A high molecular weight acrylonitrile copolymer prepared by mixed solvent polymerization: I. effect of monomer feed ratios on polymerization and stabilization

Article abstract of DOI:10.1039/c4ra10779a

A bifunctional comonomer β-methylhydrogen itaconate (MHI) was employed to prepare high molecular weight poly[acrylonitrile-co-(β-methylhydrogen itaconate)] [P(AN-co-MHI)] copolymers by radical polymerization in a mixed solvent of dimethyl sulfoxide/deionized water = 60/40 (wt/wt), which was used as a carbon fiber precursor instead of acrylonitrile terpolymers for improving the stabilization and spinnability simultaneously. The structure and stabilization of P(AN-co-MHI) copolymers with different monomer feed ratios were characterized by elemental analysis (EA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that both the polymerization conversion and molecular weight reduce with the increase of MHI content in the feed due to the larger molecular volume of MHI. The monomer reactivity ratios were calculated by Fineman-Ross and Kelen-Tüds methods and the reactivity of MHI is higher than that of AN. Two parameters E_s = A₁₆₁₈ cm^-1/A₂₂₄₄ cm^-1 and SI = (I₀ - I_s)/I₀ were defined to evaluate the extent of stabilization and the activation energy (E_a) of the cyclization. The FTIR, XRD and DSC results show that P(AN-co-MHI) copolymers exhibit significantly improved stabilization characteristics than the PAN homopolymer and poly(acrlonitrile-methyl acrylate-acrylic acid) terpolymer, such as a larger extent of stabilization, lower initiation temperature and smaller E_a of cyclization, which is attributed to the ionic initiation of the MHI comonomer and is beneficial for preparing high performance carbon fiber.

Full text of DOI:10.1039/c4ra10779a

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A high molecular weight acrylonitrile copolymer
prepared by mixed solvent polymerization: I. effect
of monomer feed ratios on polymerization and
stabilization
Cite this: RSC Adv., 2014, 4, 64043
a
a
b
a
a
a
*
*
Anqi Ju, Yafeng Yan, Dawei Wang, Jun Luo, Mingqiao Ge and Mengjuan Li
A bifunctional comonomer b-methylhydrogen itaconate (MHI) was employed to prepare high molecular
weight poly[acrylonitrile-co-(b-methylhydrogen itaconate)] [P(AN-co-MHI)] copolymers by radical
polymerization in a mixed solvent of dimethyl sulfoxide/deionized water ¼ 60/40 (wt/wt), which was
used as a carbon fiber precursor instead of acrylonitrile terpolymers for improving the stabilization and
spinnability simultaneously. The structure and stabilization of P(AN-co-MHI) copolymers with different
monomer feed ratios were characterized by elemental analysis (EA), Fourier transform infrared
spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show
that both the polymerization conversion and molecular weight reduce with the increase of MHI content
in the feed due to the larger molecular volume of MHI. The monomer reactivity ratios were calculated
by Fineman–Ross and Kelen–Tudos methods and the reactivity of MHI is higher than that of AN. Two
¨ ˜
parameters Es ¼ A1618
/A2244
cmꢀ1
and SI ¼ (I0 ꢀ Is)/I0 were defined to evaluate the extent of
cmꢀ1
stabilization and the activation energy (Ea) of the cyclization. The FTIR, XRD and DSC results show that
P(AN-co-MHI) copolymers exhibit significantly improved stabilization characteristics than the PAN
homopolymer and poly(acrlonitrile–methyl acrylate–acrylic acid) terpolymer, such as a larger extent of
stabilization, lower initiation temperature and smaller Ea of cyclization, which is attributed to the ionic
initiation of the MHI comonomer and is beneficial for preparing high performance carbon fiber.
Received 19th September 2014
Accepted 19th November 2014
DOI: 10.1039/c4ra10779a
www.rsc.org/advances
stabilization of PAN which plays an important role in the
properties of the carbon ber.16 Generally, acidic comonomers,
1. Introduction
such as acrylic acid (AA), methacrylic acid (MA) and itaconic
acid (IA), are introduced to reduce the initiation temperature of
cyclization and improve the stabilization of PAN.17,18 Neutral
comonomers like methyl acrylate (MA) and methyl methacrylate
(MMA) are used to improve the solubility, drawability and
spinnability of PAN.19,20 Many acrylonitrile terpolymers, such as
P(AN–MA–IA), P(AN–MMA–IA) and P(AN–MA–AA), are used as
precursor to prepare carbon ber.21 The spinnability of acrylo-
nitrile terpolymers has been improved by the incorporation of
neutral comonomers which act as a lubricant. However, the
initiation temperature of acrylonitrile terpolymers is almost as
high as PAN homopolymer's and the heat release is still
centralized, which breaks molecular chains during stabiliza-
tion.22 The reason is that the reactivity of acidic comonomers is
lower than that of neutral comonomers, which results in less
acidic comonomers incorporated into the polymer chains
during polymerization. In addition, the sequence structure of
terpolymer chains is difficult to control. For the above reasons,
the acrylontitrile terpolymers are not the best material for
carbon ber precursor. In our previous work a bifunctional
comonomer b-methylhydrogen itaconate (MHI) containing
Carbon ber, an ideal reinforcing material for advanced
composite materials, has been extensively applied in high-tech
aerospace, defense areas, and civil engineering (e.g., trans-
portation industry, architecture industry, medical industry, and
sports equipment).1–3 It has been documented that 90% of the
carbon bers used today are made from a polyarylonitrile (PAN)-
based precursor. In recent years, carbon nanobers, porous
carbons, nanocarbons made from acrylonitrile polymers have
been extensively studied, and can be used for catalysts,4–6
sensors,7 gas capture8 and supercapacitors.9–12 PAN homopol-
ymer has hardly been used as a precursor for carbon ber
because of its poor spinnability and stabilization such as high
stabilization temperature and centralized heat release.13–15
Therefore, a small amount of comonomers are usually incor-
porated into PAN chains to enhance solubility, spinnability,
hydrophilicity and drawability, and especially to promote the
aCollege of Textile & Clothing, Jiangnan University, Lihu road 1800, 214122 Wuxi,
China. E-mail: anqiju@163.com
bThe Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School
of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China.
