Promoting effect of methyne/methylene moiety of bisphenol E/F on phthalonitrile resin curing: Expanding the structural design route of phthalonitrile resin

Article abstract of DOI:10.1016/j.polymer.2020.123001

In this paper, three phthalonitrile (PN) model compounds containing benzene ring-linked alkyl units, bisphenol A (methyl) phthalonitrile monomer (BPAh), bisphenol E (methyne) phthalonitrile monomer (BPEh) and bisphenol F (methylene) phthalonitrile monomer (BPFh), were synthesized. It was confirmed that methylene/methyne could effectively promote the curing reaction of phthalonitrile. Rheology analysis showed that the promotion effect of methylene is stronger than that of methyne, but methyl has no obvious promotion effect. The effect of DPPH on the reaction rate, added considerable evidences that the curing reaction of BPEh and BPFh involved a free radical process. Qualified thermal properties were investigated by Dynamic mechanical analysis (DMA) and thermal gravimetric analysis (TGA). For post-cured resin of BPEh and BPFh, the Tg is more than 500 °C, the T_5% were observed at 495 °C and 491 °C, and the char yield in 800 °C reached 73.1% and 71.9% respectively.

Full text of DOI:10.1016/j.polymer.2020.123001

Polymer 210 (2020) 123001
Contents lists available at ScienceDirect
Polymer
journal homepage: http://www.elsevier.com/locate/polymer
Promoting effect of methyne/methylene moiety of bisphenol E/F on
phthalonitrile resin curing: Expanding the structural design route of
phthalonitrile resin
Shenjun Liao, Hao Wu, Xian He, Jiang-huai Hu, Renke Li, Yao Liu, Jiangbo Lv, Yang Liu,
Zhengzhou Liu, Ke Zeng*, Gang Yang**
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this paper, three phthalonitrile (PN) model compounds containing benzene ring-linked alkyl units, bisphenol A
(methyl) phthalonitrile monomer (BPAh), bisphenol E (methyne) phthalonitrile monomer (BPEh) and bisphenol
F (methylene) phthalonitrile monomer (BPFh), were synthesized. It was confirmed that methylene/methyne
could effectively promote the curing reaction of phthalonitrile. Rheology analysis showed that the promotion
effect of methylene is stronger than that of methyne, but methyl has no obvious promotion effect. The effect of
DPPH on the reaction rate, added considerable evidences that the curing reaction of BPEh and BPFh involved a
free radical process. Qualified thermal properties were investigated by Dynamic mechanical analysis (DMA) and
thermal gravimetric analysis (TGA). For post-cured resin of BPEh and BPFh, the Tg is more than 500 ◦C, the T5%
were observed at 495 ◦C and 491 ◦C, and the char yield in 800 ◦C reached 73.1% and 71.9% respectively.
Phthalonitrile
Methylene
Methyne
Promoting effect
Curing mechanism
1. Introduction
system and broaden the application by cyano-based reactive chemistry
and the design of the curing system. For instance, based on the design of
Phthalonitrile resin (PN), as a high-performance polymer with plenty
of excellent properties, has properties like heat-resistance, chemical
resistance, low water absorption, flame retardancy, etc. [1–7]. These
excellent properties endow phthalonitrile resin application potential in
the fields of aerospace industry, composite materials and carbon pre-
cursors, which receive wide attention from researchers [8,9].
hydroxy-PN curing system of benzoxazine-PN [16], phenolic-PN [18],
allyl-PN [19], alkynyl-PN [20] have been developed. From a scientific
point of view, the reaction path of the current active hydrogen curing
system is generally considered to involve ionic intermediates, such as
ammonium salts [10]. However, the knowledge of the cyano reaction
chemistry and curing mechanism of phthalonitrile resin is very limited.
This restricts the scope of molecular design and material modification to
a certain extent. In fact, in the field of foundation chemistry, the free
radical reactivity of cyano groups has received extensive attention [21].
At the same time, early research has shown another path of thermal
polymerization of phthalonitrile-free radical course [21]. The study
found that free radical initiator tert-butanol peroxide could promote the
polymerization of cyano groups, and the product contains phthalocya-
nine ring structure.
However, related studies have shown that [5–7], the intrinsic curing
rate of phthalonitrile resin is slow, and it takes a long time to gelate
◦
above 300 C. Adding curing agent is a prevailing route to solve the
problem. Traditional curing agents include Lewis acid/base compounds,
such as metal salts, strong organic acid/amine salts, amines, phenols,
and some heterocyclic structures containing active hydrogen [10–13].
These curing agents could significantly reduce the initial curing tem-
perature of the nitrile group, and the cured resin has excellent thermal,
thermo-oxidative stability and outstanding thermomechanical
properties.
