Strategies for improving the performance of diallyl bisphenol A-based benzoxazine resin: Chemical modification via acetylene and physical blending with bismaleimide

Article abstract of DOI:10.1016/j.reactfunctpolym.2021.104958

The benzoxazine resin (DBA-a) synthesized from raw materials of 2,2′-diallyl bisphenol A (DBA), aniline and paraformaldehyde has been well-commercialized, but a reduction in its polymerization temperature and improved comprehensive properties are highly challenging to satisfy the requirements of various composites. Here, we have developed three strategies for improving the performance of DBA-a resin: the first is the chemical modification, synthesizing a novel DBA-based bisbenzoxazine containing acetylene functionality (DBA-ac); the second is the physical blend of bismaleimide (DDM-BMI) and DBA-a, forming a homogenous copolymer; and the third is combining both chemical and physical methods, achieving the DBA-ac/DDM-BMI blend as a new precursor for high-performance thermosets. The newly obtained DBA-ac possesses the advantages of relatively lower polymerization temperature and good thermal stability during the polymerization process. In addition, the polymerization temperature of DBA-ac can be further lowered by blending with DDM-BMI via multiple polymerization mechanisms. Specifically, the resulting copolymeric thermoset derived from DBA-ac/DDM-BMI blend is found to show fascinating properties such as exceptionally low coefficient of thermal expansion (CTE of 37.1 ppm/^oC), high thermal stability (T_g of 370 °C), outstanding flame retardancy (THR of 17.6 KJg^?1), and low dielectric constant (2.86 at 1 MHz). The current study delivers readily approaches to enhance the performance of thermosetting resins via both chemical modification and physical blending.

Full text of DOI:10.1016/j.reactfunctpolym.2021.104958

Reactive and Functional Polymers 165 (2021) 104958
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Perspective Article
Strategies for improving the performance of diallyl bisphenol A-based
benzoxazine resin: Chemical modification via acetylene and physical
blending with bismaleimide
Rui Yang, Kan Zhang*
Research School of Polymeric Materials, School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
A R T I C L E I N F O
A B S T R A C T
The benzoxazine resin (DBA-a) synthesized from raw materials of 2,2′-diallyl bisphenol A (DBA), aniline and
paraformaldehyde has been well-commercialized, but a reduction in its polymerization temperature and
improved comprehensive properties are highly challenging to satisfy the requirements of various composites.
Here, we have developed three strategies for improving the performance of DBA-a resin: the first is the chemical
modification, synthesizing a novel DBA-based bisbenzoxazine containing acetylene functionality (DBA-ac); the
second is the physical blend of bismaleimide (DDM-BMI) and DBA-a, forming a homogenous copolymer; and the
third is combining both chemical and physical methods, achieving the DBA-ac/DDM-BMI blend as a new pre-
cursor for high-performance thermosets. The newly obtained DBA-ac possesses the advantages of relatively lower
polymerization temperature and good thermal stability during the polymerization process. In addition, the
polymerization temperature of DBA-ac can be further lowered by blending with DDM-BMI via multiple poly-
merization mechanisms. Specifically, the resulting copolymeric thermoset derived from DBA-ac/DDM-BMI blend
is found to show fascinating properties such as exceptionally low coefficient of thermal expansion (CTE of 37.1
ppm/oC), high thermal stability (Tg of 370 ◦C), outstanding flame retardancy (THR of 17.6 KJgꢀ 1), and low
dielectric constant (2.86 at 1 MHz). The current study delivers readily approaches to enhance the performance of
thermosetting resins via both chemical modification and physical blending.
Keywords:
Benzoxazine resin
Diallyl bisphenol A
Acetylene
Bismaleimide
High performance
1. Introduction
thermosetting resins with improved properties to satisfy more high-
performance applications over the past three decades. For instance,
Thermosets with high thermal stability and outstanding mechanical
performance have been widely applied as resin matrix for composites in
electrical insulation, aerospace and ballistic industries [1]. Among the
many well-known thermosetting resins, benzoxazines have attracted
significant attention in recent years because of their excellent compre-
hensive properties [2–5]. In addition, the thermal polymerization of
benzoxazine usually takes a long time above 200 ◦C, and the higher
curing temperature and long curing period limit the application of
benzoxazine resin. The polymerization temperature can be reduced by
molecular tailoring at the structure level or by the addition of external
auxiliaries and/or copolymerization with other polymers [6–8]. Typi-
cally, their polymerized polymers, polybenzoxazines, possess high glass
transition temperatures [9,10], low surface energy [11–13], high me-
chanical strength [14], and unique electric properties [15–17]. Never-
theless, many efforts have been made to synthesize new benzoxazine
the performance of polybenzoxazine thermosets can be further
enhanced by incorporating additional cross-linkable or rigid function-
alities, such as nitrile [18], acetylene [19,20], allyl [21], maleimide
[20,22,23], norbornene [24–26], amide [27–29], imide [30,31], and
benzoxazole groups [32]. Among them, we are especially interested in
the acetylene functional benzoxazines since the acetylene can be easily
thermally activated to polymerize. Previous works have clearly shown
that the acetylene group can lower the ring-opening polymerization
temperature of the oxazine ring [19]. In addition, it can also signifi-
cantly reduce the polymerization temperatures of norbornene- or
phthalonitrile-containing thermosetting systems, which general require
the polymerization temperature over 300 ◦C [33,34].