E-mail: wangdw@jiangnan.edu.cn
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carboxyl group and ester group was synthesized to prepare poly
[acrylonitrile-co-(b-methylhydrogen itaconate)] [P(AN-co-MHI)]
copolymer by solution polymerization, which was used as
carbon ber precursor instead of acrylonitrile terpolymers.23 It
was found out that the stabilization and spinnability of PAN
were improved effectively by the bifunctional MHI. However,
the highest molecular weight of P(AN-co-MHI) copolymer
prepared by solution polymerization in our previous work is
only 9.85 ꢁ 104 g molꢀ1, which is not benecial to preparing
high performance carbon ber due to the low molecular weight
of P(AN-co-MHI).
In order to increase the molecular weight of P(AN-co-MHI), a
mixed solutions of dimethyl sulfoxide/deionized water ¼ 60/40
(wt/wt) was used as reaction media to synthesize P(AN-co-MHI)
copolymer in this work. The chain transfer constant of deion-
ized water is 0 for radical ꢂꢂꢂANc, which can avoid the chain
transfer of radical ꢂꢂꢂANc and improve the molecular weight
of P(AN-co-MHI) signicantly. The copolymerization of AN with
different amounts of MHI in the mixed solutions of dimethyl
sulfoxide/deionized water were investigated in detail. The effect
of monomer feed ratios on polymerization and stabilization
were studied by elemental analysis (EA), Fourier transform
infrared spectroscopy (FTIR), X-ray diffraction (XRD) and
differential scanning calorimetry (DSC) sufficiently.
2.3 Preparation of P(AN-co-MHI) by radical polymerization
in mixed solvents
Copolymerization of AN with different amounts of MHI were
carried out in a three-necked ask at 60 ꢃC under nitrogen
atmosphere using mixed solvents of DMSO/deionized water ¼
60/40 (wt/wt) as the reaction medium. In the reaction system,
the total monomer concentration was 24 wt%, and the
concentration of AIBN was 0.8 wt% on the basis of monomer.
Good agitation was used to make sure the heat of polymeriza-
tion throw off sufficiently and the reaction was terminated aer
12 h. The resultant precipitate was ltered and washed with
methanol for three times. The isolated precipitate was dried at
75 ꢃC under vacuum to a constant weight. For comparison, PAN
homopolymer and poly(acrlonitrile–methyl acrylate–acrylic
acid) [P(AN–MA–AA)] with molar feed ratios AN/MA/AA ¼ 98/2/2
was also prepared in this work.
In order to determine the monomer reactivity ratios, similar
polymerization conditions were used to prepare P(AN-co-MHI)
with the mole fraction of MHI in the feed varying from 1 to 5
mol% and the copolymerization was terminated at a low
conversion of #10%.
2.4 Characterization
Viscosity average molecular weight (Mv) of the acrylonitrile
polymers was measured in a (50 ꢄ 0.5) ꢃC water bath using
Ubbelolohde viscometer method. The equation is expressed as
follows (eqn (1)):
2. Experimental
2.1 Materials
Acrylonitrile (AN, analytical grade) was purchased from
Shanghai Boer Chemical Reagent Co., Ltd. Shanghai, China and
distilled twice before polymerization. Azodiisobutyronitrile
(AIBN, analytical grade) was commercially provided by Aladdin
Industrial Corporation, Shanghai, China and recrystallized
twice by methanol. Dimethyl sulfoxide (DMSO, analytical grade)
was received from Shanghai Huadong Reagent Company,
Shanghai, China and distilled under reduced pressure before
use. Itaconic acid (IA), petroleum ether, methanol, benzene and
benzoyl chloride (analytical grade) were commercially supplied
by Sinopharm Chemical Reagent Co., Ltd. Shanghai, China and
used as received.
