Thermal cross-link behavior of an amorphous poly (para-arylene sulfide sulfone amide)

Article abstract of DOI:10.1016/j.polymer.2011.01.001

Cross-link behavior of an amorphous poly (para-arylene sulfide sulfone amide) synthesized via low temperature solution polycondensation was observed for the first time, when the polymer was subject to a series of thermal curing at 260 °C in air condition. The formation of cross-link network was demonstrated by the DSC and TGA results that T_g of the polymer enhanced from 259.17 °C to 268.89 °C, and the 1% weight loss temperature increased remarkably from 243.75 °C to 345.87 °C. EPR analysis further suggested that two kinds of free radicals, CO and C, induced by thermal curing were responsible for this cross-link behavior. According to FT-IR spectrum, the origin of these free radicals was confirmed as amide CO group in the polymer backbone. The cross-linking type was attributed to conventional radical cross-link reaction and the cross-link mechanism was discussed in detail subsequently.

Full text of DOI:10.1016/j.polymer.2011.01.001

Polymer 52 (2011) 1013e1018
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Thermal cross-link behavior of an amorphous poly (para-arylene sulfide
sulfone amide)
,b,
Yaqi Yang a, Gang Zhang b, Jing Liu c, Shengru Long b, Xiaojun Wang b, Jie Yang a
*
a College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering of China, Sichuan University, Chengdu 610065, PR China
b Institute of Materials Science and Technology, Sichuan University, Chengdu 610064, PR China
c College of Chemistry, Sichuan University, Chengdu 610064, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Cross-link behavior of an amorphous poly (para-arylene sulfide sulfone amide) synthesized via low
temperature solution polycondensation was observed for the first time, when the polymer was subject to
a series of thermal curing at 260 ꢀC in air condition. The formation of cross-link network was demon-
strated by the DSC and TGA results that Tg of the polymer enhanced from 259.17 ꢀC to 268.89 ꢀC, and the
1% weight loss temperature increased remarkably from 243.75 ꢀC to 345.87 ꢀC. EPR analysis further
suggested that two kinds of free radicals, COꢁ and Cꢁ, induced by thermal curing were responsible for this
cross-link behavior. According to FT-IR spectrum, the origin of these free radicals was confirmed as amide
C]O group in the polymer backbone. The cross-linking type was attributed to conventional radical
cross-link reaction and the cross-link mechanism was discussed in detail subsequently.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 13 September 2010
Received in revised form
10 December 2010
Accepted 1 January 2011
Available online 9 January 2011
Keywords:
Poly (para-arylene sulfide sulfone amide)
Thermal curing
Cross-link behavior
1. Introduction
physical properties, which may occur during thermal histories. In
recent years, a large number of experimental data have established
Poly (phenylene sulfide) (PPS) is a semi-crystalline thermo-
plastic engineering polymer, which has attracted much attention
from researchers due to its excellent mechanical properties, good
solvent resistance, high temperature resistance and so on [1e10].
As a structurally modified material of PPS, an amorphous poly
(arylene sulfide sulfone amide) (PASSA) was synthesized via low
temperature solution polycondensation in our laboratory [11]. It
possesses the desirable dimensional stability and chemical resis-
tance of PPS as well as the high strength of polyamide (PA); in
addition, it has higher glass-transition temperature than PPS which
makes up for the disadvantage of PPS. PASSA is soluble in some
polar solvents due to its improved solubility compared to PPS, such
as N-methyl-2-pyrrolidone (NMP) and N,N-dimethylformamide
(DMF), but it still has much higher chemical resistance than PA. The
superior characteristics of PASSA make it valuable and potentially
applicable in polymer electrolyte membranes, composite bipolar
plates, conductive materials, surface mounted devices, vehicle
sensors and so on [12e15].
the thermal properties of PPS [16e19]. Gies et al. [20] reported that
PPS thermal curing in the presence of oxygen leaded to oxygen
uptake in the form of sulfoxides in the PPS backbone. Perng [21]
researched the mechanism and kinetic model for thermal decom-
position behavior of PPS by stepwise pyrolysis/gas chromatography/
mass spectrometry and thermogravimetry analysis/mass spec-
trometry techniques. However, there were few literatures that dis-
cussed the thermal properties of amorphous structurally modified
materials of PPS. Wang [22] studied the thermal degradation of poly
(phenylene sulfide sulfone) (PPSS) by thermogravimetric analysis,
and suggested the reaction mechanism was a Dn deceleration type.