Another attractive incorporated functionality is allyl group, which
can react with hydride-terminated siloxane to form main-chain type
benzoxazine-containing polymers [35]. 2,2′-Diallyl bisphenol A (DBA)
* Corresponding author.
E-mail address: zhangkan@ujs.edu.cn (K. Zhang).
https://doi.org/10.1016/j.reactfunctpolym.2021.104958
Received 10 May 2021; Received in revised form 31 May 2021; Accepted 2 June 2021
Available online 5 June 2021
1381-5148/© 2021 Elsevier B.V. All rights reserved.

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
Scheme 1. Chemical Structures of 2,2′-Diallyl Bisphenol A (DBA) and Bismaleimide Resin (DDM-BMI).
(Scheme 1) was widely used as the phenol resource to synthesize allyl-
containing benzoxazine through Mannich condensation. Although the
DBA-based benzoxazine resin has been well-commercialized, a reduc-
tion in its polymerization temperature and improved comprehensive
properties are highly challenging to satisfy the requirements of various
applications. Wang et al. prepared DBA-a (DBA and aniline-based bis-
benzoxazine) resin, and discovered that the exothermic peak for the
ring-opening polymerization of this allyl-containing benzoxazine was
over 260 ◦C [36]. Besides, a much higher polymerization temperature
(348 ◦C) was required for the cross-linking reaction of allyl group [21].
On the other hand, previous studies showed that DBA is a powerful
modifier used to improve the processability of bismaleimide (BMI)
resins [21,37]. In general, the aromatic nature and the high crosslink
density of the thermosets based on unmodified BMI resins make them
brittle and hence limit their extended application. The copolymerization
of BMI and DBA could proceed at elevated temperatures through ene-
addition type reaction, thereby leading the high thermal stability and
excellent toughness of the resulting BMI-based thermosets [37].
The above findings inspired us to develop new approaches to
enhance the performance of DBA-based benzoxazine resins. In this
study, we report efficient strategies for improving the comprehensive
properties of DBA-a benzoxazine resin. Firstly, a DBA-based benzoxazine
containing acetylene was successfully synthesized. We found that
incorporation of acetylene in benzoxazine significantly decreases the
polymerization temperature of both oxazine ring and allyl group. Sec-
ondly, we used 4,4′-diaminodiphenyl methane-based bismaleimide
(DDM-BMI) (Scheme 1) to blend with DBA-a. The blend thermosetting
system also showed relatively lower polymerization temperature and
higher thermal stability compared with DBA-a during the polymeriza-
tion process. Lastly, a new thermosetting system was encouraged to
achieve by blending acetylene-containing DBA-based bisbenzoxazine
with DDM-BMI via combined chemical modification and physical
blending approaches. The thermosetting system developed via both
chemical and physical methods showed multiple polymerization
mechanisms, excellent thermal and dielectric properties, evidencing its
potential applications in microelectronic industries and other high
performance areas. The detailed strategies of chemical synthesis and
physical blends, the multiple polymerization mechanisms involved in
the polymerization process, and the properties of the resulting thermo-
sets have been discussed in the current work.
recorded on a Bruker AV 400 MHz NMR spectrometer using tetramethyl
silane (TMS) as internal standard. Two dimensional (2D) NMR analysis
1
of H-13C HMQC spectrum was also performed to further confirm the
assignments on the same NMR spectrometer. Fourier-transform infrared
(FT-IR) measurements for both benzoxazines and thermosets were per-
formed on a Nicolet Nexus 670 spectrophotometer in the frequency
range of 4000–400 cmꢀ 1 at a resolution of 4 cmꢀ 1. High resolution mass
spectrometry (HRMS) was carried out on a mass spectrometer (Bruker
solanX 70 FT-MS). Elemental analysis of benzoxazine samples were
performed on an Elementar Vario EL-III analyzer. Differential scanning
calorimeter (DSC) analysis was conducted in a nitrogen atmosphere (60
mL/min) on a NETZSCH Model 204f1 instrument with a heating rate of
◦
10 C/min. Dynamic mechanical analysis (DMA) for polybenzoxazine
thermosets was carried out on NETZSCH DMA/242E analyzer, and a
tension mode with an amplitude of 10 μm with a frequency of 1 Hz and
the heating rate of 3 ◦C/min during the measurement. The fracture
surfaces of thermosets were determined by scanning electron micro-
scope (SEM) (JEOL JSM 6700F). Each thermoset sample was sputtered
with a thin layer of gold before starting the measurement. Thermogra-
vimetric analysis (TGA) was performed using a NETZSCH STA449-C
analyzer. Each sample was heated from room temperature to 800 ◦C
at a heating rate of 10 ◦C/min under N2 (60mL/min). Microscale com-
bustion calorimeter (MCC, Fire Testing Technology) was adopted to
obtain the specific heat release rate (HRR, Wgꢀ 1). The total heat release
(THR, KJgꢀ 1) was calculated based on the HRR curves. About 5 mg of
dried thermoset sample was measured each time from 100 to 750 ◦C at a
heating rate of 1 K/s, under a stream of N2 (80mL minꢀ 1). The dielectric
constants and dielectric losses of thermoset films were tested by the
capacitance method using a TZDM-RT-300 Dielectric Testing System
Analyzer. Both surfaces of the sample were coated by silver paste for
dielectric testing.
