Synthesis of novel allylamine-fluorene based benzoxazine and its copolymerization with typical benzoxazine: Curing behavior and thermal properties

Article abstract of DOI:10.1039/d0nj04262e

In this paper, 2,7-dihydroxy-9,9-bis(4-aminophenyl)-fluorene (BADHF) monomer was synthesized via direct condensation reaction catalyzed by methylsulfonic acid. A novel allylamine-based benzoxazine monomer containing fluorene rings and polymerizable groups

Full text of DOI:10.1039/d0nj04262e

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Synthesis of novel allylamine-fluorene based benzoxazine and its
copolymerization with typical benzoxazine: Curing behavior and
thermal properties
Received 00th January 20xx,
Accepted 00th January 20xx
Ting Wang,a Zhi-cheng Wang,a Zhong-cheng Pan,*a Abdul Qadeer Dayo,b Wen-bin Liu,*a and Jun
DOI: 10.1039/x0xx00000x
Wang*a
In this paper, 2,7-dihydroxy-9,9-bis(4-aminophenyl)-fluorene monomer (BADHF) was synthesized by direct condensation
reaction catalyzed by methylsulfonic acid. A novel allylamine-based benzoxazine monomer containing fluorene group and
polymerizable group (t-BF-sa-a) was obtained via o-hydroxy-benzyl amine method combining with one stage Mannich
condensation reaction. The result showed that the introduction of fluorene rings and polymerizable groups and the
increase of oxazine rings in the polymer network structure can result in an obvious increase in the crosslinking density of
polymers. Therefore, the t-BF-sa-a polybenzoxazine (poly(t-BF-sa-a)) exhibited a higher glass transition temperature (Tg)
and better thermal stability.
The synthesized t-BF-sa-a monomer was used to modify traditional phenol-aniline-based mono-functional phenol-aniline
(P-a) and bisphenol A-aniline-based bifunctional (BA-a) benzoxazines. Due to the introduction of heat-resistant fluorene
rings and the increase of crosslinking degree of copolymers, the copolymers (poly(P-a/t-BF-sa-a) and poly(BA-a/t-BF-sa-a))
showed excellent thermal properties. Among them, the Tg, T5, T10, and Yc of poly(P-a/t-BF-sa-a) were increased by 34 oC, 58
o
oC, 35 C, and 8%, respectively, even higher than that of bisphenol fluorene aniline-based polybenzoxazine. Furthermore,
due to the long alkyl chain in the t-BF-sa-a monomer, the E'of the copolymers was decreased significantly, which greatly
improved the shortcomings of traditional polybenzoxazines, such as high brittleness and poor toughness.
seriously hinder the further application of such products.
Therefore, the preparation of polybenzoxazines with the
excellent process and mechanical properties is a hot issue in
1. Introduction
Benzoxazine monomers have some excellent properties, for
example, no generation of by-products during the curing
the applied research.
By introducing some polymerizable groups into the structure
of the benzoxazine monomer, the crosslinking density of
polybenzoxazine can be effectively increased, and the
mechanical properties and thermal stability of the polymer can
be significantly improved. Polymerizable groups include allyl,
allyl ether, acetylene, propynyl, cyano, norbornene, and
maleimide, etc [12-14]. Agag et al. synthesized a series of
benzoxazine monomers containing polymerizable groups [15, 16]
The experimental results showed that the thermal properties
of the polymers were greatly improved. Later on, Kumar et al.
synthesized benzoxazine monomers from the reaction of 2,2'-
diallyl-bisphenol-A, aniline, and formaldehyde [17]. The results
showed that the Tg of the polymer was 300 oC, and the thermal
decomposition temperature of 5% and 10% was as high as 395
and 425 oC, respectively, with excellent thermal performance.
Fluorene-based polymer materials have excellent heat-
resistance and flame-retardant properties, high carbon residue
rate, limiting oxygen index, light stability, and chemical
stability [18-21]. However, fluorene exists in the side chain with
the form of overhang in such fluorene-based monomers,
which results in the larger rigidity of the polymer segment, the
higher curing temperature, and the poor toughness of the
process, low porosity, and near-zero shrinkage, etc[1-3]
.