As we all know, the methyl/methylene/methyne groups in the alkyl
group are the building blocks of aliphatic structures that make up a basic
organic material system, such as synthetic polymer materials and nat-
ural polymers [24]. On the one hand, from the perspective of polymer
Meanwhile, previous studies have also shown that it is expected to
introduce the excellent properties of PN resins into other materials
* Corresponding author.
** Corresponding authors.
E-mail addresses: zk_ican@scu.edu.cn (K. Zeng), yangg@scu.edu.cn (G. Yang).
https://doi.org/10.1016/j.polymer.2020.123001
Received 4 June 2020; Received in revised form 26 August 2020; Accepted 31 August 2020
Available online 4 September 2020
0032-3861/© 2020 Elsevier Ltd. All rights reserved.

S. Liao et al.
Polymer 210 (2020) 123001
Scheme 1. Synthesis of BPAh, BPFh, BPEh monomer.
chemistry, the free radical activity derived from such classic carbon
chain units has been established [25,26]. On the other hand, compared
to traditional active hydrogen curing systems (amino group-
s/phenols/acids, etc.), the carbon chain unit is relatively inert, and its
structure has better chemical resistance and stability during the func-
tionalization process [22]. Therefore, we assume whether the alkyl
chain can effectively initiate the thermal polymerization of phthaloni-
trile. The study on this scientific issue is expected to provide a new route
for the design of PN resin system and a new opportunity for its modifi-
cation, such as the high-performance of biomass [17]. However, as far as
we know, there are few systematic and in-depth studies on this issue.
Recently, our laboratory has developed a new curing system that pro-
motes the polymerization of phthalonitrile resins with imide cycloali-
phatic moiety. The research provides essential cases for curing PN resins
with alkyl units. The study found that a unique synergistic curing
behavior existed between imide cycloaliphatic moiety and phthaloni-
trile unit [23], and the cured resins showed excellent thermo-oxidative
stability and thermomechanical properties. It is believed that the
mechanism of thermal polymerization is related to the free radical in-
termediate stabilized by the conjugation effect at high temperature [23].
However, from the perspective of basic research, relatively complex
alkyl radicals (including methyl-methylene-methyne radicals) limit the
understanding of reaction sites and mechanisms. From the perspective
of applied research, it restricted the design flexibility of the PN resin
system [22].
2. Experimental
2.1. Materials
4-Nitrophthalonitrile was purchased from Jiangsu Taixing Shengm-
ing Fine Chemical Co. Ltd. 4,4-ethylenebisphenol was purchased from
Shanghai Titan Technology Co. Ltd. Dimethyl sulfoxide (DMSO), N, N-
dimethylformamide (DMF), potassium carbonate, bisphenol A, bisphe-
nol F, acetonitrile and methanol were purchased from Chengdu Kelong
Chemical Reagent Co. Ltd. For direct use after purchase. All the mate-
rials were used without further purification.
2.2. Synthesis of monomer (BPAh, BPFh, BPEh)
The synthesis of bisphenol A phthalonitrile monomer (BPAh) is
shown in Scheme 1. Firstly, 4-nitrophthalonitrile (9.12 g, 0.053 mol),
bisphenol A (5.01 g, 0.022 mol) and 35 mL of dimethyl sulfoxide were
added to a 50 mL three-necked bottle in sequence under nitrogen con-
dition. Then, stirred 4-nitrophthalonitrile and bisphenol A at room
temperature until completely dissolved. Subsequently, the grounded
potassium carbonate powder (4.32 g, 0.032 mol) was added to the so-
◦
lution. Reaction carried out at 30 C for 12 h. At the end of reaction,
deionized water was empolyed to wash solution until the PH of the
filtrate became netural. Finally, the crude product was stirred with 80
mL of methanol, washed and filtered, and the filter cake was dried in a
rotary evaporator at 70 ◦C for 4 h to obtain a yellow product BPAh (9.43
g, yield 89.2 wt%).
According to previous reports, free radicals derived from benzene
ring methylene/methyne groups have relative stability [27a-c], and the
modifiability of benzene ring units can also provide opportunities for
material design [17] [28]. In this work, the simple structural units were
introduced into monomer to verify whether it could effectively accel-
erate the polymerization of PN. The curing system is expected to provide
a basic model system for the research on the free radical polymerization
mechanism of phthalonitrile resin and a flexible and rich design
approach for PN resin. Based on this assumption, three PN model
compounds containing benzene ring-linked alkyl units, bisphenol A
(methyl) phthalonitrile monomer (BPAh), bisphenol E (methyne)
phthalonitrile monomer (BPEh) and bisphenol F (methylene) phthalo-
nitrile monomer (BPFh) were designed and synthesized. By comparing
the curing behavior, it is proved that methylene/methyne could effec-
tively promote the reaction of phthalonitrile, in which the promotion
effect of methylene is stronger than that of methyne, and the cured
product has qualified thermal properties. This research greatly
expanded the design scope of PN resin and its curing system, and laid the
foundation for the subsequent mechanism research and application
research.