0.759
[h] ¼ 2.83 ꢁ 10ꢀ4Mv
(1)
where [h] is the intrinsic viscosity calculated by linear extrapo-
lation. Five concentrations (0.008, 0.0054, 0.0040, 0.0032 and
0.0026 g mLꢀ1) were used for molecular weight calculation.16,24
Proton nuclear magnetic resonance 1H NMR (400 MHz) spectra
were recorded on a Bruker DMX-400 NMR spectrometer using
dimethyl sulfoxide (DMSO-d6) as a solvent at room temperature.
FTIR spectra of KBr disks were measured on a Nicoletis™10™
FTIR spectrophotometer at room temperature; 32 scans were
collected at a resolution of 1 cmꢀ1. 1 mg powder sample was
mixed thoroughly with 100 mg KBr and pelletized for FTIR
characterization. DSC curves were carried out on a TA instru-
2.2 Synthesis of bifunctional comonomer MHI
ment Q200. The samples were scanned at different heat rates
ꢃ
To a 100 mL round-bottomed ask was added IA (13.00 g, 100.00 from ambient temperature to 375 C under N2 atmosphere (40
mmol), methanol (14.20 mL, 350.00 mmol) and benzoyl chlo- mL minꢀ1). The content of carbon (C), nitrogen (N), hydrogen
ride (0.50 mL, 4.30 mmol). The mixture was reuxed at 65 ꢃC for (H) and oxygen (O) in P(AN-co-MHI) copolymers were deter-
0.5 h and then cooled to room temperature. The reaction mined by a Elementar Vario EL III elemental analyzer. The
mixture was distilled under reduced pressure to remove excess monomer reactivity ratios were calculated from the elemental
methanol and then followed by standing to get precipitation. analysis results.
The precipitation was recrystallized from benzene–petroleum
ether (v/v ¼ 1 : 1).
X-ray diffraction (XRD) patterns of power P(AN-co-MHI)
copolymers were measured on a Rigaku D/max-2550 diffrac-
White crystal, 84.52% yield. IR (KBr) nmax cmꢀ1: 3004, 2955 tometer between 3–50ꢃ at the rate of 3ꢃ minꢀ1. All the XRD
1
(C–H), 1726, 1691 (C]O), 1636 (C]C), 1237, 1170 (C–O). H- measurements were carried out under the same conditions. The
NMR (400 Hz, DMSO-d6, RT, TMS) d ppm: 12.616 (s, 1H, interplanar spacings (d) and crystallite size (Lc) of the crystal
COOH), 6.149 (d, J ¼ 1.20 Hz, 1H, CH2]), 5.763 (d, J ¼ 1.20 Hz, were estimated by the Scherrer equation and Bragg equation
1H, CH2]), 3.580 (s, 3H, OCH3), 3.336 (s, 2H, CH2).
respectively as follows:25
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According to the Mayo–Lewis equation, the Fineman–Ross
method is expressed as follows:
d ¼ l/2 sin q
(2)
(3)
Lc ¼ Kl/B cos q
G ¼ r1H ꢀ r2
G and H are represented by
(6)
where l ¼ 0.154056 nm is the wavelength of CuKa X-ray, q is the
Bragg angle, B is the full width at half maximum intensity
(FWHM) of the peak around 2q ¼ 17ꢃ, and K is a constant 0.89.
The crystallinity index (CI) was determined by Bell and Dumb-
leton method:22
G ¼ X(Y ꢀ 1)/Y, H ¼ X2/Y
(7)
where X and Y are the molar fraction ratios of AN/MHI in the
feed and in the P(AN-co-MHI) copolymers, respectively:
CI ¼ Ac/(Ac + Aa)
(4)
where Ac is the integral area of crystalline zone around 2q ¼ 17ꢃ
in XRD patterns, and Aa is the integral area of amorphous zone.
Ac was calculated using straight line segments from 2q ¼ 11ꢃ to
2q ¼ 21ꢃ as the baseline, while the total integral area A (A ¼ Aa +
Ac) used straight line segments from 2q ¼ 11ꢃ to 2q ¼ 32ꢃ as the
baseline.
X ¼ [M1]/[M2], Y ¼ d[M1]/d[M2]
(8)
The plot of G versus H gives a straight line with the slope as r1
and the intercept of the y axis as r2.
The linear relationship equations proposed by Kelen and
˜
¨
Tudos are as follows:
ꢀ
ꢁ
r2
r2
For the stabilization of P(AN-co-MHI), the powder samples
were heated at 200 ꢃC in an air oven with a temperature accuracy
h ¼ r1 þ
x ꢀ
(9)
a
a
ꢃ
of 1 C for 30 minutes.
h and x are represented by
h ¼ G/(a + H), x ¼ H/(a + H)
a is an arbitrary constant and represented by
a ¼ (HmHM)ꢀ1/2
(10)
(11)
3. Results and discussion
3.1 Reactivity ratio studies
The molecular weights of MHI and AN are 144 and 53 g molꢀ1
respectively. According to the mechanism of radical polymeri-
zation, the molar fraction of MHI in P(AN-co-MHI) can be
calculated by the following equation with the end-group effect
on the polymer ignored:
,
where Hm and HM is the lowest and highest values calculated
from the series of measurements. The plot of h versus x gives a
straight line. The extrapolation of the line to x ¼ 1 gives r1 and to
0 gives r2/a (Fig. 1).