Therefore, polybenzoxazines have a promising prospect in the
application of insulating materials, electronic encapsulating
materials, ablative resins, etc [2,4-8]. Benzoxazine monomers
have extraordinary molecular design flexibility with different
structures, by combining phenols and amines with different
structures. This allows the designing of benzoxazine monomer
structures with a wide range of properties [9-11]. At present, the
reported benzoxazine monomers are mostly bisphenol type
and diamine type bifunctional benzoxazine monomers. The
studies on the synthesis and properties of asymmetric multi-
functional benzoxazine monomers are relatively backward.
Moreover, the high curing temperature, poor processability,
high brittleness, and poor toughness of polybenzoxazines
.
a. Key Laboratory of Superlight Material and Surface Technology of Ministry of
Education, College of Materials Science and Chemical Engineering, Harbin
Engineering University, Harbin 150001, China.
b. Department of Chemical Engineering, Balochistan University of Information
Technology, Engineering and Management Sciences, Quetta 87300, Pakistan.
* Corresponding author E-mail:Zhong-cheng Pan (421336390@herbeu.edu.cn),
Wen-bin Liu (wjlwb@163.com),Jun Wang (wj6267@sina.com).

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polymer. Consequently, it is essential to redesign the fluorene BADHF (0.02 mol), concentrated sulfuric acid (0.2 mL), and o-
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molecular structure for improving polymer properties [22-26].In hydroxybenzaldehyde (0.04 mol) were dissolved in 100 mL of
DOI: 10.1039/D0NJ04262E
this paper, BADHF was prepared via introducing phenolic ethanol and heated to reflux temperature for 10 h. 0.06 mol of
hydroxyl group and amino group into 2,7,9 positions of sodium borohydride was added in batches into the mixture
fluorene through molecular structure design, using the after cooling the mixture to room temperature, and then the
excellent heat resistance and the active sites of fluorene. The system was maintained and stirred for another 15 min. Then,
t-BF-sa- monomer was synthesized by the o-hydroxy- dichloromethane was added for extraction, and the organic
benzylamine method combined with one-stage Mannich phase was separated. Finally, the solvent was detached by
condensation reaction with allylamine as raw material. rotating evaporation, and the intermediate product (b-SAH)
Compared with the traditional three-step method, the was obtained with a yield of 78%. FTIR (KBr, cm-1): ν = 3600-
synthesis of asymmetric benzoxazine monomer reduces the 3100 (N–H and O–H stretching overlap), 3032 (unsaturated C-
treatment process of the intermediate product, and a small H stretching), 2910 (saturated alkyl C-H stretching), 1368,1338
amount of sulfuric acid is added into the reaction system to (CH2 wagging), 1232 (C−N stretching), 1182 (asymmetric
catalyze the reduction of double bonds by sodium borohydride, stretching of C–N–C), 950 (out-of-plane C–H). 1HNMR (500
greatly shortening the reaction time and experimental period, MHz, DMSO-d6, ppm): 9.45 (s, 2H, –OH), 9.21 (s, 2H, –OH),
improving the yield of the intermediate product and reducing 6.42-7.44 (m, 22H, Ar–H), 5.88(s, 2H,–NH), 4.11(s, 4H,–CH2).
the production cost. In traditional three-step synthesis method, 13C NMR (100 MHz, DMSO-d6, ppm): 156.5 (C–OH), 149.8 (C–
the double bond reduction process takes 8 to 12 hours, but in NH–), 61.2 (the quaternary carbon atom in fluorene ring), 51.6
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this paper, it only takes 15 minutes.
(Ar–CH2–N). Elem. Anal. Calcd for C39H32N2O4: C%: 79.03, H%:
By adjusting the number of rigid and flexible groups and 5.44, N%: 4.73. Found: C%: 78.83, H%: 5.58, N%: 4.65.
functional groups in benzoxazine monomer, the problems of 2.2.3 Preparation of t-BF-sa-a monomer
small molecular weight of the resulting polymers, low
crosslinking density, poor toughness, and the decrease of
thermal properties due to the introduction of flexible groups
can be solved. The typical P-a and BA-a benzoxazine
Allylamine (0.01 mol), paraformaldehyde (0.03 mol), b-SAH
(0.005 mol), and chloroform (50 mL) were stirred into a 250
o
mL three-necked bottle at 60 C for 24 h and cooled down to
the room temperature. Organic solvents were removed by
monomers were modified by the prepared polyfunctional
rotating evaporation, and the crude product was collected.
fluorenyl benzoxazine monomer, and the influence mechanism
Then it was dissolved in the dichloromethane solvent, and the
of benzoxazine monomer structure on the polymerization
organic phase was separated by repeated alkali and water
behavior and properties of the blends was systematically
washing. Finally, the dichloromethane solvent was removed by
recognized, which provided
a scientific basis for the
rotating evaporation, and t-BF-sa-a was obtained (yield 89%).
