponding structures were
detected via HPLC and HPLC-MS. In the cases of AMTU and
ZnCl2, products of TCPT and compound 2 with molecular
weight of 384 and 402 were identied, respectively. Moreover,
the mole ratio of TCPT/compound 2 was 1/1.68 in the system
with AMTU, whereas this value was 1.38/1 with ZnCl2. This
result indicated that AMTU was more benecial to afford
compound 2 than ZnCl2 under the present reaction conditions.
Burchill et al.30 have reported that TCPT was produced from
phthalonitrile through route A (Scheme 2) under the presence of
organic amine or ammonium salt. In the case of AMTU, the
phenomenon probably can be explained by the existence of
weak base urea, which induced the reaction of route B to afford
compound 2 from 1, and the dehydration of compound 2, acted
as the rate-determining step, should need higher temperature
to promote the reaction. It is noted that neither TCPT nor
compound 2 was detected in the reaction with BAS, which
illustrated that trimerization of phthalonitrile can't be pro-
ceeded smoothly under the present reaction conditions. Based
on these results, it is worth to make attempt to employ AMTU as
a curing agent for phthalonitrile-terminated oligomers.
Scheme 2 Reaction route to afford TCPT and compound 2 over
AMTU.
AMTU were close to those with BAS and ZnCl2. However,
compared with the IR spectrum of pristine PPEN-Ph, the
intensity of peak around 2234 cmꢁ1 attribute to C^N was
3.2. Preparation of PPEN-Ph
As mentioned above, phthalonitrile-terminated PPEN has been
synthesized successfully in our lab (Scheme 3). Higher density
of terminal cyano groups will be benet for thermal curing, in
this work, the chain length was tuned by regulating the feed
ratio, and the molecular weight of PPEN-Ph was xed around
800. The structure of obtained PPEN-Ph was conrmed by FT-IR
and 1H NMR. In the FT-IR spectrum (Fig. 1a), the band for Ph–
O–Ph at 1249 cmꢁ1 was the evidence for reaction between DFBN
and DHPZ. And the peaks attribute to N–H (d ¼ 12.64) and O–H
(d ¼ 9.70) in DHPZ disappeared in the 1H NMR spectrum of
resulting oligomer (see Fig. S1 in ESI†). Combined these results,
it can be concluded that PPEN was completely terminated by
NPh.24 The thermal behavior of PPEN-Ph was investigated by
DSC. As seen in Fig. 2, the Tg of PPEN-Ph was 165 ꢀC
(10 ꢀC minꢁ1, second scan), and this supply the feasibility of
conducting the curing reaction at low temperature.
3.3. Thermal curing of PPEN-Ph
With the designed molecular weight PPEN-Ph in our hands, two
common curing agents BAS and ZnCl2 were employed as
reference to compare the curing behavior over AMTU. As the
products in curing of PPEN-Ph were insoluble in the tested
solvents, FT-IR has been conrmed to be a powerful character-
ization approach to detect the structure. As shown in Fig. 1b–d,
the spectra of the mixture aer curing over three curing agents
were similar, which indicated that the curing products over Scheme 3 Preparation of PPEN-Ph and its curing route.
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Fig. 3 TGA curves (20 ꢀC minꢁ1, N2) of cured PPEN-Ph over different
curing agents.
Fig. 1 IR spectra of PPEN-Ph (a) and cured PPEN-Phs with various
curing agents ((b) BAS; (c) ZnCl2; and (d) AMTU).
Table 1 The main data of TGA curves of resulting PPEN-Ph over
various curing agents
Curing agents
Td5%/ꢀC
Td10%/ꢀC
Cy800/%
BAS
ZnCl2
AMTU
483
488
487
512
536
540
79
76
75
under identical curing conditions. Therefore, it is reasonable to
assume that AMTU should be a competitive curing agent for
phthalonitrile resins compared with other agents.
3.4. Curing kinetics
To further study the performance of curing PPEN-Ph over
AMTU, curing kinetics were determined by DSC. PPEN-Ph was
blended with different curing agents with a mass ratio of 20 : 1,
then the mixtures were successively scanned at different heat-
ing rates (5 ꢀC minꢁ1, 10 ꢀC minꢁ1, 15 ꢀC minꢁ1, and 20 ꢀC
Fig. 2 DSC curve of PPEN-Ph.
declined, and some new peaks appeared. For example, two
peaks at 1773 cmꢁ1 and 1728 cmꢁ1 were assigned to the new
bonds of C]N and C–N, respectively, while peaks at 1405 cmꢁ1
and 1358 cmꢁ1 can be considered as evidence for formation of
aryl-s-triazine moieties. Thus, it can be said that the thermal
curing of PPEN-Ph can be proceeded smoothly over AMTU as
other common curing agents.
minꢁ1) from 100 C to 400 C on DSC. The DSC curves of the
reactions with different curing agents were presented in Fig. S2–
S4 (ESI†), and the main data were listed in Tables S1–S3 (ESI†).
It is noted that two distinct curing peaks appear in the DSC
ꢀ
ꢀ
ꢀ
curves: the rst one (Tp) was around 230–300 C and another
ꢀ
one (T0p) was in the range of 340–400 C. The curing reaction
was began from 230–300 ꢀC, as the reaction proceeded, the
movement of molecular chain segments was blocked, and the
collision rate between terminal cyano groups on the molecular
chain was declined. However, the molecular chain segments
can be continued to move with increasing curing temperature
and the curing reactions can be proceeded again. Therefore, the
curing reaction was divided into two steps, and there were two
distinct curing peaks in the DSC curves. Herein, the DSC curves
were analyzed based on the following assumption: the
exothermic heat evolved during curing is proportional to the
conversion of cyano groups.31 In the cases of BAS and AMTU, the
Thermal stability of the resulting thermosetting resin also is
the criteria to evaluate the performance of curing agent. TGA
curves of the cured PPEN-Ph over various curing agents were
illustrated in Fig. 3. The main data including the temperature at
weight loss 5% (Td5%) and 10% (Td10%), as well as the char yields
at 800 ꢀC (Cy800), were summarized in Table 1. Although the
Cy800 of cured PPEN-Ph over AMTU (75%) was little lower than
that over BAS (79%), the values of Td5% and Td10% over former
were higher than those over latter, respectively, and the Td10%
was enhanced by 28 ꢀC. Actually, the thermal properties of
cured PPEN-Ph with AMTU were very close to those with ZnCl2
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Table 2 Tp0 and T0 of curing reactions with different curing agents
resulting thermosetting resins are studied currently under way,
this work still can supply some valuable information for the
researchers in this eld to explore high-performance curing
agent on phthalonitrile resins.
p0
Curing agents
Tp0/ꢀC
T0p0/ꢀC
BAS
ZnCl2
AMTU
245.5
268.5
223.0
332.0
360.5
338.5
Acknowledgements
We gratefully acknowledge the nancial support of National
Natural Science Foundation of China (21074017, 51273029), the
Fundamental Research Funds for the Central Universities
(DUT15RC(3)032) and General Project of Scientic Research of
the Education Department of Liaoning Province (L2015116).
Table 3 Ea of curing reactions with different curing agents
Ea/kJ molꢁ1
Curing agents
Kissinger equation
Ozawa equation
BAS
ZnCl2
AMTU
66.7
201.5
78.4
72.2
200.3
82.9
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