Â
Ã
CO=ð4 ꢁ 16Þ
1 ꢀ ðCO ꢁ 144Þ=ð4 ꢁ 16Þ
53
53CO
The reactivity ratios of AN and MHI determined by Fineman–
MHI ¼
¼
(5)
CO
64 ꢀ 91CO
˜
¨
Ross and Kelen–Tudos methods are compiled in Table 2.
Obviously, the reactivity ratios obtained from these two
methods show good agreement and the reactivity ratio value of
MHI is larger than that of AN, indicating the comonomer MHI
possesses higher reactivity than AN. Therefore, the propagating
radicals of ꢂꢂꢂANc and ꢂꢂꢂMHIc have a preference for
comonomer MHI and the comonomer MHI has better chance to
incorporate into the polymer chains than AN. It is expected the
content of MHI is richer in the P(AN-co-MHI) copolymers than
in the feed, the evidence can be found in following element
analysis. Comparing with the solution polymerization of AN
and MHI in DMSO in our previous work,23 the reactivity ratios of
MHI is larger in the mixed solvents polymerization (2.52) than
that in solution polymerization of DMSO (2.43) under the same
reaction conditions, on the contrary the reactivity of AN is
smaller in mixed solvents polymerization (0.52) than in solution
polymerization (0.54). The solubility of AN is smaller in DMSO/
deionized water ¼ 60/40 (wt/wt) than in DMSO due to the low
þ
4 ꢁ 16
where CO is the oxygen content of P(AN-co-MHI) copolymers.
The molar fraction ratios of AN/MHI in the P(AN-co-MHI)
copolymers can be obtained from the elemental analysis of
oxygen correspondingly, which are used to calculate the
monomer reactivity ratios by Fineman–Ross26 and Kelen–
27
˜
¨
Tudos methods. The parameters for copolymerization of AN
with MHI in mixed solvents DMSO/deionized water ¼ 60/40 (wt/
wt) under low conversion are shown in Table 1. On the basis of
˜
¨
Table 1, the Fineman–Ross and Kelen–Tudos methods are used
to calculate the monomer reactivity ratios.
Table 1 Parameters for copolymerization of AN with MHI in DMSO/
deionized water ¼ 60/40 (wt/wt) under low conversion
X
Conversion
(%)
O content in the
copolymers (wt%)
(mol/mol)
Y (mol/mol)
ꢃ
solubility of AN in water (9 wt% at 60 C), which reduces the
actual concentration of AN in the reaction media of DMSO/
deionized water ¼ 60/40 (wt/wt). While the comonomer MHI
can dissolve completely in both DMSO and mixed solvents of
DMSO/deionized water. For the above reason the reactivity
ratios of MHI in the mixed solvents polymerization is larger
99/1
98/2
97/3
96/4
95/5
6.21
6.43
6.25
6.04
5.98
2.240
4.336
6.251
8.101
9.812
51.191
25.132
16.601
12.189
9.590
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Fig. 2 Effect of monomer feed ratios on the conversion of polymer-
ization and molecular weight.
twice of that prepared in the solvent DMSO under the same
conditions because of the small chain transfer constant of
deionized water. It is well known that the molecular weight of
acrylonitrile copolymers has great effect on the performance of
resulting carbon ber. For the P(AN-co-MHI) copolymer
prepared in the mixed solvents of DMSO/deionized water in this
work, the molecular weight of P(AN-co-MHI) can meet the
requirement of high performance carbon ber when the
monomer feed ratios of AN/MHI is larger than 97/3 (mol/mol).
In industry production the polymerization conversion should
also be taken into account, the amounts of MHI in the feed
should be controlled less than 2.0 mol% based on total mono-
mers in order to get high conversion of polymerization.
Fig. 1 Monomer reactivity ratios calculated by Fineman–Ross and
Kelen–Tudos methods.
¨ ˜
Elemental analysis of P(AN-co-MHI) copolymers with
different molar feed ratios of AN/MHI are shown in Fig. 3. The
oxygen element in the copolymers is only provided by the
comonomer MHI. As shown in Fig. 3, the O content of P(AN-co-
MHI) copolymers increases with the increase of MHI amounts
in the feed, while the contents of C, N and H reduce, indicating
the increase of MHI content in P(AN-co-MHI) copolymers. In
addition, the content of O in the feed (Of) is also shown in Fig. 3.
It is found that the content of O is much larger in P(AN-co-MHI)
than in solvent polymerization under the same reaction
conditions.