FTIR (KBr, cm-1): ν=3396 (O–H stretching), 3036 (unsaturated
C-H stretching), 2847 (saturated alkyl C-H stretching),
1383,1337 (CH2 wagging), 1251, 1226 (C–O–C asymmetric
stretching), 1160 (C–N–C asymmetric stretching), 1113 (C-O-C
symmetrical stretching), 928-945 (C–H out-of-plane bending),
1641(C=C stretching of allyl), 990 (=C-H out of plane
wagging).1HNMR (500 MHz, DMSO-d6, ppm): 6.41-7.44 (m,
20H, Ar-H), 5.81 (m, 2H, –CH=), 5.81 (m, 4H, =CH2), 3.35 (s, 4H,
–CH2–), 3.96, 4.58 (s, 8H, Ar–CH2–N), 4.76, 5.36 (s, 8H, O–CH2–
N).13C NMR (100 MHz, DMSO-d6, ppm): 85.6-91.8 (O–CH2–N),
53.3-58.2 (Ar–CH2–N), 63.2 (the quaternary carbon atom in
fluorene ring), 134.5 (−CH= of allyl), 116.7 (=CH2 of allyl), 56.3
(−CH2− of allyl). Elem. Anal. Calcd for C51H46N4O4: C%: 78.64,
H%: 5.95, N%: 7.19. Found: C%: 78.43, H%: 5.68, N%: 7.25.
application of polybenzoxazine in advanced resin matrix
composites, ablative materials, electronic packaging materials,
and other fields, and promoted the development and
perfection of polybenzoxazine application research.
2. Experimental
2.1. Raw materials
2,7-Dihydroxy-fluorene-9-one, o-hydroxybenzaldehyde, and
paraformaldehyde were purchased from Kermel Chemical
Reagent Co., Ltd. Tianjin, China. Allylamine methylsulfonic acid,
trimethylamine, and sodium borohydride were supplied by
Jinshan Chemical Reagent Co., Ltd. Chengdu, China. Aniline,
concentrated sulfuric acid, ethanol and trichloromethane were
purchased from Jingchun Chemical Reagent Co., Ltd. Shanghai,
China. The P-a (98.5%) and BA-a (98.0%) monomers were
gifted by the Huacui Advanced Materials Co. Ltd. Jiangxi, China.
2.2 Preparation of t-BF-sa-a monomer and its intermediates
The synthesis route of t-BF-sa-a monomer is shown in Scheme
1. The method was discussed in the sub-sections.
2.2.1 Preparation of BADHFmonomer
The BADHF was prepared by the reaction of 2,7-dihydroxy-9-
fluorenone with aniline. The followed synthesis method was
discussed in detail in an earlier published article [36]
.
2.2.2 Preparation of reduction products of bisphenol bis amine-
salicylaldehyde monomer (b-SAH)
Scheme 1-The synthesizing route of t-BF-sa-a monomer
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2.3 Curing of t-BF-sa-a monomer and copolymers
overlapping stretching vibrations of N-H and O-H. The band at
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1232 cm-1 is assigned to the stretching vDibOrIa: t1i0o.1n03o9f/DC0-NNJ.0T4h2e62reE
is no absorption band of C=N vibration near 1615 cm-1, which
indicates that the C=N bond has been completely reduced by
sodium borohydride. For the t-BF-sa-a monomer, it can be
seen that the band at 3405 cm-1 is the hydroxyl group
stretching vibration, which may be caused by the ring-opening
of a small number of oxazine rings. The stretching vibration
band at 3034 cm-1 is attributed to the unsaturated C-H in the
benzene ring, while the stretching vibration band at 2846 cm-
1belongs to the saturated alkyl C-H. The bands at 1369 and
1337 cm-1 are attributed to the oscillating vibrations of
methylene in the oxazine rings. The stretching vibration bands
near 1225, 1252, and 1110 cm-1 are distributed to the
asymmetric and symmetric C-O-C, while the stretching
vibration of the asymmetric C-N-C can be observed at 1158 cm-
1. We can notice that the absorption peaks of the oxazine ring
appear at 926-944 cm-1 corresponding to the out-of-plane
bending vibration of C-H in the benzene ring. The C=C
stretching vibration and =C-H out-of-plane oscillating vibration
peaks of allyl groups are observed at 1641 and 971 cm-1.