It is not yet clear about the thermal behavior before degradation of
these amorphous polymers. For the purpose of theoretical research
and application, it is necessary to study the thermal properties of
this PASSA resin.
In our previous work, the effect of annealing treatment on meta-
PASSA in nitrogen atmosphere was studied [23]. The result showed
that decarboxylation of the terminal carboxylic acid group occurred
during the annealing treatment, and annealed m-PASSA exhibited
different thermal properties compared to the untreated one.
However, no cross-link behavior of the polymer was discovered in
this work.
Thermal behavior is one of the most significant characteristics of
polymers due to its crucial influence in application. So it is of great
importance to understand the changes in chemical structures and
In the recent study, we present experimental results on thermal
curing of para-PASSA in air condition. Our aim is to learn the
thermal property of this polymer and the changes of chain
* Corresponding author. Institute of Materials Science and Technology, Sichuan
University, Chengdu 610064, PR China. Tel./fax: þ86 28 85412866.
E-mail address: ppsf@scu.edu.cn (J. Yang).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.01.001

1014
Y. Yang et al. / Polymer 52 (2011) 1013e1018
structure in different thermal histories. Thermal cross-link
behavior of the polymer is demonstrated for the first time via the
enhancement of glass-transition temperature and 1% weight loss
temperature. Amide C]O group in the polymer backbone produces
two kinds of free radicals which are responsible for this cross-link
behavior, and the reaction mechanism is discussed.
2. Experimental
2.1. Materials
4,40-Dichloro diphenyl sulfone (DCDPS) was supplied by Jinsheng
Chemical Reagents Company (Huaian, China). 4-Aminothiophenol
was acquired from Shouerfu Chemical Reagent Corporation (Jinyun,
China). 1,4-Benzenedicarbonyl chloride was purchased from Lianda
Chemical Reagent Corporation (Nanchang, China). Sodium hydroxide
(NaOH) and potassium carbonate (K2CO3) were gained from Chan-
glian Chemical Reagent Corporation (Chengdu, China). N,N-dime-
thylformamide (DMF) and anhydrous alcohol were obtained from
Kelong Chemical Reagent Corporation (Chengdu, China). N-methyl-
2-pyrrolidone (NMP) was provided by Jinlong Chemical Industry
Company (Nanjing, China). All the above chemicals were commercial
products and used as received.
Fig. 2. 1H NMR spectrum of BAPS.
by the cooling in air until the temperature drops to the indoor one.
The samples for ESR measurement should be quenched into ice
water after the treatment to freeze the unreacted free radicals.
2.5. Thermal property measurements
Differential scanning calorimetry (DSC) was performed using
the NETZSCH DSC 200 PC thermal analysis equipment with poly-
mer powders, fitted with a cooler system using liquid nitrogen. It
was operated at a gas rate of 10 ml/min in nitrogen atmosphere.
The heating rate of DSC measurement was 10 ꢀC/min.
2.2. Synthesis of p-PASSA
The monomer, 4,40-bis(4-aminophenylthio) diphenyl sulfone,
was synthesized using DCDPS via an aromatic nucleophilic substi-
tution reaction. The typical reaction condition was at 150 ꢀC in
nitrogen atmosphere.
Thermogravimetric analysis (TGA) measurements were con-
ducted using the TGA Q500 V6.4 Build 193 thermal analysis equip-
ment with a heating rate of 10 ꢀC/min in nitrogen atmosphere.
The synthesis of para-PASSA was performed following the poly-
condensation method in a glassbatch reactor. The proper amounts of
4,40-bis(4-aminophenylthio) diphenyl sulfone and appropriate
1,4-benzenedicarbonyl chloride with a molar ratio of 1.01/1.00 were
charged into the reaction tube of the polycondensation apparatus.
The reaction mixture was dissolved in NMP at freezing temperature
remaining for hours [11]. The product was washed repeatedly with
hot water and then dried at 120 ꢀC for 48 h. The synthesis route is
shown in Fig. 1.
2.6. Characterization of the polymer structure and molecular
weight
Fourier-transform infrared (FT-IR) spectroscopic measurements
were characterized by examining thin, homogeneous membranes
in the NEXUS670 FT-IR instrument.
Nuclear magnetic resonance (1H NMR) instrument was used to
provide the chain structure with a BRUKER-400 NMR Spectrometer.
The samples were dissolved in deuterated dimethyl sulfoxide.