3.2 Molecular weight and elemental analysis studies
The effect of monomer feed ratios (AN/MHI) on the conversion
of polymerization and molecular weight of P(AN-co-MHI) are
shown in Fig. 2. Both the conversion of polymerization and
molecular weight of P(AN-co-MHI) reduce with the increase of
MHI amounts in the feed as shown in Fig. 2. The possible
reason is that MHI has larger molecular volume than AN, which
counteracts the growth of polymer chains and reduces the
polymerization conversion and molecular weight.24 The similar
results can be found in our previous work for the solution
polymerization of AN with MHI in DMSO. It is worth to point
out that the molecular weight of P(AN-co-MHI) prepared in the
mixed solvents of DMSO/deionized water ¼ 60/40 (wt/wt) is
Table 2 Reactivity ratios calculated by different methods in mixed
solvents polymerization
Method
r1 (AN)
r2 (MHI)
Fineman–Ross
0.52
0.52
2.52
2.34
Fig. 3 Elemental analysis of P(AN-co-MHI) copolymers with different
˜
¨
Kelen–Tudos
molar feed ratios of AN/MHI: Of, O content in the feed.
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copolymers than in the feed, suggesting the richer content of
The FTIR spectra of PAN and P(AN-co-MHI) copolymers with
MHI in P(AN-co-MHI) copolymers, which conrms the above different compositions stabilized in air at 200 ꢃC for 30 minutes
reactivity ratio results.
are shown in Fig 4(b). It is well known that a variety of
exothermic chemical reactions, such as cyclization, dehydro-
genation and oxidation reactions, occur during the stabiliza-
tion. The cyclization reactions turn –C^N groups into –C]N–
structure and the dehydrogenation reactions convert –CH2–
CH2– structure into –CH]CH– structure, thus the intensity
changes of bands at 2244 cmꢀ1 assigned to stretching vibration
of C^N and bands at 1629 cmꢀ1 assigned to stretching vibra-
tion of C]N conjugated with C]C can be used to evaluate the
extent of stabilization. In order to evaluate the extent of stabi-
lization, a parameter (Es) is dened as follows:
3.3 FTIR studies
The FTIR spectra of PAN and P(AN-co-MHI) copolymers with
different molar feed ratios of AN/MHI are shown in Fig. 4(a). As
shown in Fig. 4(a), the bands at 1731 cmꢀ1 assigned to
stretching vibration of C]O in MHI appears in the FTIR spectra
of P(AN-co-MHI) copolymers,28 suggesting the comonomer MHI
has copolymerized with AN successfully. Furthermore, the
intensity of C]O absorption bands becomes stronger with the
increase of MHI amounts in the feed, hinting the MHI content
in the P(AN-co-MHI) copolymers increases, which is consistent
with the above elemental analysis results. While the intensity of
bands at 2244 cmꢀ1 assigned to stretching vibration of C^N
does not show obvious change in all the polymers, indicating
the presence of long uninterrupted sequences of AN units in all
acrylonitrile copolymers.29 The intensity of the bands at 2939
cmꢀ1 assigned to the C–H stretching vibration of CH, CH2, and
CH3 (ref. 30 and 31) and the bands at 1454 assigned to bending
vibration of CH2 keep unchanged in all polymers.
Es ¼ A1618 cmꢀ1/A2244 cmꢀ1
(12)
where A is the absorbance intensity dened as A ¼ log(T0/T), T0
and T are the transmittances at baseline and maximum,
respectively. According to Lambert–Beer's law: A ¼ abc, where a
is the molar absorption coefficient, b is the thickness of the KBr
pellet (cm), and c is the molar concentration of sample solution
(mol cmꢀ1). The absorbance intensities of bands at 1618 and
2244 cmꢀ1 are determined on the same KBr pellet, namely with
the same value of b, therefore the absorbance intensity ratio of
these two bands is approximately equal to the content ratio of
the generated C]N and C]C groups to the residual C^N
groups.32 The calculated values of Es can be used to represent
the extent of stabilization. The molar ratio of MHI/AN in the
P(AN-co-MHI) copolymers can be calculated from the elemental
analysis of O in Fig. 3, and the plot of Es versus molar ratio of
MHI/AN in P(AN-co-MHI) copolymers is shown in Fig. 5.
As shown in Fig. 5, the extent of stabilization of P(AN-co-
MHI) increases with the increase of MHI content in P(AN-co-
MHI) copolymers. The extent of stabilization plays an important
role in the performance of resulting carbon ber and the
stabilization reactions should proceed as fully as possible to get
ideal performance of carbon ber.33,34 Based on Fig. 5, it can be
concluded that the incorporation of MHI into PAN chains
improves the stabilization of PAN effectively, which is benecial
to preparing high performance carbon ber. It has been widely
Fig. 4 FTIR spectra of PAN and P(AN-co-MHI) copolymers with
different molar feed ratios of AN/MHI (a) original and (b) stabilized 200
ꢃC for 30 min: (a) PAN; (b) AN/MHI ¼ 99/1 (mol/mol); (c) AN/MHI ¼ 98/
2 (mol/mol), (d) AN/MHI ¼ 97/3 (mol/mol); (e) AN/MHI ¼ 96/4 (mol/ Fig. 5 Effect of molar ratio of MHI/AN in P(AN-co-MHI) on the extent
mol); (f) AN/MHI ¼ 95/5 (mol/mol).
of stabilization (Es).