Figure 2 shows the 1HNMR spectrum of the intermediates and
t-BF-sa-a monomer. For BADHF, the hydroxyl proton on the
benzene ring is discovered at 9.26 ppm. The chemical shifts at
6.40-7.45 ppm are attributed to the phenyl protons on the
benzene rings. The peak at 4.93 ppm is attached to the
hydrogen protons on the amino group. For b-SAH, the hydroxyl
proton on the fluorene ring is observed at 9.45 ppm. The
chemical shift at 9.21 ppm represents the hydroxyl protons on
the structure of raw material salicylaldehyde. δ=6.42-7.44 ppm
are the phenyl protons on the benzene ring. The chemical shift
at 5.88 ppm is ascribed to the hydrogen proton of the
secondary amine group, and the shift at 4.11 ppm is
distributed to the hydrogen protons of the methylene group.
For t-BF-sa-a monomer, the 6.41-7.44 ppm are corresponding
to hydrogen protons on the benzene ring. The multiple peaks
of 5.81 and 5.11 ppm are assigned to hydrogen protons on -
CH= and =CH2 of allyl group, and the peak at 3.35 ppm is the
hydrogen protons of -CH2- of allyl group. The chemical shifts at
3.96 and 4.76 ppm belong to hydrogen protons of N-CH2-Ar on
the bisphenol oxazine ring, and 4.58 and 5.36 ppm belong to
the hydrogen protons of O-CH2-N on the diamine oxazine ring.
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The t-BF-sa-a monomer was added to a definite shape of steel
mold and cured in an air-dry oven on isothermal heating at
150, 180, 210, 240, and 260 °C for 2 h at each stage. The BA-a
(P-a) and t-BF-sa-a monomers with appropriate mass share
were mixed by melt blending method on the heating plate and
transferred to the steel molds. The mass ratio of BA-a (P-a) and
t-BF-sa-a monomers was 4:1.The BA-a(P-a)/t-BF-sa-a blends
were also cured in isothermal heating, following procedure
was followed 150 °C/2h + 180 °C/5h + 200 °C/1h + 220 °C/1h.
2.4 Characterization methods
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The chemical structures of the t-BF-sa-a monomer and its
intermediates were confirmed by Fourier transform infrared
spectroscopy (FTIR) analysis performed on a Bruker Tensor-27
with KBr disks. Nuclear magnetic resonance spectra of
hydrogen and carbon spectrum (1HNMR and 13CNMR) were
investigated in DMSO-d6 solution as
a solvent keeping
tetramethylsilane as internal standard on a Bruker Avance-III
400 HD spectrometer. Elemental analysis was carried out with
a
Vario EL cube (Elementar Analysensysteme GmbH).
Differential scanning calorimetry (DSC) was studied on a TA
Instruments Q200 at a heating rate of 20 °C·min-1 from 50 to
400 °C in flowing nitrogen (50 ml·min-1). Thermal-gravimetric
analysis (TGA) was employed to evaluate the thermal stability
of the polymers, which was carried out from 50 to 800 °C on a
TA Instruments Q50 with a heating rate of 10 °C·min-1 under
the atmosphere of nitrogen. The rheological behaviors were
recorded by using a TA Instruments (AR-2000rheometer) with
a 25 mm diameter parallel plate fixture at 10 s relaxation time
and 3.259 Pa pressure. Tg temperatures of the cured polymers
and copolymers were investigated by a dynamic thermal
mechanical analyzer (DMA) using dynamic mechanical analyzer
TA Q800 at a scanning speed of 5 °C·min-1 in a nitrogen
atmosphere. The morphological structures of the samples
were recorded on a HITACHI S-4300. First, the samples were
frozen in liquid nitrogen for a while, then they were taken out
and quickly broken; subsequently, the samples adhered to the
conductive adhesive keeping cross-section in the upward
direction.