1H NMR (400 MHz, DMSO-d6): 8.14–8.07 (d, 2H, Ar–H), 7.82–7.78
(d, 2H, Ar–H), 7.42–7.30 (dd, 6H, Ar–H), 7.17–7.09 (d, 4H, Ar–H),
1.76–1.62 (s, 6H, aliphatic-CH). FTIR (KBr, cmꢀ 1), 2970 (-CH3), 2231
–
–
(-C N), 1502 (benzene), 1281 (Ar-O-Ar).
–
The synthesis of bisphenol F phthalonitrile monomer (BPFh) is
shown in Scheme 1. Firstly, 4-nitrophthalonitrile (6.92 g, 0.040 mol),
bisphenol F (3.01 g, 0.015 mol) and 20 mL of dimethyl sulfoxide were
added to a 50 mL three-necked flask under nitrogen condition. Then,
stirred 4-nitrophthalonitrile and bisphenol F at room temperature until
completely dissolved. Subsequently, the grounded potassium carbonate
powder (2.59 g, 0.019 mol) was added to the solution. Reaction carried
out at 30 ◦C for 12 h. At the end of reaction, deionized water was
empolyed to wash solution until the PH of the filtrate became netural.
Finally, the crude product was purified by recrystallization from
acetonitrile, filtered, and the filter cake was dried in a rotary evaporator
at 70 ◦C for 4 h to obtain a brownish yellow product BPFh (5.23 g, yield
77.1 wt%).
1H NMR (400 MHz, CDCl3, δ): 7.75–7.70 (d, 1H, -C6H3), 7.32–7.30
(s, 2H, -C6H3), 7.32–7.25 (t, 2H, Ar–H), 7.02–7.06 (d, 2H, -C6H4), 4.06
(s, 1H, –CH2-).
FTIR (KBr, cmꢀ 1) of BPFh: 3080 and 3040 (aromatic-CH), 2930
–
–
(aliphatic-CH), 2232 (C N), 1089 and 1018 (Ar-O-Ar).
–
2

S. Liao et al.
Polymer 210 (2020) 123001
Fig. 1. TGA curves of BPAh, BPEh and BPFh monomers.
The synthesis of bisphenol E phthalonitrile monomer (BPEh) is
shown in Scheme 1. Firstly, 4,4-ethylenebisphenol (7.006 g, 0.033 mol)
and 60 mL of DMF were added to a 250 mL three-necked bottle. The
grounded potassium carbonate powder (9.12 g, 0.066 mol) was added
and stirred at room temperature under nitrogen condition, then, added
toluene and stirred. When the temperature was raised to 145 ◦C, toluene
begins to reflux obviously. After the water in the system is removed,
continue to reflux for 2 h and then evaporate the toluene to cool to 70 ◦C,
the 4-nitrophthalonitrile (14.10 g, 0.082 mol) was added to the solution.
Reaction carried out at 70 ◦C for 12 h. At the end of reaction, deionized
Table 1
Thermal properties of compounds BPFh, BPEh and BPAh.
Sample
T2% (◦C)
T5% (◦C)
Char yield (%) at 800 ◦
C
Tm (◦C)
BPAh
BPEh
BPFh
353.2
355.2
339.0
377.3
384.8
362.8
3.8
197.7
203
35.3
44.3
197.5
Fig. 2. DSC curves of BPAh, BPEh and BPFh monomers.
3

S. Liao et al.
Polymer 210 (2020) 123001
Fig. 3. (a) 310 ◦C isothermal rheological curve of BPAh, BPEh and BPFh monomers; (b) 270 ◦C isothermal rheological curve of BPEh, BPEh monomers.
Fig. 4. (I) TGA curves of BPEh monomer in nitrogen; (II) DSC secondary scan curves of BPEh; (III) TGA curves of BPFh monomer in nitrogen; (Ⅳ) DSC secondary scan
curves of BPFh; a:270 ◦C/7 h,310 ◦C/5 h; b:270 ◦C/7 h,310 ◦C/5 h,330 ◦C/3 h; c:270 ◦C/7 h,310 ◦C/5 h,330 ◦C/3 h, 350 ◦C/3 h.
water was empolyed to wash solution until the PH of the filtrate became
netural. The product obtained twice was dried in a vacuum oven at
100 ◦C for 5 h to obtain 11.33 g of the product BPEh (yield 74.28 wt%).