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Table 4 XRD analysis results of PAN and P(AN-co-MHI) copolymers
with different compositions according to Fig. 6
Molar ratio Peak center Peak intensity Peak intensity
of MHI/AN (o)
I0 (CPS)
Is (CPS)
SI (%)
PAN
17.02
5078
3679
3352
2386
1631
846
12 870
2891
2554
1324
752
ꢀ153.45
21.42
23.81
44.51
53.89
54.85
0.020
0.045
0.070
0.095
0.126
16.54
16.67
16.78
16.71
16.70
382
3.4 XRD studies
XRD patterns of PAN and P(AN-co-MHI) copolymers with
different compositions are shown in Fig. 6(a). The strongest
diffraction peak at about 2q ¼ 17ꢃ, corresponded to a crystalline
planar spacing of d ¼ 0.522 nm, is attributed to (100) the crys-
talline plane of the pseudohexagonal cell.36 The weak diffraction
peak at about 2q ¼ 29ꢃ (d ¼ 0.30 nm) is assigned to the (101)
crystalline plane of the pseudohexagonal cell.36 XRD patterns of
acrylonitrile polymers were processed via Origin 8.0 soware to
analyze the peak center and the full width at half maximum
height (FWHM) around 2q ¼ 17ꢃ. Based on the above data, the
corresponding crystalline planar spacing d, crystallite size Lc
and crystallinity index CI can be calculated. All the data are
tabulated in Table 3. Obviously, the crystallinity index CI of PAN
is larger than that of P(AN-co-MHI) copolymers and decreases
with increase of MHI content in P(AN-co-MHI) copolymers. The
incorporation of MHI into PAN chains blocks the interactions
between intermolecular C^N groups, which reduces the crys-
tallinity of P(AN-co-MHI). While in PAN homopolymer the
crystal structure is not destroyed with no MHI incorporated into
PAN chains, therefore PAN homopolymer has larger CI. As
shown in Fig. 6(a), the peak center around 2q ¼ 17ꢃ hardly
moves with the increase of MHI content in P(AN-co-MHI), so the
d values of PAN and P(AN-co-MHI) copolymers almost keep
unchanged. Additionally, the Lc of P(AN-co-MHI) copolymers is
larger than that of PAN homopolymer, which may attribute to
the large volume of MHI.
Fig. 6 XRD patterns of PAN and P(AN-co-MHI) copolymers with
different compositions (a) original and (b) stabilized 200 ꢃC for 30 min:
(a) PAN; (b) MHI/AN ¼ 0.020; (c) MHI/AN ¼ 0.045; (d) MHI/AN ¼ 0.070;
(e) MHI/AN ¼ 0.095; (f) MHI/AN ¼ 0.126.
accepted that the cyclization of nitrile groups in PAN homo-
polymer can only be initiated through a free radical mecha-
nism. While the cyclization in P(AN-co-MHI) copolymers can be
initiated through both a radical mechanism and an ionic
mechanism,35 because the hydroxyl oxygen of carboxyl (–COOH)
in MHI can make a nucleophilic attack on the carbon atom of
adjacent nitrile groups and then induce molecules to cyclize,
which can promote the cyclization of P(AN-co-MHI)
signicantly.
Table 3 XRD analysis results of PAN and P(AN-co-MHI) copolymers
with different compositions corresponding to Fig. 6(a)
Molar ratio Peak center
of MHI/AN
(o)
FWHW (o) D (nm) Lc (nm) CI (%)
PAN
17.16
17.02
17.10
17.04
17.02
17.10
0.93
0.88
0.87
0.83
0.82
0.89
0.522
0.526
0.524
0.526
0.526
0.524
8.840
9.336
9.447
9.899
10.019
9.235
51.04
43.78
42.51
41.78
41.27
40.82
0.020
0.045
0.070
0.095
0.126
Fig. 7 DSC curves of PAN, P(AN–MA–AA) with molar feed ratios AN/
MA/AA ¼ 98/2/2 and P(AN-co-MHI) copolymers with compositions.