3. Results and discussion
3.1 Structural characterization of t-BF-sa-a monomer and its
intermediates
Figure 1 shows the FTIR spectrum of the t-BF-sa-a monomer
and its intermediates. For BADHF, the characteristic stretching
vibration band of primary amine (-NH2) is observed at 3454
cm-1. It can be found that the absorption band at 3381 cm-1 is
typically for hydroxy (O-H) stretching. The characteristic
absorption bands at 1611 and 1511 cm-1 are corresponding to
the benzene ring skeleton. The C-N bending absorption peak
can be noticed near 1180 cm-1. However, there is no stretching
vibration peak of C=O at 1700 cm-1, which indicates that the
BADHFhas been successfully synthesized. For b-SAH, the C-N
stretching vibration is observed at 1232 cm-1, while the
broadband absorption at 3100-3600 cm-1 is designated to the
Figure 1-FTIR spectrum of t-BF-sa-amonomer along with its intermediates
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Figure 4-Rheological curve of t-BF-sa-a monomer
Figure 2-1HNMR spectrum of t-BF-sa-a monomer along with its
intermediates
The study of the rheological properties can provide more
intuitive data support and theoretical guidance for the
processing and forming process of synthesized benzoxazine
monomer. Therefore, for a better understanding of the
processing, the rheological behavior of t-BF-sa-a monomer was
studied by means of temperature scanning. The temperature
dependence of the viscosity of t-BF-sa-a monomer is
performed from 100 to 200 °C and produced as Figure 4.
As can be seen from Figure 4, the t-BF-sa-a monomer is in
solid-state at room temperature and begins to melt
progressively with increasing temperature, which results in
that the molecular motion is enhanced and the system
viscosity is reduced obviously. As the temperature is
constantly rising, the t-BF-sa-a monomer can form a large
spatial network structure through the ring-opening
polymerization, which limits the molecular movement and
increases the viscosity of the system.
In this paper, when the viscosity value is less than 1000 Pa·s,
the intersection point of the low-temperature zone is the
softening point temperature of the system. The junction point
of the high-temperature zone is the gel point temperature of
the system, the temperature range between the softening
point temperature and the gel point temperature is the
processing window temperature of the system [46,47]. The t-BF-
sa-a monomer has a lower softening point temperature, which
is mainly related to the number of aromatic rings and the alkyl
chain length. The minimum viscosity of t-BF-sa-a monomer
was only 66.4 Pa·s. The t-BF-sa-a monomer has a lower gel
point temperature, which shows that t-BF-sa-a monomer can
take place the crosslinking reaction at low temperature and
has high reactivity.
In order to further determine the chemical structures of t-BF-
sa-a monomer and its intermediates, the 13CNMR spectra are
shown in Figure 3. For BADHF, the single peaks located at
156.5 and 149.8 ppm are assigned to the carbon atom
resonances of C–OH and C–NH2, respectively. The peak around
at 61.2 ppm is ascribed to the quaternary carbon atom in
fluorene ring. For b-SAH, the chemical shift at 51.6 ppm is
described to the carbon atom resonances of Ar–CH2–N of
oxazine ring. The other peaks are similar to that of the BADHF
monomer.
For t-BF-sa-a monomer, the chemical shifts at 85.6-91.8 and
53.3-58.2 ppm are assigned to the carbon atom resonances of
O–CH2–N and Ar–CH2–N of oxazine ring, respectively. The
carbon atom of−CH2−on allyl appears at 56.3 ppm, while the
carbon atoms of −CH= and =CH2 on allyl are observed at 134.5
and 116.7 ppm, respectively. All expected characteristic
absorption peaks were found in the 13CNMR spectra. The
spectra data of FTIR, 1HNMR, and 13C NMR show that the t-BF-
sa-a monomer and its intermediates are synthesized
successfully.
3.2 Polymerization behavior of monomer
3.2.1 Rheological properties
3.2.2 DSC study on thermal curing behaviour
The curing behavior of t-BF-sa-a monomer is determined by
DSC analysis after curing on each stage and illustrated as
Figure 5.