3. Methods
3.1. Synthesis of 6,6′-(propane-2,2-diyl)bis(8-allyl-3-phenyl-3,4-
dihydro-2H-benzo[e][1,3]oxazine) (abbreviated as DBA-a)
2,2′-Diallyl bisphenol A (DBA) (3.08 g, 0.01 mol), paraformaldehyde
(1.32 g, 0.044 mol), aniline (1.86 g, 0.02 mol) and 30 mL of toluene
were added into a 100 mL single-neck flask. The reaction mixtures were
stirred and then heated under reflux for 5 h. Then the reaction mixture
was washed with 3 N NaOH aqueous solution and distilled water for 3
times, respectively. The solvent was then removed by a rotary evapo-
rator. Finally, the liquid product was dried under vacuum at 60 ◦C for 48
h (yield ca. 83%). 1H NMR (400 MHz, CDCl3), ppm: δ = 7.36–6.77 (14H,
2. Experimental
2.1. Materials
2,2′-Diallyl bisphenol A (DBA) (98%), 4,4′-bismaleimidodiphenyl-
methane (DDM-BMI) (99%), paraformaldehyde (99%), and 3-amino-
phenylacetylene (98%) were purchased from Aladdin Reagent, China.
Aniline, sodium hydroxide (NaOH), toluene and acetone were pur-
chased from Energy Reagent, China, and used without further
purification.
–
–
2
– –
Ar), 6.06 (m, 2H, CH CH ), 5.42 (s, 4H, O CH2 N, oxazine), 5.08
–
2
–
–
–
– –
(d, 4H,
CH ), 4.63 (s, 4H, Ar CH2 N, oxazine), 3.38 (d, 4H,
– –CH
–
N
CH2 CH ), 1.65 (s, 6H, CH3). FT-IR spectra (KBr), cmꢀ 1: 1637
–
–
–
–
– –
O
(olefinic C C stretching vibration of allyl), 1230 (C
C antisym-
metric stretching), 946 (benzoxazine related band). Anal. calcd for
C
37H38N2O2: C, 81.88%; H, 7.06%; N, 5.16%. Found: C, 81.81%; H,
7.09%; N, 5.14%. HRMS-ESI (m/z): [M
+
H]+ calculated for
C
37H39N2O+2 , 543.3006; found, 543.2994.
2.2. Characterization
1H and 13C nuclear magnetic resonance (NMR) spectra were
2

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
Scheme 2. Synthesis of DBA-a and DBA-ac.
3.2. Synthesis of 6,6′-(propane-2,2-diyl)bis(8-allyl-3-(3-ethynylphenyl)-
3,4-dihydro-2H-benzo[e][1,3]oxazine) (abbreviated as DBA-ac)
2,2′-Diallyl bisphenol A (DBA) (3.08 g, 0.01 mol), paraformaldehyde
(1.32 g, 0.044 mol), 3-aminopheylacetylene (2.34 g, 0.02 mol), and 30
mL of toluene were added into a 100 mL round-bottom flask. The re-
action mixtures were heated and under reflux for 6 h. Afterwards the
reaction mixture was washed with 3 N NaOH aqueous solution and
distilled water for 3 times each, and the solvent was removed by a rotary
evaporator. The final product was dried in a vacuum oven at 60 ◦C for
48 h (yield ca. 80%). 1H NMR (400 MHz, CDCl3), ppm: δ = 7.30–6.70
–
–
2
– –
(12H, Ar), 6.00 (m, 2H, CH CH ), 5.36 (s, 4H, O CH2 N, oxazine),
–
2
–
–
–
– –
5.03 (d, 4H, CH CH ), 4.58 (s, 4H, Ar CH2 N, oxazine), 3.32 (d,
–
–
–
–
–
–
–
4H, N CH -CH ), 3.07 (s, 2H, C CH), 1.59 (s, 6H, CH3). FT-IR
2
spectra (KBr), cmꢀ 1: 3287 (
H stretching), 2106 (C C stretch-
–
–
–
– –
C
–
–
–
–
– –
O C
ing), 1638 (olefinic C C stretching vibration of allyl), 1231 (C
asymmetric stretching), 950 (oxazine ring related mode). Anal. calcd for
C41H38N2O2: C, 83.36%; H, 6.48%; N, 4.74%. Found: C, 83.30%; H,
6.51%; N, 7.69%. HRMS-ESI (m/z): [M
+
H]+ calculated for
C
41H39N2O+6 , 591.3006; found, 591.2995.