1H NMR (400 MHz, DMSO-d6): 8.15–8.08 (d, 2H, Ar–H), 7.81–7.76
(d, 2H, Ar–H), 7.46–7.39 (d, 4H, Ar–H), 7.38–7.32 (dd, 2H, Ar–H),
7.18–7.10 (d, 4H, Ar–H), 4.32–4.24 (q, H, aliphatic-CH), 1.67–1.58 (d,
3H, aliphatic-CH3).
FTIR (KBr, cmꢀ 1): 3080 and 3040 (aromatic-CH), 2885 (aliphatic-
–
–
CH), 2940 (-CH3), 2230 (C N), 1089 and 1014 (Ar-O-Ar).
–
2.3. Preparation of resin
According to the TGA and rheological curves of BPEh and BPFh
monomers and related literature [17,23], the curing procedures of BPEh
4

S. Liao et al.
Polymer 210 (2020) 123001
2.4. Characterization
Table 2
TGA of BPEh, BPFh resin compared with conventional PN resin.
1H NMR (400 MHz) was measured by a Bruker Avance-400 nuclear
magnetic spectrometer, with DMSO-d6 or CDCl3 solvent and tetrame-
thylsilane (TMS) as internal standard.
Sample
T5 (oC)
Char yield (%) at 800 ◦
C
Reference
BPEh(a)
BPEh(b)
BPEh(c)
BPFh(a)
BPFh(b)
BPFh(c)
446
474
495
475
485
491
400
466.8
537
73.3
71.9
73.1
73.8
72.5
71.9
63
this work
this work
this work
this work
this work
this work
[43]
Thermal gravimetric analysis (TGA) was tested by a TA Q500 ther-
mogravimetric analyzer at a heating rate of 10 ◦C minꢀ 1 under nitrogen
atmosphere (60 mL minꢀ 1). and the experimental temperature range
from 40 to 800 ◦C. The quality for monomer testing is around 5.0 mg,
and the quality for resin testing is around 6.0 mg. All the testing samples
were grinded to powder before loaded in the TGA chamber.
Differential scanning calorimetry (DSC) is performed on TA Q200at a
heating rate of 10 ◦C minꢀ 1 under nitrogen atmosphere (60 mL minꢀ 1).
Rheological experiments were conducted on a TA instruments AR-
2000ex rheometer in conjunction with an environmental testing
chamber for temperature control. The measurements were made using
25 mm diameter parallel plates at a constant strain value of 2.5 × 10ꢀ 4
and a frequency of 1 Hz. The temperature of isothermal time scan is
270 ◦C and 310 ◦C respectively, and the experimental atmosphere is air
atmosphere.
3BOCN/MI
DPBA-Ph
BPH/APN
73.4
70
[45]
[44]
and BPFh are determined as:
a: 270 ◦C/7 h, 310 ◦C/5 h;
b: 270 ◦C/7 h, 310 ◦C/5 h, 330 ◦C/3 h;
c: 270 ◦C/7 h, 310 ◦C/5 h, 330 ◦C/3 h, 350 ◦C/3 h;
Put a certain amount of prepared BPEh and BPFh powder monomers
into an aluminum grinding tool, and perform reaction curing in a ni-
trogen atmosphere in a muffle furnace in three procedures of a, b and c
respectively.
Fourier transform infrared (FT-IR) was recorded on a Nicolet FT-IR-
380 Fourier transform infrared spectrometer using KBr pellets from
4000 to 400 cmꢀ 1 at a resolution of 4 cmꢀ 1 with 16 scans. In situ IR was
measured with the identical method on the same spectrometer with a
Fig. 5. (a) DMA curves of cured BPEh; (b) DMA curves of cured BPFh.
Fig. 6. (a) FTIR spectra of BPEh monomer and BPEh polymer; (b) FTIR spectra of BPFh monomer and BPFh polymer.
5

S. Liao et al.
Polymer 210 (2020) 123001
Fig. 7. The UV–Vis spectra of BPEh and BPFh polymer.
microheater. We use the area of the chemical stable benzene ring
structure (1530 cmꢀ 1 to 1450 cmꢀ 1) as a benchmark to calculate the
conversion of cyano groups.
5%weight loss temperature 355.2 ◦C and 384.8 ◦C, which is significantly
higher than that of compound BPFh. The reason may be that earlier
decomposition of BPFh than BPAh and BPEh. However, as the temper-
ature continues to rise, when the experimental temperature goes up to
800 ◦C, the char yield of the BPFh and BPEh compounds at 800 ◦C
reaches 44.3% and 35.3%, respectively, and the control group BPAh
almost completely decomposes. And only less than 4% of the quality
remains. This indicates that the BPFh and BPEh samples may have un-
dergone curing reaction during the temperature increase. According to
previous reports [17], BPAh does not have a self-promoting effect and
mainly used for control group in this study.