X-ray diffraction (XRD) analyses were carried out with a Philips
electronic instrument (X’pert Pro MPD) using CueKa radiation at
40 kV and 150 mA with scanning from 0ꢀ to 50ꢀ.
2.3. Preparation of the p-PASSA membranes
The p-PASSA membranes were prepared by dissolving the
polymer powders in NMP, and then casting the solution on
microscopy slides directly. The films were subsequently dried in an
oven at 80 ꢀC for 24 h to remove the residual solvent.
Molecular weight distributions were measured by conventional
gel permeation chromatography (GPC) system equipped with
a Waters 515 Isocratic HPLC pump, a Waters 2414 refractive index
detector, GPC measurements were carried out at 60 ꢀC using DMF
as eluent with a flow rate of 1.0 mL/min. The system was calibrated
with linear polystyrene standards.
2.4. Thermal curing
The prepared polymer (powder or membrane) was subject to
a series of thermal histories by being heated at 260 ꢀC for 2 h, 6 h and
10 h, respectively, in air, oxygen and nitrogen environment, followed
2.7. Characterization of free radicals
Electron Paramagnetic Resonance (EPR) measurements were
conducted at 15 ꢀC on an X-band JEOL JES-FA200 EPR spectrometer,
and operated at a 9.4 GHz modulation field with microwave power of
1 mW. When the microwave power saturation method was used, the
microwave power was changed from 0.04 mW to 49 mW. The radical
concentration was characterized by the ESR spectrum intensity
compared to a standard sample of weak pitch. The sample powders
or membranes (18.5 ꢂ 0.2 mg) were examined in a quartz cell.
Table 1
Chemical shifts (d) of the monomer.
Chemical shifts
BAPS
d
(ppm)
Ha
Hb
Hc
Hd
He
7.661
7.134
7.038
6.615
5.620
Fig. 1. Schematic illustrations of synthesis route of p-PASSA.

Y. Yang et al. / Polymer 52 (2011) 1013e1018
1015
appear in the NMR spectrum, and the ratio of corresponding inte-
gral curves is 1:1:1:1:1, which implies the symmetric disubstitution
of the benzene ring. This structure can also be verified by the
representative FT-IR spectrum of the polymer (Fig. 3).
The presence of sulfuryl S]O stretch in the backbone of the
polymer is indicated by absorption at 1313 cmꢃ1 and 1155 cmꢃ1
[24], and the broad band in the 3245e3341 cmꢃ1 region corre-
sponds to the NeH stretch [25,26]. The absorption peaks around
763 cmꢃ1 and 827 cmꢃ1 are attributed to the CeH out of plane
stretch of 1,4-disubstituted benzene, which indicate a linear or
1,4-conjugated phenyl structure. This fact is in agreement with
Wang’s work on synthesis of poly (phenylenesulfidephenylen-
amine) by self-polycondensation of methyl-(4-anilinophenyl)
sulfide [27].
The aggregative structure of p-PASSA was determined by the
XRD analysis. The presence of strong polar O]S]O bonds in
a repeated unit of the polymer backbone imparts a highly amor-
phous character to the polymer as is evident from the presence of
a broad amorphous diffraction peak (Fig. 4) [28]. The position of
a quite wide peak in the range of 15e25ꢀ is taken as a measure
of the most probable intersegmental spacing [29], and the narrow
peak of p-PASSA after curing compared to the untreated one results
from the reduction of distances among molecules.
Fig. 3. FT-IR spectrum of p-PASSA before curing (black line), and after curing in air at
260 ꢀC for 10 h (dash line).
Molecular weight of the polymer was determined by GPC
analysis with Mn value of 59,565, and Mw value of 215,930.
3.2. TGA measurements
TG and DTG curves of the polymer, measured in nitrogen, are
presented in Fig. 5 and details are listed in Table 2. The onset
degradation temperature is as high as 456.37 ꢀC, which is a good
indication of the polymer thermal stability.
The comparison in Table 2 indicates that thermal stability of the
polymer is improved after curing. The 1% weight loss temperature
rises remarkably from 243.75 ꢀC to 345.87 ꢀC, and weight loss at
800 ꢀC decreases by about 2%. The mass loss is due to the elimi-
nation of volatile species from the polymer. The low 1% weight loss
temperature of p-PASSA before treatment implies that some vola-
tiles and low molecular weight oligomers are contained in the
samples [30]. The enhanced temperature of 1% weight loss and
reduced weight loss at 800 ꢀC after curing suggest that thermal
cross-link behavior of the polymer could exist and the oligomers
are efficiently incorporated into the cross-linked structure during
the treatment process. To identify the existence of cross-link
behavior, DSC measurements were employed as described below.