3.3. Polymerization of DBA-based benzoxazine monomers
Polymerization of DBA-a was carried out by thermal treatments at
140, 160, 180, 200, 220, 240, 260 and 280 ◦C for 1 h at each step, to
obtain poly(DBA-a). Poly(DBA-ac) was also obtained following the same
stepwise polymerization procedure with a lower terminal temperature
at 240 ◦C.
3.4. Preparation and polymerization of resin blends
DBA-a or DBA-ac was blended with DDM-BMI in the mole ratio of 1:1
to form DBA-a/DDM-BMI and DBA-ac/DDM-BMI blends. Each blend
◦
was stirred in a beaker at 100 C until the homogenous liquid sample
was obtained. Polymerization of DBA-a/DDM-BMI blend was carried out
by thermal treatments at 140, 160, 180, 200, 220, 240 and 260 ◦C for 1 h
at each step, obtaining poly(DBA-a-co-DDM-BMI). The blend of DBA-ac/
DDM-BMI was also polymerized stepwise, with every step carried out at
140, 160, 180, 200 and 220 ◦C for 1 h each, thus obtaining poly(DBA-ac-
co-DDM-BMI).
Fig. 1. 1H (a) and 13C (b) NMR spectra of DBA-a and DBA-ac in CDCl3.
3

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
Fig. 2. FT-IR spectra of DBA-a and DBA-ac.
Fig. 3. DSC thermograms of DBA-a, DBA-ac, DBA-a/DDM-BMI and DBA-ac/
DDM-BMI.
4. Result and discussion
determine the existence of characteristic functionalities in DBA-a and
4.1. Synthesis of DBA-based bisbenzoxazines
–
–
–
– –
C H stretching and C C stretching modes
–
DBA-ac. For example, the
–
of acetylene group in DBA-ac are observed at 3287 and 2106 cmꢀ 1
,
In the current work, two bisbenzoxazine monomers were success-
fully synthesized using 2,2′-diallyl bisphenol A (DBA), aniline/3-
aminophenylacetylene, and paraformaldehyde as shown in Scheme 2.
We specifically purified both monomers since the previous studies have
indicated that the involved impurities in benzoxazine resin show the
promoting effect on the ring-opening polymerization of the oxazine ring
[38,39]. The structures and purities of bisbenzoxazine monomers were
confirmed by NMR, FT-IR, HRMS and elemental analysis.
respectively [40]. The characteristic bands at 1637 and 1638 cmꢀ 1 are
–
assigned to the olefinic C C stretching vibration of allyl in DBA-a and
–
DBA-ac, respectively [36]. Additionally, the aromatic ether of oxazine
ring in DBA-a and DBA-ac is determined by the bands centered at 1230
– –
and 1231 cmꢀ 1, respectively, which can be assigned to the C
O C
antisymmetric stretching modes [41,42]. Furthermore, the oxazine ring
related modes for DBA-a and DBA-ac are found at 946 and 950 cmꢀ 1
,
Fig. 1 shows the 1H and 13C NMR spectra of DBA-a and DBA-ac. The
nomenclatures used for NMR assignments are also indicated in Fig. 1. As
shown in Fig. 1a, the characteristic signals for the benzoxazine, corre-
respectively [43]. The above results combined with the elemental
analysis and HRMS give sufficient evidences for the excellent purity of
both bisbenzoxazine monomers obtained in this study.
– –
sponding to the two methylenes in the oxazine ring (Ar CH2 N- and
– – – –
CH2 N ) for DBA-a are found at 4.63 and 5.42 ppm, respec-
O
4.2. Polymerization and processability of DBA-based benzoxazines and
benzoxazine/bismaleimide blends
tively. These resonances for DBA-ac are observed at 4.58 and 5.36 ppm,
respectively. Besides, the signals attributed to methyl in the DBA
structure appear at 1.65 and 1.59 ppm for DBA-a and DBA-ac, respec-
tively. In addition, the characteristic proton resonances of the allyl
group in DBA-a and DBA-ac can be observed around 3.38, 5.08 and 6.06
ppm, and 3.32, 5.03 and 6.00 ppm, respectively. Moreover, the presence
of acetylene group in DAB-ac is confirmed by the proton signal of
The polymerization profile of benzoxazine monomers and benzox-
azine/bismaleimide blends was studied by DSC as shown in Fig. 3. As
can be seen from the figure, among the four thermosetting resins stud-
ied, DBA-a shows the highest exothermic peak temperature at 281 ◦C.
Besides, another exothermic peak which is possible attributed to the
polymerization of allyl group can be found from 300 to 350 ◦C. The
exotherm that is due to the polymerization of DBA-ac has its maximum
located at 242 ◦C, which is much lower than that of DBA-a. As a result,
the modification of acetylene group in DBA-a can greatly lower its
polymerization temperature. Such reducing in polymerization temper-
ature may be caused by the heat releasing from the cross-link of acety-
lene. In the case of the thermosetting blend of DBA-a/DDM-BMI, two
separated exothermic peaks centered at 216 and 268 ◦C are observed.