Solid-state UV–Vis reflectance spectra was analyzed with a UV–Vis
spectrophotometer (UV-3600, Shimadzu, Japan) from 1000 to 200 nm.
Dynamic mechanical analysis (DMA) was recorded on a TA instru-
ment DMA Q800 at a heating rate of 5 ◦C minꢀ 1 from 50 to 500 ◦C with a
load frequency of 1 Hz in nitrogen atmosphere. The curing procedures of
blend sample is: a: 270 ◦C/7 h, 310 ◦C/5 h; b: 270 ◦C/7 h, 310 ◦C/5 h,
330 ◦C/3 h; c: 270 ◦C/7 h, 310 ◦C/5 h, 330 ◦C/3 h, 350 ◦C/3 h; The size
of the blends is polished to: 33.0 mm× 14.0 mm× 3.5 mm.
MALDI-TOF MS was performed on a Germany Bruker autoflex III
equipped with an N2 laser (337 nm). BPEh and BPFh samples were made
under the conditions of 270 ◦C/2 h and 270 ◦C/0.5 h respectively,
samples were partially dissolved in THF, and then mixed with the ma-
trix. Spectra were obtained in the linear positive mode. Except for a few
cases, methyl-cyano-4-hydroxycinnamic acid (CHCA) was used as the
matrix, there are no additives in most cases.
DSC was used to study the thermal behavior of the BPFh, BPEh and
BPAh samples during heating process, as shown in Fig. 2. It can be seen
that the BPFh, BPEh and BPAh samples all have obvious endothermic
peaks around 200 ◦C, which may be caused by the melt flow of the
sample around this temperature, and their melting points are 197.5 ◦C,
203.0 ◦C and 197.7 ◦C respectively, which indicated BPAh compounds
have obvious advantages for the comparative study of BPFh and BPEh
compounds. However, it’s easy to noticed that the BPFh, BPEh and BPAh
samples showed only one endothermic peak in the measured tempera-
ture range, but no obvious exothermic peak. Combined with the results
of TGA and the rheological results below, it is speculated that for the
phthalonitrile compound derived from bisphenol A, this may be due to
the fact that the sample cannot accelerate the polymerization of the
monomer by self-curing during the heating process, so the location of its
exothermic peak cannot be detected. For phthalonitrile compounds
derived from bisphenol F and E, this may be due to the slow curing rate
of the sample and the mild reaction in the temperature range tested, The
curing exothermic peak of the BPFh and BPEh samples is higher than the
temperature range that can be tested by DSC, so the location of its
exothermic peak is not detected.
Molecular Electrostatic Potential (MEP) is simulated by molecular
simulation technology for free radicals derived from BPEh and BPFh
monomer (R-BPEh and R-BPFh). All simulations were performed by
Materials Studio 8.0 software. All the Geometry Optimization of
monomer by Dmol3 (Quality: fine, Functional: GGA-PBE, basis set: DNP,
Basis file: 4.4, Properties: Electron density, Electrostatics).
3. Results and discussion
3.1. Study on thermal performance and curing behavior of monomer
As shown in Fig. 1, TGA was carried out to study the thermal prop-
erties of three monomers introduced in this paper. The temperature at
2% weight loss (T2%), the temperature at 5% weight loss (T5%) and the
char yield at 800 ◦C are listed in Table 1. It can be seen from Fig. 1 that
the thermal stability of the compounds BPFh, BPEh and BPAh in a ni-
trogen atmosphere showed a clear difference. The temperature of 2%
and 5% of BPFh weight loss is 339 ◦C and 362.8 ◦C, while the 2% and 5%
weight loss Temperature of BPAh is 353.2 ◦C and 377.3 ◦C, BPEh 2% and
The molecular structure of BPEh and BPFh does not contain the
traditional active hydrogen structure that can accelerate PN curing.
Meanwhile, combined with the previous research of our research group
on alicyclic imide compounds/phthalic acid resin [23] and the related
Lignin-based PN research [17], we speculated that the reactive “active
hydrogen” generated by the alkyl group in the BPEh and BPFh molecules
6

S. Liao et al.
Polymer 210 (2020) 123001
Fig. 8. In situ FTIR of BPEh monomer heated from 200 to 310 ◦C. (a): the stretching vibration of nitrile (-CN) (b): the stretching vibration of –C¼N in polytriazine.;
(c): the stretching vibration of N–H isoindoline; (d): the stretching vibration of –CH–.
at high temperature accelerates the polymerization of the nitrile group.