Fig. 4. Wide-angle X-ray diffraction of p-PASSA before curing (black line), and after
curing in air at 260 ꢀC for 10 h (red line). (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article).
3. Results and discussion
3.1. Structure and molecular weight characterization
The chemical structures of monomer and polymer were char-
acterized with 1H NMR and FT-IR spectrometer. 1H NMR spectra for
4,40-bis(4-aminophenylthio) diphenyl sulfone (BAPS) is shown in
Fig. 2 and details are tabulated in Table 1. Five groups of peaks
3.3. DSC measurements
Fig. 6 represents the DSC curves of p-PASSA samples which were
subject to different thermal histories. The endothermic peaks
Fig. 5. TG and DTG curves of p-PASSA before curing (a), and after curing in air at 260 ꢀC for 10 h (b) heated at 10 ꢀC/min in nitrogen.

1016
Y. Yang et al. / Polymer 52 (2011) 1013e1018
Table 2
Thermal properties of p-PASSA: results of TGA measurements.
Samples
1% weight loss
temperature (ꢀC)
Onset degradation
temperature (ꢀC)
Maximal degradation
temperature (ꢀC)
Weight loss at 800 ꢀC (%)
p-PASSA before curing
p-PASSA after curing
243.75
345.87
456.37
457.84
490.69
490.84
59.12
57.54
Fig. 6. DSC thermograms of p-PASSA before curing (a), and after curing in air at 260 ꢀC for 10 h (b) measured with a 10 ꢀC/min heatecooleheat cycle in nitrogen.
revealed in the scans are attributed to the glass transition of the
polymer [23]. In Fig. 6a, the Tg value increases by nearly 4 ꢀC in the
second heating curve. This is indicative of curing and indicating the
presence of cross-link structure [31]. After thermal curing at 260 ꢀC
for 10 h in air, the Tg value increases from 259.17 ꢀC to 268.64 ꢀC
(Fig. 6b), and the Tg values in the first and second heating curves are
almost the same. This fact implies that the cross-link structure has
already formed. Furthermore, the second sample was put in a vial
with w3 mL of NMP. After 1 week, the sample was not dissolved in
the solvent. This also illustrates that thermal curing has occurred to
a sufficient extent to form the cross-link material.
the line feature clearly shows an enhancement of the peaks with
the increase of curing time, which indicates a rise of R1 free radical
concentration.
The variation of radical concentrationwith time is shown in Fig. 8.
The curve of concentration versus treatment timecan be dividedinto
three stages. The first 3-h is a stage of activating free radicals. In this
stage, free radicals have little chance to impact each other, which is
caused by their few existences, so the concentration rises with time
linearly from 0.007 ꢄ 1013 spins mgꢃ1 to 4.36 ꢄ 1013 spins mgꢃ1. The
radicals increase slightly in the next stage from 3 to 6 h, only from
4.36 ꢄ 1013 spins mgꢃ1 to 6.62 ꢄ1013 spins mgꢃ1. It is considered
that the free radicals are initiated and consumed via the cross-link-
ing reaction in this stage [33]. The rate of radical generation is only
a little above the rate of consumption, which results in the mildly rise
of concentration. When the treatment time is above 6 h, the
concentration enhances linearly again. It is inferred that a network
structure has already formed in this stage, and the free radicals
produced are not consumed anymore. This fact corresponds to the
DSC result at the same curing temperature.
3.4. EPR measurements
EPR analysis is used to identify the generation of free radicals
[32], and confirm the cross-link reaction type. The effect of thermal
curing on p-PASSA powders in a time span of 0e10 h was studied by
using the EPR spectrum. As it can be seen from Fig. 7, a typical single
line shape (g ¼ 2.0023) is exhibited in the spectrum. R1 is used
hereinafter to represent this kind of free radicals. A comparison of
From the line shape in Fig. 7 one can see that the left side of the
peak is a little wider than the right one, which is highlighted by an
Fig. 7. EPR spectrum of p-PASSA samples for different thermal curing times at 260 ꢀ
C
in air.
Fig. 8. Concentration of R1 free radicals vs. time of thermal curing.

Y. Yang et al. / Polymer 52 (2011) 1013e1018
1017
Fig. 9. EPR spectrum of p-PASSA samples cured in air at 260 ꢀC for 10 h with different power.