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Table 5 Parameters for DSC curves of PAN and P(AN-co-MHI) copolymers with different compositions measured under N2 atmosphere
DH/DT
Molar ratio of MHI/AN
Ti (ꢃC)
Tp1 (ꢃC)
Tp2 (ꢃC)
Tf (ꢃC)
DT (ꢃC)
DH (J gꢀ1
)
(J gꢀ1 ꢃCꢀ1
)
PAN
P(AN–MA–AA)
0.020
0.045
0.070
244.16
216.89
177.53
176.59
175.58
170.72
167.61
278.99
287.46
213.35
214.00
210.39
213.68
215.80
303.11
318.05
337.36
345.61
355.01
366.90
373.19
58.95
101.16
159.83
169.02
179.43
196.18
205.58
2004.67
2006.46
2146.50
2227.22
2312.75
2490.81
2513.10
34.01
19.83
13.43
13.18
12.89
12.70
12.22
277.81
280.02
286.58
292.03
298.20
0.095
0.126
XRD patterns of PAN and P(AN-co-MHI) copolymers with
The DSC curves were obtained under N2 atmosphere and no
ꢃ
different compositions heated in air at 200 C for 30 min are oxidative reactions occurred during this process, so the DSC
shown in Fig. 6(b). The stabilization is a process transferring exotherms of acrylonitrile polymers are attributed to the cycli-
linear structure into ladder structure, and the structural zation reactions. As shown in Fig. 7, there is only one
changes during the stabilization agree well with the intensity exothermic peak in PAN homopolymer and the cyclization
changes of the peak around 2q ¼ 17ꢃ. Thus, the intensity reactions can only be initiated through a free radical mecha-
changes around 2q ¼ 17ꢃ can be used to evaluate the extent of nism. This causes a large amount of heat to be released
stabilization, the equation is expressed as follows:36
simultaneously, which leads to the breakage of molecular
chains and nally results in structural defects in the resulting
carbon ber. Although the initiation temperature of P(AN–MA–
SI ¼ (I0 ꢀ Is)/I0
(13)
ꢃ
ꢃ
AA) decreases from 244 C to 216 C aꢀn1d the DH/DT of P(AN–
MA–AA) reduces from 34.01 J gꢀ1 ꢃC to 19.83 J gꢀ1 ꢃCꢀ1
compared with PAN homopolymer as shown in Table 5, the heat
release of P(AN–MA–AA) is still concentrative and expeditious
because there is only one exothermic peak in P(AN–MA–AA) as
shown in Fig. 7, which is not benecial to making high
performance carbon ber. Whereas in P(AN-co-MHI) copoly-
mers, there are two exothermic peaks and the cyclization reac-
tions can be initiated through radical and ionic mechanism.35
The lower exothermic peak (peak 1) is assigned to cyclization
reactions initiated by MHI through ionic mechanism, which
broadens the exothermic peak and avoids centralized heat
release. Furthermore, the area of peak 1 increase with the
increase of MHI content in P(AN-co-MHI) copolymers as shown
in the enlarged DSC curves between 180 and 124 ꢃC on the top
le corner of Fig. 7, suggesting that more cyclization reactions
are initiated by MHI through ionic mechanism, which promotes
the stabilization of P(AN-co-MHI).
where I0 is the intensity of peak around 2q ¼ 17ꢃ from the
original polymer, and Is is the intensity of peak around 2q ¼ 17ꢃ
from the polymer stabilized at 200 ꢃC for 30 min. SI values
determined by the equation are listed in Table 4. The (100) peak
intensity of PAN homopolymer stabilized at 200 ꢃC is higher
than that of original PAN homopolymer and the SI value is
negative, which can be illustrated by further crystallization in
PAN homopolymer. From the following DSC analysis, it is
known that the stabilization reactions of PAN homopolymer
ꢃ
have not yet started at 200 C, however the energy provided by
heating is enough to break the boundaries between crystalline
zones and amorphous zones,36 which results in the increase of
peak intensity around 2q ¼ 17ꢃ. While in the P(AN-co-MHI)
copolymers, the SI values are positive and become larger with
increase of MHI content in P(AN-co-MHI) copolymers, indi-
cating the increase of extent of stabilization. It can be concluded
that the stabilization of P(AN-co-MHI) has been effectively
improved by the incorporation of MHI, the more the better.
Aꢃs shown in Table 5, the Ti of P(AN-co-MHI) copolymers (ca.
175 C) is much lower than that of PAN homopolymer (ca. 244
ꢃC) and P(AN–MA–AA) (ca. 216 ꢃC), suggesting a greater ease of
initiation of cyclization reactions in P(AN-co-MHI) copolymers.
However, the Tf becomes higher with the increase of MHI
content in P(AN-co-MHI), indicating the separation of ionic
cyclization reactions and radical cyclization reactions. PAN
homopolymer has the smallest DT and the largest DH/DT as
shown in Table 5, implying that the heat release is concentrative
and expeditious. The incorporation of MHI into PAN chains can
slows the velocity of evolving heat, as evidenced from the larger
DT and smaller DH/DT in the P(AN-co-MHI) copolymers. In
addition, both DH and DT has a tendency to become larger with
the increase of MHI content in P(AN-co-MHI) copolymers, while
DH/DT has a tendency to become smaller. Apparently, the
synthesized comonomer MHI has a function of relaxing heat
3.5 DSC studies
Fig. 7 show the DSC curves of PAN, P(AN–MA–AA) with molar
feed ratio of AN/MA/AA ¼ 98/2/2 and P(AN-co-MHI) copolymers
with different compositions heated at 10 ꢃC minꢀ1 from
ambient temperature to 375 C under N2 (40 mL minꢀ1), and
ꢃ
the enlarged DSC curves between 180 and 124 ꢃC was inserted in
the top le corner of Fig. 7. The parameters obtained from the
exotherms, including the temperature of initiation (Ti), the
temperature of termination (Tf) and their difference (DT ¼ Tf ꢀ
Ti), the rst peak temperature (Tp1, the peak at lower tempera-
ture), the second peak temperature (Tp2, the peak at higher
temperature), the evolved heat (DH), and the velocity of evolving
heat (DH/DT), are listed in Table 5.