In order to further analyze the possible alkyl (methyne or methylene)
reaction sites, we studied the isothermal rheological behavior of BPEh
and BPFh monomers. As shown in Fig. 3 (a), under the isothermal
conditions of BPEh and BPFh at 310 ◦C, the viscosity of BPFh increases
rapidly at 30 min, and the viscosity of BPEh increases rapidly at 85 min.
Combined with the char yield of BPAh in TGA, BPEh and BPFh are
further verified a rapid curing reaction occurred at 310 ◦C. In addition,
the isothermal time scan at 310 ◦C that BPFh has a faster curing effect
than BPEh. At the same time, in order to further understand the differ-
ence between the curing effects of BPEh and BPFh, We performed a
270 ◦C isothermal time scan of BPEh and BPFh, as shown in Fig. 3 (b),
the viscosity of BPFh increased rapidly at about 3 h, and the viscosity of
BPFh increases significantly at 7 h. Two sets of isothermal rheological
data indicate that the curing efficiency of BPFh is higher than that of
BPEh.
increased, and the degree of crosslinking of the resin is enhanced, so that
the resin shows good base heat and thermal stability. In the DSC sec-
ondary scanning curves of BPEh and BPFh resins, there is no obvious
exothermic and endothermic peaks, it is speculated that the monomers
may basically react completely at this time. It is worth noting that there
is no obvious turning point in the DSC curve, which implies that no
◦
obvious Tg is observed in the range of 350 C, which proved that the
resin has good basic thermal properties. Table 2 lists the basic thermal
performance data of BPEh and BPFh resins under different curing pro-
cedures and PN resins promoted by traditional curing agents. It can be
seen that compared with 3BOCN/MI system, BPEh and BPFh resins have
different curing procedures the thermal performance below shows
certain advantages. Compared with DPBA-Ph and APN/BPH systems,
BPEh and BPFh resins also have comparable thermal properties. This
shows indirectly that BPEh and BPFh have undergone polymerization.
Dynamic mechanical analysis (DMA) was employed to analyze
thermo-mechanical properties of BPEh and BPFh resins. As shown in
Fig. 5, It can be observed from tan δ that both resins have two secondary
transitions with a temperature range from 100 ◦C to 300 ◦C. The
maximum testing temperature of DMA was set to 500 ◦C because of the
3.2. Thermal properties of resin
As shown in Fig. 4, TGA shows that the char yield of the two
monomers after curing BPEh and BPFh is significantly higher than that
of the monomer. The carbon residue under different curing procedures is
greater than 70%. Compared with monomers, as curing time increases,
the T5% of the two resins also increases. Under the curing conditions of
procedure c, the T5% of the two resins is close to 500 ◦C, indicating that
as the temperature increases, the cyanide conversion rate of the base is
T
5% of the resin. Meanwhile, the tan δ of the two resins does not form a
complete peak at around 500 ◦C. Thus, according to the peak of tan δ, it
can be rationally estimated that the Tg of the two resins is more than
500 ◦C. In addition, the storage modulus of the resin did not decrease in
the order of magnitude, suggesting again that the resin has qualified
thermo-mechanical properties.
7

S. Liao et al.
Polymer 210 (2020) 123001
Fig. 9. In situ FTIR of BPFh monomer heated from 200 to 310 ◦C. (a): the stretching vibration of nitrile (-CN) (b): the stretching vibration of –C¼N in polytriazine.;
(c): the stretching vibration of N–H isoindoline; (d): the stretching vibration of –CH2-.
3.3. Discussion of curing mechanism
the FTIR. UV–Vis is a classic method to confirm the phthalocyanine ring,
as shown in Fig. 7, the peak of phthalocyanine rings at around 603 nm
[23,47,48] were observed for the cured resins. It is worth noting that the
methyne group (2878 cmꢀ 1) in the BPEh monomer structure [27] and
the methylene group (2915 cmꢀ 1) in the BPFh monomer [37] could be
observed in Fig. 6. There is a significant decrease before and after curing
reaction, which suggests that methine and methylene may participate in
the curing reaction of the nitrile group, and even be the key factor to
initiate the polymerization of the nitrile group.