Clearly, blending with bismaleimide can also lower the polymerization
temperature of DBA-a. However, the assignments of these two
exothermic peaks involved in the polymerization process of DBA-a/
DDM-BMI need to be further confirmed. In the case of DBA-ac/DDM-
BMI blend, an exothermic peak centered at 213 ◦C is observed, which
shows the lowest polymerization temperature among these four ther-
mosetting systems. Besides, the relatively higher exothermic peak tem-
perature observed at 268 ◦C in DBA-a/DDM-BMI has completely
–
– –
C
CH at 3.07 ppm.
–
Fig. 1b shows the 13C NMR spectra of DBA-a and DBA-ac. The typical
– – – – – – –
groups from the
carbon signals for Ar CH2
N
and
O
CH2
N
oxazine ring in DBA-a and DBA-ac are observed at 50.84 and 79.07 ppm,
and 50.77 and 78.64 ppm, respectively. The carbon resonances of the
methyl functionality in DBA-a and DBA-ac appear at 31.24 and 31.27
ppm, respectively. Besides, the carbon resonances attributed to the allyl
group in DBA-a and DBA-ac are found at 33.96, 115.26 and 142.73 ppm,
and 34.03, 115.47 and 142.92 ppm, respectively. The carbon resonances
assigned to acetylene group in DBA-ac were confirmed by using H 13C
1
–
HMQC NMR analysis (Fig. S1). As also shown in Fig. 1b, the signal of
–
– –
C
CH group in DAB-ac is highly overlapped with the carbon signals
–
in CDCl3, which is located at 77.15 ppm. Furthermore, another carbon
–
– –
C CH in acetylene is observed at 84.00 ppm.
–
resonance attributed to
The chemical structure of bisbenzoxazine monomers was further
confirmed by FT-IR. As shown in Fig. 2, a lot of bands are highlighted to
4

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
well as blending bismaleimide in DBA-based benzoxazine can signifi-
cantly lower the polymerization temperature of thermosetting systems.
These in situ FT-IR spectral differences among the benzoxazine mono-
mers and resin blends are a strong evidence of the proposed accelerating
mechanism and in accordance with the conclusion obtained from the
above DSC analyses. Therefore, we propose structural transformation
behaviors for DBA-a, DBA-ac, DBA-a/DDM-BMI and DBA-ac/DDM-BMI
as shown in Scheme 3.
The processability of these thermosetting resins was also evaluated.
The above DSC thermograms of these resins all exhibit no melting peaks.
This is of great interest, as DBA-based benzoxazine systems show
excellent processability by taking their amorphous forms into consid-
eration. Besides, the dependence of viscosity on temperature for these
resins are shown in Fig. 5. The viscosities of these resins significantly
decrease with increasing the temperature. In particular, the viscosity
values of DBA-a and DBA-ac are as low as 10 Pa.s at 73 and 85 ◦C,
◦
respectively, at a heating rate of 3 C/min, which exhibits the advan-
tages of being DBA-based benzoxazine thermosetting resins with low
viscosity. Although the viscosities of both DBA-a/DDM-BMI and DBA-
ac/DDM-BMI blends are much higher than intrinsic DBA-based ben-
zoxazine resins, they still can transfer into liquid forms as the temper-
Fig. 4. In situ FT-IR spectra of DBA-ac/DDM-BMI blend after various thermal
treatments at the designated temperature for 1 h.
◦
ature increases to 100 C. In addition, the viscosities of DBA-a/DDM-
BMI and DBA-ac/DDM-BMI increase rapidly after 150 ◦C, indicating
that both blends possess very low initial polymerization temperature.
Such low onset temperatures for both thermosetting blends can be
attributed to the acceleration of the polymerization of the olefinic bond
via the exothermic heat generated by maleimide and/or acetylene
functionalities.
disappeared in DBA-ac/DDM-BMI blend. This indicates that the acety-
lene group and bismaleimide in DBA-ac/DDM-BMI should paly accel-
erating roles on different polymerizable groups in DBA-a during the
polymerization process. In general, benzoxazine resins show a single
exothermic peak although it possesses different polymerization tem-
peratures which caused by various mechanisms [40]. The newly ob-
tained DBA-ac/DDM-BMI thermosetting system also exhibits a similar
trend of involving multiple polymerization processes into one relatively
lower polymerization temperature range, possibly due to the accelera-
tion of the polymerization of oxazine ring and the olefinic bond via the
exothermic heat released by both acetylene and maleimide
functionalities.
The thermal stability of the thermosetting resins during the pro-
cessing is another key factor for evaluating the processability of ther-
mosets. The thermal stability of DBA-a, DBA-ac, DBA-a/DDM-BMI and
DBA-ac/DDM-BMI resins were studied by TGA, and the thermogram of
each thermosetting resin was obtained. As shown in Fig. 6, DBA-a ex-
hibits an excessive amount of weight-loss (~10%) before the onset of the
polymerization. Besides, a further ~18% of weight loss can be found in
its polymerization process, which is possibly due to the cleavage of
zwitterionic intermediate, forming extraordinary unstable phenolic
species and N-methyleneaniline [44]. Since both the ortho- and para-
positions of the oxygen atom in the oxazine ring of DBA-a are
substituted, a cross-linking network with very low density is expected to
form despite some reactivity in the aromatic ring of the aniline group.