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release during stabilization, which is benecial to making high 375 ꢃC under N2 (40 mL minꢀ1). For all acrylonitrile copolymers,
performance carbon ber.
the exotherms wholly shi to higher temperature with the
increase of heating rate and the exothermic peaks become
stronger as shown in Fig. 8. Kissinger37 and Ozawa's methods38
are mostly used to calculate Ea in literatures, because they just
require a series of DSC curves heated at different rates to
quantify Ea without any prior knowledge of reaction mecha-
nism. These two methods were also used to calculate Ea of the
cyclization reactions in this work. The peak temperature of DSC
(Tm) used to evaluate Ea shows a regular increase with the
increase of heating rate (4).
3.6 Evaluation of activation energy (Ea) of cyclization
reactions
Fig. 8 shows DSC curves of PAN homopolymer, P(AN–MA–AA)
with molar feed ratios of AN/MA/AA ¼ 98/2/2 and P(AN-co-MHI)
copolymer with MHI/AN ¼ 0.070 (mol/mol) heated at different
rates (5, 10, 15, 20, 30 C minꢀ1) from ambient temperature to
ꢃ
The equation used by Kissinger's method are as follows:37
Â
À ꢂ
ÁÃ
2
d ln 4 Tm
Ea
R
ꢀ
¼
(14)
dð1=TmÞ
Ea was calculated from the slope of the linear plot of ln(4/
Tm2) versus 1000/Tm as shown in Fig. 9(a).
Ozawa's method uses the following equation:38
Ea
R
dðlog 4Þ
dð1=TmÞ
ꢀ
¼ 2:15
(15)
Fig. 8 DSC curves of PAN (a), P(AN–MA–AA) with molar feed ratio of Fig. 9 Kissinger method (a) and Ozawa method (b) used to quantify Ea:
AN/MA/AA ¼ 98/2/2 (b) and P(AN-co-MHI) with MHI/AN ¼ 0.070 (c) (a) peak 1 of P(AN-co-MHI), (b) peak 2 of P(AN-co-MHI), (c) PAN, (d)
heated at different rates.
P(AN–MA–AA).
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Table 6 Ea determined by Kissinger and Ozawa methods
˜
¨
Fineman–Ross and Kelen–Tudos methods show good agree-
ment and the comonomer MHI possesses higher reactivity than
AN, which results in higher content of MHI in P(AN-co-MHI)
copolymers than in the feed evidenced by elemental analysis.
From the FTIR, DSC and XRD results, it can be found the
stabilization of P(AN-co-MHI) copolymers has been signicantly
improved by MHI with lower initiation temperature, broadened
exothermic peak and larger extent of stabilization, which is
mainly attributed to the ionic cyclization reactions initiated by
MHI. It is further conrmed by the calculation of Ea based on
Kissinger method and Ozawa method.
Kissinger
(kJ molꢀ1
)
Ozawa (kJ molꢀ1
)
Molar ratio of MHI/AN
Peak 1
Peak 2
Peak 1
Peak 2
PAN
168.39
128.06
105.67
108.05
103.22
104.04
100.38
165.89
128.33
106.32
108.53
103.95
104.85
101.39
P(AN–MA–AA)
MHI/AN ¼ 0.020
MHI/AN ¼ 0.045
MHI/AN ¼ 0.070
MHI/AN ¼ 0.095
MHI/AN ¼ 0.126
184.22
181.34
283.82
236.63
209.64
180.64
177.93
273.76
229.81
204.71
Acknowledgements
Ea was calculated from the slope of the linear plot of log 4
versus 1000/Tm as shown in Fig. 9(b).
Financial support of this work from Natural Science Foundation
of Jiangsu Province (no. BK20140159), China Postdoctoral
Science Foundation (no. 2014M561570), Jiangsu Planned
Projects for Postdoctoral Research Funds (no. 1402195C),
National Natural Science Foundation of China (no. 21401080)
and Important National Research Program “863” (no.
2012AA030313-1) was gratefully acknowledged.
The Ea of cyclization reactions of P(AN-co-MHI) copolymers
with MHI/AN ¼ 0.020, 0.045, 0.095 and 0.126 (mol/mol) were
also calculated by Kissinger and Ozawa's methods and all the
results are compiled in Table 6. As shown in Table 6, the Ea
determined by Kissinger method agrees well with that obtained
by Ozawa method. All DSC curves were obtained under N2
atmosphere, only cyclization reactions occurred during this
process. The Ea values of cyclization reactions for PAN homo-
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