Compared with calorimetry and rheology, infrared could show the
reaction of functional groups. We analyzed the structural evolution of
BPEh and BPFh before and after curing by off-situ FTIR. As shown in
Fig. 6, there is a strong nitrile peak (2232 cmꢀ 1,2230 cmꢀ 1) for both
monomers; compared to the monomer, the nitrile peak in the cured
product has a significant decrease. Relative reports pointed out that
[29–31], isoindoline also has characteristic diffraction peaks at 1720
cmꢀ 1, 1660 cmꢀ 1, and from the results in Fig. 6, it can be seen that BPEh
and BPFh resins are at 1720 cmꢀ 1 The 1660 cmꢀ 1 have the diffraction
peak attributed to isoindoline, which further indicates that the BPEh and
BPFh samples may generate isoindoline structure during the curing re-
action. At the same time, it is generally believed that if the phthalonitrile
group reacts to form a triazine ring during the curing reaction, there will
be more obvious peaks at 1520 cmꢀ 1 and 1360 cmꢀ 1 in the infrared
spectrum [10,32], and it can be seen from Fig. 6 that under the exper-
imental conditions, a clear sharp peak is indeed observed at the position
assigned to the characteristic peak of the triazine ring, so it is speculated
that BPEh and BPFh samples may generate three during the curing re-
action. Azine ring structure. Interestingly, this is different from our
previous studies on the acceleration of the polymerization of phthalo-
nitrile resins with imide cycloaliphatic structural elements [23] and the
PN resin (BPN) derived from creosol [17]. The characteristic peak of the
phthalocyanine ring (1010 cmꢀ 1) and the characteristic peak position of
the ether bond in BPEh and BPFh (1017 cmꢀ 1 [33]) have a certain
overlap, it is difficult to determine the formation of phthalocyanine by
Because the off-situ infrared sample preparation and other processes
have a great influence on the test results, and in-situ infrared can
eliminate the interference of these aspects on the test results [35,36], so
further observation of BPEh and BPFh samples through in-situ infrared
changes in groups during the heating process. Fig. 8 (a) and 9 (a) are the
in-situ infrared spectra of the BPEh and BPFh samples at 2250-2200
cmꢀ 1, it’s easy to find that as the temperature increases, the character-
istic diffraction peak (2228 cmꢀ 1) belongs to the nitrile is obviously
weakened, indicating that the nitrile group participated in the curing
process of the BPEh and BPFh. In order to further verify whether the
triazine ring was formed during the reaction, Fig. 8 (b) and 9 (b) gave
the in-situ infrared spectra of the BPEh and BPFh samples at 1550-1300
cmꢀ 1 respectively. As the temperature rises, new absorption peaks
gradually appear at the positions of the characteristic diffraction peaks
of the triazine ring (1520 cmꢀ 1 and 1360 cmꢀ 1), which is consistent with
the results of off-situ infrared, further verifying the triazine Ring for-
mation. It has been documented that [29–31], the formation of iso-
indoline also shows significant absorption peaks near 1720 cmꢀ 1 and
8

S. Liao et al.
Polymer 210 (2020) 123001
Scheme 2. The proposed reaction mechanism of BPEh and BPFh.
1660 cmꢀ 1. Fig. 8 (c) and 9 (c) are the in-situ infrared spectra of the
BPEh and BPFh samples at 2000-1300 cmꢀ 1 respectively. Obviously, as
the temperature increases, a new absorption peaks of BPEh samples
appeared at 1740 cmꢀ 1 and at 1660 cmꢀ 1, new absorption peaks of BPFh
samples appeared at 1720 cmꢀ 1 and 1660 cmꢀ 1, indicating that the
BPEh and BPFh samples formed an isoindolin structure during the
curing process.
(calculated values are: 1887.56 and 1809.52), this tetramer is probably
related to the formation of phthalocyanine ring. In addition, the dimer
could be attributed to the copolymer/homopolymer isoindoline struc-
ture, and the trimer may be related to the triazine ring or the
polyisoindoline.
In order to further investigate the basis for the two cured model
compounds to be cured, we analyzed the possible structures BPEh
(–CH–, 2878 cmꢀ 1) and BPFh (-CH2-, 2915 cmꢀ 1) in situ infrared. As
shown in Fig. 8 (d) and Fig. 9 (d), the methyne absorption peak of the
BPEh sample and the methylene absorption peak of the BPFh sample,
respectively, it can be seen that as the temperature increases, the
methyne absorption peak The absorption peak of (2878 cmꢀ 1) methy-
lene group (2915 cmꢀ 1) was significantly weakened, suggesting that
In order to further study the structure of the polymerization product,
BPEh and BPFh samples were performed on MALDI-TOF MS. The test
results of BPEh are shown in Figure S1 and Table S1, and that of BPFh
are shown in Figure S2 and Table S2. It could be observed that two pre-
cured resins show typical oligomer characteristics, the m/z at 1887.12
and 1810.95 correspond to the tetramers of BPEh and BPFh respectively
9

S. Liao et al.
Polymer 210 (2020) 123001
Fig. 10. Cyano conversion as a function of times at 250 ◦C and 270 ◦C.
methyne group and methylene group are likely to participate in the
curing process of nitrile group. This is also consistent with offline
infrared.