Fig. 10. Schematic illustrations of EPR signals at different microwave power: (a) 2 mW; (b) 36 mW.
arrow. It is possible that another weak peak (R2) covered by R1
exists, so the microwave power saturation method is employed to
confirm and differentiate them [34].
In conclusion, the different saturated characteristics of R1 and R2
are the reason for broadened left side of the peak in EPR signals.
The phenomenon that EPR signal enhanced with the increase of
thermal curing time suggested that R1 may be oxygen free radicals
because of the treatment environment in air. In order to analyze the
origin of these free radicals, a group of contrast experiments were
carried out in N2, air and O2 atmosphere, respectively, with
a thermal curing at 260 ꢀC for 10 h.
A comparison of the three signals in EPR spectrum (Fig. 11a)
clearly shows that the radical concentration produced in N2 is much
lower than that in air, and close to the untreated one, meanwhile O2
environment induces the highest radical concentration. Fig. 11b is
the amplifying signal of the sample cured in N2, from which one can
see that the R1 signal decreased sharply due to the curing envi-
ronment, and R2 signal is clearer in this picture (highlighted by an
arrow). So it is considered that the production of R1 and R2 is closely
related to O2, and R1 could be ROꢁ free radicals.
With the increase of microwave power in Fig. 9a, R2 signal
appeared gradually, which could be observed in the amplifying
signal of 8 mW (Fig. 9b). Due to the few number of R2 free radicals,
the R2 signal is so weak that covered the R1 signal. Meanwhile, the
EPR signal enhanced to maximum at 2 mW, and then weakened
gradually. The variations of signal intensities are attributed to the
saturated characteristic of R1 radicals [33], which is easily saturated
by microwave power at a level around 2 mW. The phenomenon
that left side of the peak widened with the increase of power could
be explained by the existence of R2 free radicals. As shown in Fig.10,
the EPR signal is the superposition of R1 (blue line) and R2 (red line).
When microwave power grows above 2 mW, R1 signal is saturated
and weakens gradually. However, R2 signal does not saturate with
power easily, which increases slightly in the power range (Fig. 10b).
Fig. 11. EPR spectrum of p-PASSA samples cured in different atmosphere at 260 ꢀC for 10 h.

1018
Y. Yang et al. / Polymer 52 (2011) 1013e1018
regarding the processing and mechanical properties of this material
are in active progress and the results will be presented in our future
articles.
Acknowledgements
This work is financially supported by the National High Tech-
nology Research and Development Programme of China (863
program) with the project No. 2007AA03Z561. We would like to
express our sincere thanks to Dr. Guodong Li of Jilin University for
his kind assistance in Electron Paramagnetic Resonance (EPR)
measurement.
Fig. 12. Schematic illustrations of the cross-link reaction process.
According to the FT-IR spectrum of the polymer (Fig. 3), an
obvious decrease in the amide C]O peak at 1644 cmꢃ1 after curing
is observed, which coincides with our inference. Therefore it can be
concluded that the origin of R1 and R2 is the amide C]O group.
Combined with EPR analysis it is reasonable that R1 are COꢁ free
radicals, and R2 are Cꢁ free radicals. The cross-linking reaction
mechanism is suggested as below.
References
[1] Schuster M, de Araujo CC, Atanasov V, Andersen HT, Kreuer KD, Maier J.
Macromolecules 2009;42:3129e37.
[2] Naffakh M, Marco C, Gomez MA, Gomez-Herrero J, Jimenez I. J Phys Chem B
2009;113:10104e11.
[3] Jeon IY, Lee HJ, Choi YS, Tan LS, Baek JB. Macromolecules 2008;41:7423e32.
[4] Naffakh M, Marco C, Gomez MA, Gomez-Herrero J, Jimenez I. J Phys Chem B
2009;113:7107e15.
[5] Zou H, Wang K, Zhang Q, Fu Q. Polymer 2006;47:7821e6.
[6] Naffakh M, Marco C, Gomez MA, Gomez-Herrero J, Jimenez I. J Phys Chem B
2008;112:14819e28.
[7] Sasanuma Y, Hayashi Y, Matoba H, Touma I, Ohta H, Sawanobori M, et al.
Macromolecules 2002;35:8216e26.
[8] Guo YL, Bradshaw RD. Polymer 2009;50:4048e55.
[9] Zhang RC, Xu Y, Lu A, Cheng KM, Huang YG, Li ZM. Polymer 2008;49:2604e13.
[10] Feller LM, Cerritelli S, Textor M, Hubbell JA, Tosatti SGP. Macromolecules
2005;38:10503e10.