The DSC curve of t-BF-sa-a monomer is separately plotted in
Figure 5(A) for the better visual of readers. There are two
exothermic peaks in t-BF-sa-a monomer around 230 and
300 °C. There are two reactive groups in t-BF-sa-a monomer,
i.e. oxazine ring and carbon-carbon double bond in the allyl
group. By consulting the relevant literature[49], we infer that
Figure 3-13CNMR spectra of t-BF-sa-a monomer and itsintermediates
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the extremely strong exothermic peak near 230 °C is the With the curing temperature rising to 260 °C, no exothermic
exothermic peak of ring-opening polymerization of the oxazine peak is found on the DSC heat flow curve, which indicates that
ring, while the weak exothermic peak at 300 °C is caused by there is no residual uncured benzoxazine group after thermal
the exothermic polymerization of carbon-carbon double bond curing in the t-BF-sa-a monomer. Furthermore, with the
in allyl group. The initial temperature of the ring-opening increase of curing temperature, the peak temperature moves
polymerization is 180-190 °C, which indicates that the t-BF-sa- to high temperature in DSC curves. This is because of the
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DOI: 10.1039/D0NJ04262E
a
monomer follows the mechanism of 1,3-benzoxazine increase in curing degree, the more compact crosslinking
thermal ring-opening polymerization. From the curve, we can network hinders the activity of functional groups in the system.
observe that the exothermic enthalpy of the t-BF-sa-a Therefore, only by increasing the temperature, the viscosity of
monomer is higher, and there is an obvious shoulder peak in the system can be reduced and the reactivity of functional
the spectrum. This is mainly due to the existence of groups can be increased.
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polymerizable groups in the molecular structure of the t-BF-sa- 3.2.3 FTIR study on thermal curing behavior
a monomer. At a certain temperature, the t-BF-sa-a monomer
can undergo addition polymerization, and the addition
polymerization peak overlaps partly with the exothermic peak
FTIR spectrums of t-BF-sa-a monomer after curing at different
temperatures are exhibited in Figure 6.
It can be seen from Figure6 that, in t-BF-sa-a monomer, the
of the ring-opening polymerization. Moreover, the initial
peak at 3034 cm-1 is attributed to the characteristic absorption
curing temperature of the t-BF-sa-a monomer is lower, which
of Ar−OH formed after the ring-opening of the oxazine ring,
which shows that the ring-opening polymerization of oxazine
ring with the rupture of C−O bond can take place at high
temperature. The reaction mechanism of the ring-opening
polymerization of the oxazine ring is shown in Scheme 2. The
characteristic absorption peaks of the allyl group are located at
indicates that the t-BF-sa-a monomer has high reactivity.
DSC curves of t-BF-sa-a monomer at different curing
temperatures are displayed in Figure 5(B). The curing degree
of t-BF-sa-a monomer at different curing temperatures are
27.8% (at 150 °C ), 60.6% (at 180 °C), 79.5% (at 210 °C), 92.9%
(at 240 °C), respectively. The curing degree reaches to 27.8% at
1641 cm-1 (C=C stretching vibration peak), 971 cm-1 (=C-H out-
150 °C, which can be attributable to the addition reaction
of-plane oscillation vibration peak), and 3082 cm-1 (C-H
mechanism of polymerizable groups. With the increase of
symmetric stretching vibration peak). The decreasing trend of
curing temperature, the exothermic enthalpy of the t-BF-sa-a
three absorption peaks (1641, 971, and 3082 cm-1) intensities
with the increase of curing temperature confirms the
monomer decreases, and the degree of curing increases, which
indicates that the number of unreacted functional groups in
participation of the allyl group in the curing reaction [48-50]. The
the t-BF-sa-a monomer is decreased.
C=C stretching vibration peak of the allyl group disappears
completely after curing at 180 °C, which confirms the addition
polymerization of the allyl group at this stage. The allyl group
reaction mechanism inferred from the test results is shown in
Scheme 3.
With the increasing temperature, the oxazine ring
characteristic absorption peaks (926-944 cm-1), the rocking
vibration absorption peaks of methylene (1369 and 1337 cm-1),
C−O−C stretching vibrations (1252 and 1225 cm-1), and C−N−C
stretching vibrations (1158 and 1110 cm-1) on the oxazine ring
are weakened gradually, and disappear completely after curing
on 260 °C. This is due to the addition polymerization of allyl
group, increasing the crosslinking density and the system
viscosity, and the polymerization process is changed to
diffusion control. Therefore, the higher temperature is
required for the complete oxazine ring opening. The t-BF-sa-a
monomer has been completely cured at 260 °C, which is
consistent with DSC test results.