BPFh were determined from data of in-situ FTIR, and compared with
these of resin system DPPH-containing resin system (20 wt%), as shown
in Fig. 10. We use the area of the chemical stable benzene ring structure
(1530 cmꢀ 1 to 1450 cmꢀ 1) as a benchmark to calculate the conversion of
cyano groups. The conversion of cyano groups of the DPPH-containing
resin system is much lower at initial stage of the duration both at 250
Based on the experimental results above, we preliminarily speculate
that the “active hydrogen” generated by the methylene units in the BPFh
and BPEh under high temperature conditions promotes the polymeri-
zation reaction of the nitrile group, that is, the mechanism of “methy-
lene/methyne activated phthalonitrile thermal polymerization”. As
shown in Scheme 2. Because the methyne structure can homogenize the
covalent bond under high temperature conditions, forming a relatively
stable tertiary carbon radical and a highly reactive hydrogen radical,
while the methylene structure under high temperature conditions,
relative to methyl groups are more prone to homogenous cleavage of
covalent bonds, forming a relatively stable carbon radical and a highly
reactive hydrogen radical [34]. Among them, hydrogen radicals or ter-
tiary carbon radicals may promote the polymerization of phthalonitrile.
This phenomenon of free radicals generated by the homolysis of cova-
lent bonds has been reported in the research of lignin and polymers
containing tertiary carbon structures (such as polystyrene, poly-
acrylonitrile, polyvinyl chloride, etc.) [38–40]. In summary, we specu-
late that this phenomenon of accelerating the polymerization of
phthalonitrile groups based on the structure of diphenylmethane may
achieve the curing reaction of the monomer through the free radical
pathway.
◦
and 270 C compared with these of BPEh and BPFh. The results indi-
cated that there is a certain inhibitory effect. Unexpectedly, the con-
version of cyano groups gradually increased for an extended period of
time, which probably indicated that the DPPH also played an acceler-
ated role in the later isothermal stage. It is worth noting that the cyano
conversion of the BPFh-DPPH blending has two different acceleration
stages for 270 ◦C duration, which could be related to the presence of two
hydrogen protons in BPFh. The reaction mechanism of cyano polymer-
ization accelerated by DPPH could be complicated, but the results added
considerable evidences that the curing reaction of BPEh and BPFh
involved a free radical process.
In addition, the Molecular Electrostatic Potential (MEP) is simulated
by molecular simulation technology for free radicals derived from BPEh
and BPFh monomer (R-BPEh and R-BPFh), as shown in Fig. 11. For R-
BPEh and R-BPFh, the electrons are mainly distributed near the cyano
group. Except for the hydrogen atom, the electron distribution near the
aliphatic carbon after radicalization is relatively low. Therefore, it is
reasonable to consider that the aliphatic carbon is a potential reaction
site to cyano group. Although this could not be a direct evidence of its
radical polymerization, it can provide more understanding of the reac-
tion site.
2, 2-Diphenyl-1-picryl hydrazine (DPPH), a well-known high tem-
perature free radical scavenger [23,46], was selected to further under-
stand the role of free radicals involving in the polymerization of
phthalonitrile unit in our cases. The cyano conversions of BPEh and
10

S. Liao et al.
Polymer 210 (2020) 123001
Fig. 11. The electrostatic potentials mapping of free radicals derived from R-BPEh and R-BPFh.
4. Conclusion
interests or personal relationships that could have appeared to influence
the work reported in this paper.
In this paper, a bis-phthalonitrile-terminated compound bisphenol A,
F, E with a symmetric molecular structure was synthesized. The rheo-
logical research results show that: BPEh and BPFh could undergo curing
reaction, the promotion effect of methylene is stronger than methyne,
and methyl has no obvious promotion effect. DMA shows that the Tg of
the cured resins is more than 500 ◦C and the storage modulus of the resin
did not decrease in the order of magnitude, indicating that the resin has
qualified thermo-mechanical properties. According to FTIR and UV–Vis,
isoindoline and triazine ring structures exist in the cured products.
During the curing reaction, infrared showed that the content of methyne
and methylene in BPEh and BPFh decreased, which further implied that
the methyne and methylene participated in the curing reaction of the
nitrile group. The effect of DPPH on the reaction rate, added consider-
able evidences that the curing reaction of BPEh and BPFh involved a free
radical process. The research further expands the design scope of
phthalonitrile resin curing system. The new resin systems based on BPEh
and BPFh also lay a foundation for the subsequent mechanism research
and application research. In the future, theoretical simulation and
experimental science will be combined to explore the reaction mecha-
nism from the perspective of thermodynamics and kinetics.
Acknowledgments
The authors thank the Analytical & Testing Center, Sichuan Uni-
versity, P. R. China for supplying the Materials Studio 8.0 program.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.polymer.2020.123001.
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The authors declare that they have no known competing financial
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