Hence good testing film samples of poly(DBA-a) were failed to obtain by
using the general casting polymerization method as seen from the inset
photo in Fig. 6. However, the thermal stability of DBA-a can be signif-
icantly enhanced by modification of the cross-linkable acetylene, and
through the blending of co-polymerizable bismaleimide. Thermoset film
samples with expected shape were obtained for poly(DBA-ac), poly
(DBA-a-co-DDM-BMI) and poly(DBA-ac-co-DDM-BMI). Notably, the
weight loss in the polymerization process of DBA-ac/DDM-BMI blend is
much lower compared with that of other thermosetting systems, indi-
cating the incorporation of acetylene group as well as blending bisma-
leimide in DBA-a can also greatly improve the thermal stability in
processing.
In order to qualitatively study the structural changes during the
polymerization processes of benzoxazine monomers and benzoxazine/
bismaleimide blends, in situ FT-IR analyses were then carried out.
Herein, the characteristic bands originated from the C-O-C antisym-
metric stretching and oxazine ring are used to monitor the ring-opening
–
polymerization of benzoxazine rings [42,43], and the olefinic C
C
–
stretching vibration (~1640 cmꢀ 1) are adopted to monitor the poly-
merization of allyl group. As shown in Figs. S2, the characteristic bands
of oxazine ring fully disappear after the final thermal treatment at
◦
280 C. However, the typical band attributed from the allyl group at
1637 cmꢀ 1 still keep high intensity, indicating the polymerization
temperature of allyl group in DBA-a requires much higher temperature.
Fig. S3 shows the variation of the characteristic bands in DBA-ac during
the polymerization. The bands from the acetylene at 3287 and 2106
cmꢀ 1, and the typical bands of oxazine ring as well as the band attrib-
uted to allyl group in DBA-ac all completely disappear after heating at
240 ◦C. In addition, the co-polymerization behaviors can be observed in
DBA-a/DDM-BMI blend since the bands at 826 (imide CH wagging of the
vinylene) and 690 cmꢀ 1
–
(
–
–
CH H out-of-plane bending mode) from
bismaleimide decrease with the characteristic bands from allyl and
oxazine ring in DBA-a (Fig. S4). A relatively higher temperature was
needed to fully polymerize all functionalities in this blending system.
Interestingly, the characteristic bands highlighted in Fig. 4, which are
from the polymerizable groups including oxazine ring, acetylene, allyl,
and maleimide in DBA-ac/DDM-BMI blend, all completely disappear
4.3. Thermal stability and flame retardancy of thermosets
DMA and TMA measurements were carried out for poly(DBA-ac),
poly(DBA-a-co-DDM-BMI) and poly(DBA-ac-co-DDM-BMI), respec-
tively. As mentioned in the above section, good testing film samples for
DMA and TMA measurements of poly(DMA-a) were failed to prepare
due to the existence of large amounts of sample evaporation during the
polymerization.
◦
after thermal treatment at a much lower temperature (220 C). These
results therefore confirm that the incorporation of acetylene group as
5

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
Scheme 3. Proposed Structural Transformations During the Polymerization Processes for DBA-a (a), DBA-ac (b), DBA-a/DDM-BMI (c) and DBA-ac/DDM-BMI (d).
We firstly roughly detected the Tg values of thermosets by DSC since
DDM-BMI) shows a highest Tg value at 365 ◦C. As seen from the DMA
curves in Fig. 7b, poly(DBA-ac) and poly(DBA-a-co-DDM-BMI) exhibit Tg
temperatures at 333 and 234 ◦C, respectively. Obviously, the Tg value of
poly(DBA-ac-co-DDM-BMI) is much higher than both poly(DBA-ac) and
poly(DBA-a-co-DDM-BMI) as well as the polymerization temperature of
itself. The multiple polymerization behaviors based on the ring-opening
polymerization of the oxazine ring, the polymerization of the acetylene
such testing method only requires tiny amount of samples. As shown in
◦
Fig. 7a, poly(DBA-a) exhibits a Tg value as low as 119 C, which is in
accordance with the reported value by Wang et al. [36]. However, we
also found another two reported Tg values (222 and 303 ◦C) of poly
(DBA-a) [45]. Such great difference in Tg values should be attributed
to the poor processability of DBA-a resin. Besides, an exothermic peak
over 300 ◦C can be observed for poly(DBA-a), which is due to the exis-
tence of uncured carbon‑carbon double bonds in polybenzoxazine
structure. Moreover, poly(DBA-ac) and poly(DBA-a-co-DDM-BMI) show
Tg values at 322 and 227 ◦C, respectively. Notably, poly(DBA-ac-co-
–
group, and the cross-linking of C C bonds in maleimide and allyl
–
groups significantly increase the overall cross-link density, thereby
greatly increasing the Tg value of poly(DBA-ac-co-DDM-BMI). As a
result, the strategies including the modification by acetylene group and
6

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
Fig. 5. Viscosity as a function of temperature for DBA-a, DBA-ac, DBA-a/DDM-
BMI and DBA-ac/DDM-BMI thermosetting resins.