Figure 5-DSC curves of the t-BF-sa-a monomer, before curing (A), and
after curing at different temperatures (B)
Scheme 2-The reaction mechanism of the ring opening polymerization of oxazine ring
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Scheme 3-The reaction mechanism of allyl group
Figure 8-TGA thermograms of poly(t-BF-sa-a) in a nitrogen atmosphere
3.3 Thermal and thermomechanical properties
3.3.1 Dynamic mechanical properties
The tanδ peak (Tg) of poly(t-BF-sa-a) is observed at 356.9 °C,
which is much better than that of bisphenol-A-aniline (BA-a)
and reported bifunctional fluorenyl polybenzoxazines, showing
excellent thermal properties [27-32]. This is because there are
abundant intra-and inter-molecular hydrogen bonds in its
network structure, the polymer chain is more compact, and
the thermal movement of the chain segment is greatly limited.
The cross-linking density of poly(t-BF-sa-a) is greatly increased
due to the existence of polymerizable groups. The introduction
of fluorenyl group can increase the rigidity of the molecular
chain, limit the internal rotation of the polymer chain. In
addition, poly(t-BF-sa-a) exhibits a high storage modulus (E′),
as high as 2200MPa in the glassy state. This is mainly because
the introduction of polymerizable groups in the monomer
structure increases the cross-linking density of the polymer,
which makes the polymer segments begin to move at a higher
temperature.
The Tg is a significant parameter to characterize the thermal
properties of polymers, and it is the characteristic temperature
of polymers from the glass state to the viscoelastic state [37, 38]
.
In the practical application of polymer materials, the Tg is
generally the upper limit of the continuous service
temperature for plastics and thermosetting resins, and the
lower limit of the continuous service temperature for
elastomers [39,40]. The DMA test result of poly(t-BF-sa-a) is
depicted in Figure 7. The rigidity of the material can be directly
reflected by the storage modulus, and the damping
characteristics of the material can be characterized by the loss
factor, and the peak temperature of the loss factor curve is the
Tg of the material.
3.3.2 Thermal stability
The thermogravimetric curve of poly(t-BF-sa-a) in a nitrogen
environment is displayed in Figure 8. The initial degradation
temperatures (T5 and T10) of poly(t-BF-sa-a) are recorded at
370 °C and 396 °C, respectively, and the carbon residue (Yc) at
800 °C is 58.4%, which are higher than that of bisphenol
fluorene aniline-based polybenzoxazine [28,33-35,45]
.
This is because the increase of the number of oxazine rings can
improve the crosslinking density of the polymer, and the
increase of the number of aromatic rings will lead to the high
carbon residue rate of the polymer. The introduction of large
volume rigid fluorene ring can effectively improve the heat
resistance of the polymer, and the increase of the number of
functionalities can also improve the cross-linking density of the
poly(t-BF-sa-a), effectively reduce the number of unstable end
groups in the polymer, so as to improve the thermal stability of
the polymer. The t-BF-sa-a monomer contains the
polymerizable allyl groups. The additive polymerization of allyl
groups can further improve the crosslinking density and
prevent the volatilization of amine during the initial thermal
decomposition stage.
Figure 6-FTIR spectrums of t-BF-sa-a monomer after curing at
different temperatures
3.4 Copolymerization with traditional benzoxazine monomers
Traditional monofunctional P-a benzoxazine and bifunctional
BA-a benzoxazine have excellent processing performance, but
they also have some disadvantages such as high brittleness,
Figure 7-The DMA test results of poly(t-BF-sa-a), Storage modulus (A),
Loss tangent (B)
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It can be seen from Figure 9 that there is a sharp exothermic
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R
OH
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DOI: 10.1b0l3e9n/Dd0sN, J0w42h6ic2hE
O
N
peak in P-a monomer and P-a/t-BF-sa-a
N
R
Heat
corresponds to the ring-opening polymerization of the
reaction system. The initial polymerization temperature of P-a
monomer is 210 °C, the maximum exothermic peak
temperature is 245 °C, and the polymerization enthalpy is
375.3 J/g. When t-BF-sa-a monomer is added into P-a
monomer, the initial polymerization temperature and the peak
temperature of the P-a/t-BF-sa-a blends are decreased from
210 °C and 245 °C to 195 °C and 237 °C, respectively, but the
exothermic enthalpy is increased. This is related to the lower
initial polymerization temperature of t-BF-sa-a monomer.