[11] Yang J. CN1935874.
[12] Barique MA, Wu LB, Takimoto N, Kidena K, Ohira A. J Phys Chem B 2009;113:
15921e7.
3.5. Possible reaction mechanism of cross-link
Based on the research hereinbefore, the thermal cross-link
behavior of p-PASSA could be explained by the following mechanism.
The double bonds of amide C]O are opened when they are
subject to thermal curing in air, and two kinds of free radicals,
Cꢁ and COꢁ, are produced consequently. With the effect of O2, most
Cꢁ are transformed into COꢁ free radicals, which causes the higher
concentration of R1 than R2. The cross-linking network was formed
afterward which was induced by these two kinds of free radicals.
The cross-linking reaction type belongs to conventional radical
cross-link reaction, and the reaction process is shown in Fig. 12.
[13] Xia LG, Li AJ, Wang WQ, Yin Q, Lin H, Zhao YB. J Power Sources 2008;178:
363e7.
[14] Huang J, Baird DG, McGrath JE. J Power Sources 2005;150:110e9.
[15] Mighri F, Huneault MA, Champagne MF. Polym Eng Sci 2004;44(9):1755e65.
[16] Tanthapanichakoon W, Furuuchi M, Nitta KH, Hata M, Endoh S, Otani Y. Polym
Degrad Stab 2006;91:1637e44.
4. Conclusions
[17] Hawkins RT. Macromolecules 1976;9(2):189e94.
[18] Scobbo JJ, Hwang CR. Polym Eng Sci 1994;34(23):1744e9.
[19] Gies AP, Geibel JF, Hercules DM. Macromolecules 2010;43:952e67.
[20] Gies AP, Geibel JF, Hercules DM. Macromolecules 2010;43:943e51.
[21] Perng LH. Polym Degrad Stab 2000;69:323e32.
[22] Wang HD, Yang J, Long SR, Wang XJ, Yang Z, Li GX. Polym Degrad Stab 2004;
83:1637e44.
An amorphous p-PASSA was synthesized via low temperature
solution polycondensation. The polymer with high glass-transition
temperature (259 ꢀC) and thermal stability suggests a potential use
in heat-resistant materials.
We have demonstrated, for the first time, the thermal cross-link
behavior of the polymer at 260 ꢀC in air condition. The results
showed that the formation of cross-link network enhanced the Tg
from 259.17 ꢀC to 268.89 ꢀC, and the 1% weight loss temperature of
the polymer increased remarkably from 243.75 ꢀC to 345.87 ꢀC.
Two kinds of free radicals, COꢁ and Cꢁ, existed in the polymer, and
concentration of the former was much higher than that of the latter.
These two kinds of free radicals were responsible for the inter-
molecular cross-linking behavior. The cross-linking type was
attributed to conventional radical cross-link reaction, and the
reaction mechanism was also suggested.
[23] Liu J, Zhang G, Long SR, Wang XJ, Yang J. J Macromol Sci Part B Physics 2010;
49(2):229e35.
[24] Lakshmi RTSM, Vyas MK, Brar AS, Varma IK. Eur Polym J 2006;42:1423e32.
[25] Song Z, Baker WE. J Polym Sci Part A Polym Chem 1992;30:1589e600.
[26] Semsarzadeh MA, Poursorkhabi V. Polym Degrad Stab 2009;94:1860e6.
[27] Wang LX, Wang FS, Zhang JP, Wang RS, Soczka-Guth T, Mullen K. Synthetic
Met 1999;101:320.
[28] Poojari Y, Clarson SJ. Macromolecules 2010;43:4616e22.
[29] Niemela S, Leppanen J, Sundholm F. Polymer 1996;37(18):4155e65.
[30] Iseki T, Narisawa M, Katase Y, Oka K, Dohmaru T, Okamura K. Chem Mater
2001;13:4163e9.
[31] Mukherjee I, Drake K, Berke-Schlessel D, Lelkes PI, Yeh JM, Wei Y. Macro-
molecules 2010;43:3277e85.
[32] Bhadra S, Khastgir D. Polym Degrad Stab 2007;92:1824e32.
[33] Niu YM, Wang GB, Zhang SX, Na Y, Jiang ZH. Polym Int 2005;54:180e4.
[34] Hill DJ, Choi B- K. Polym Int 1999;48:956.
This cross-linkable polymer represents a new class of super engi-
neering plastic with great thermal stability. Further investigations