Fig. 7. DSC thermograms (a) and dynamic mechanical spectra (b)
of thermosets.
compatibility between DBA-ac and bismaleimide after polymerization.
Fig. 9 shows the TMA curves of thermoset films of poly(DBA-ac), poly
(DBA-a-co-DDM-BMI) and poly(DBA-ac-co-DDM-BMI). The coefficient
of thermal expansion (CTE) of poly(DBA-ac-co-DDM-BMI) is obtained
from TMA as 37.1 ppm/oC in a temperature range of 25 to 200 ◦C. This
CTE value is much lower than poly(DBA-ac) and poly(DBA-a-co-DDM-
BMI), while the CTE values of poly(DBA-ac) and poly(DBA-a-co-DDM-
BMI) are calculated as 119.0 and 41.3 ppm/oC, respectively. The CTE
value of poly(DBA-ac-co-DDM-BMI) is also much lower than many other
reported polybenzoxazines, bismaleimides, and polyimides, indicating it
can be satisfied for dimensional stability as electronic materials
[1,46,47].
Fig. 6. TGA thermograms of DBA-a, DBA-ac, DBA-a/DDM-BMI and DBA-ac/
DDM-BMI. The inset is a photo of polymerization film sample of poly(DBA-a).
blending bismaleimide in DBA-based benzoxazine can greatly improve
the Tg of the resulting thermosets.
The above DMA thermogram shows a single Tg peak of tan δ for poly
(DBA-ac-co-DDM-BMI) with reasonable narrow width, suggesting the
homogenous network of copolymeric thermoset based on DBA-ac/DDM-
BMI blend. However, the Tg peak for the thermogram of poly(DBA-a-co-
DDM-BMI) is much wider, which is possibly attributed to the separated
polymerization processes involved in DBA-a/DDM-BMI as supported
from the above DSC thermogram. In order to make a further confirma-
tion of compatibility, SEM images for the fracture surfaces of poly(DBA-
a-co-DDM-BMI) and poly(DBA-ac-co-DDM-BMI) were obtained, and the
corresponding pictures are shown in Fig. 8. The fissure from the fracture
surface of poly(DBA-ac-co-DDM-BMI) is clearly neater than that poly
(DBA-a-co-DDM-BMI). The uniformly fractured surface without any
domains of poly(DBA-ac-co-DDM-BMI) represents a single- phase and
homogeneous cross-linked network, which also indicated the good
The thermal stability of poly(DBA-a), poly(DBA-ac), poly(DBA-a-co-
DDM-BMI) and poly(DBA-ac-co-DDM-BMI) were studied by TGA in ni-
trogen as seen in Fig. 10a. An obvious weight loss from 250 to 400 ◦C can
be observed for poly(DBA-a), which is due to the initial decomposition
of terminal defects in polybenzoxazine networks [44]. However, the
weight loss at this stage in poly(DBA-a) can be slightly retrained after
blending BMI-DDM. As shown in Fig. 10b, a relatively lower decom-
posing rate is observed in the DTG curve of poly(DBA-a-co-DDM-BMI).
Nevertheless, poly(DBA-a) and poly(DBA-a-co-DDM-BMI) show rela-
tively lower Td5 (5% weight loss temperature) of 291 and 338 ◦C,
respectively. On the contrary, poly(DBA-ac) and poly(DBA-ac-co-DDM-
BMI) possess outstanding thermal stability with much higher Td5 of 425
7

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
Fig. 8. SEM images for the fracture surfaces of poly(DBA-a-co-DDM-BMI) and poly(DBA-ac-co-DDM-BMI).
to a substantial HRC reduction. The advantages of the strategy
combining acetylene modification and blending bismaleimide in DBA-
based benzoxazine are at least two-folds: firstly, improving the ther-
mal stability of the initial weight-loss stage; secondly, lowering the fire
flammability via the highly cross-linked networks contributed from
oxazine ring, acetylene, maleimide and allyl groups. Therefore, the
newly developed DBA-ac/DDM-BMI thermosetting system also extends
an opportunity for the application that benefits from flame retardancy.
The thermal and fire-related property data from above TMA, DMA, DSC,
TGA and MCC for thermosets are summarized in Table 1.
4.4. Dielectric properties of poly(DBA-ac-co-DDM-BMI)
The above results of excellent dimensional stability, high thermal
performance and good flame retardancy for poly(DBA-ac-co-DDM-BMI)
evidence its potential applications in electronic materials. Therefore, we
further evaluated the dielectric properties of poly(DBA-ac-co-DDM-BMI)
by the capacitance method applying the following equation [48,49]:
Cl
Ak0
Fig. 9. Thermomechanical analysis of thermoset films of poly(DBA-ac), poly
(DBA-a-co-DDM-BMI) and poly(DBA-ac-co-DDM-BMI).
k =
(1)
Herein, C is the capacitance of poly(DBA-ac-co-DDM-BMI), l is the
◦
and 426 C, respectively. Clearly, the initial weight loss in DBA-based
thickness of the sample, A is the surface area of the silver coated poly
(DBA-ac-co-DDM-BMI) film, and k0 is the vacuum dielectric constant
(8.854 × 10ꢀ 12 F mꢀ 1).
thermosets can be completely eliminated after incorporation of acety-
lene group in the thermosetting system. Specifically, the temperatures of
Td5 of poly(DBA-ac) and poly(DBA-ac-co-DDM-BMI) are much higher
than other reported high-performance polybenzoxazines derived from
acetylene-containing benzoxazines [19,20]. Moreover, poly(DBA-ac-co-
DDM-BMI) shows a highest char yield value of 45.3% at 800 ◦C in ni-
trogen. This enhanced thermal stability of poly(DBA-ac-co-DDM-BMI) is
mainly due to the highly cross-linked networks formed from the multiple
polymerizations of oxazine ring, allyl, acetylene and maleimide groups
in the DBA-ac/DDM-BMI blend.
As shown in Fig. 11, the dielectric constant (k) values decrease from
2.92 to 2.86 within the frequency range from 10 Hz to 1 MHz. After the
fully polymerized, the acetylene in DBA-ac/DDM-BMI blend can in-
crease the free volume through the cyclotrimerization or copolymeri-
zation with allyl and maleimide groups, leading to a low dielectric
constant of the resulting thermoset. The k value of poly(DBA-ac-co-
DDM-BMI) is fairly low (k < 3), which is comparable to the dielectric
constants of other commercially available low-k polymers, such as pol-
yimides, polybenzoxazoles, and cyanate esters [50–53]. Fig. 11 also
shows the dielectric loss (tan δ) values of poly(DBA-ac-co-DDM-BMI).
The tan δ values of poly(DBA-ac-co-DDM-BMI) ranged from 0.002 to
0.007 within the frequency range from 100 Hz to 1 MHz. Generally, a
relatively low value of dielectric loss is always highly desired for
interlayer dielectric material applications [48]. As a result, the dielectric
measurement further supports that poly(DBA-ac-co-DDM-BMI) shows
great potential applications in the advanced electronic fields.
The heat release capacity (HRC) and total heat release (THR) values
were obtained from MCC in order to evaluate the flammability of poly
(DBA-a), poly(DBA-ac), poly(DBA-a-co-DDM-BMI) and poly(DBA-ac-
co-DDM-BMI). As shown in Fig. 10c, MCC spectra of poly(DBA-a),
poly(DBA-ac), poly(DBA-a-co-DDM-BMI) and poly(DBA-ac-co-DDM-
BMI) reveal HRC values of 169, 339, 237 and 140 Jgꢀ 1 Kꢀ 1, respec-
tively. In addition, poly(DBA-a), poly(DBA-ac), poly(DBA-a-co-DDM-
BMI) and poly(DBA-ac-co-DDM-BMI) exhibit THR values of 18.6, 30.5,
25.7 and 17.6 KJgꢀ 1 as shown in Fig. 10d. Both the HRC and THR values
of poly(DBA-ac-co-DDM-BMI) are much lower than the other three
thermosets obtained in this study. Notably, the incorporation of acety-
lene and blending maleimide in the DBA-based benzoxazine resin leads
It has been concluded that strategies of incorporating of acetylene
and blending with bismaleimide can significantly lower the polymeri-
zation temperature and improve the comprehensive properties of DBA-a
resin; a more detailed study on the relationship between the mole ratio
8

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
Fig. 10. TGA thermograms (a) and derivative weigh loss curves (b) of each thermoset, and heat release rate (HRR) (c) and total heat release (THR) (d) verses
temperature for each thermoset.
Table 1
Thermal stability and flame retardancy of poly(DBA-a), poly(DBA-ac), poly
(DBA-a-co-DDM-BMI) and poly(DBA-ac-co-DDM-BMI).
Sample
CTEa
Tgb
Tgc
Td5
Yc
HRC
THR
ppm/oC
(◦C)
(◦C)
(◦C)
(wt
%)
(Jgꢀ 1
(KJgꢀ 1
)
Kꢀ 1
)
poly(DBA-
a)
–
–
119
322
227
291
425
338
32.8
43.2
36.3
169
339
237
18.6
30.5
25.7
poly(DBA-
ac)
41.3
119.0
333
234
poly(DBA-
a-co-
DDM-
BMI)
poly(DBA-
ac-co-
37.1
370
365
426
45.3
140
17.6
DDM-
BMI)
a
Coefficient of thermal expansion is recorded from 25 to 200 ◦C.
Fig. 11. Dielectric constant and dielectric loss versus frequency of poly(DBA-ac-
co-DDM-BMI) at room temperature.
b
c
Tg temperatures were measured from DMA.
Tg temperatures were obtained by DSC.
9

R. Yang and K. Zhang
Reactive and Functional Polymers 165 (2021) 104958
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In summary, we have successfully developed a new thermosetting
system, DBA-ac/DDM-BMI blend, which shows combined advantages of
low polymerization temperature, high thermal stability, low flamma-
bility and low dielectric properties. The resulting thermoset, poly(DBA-
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Acknowledgements
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The authors acknowledge the National Natural Science Foundation
of China (52073125 and 51603093) for financial support.
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