When t-BF-sa-a monomer is added into P-a monomer, the
thermal ring-opening polymerization of t-BF-sa-a monomer in
the P-a/t-BF-sa-a blends will take place in advance, and the
phenol hydroxyl with catalytic activity will be produced, and
then the thermal ring-opening polymerization of P-a monomer
is initiated. In addition, due to the presence of polymerizable
group in t-BF-sa-a monomer, there are more crosslinking sites in t-
BF-sa-a monomer, which results in a higher exothermic enthalpy of
the P-a/t-BF-sa-a blends than that of P-a monomer.
When t-BF-sa-a monomer is added into BA-a monomer, the
initial polymerization temperature of the BA-a/t-BF-sa-a
blends is significantly decreased. This is because BA-a
monomer is solid at room temperature (melting point: 69 °C).
When the addition reaction of allyl group in t-BF-sa-a
monomer takes place, the amount of flexible alkyl in the BA-
a/t-BF-sa-a blends increases, which results in the molecular
flexibility of the BA-a/t-BF-sa-a blends increases, the collision
and transfer of functional groups becomes much easier, the
polymerization activity increases, and the initial
polymerization temperature decreases.
R
O
N
OH
OH
N
R
9
Heat
H3C
CH3
H3C
CH3
H3
C
CH3
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R
R
N
N
N
O
OH
OH
R
Scheme 4-Polymerization reaction of P-a and BA-a
poor toughness, low thermal performance, and poor thermal
stability [41, 42]. P-a monomer and BA-a monomer can undergo
the ring-opening polymerization on heating and form
linearcross-linking structure, and their structures are similar to
that of phenolic resins [42,43]. The polymerization process is
shown in Scheme 4.
The processing properties, thermal stabilities, and mechanical
properties of fluorenyl polybenzoxazines can be improved and
enhanced by adjusting the ratio of rigid and flexible groups
and introducing other functional groups [44]. However, the
introduction of large structure of fluorenyl ring still leads to
the shortcomings such as low cross-linking density, high
brittleness, and high synthesis cost [45]. Therefore, it is an
economical and effective method to modify traditional
benzoxazines by blending or copolymerization, and to improve
the comprehensive properties of resin materials by using
synthesized high-performance monomers. The synthesized t-
BF-sa-a monomer can form a highly crosslinked polymer
network structure after the ring-opening polymerization. This
is due to the fact that the existence of more cross-linking sites
and active functional groups in the monomer structure can
greatly affect the properties of traditional polybenzoxazines
(poly(P-A) and poly(BA-a)).
3.4.1DSC of P-a/t-BF-sa-a and BA-a/t-BF-sa-ablends
The DSC curves of neat P-a and BA-a monomer, P-a/t-BF-sa-a
blends and BA-a/t-BF-sa-a blends are shown in Figure 9.
Figure 9-DSC spectra of neat P-a and BA-a, and P-a/t-BF-sa-a and
BA-a/t-BF-sa-ablends
Figure 10-The DMA test results of neat and copolymers
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Table1-DMA analysis of neat and copolymers
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Tg
(°C)
E′ (MPa,
Crosslinking
Samples
50 °C)
3478
2743
3440
Density(mol·cm-3)
Poly(P-a)
Poly(P-a/t-BF-sa-a)
Poly(BA-a)
157.1
190.4
181.2
0.00415
9
0.00566
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0.00479
Poly(BA-a/t-BF-sa-a) 215.5
3178
0.00699
Figure 11-TGA thermograms of poly(P-a), poly(BA-a), poly(P-a/t-
BF-sa-a), and poly(BA-a/t-BF-sa-a)
3.4.2 Dynamic mechanical properties of copolymers
The DMA test results of poly(P-a), poly(BA-a), poly(P-a/t-BF-sa-
a), and poly(BA-a/t-BF-sa-a) are shown in Figure 10, and the
dynamic mechanical property parameters are shown in Table 1.
When t-BF-sa-a monomer is added into P-a monomer, the Tg
of poly(P-a/t-BF-sa-a) is increased from 157.1 °C to 190.4 °C.
This is related to the introduction of heat-resistant fluorene
group and increases the crosslinking degree of poly(P-a/t-BF-
sa-a). The t-BF-sa-a monomer contains a large number of
polymerizable groups, which can significantly increase the
crosslinking density, so poly(P-a/t-BF-sa-a) shows excellent
thermoproperties. However, compared with poly(P-a), the E'
of poly(P-a/t-BF-sa-a) is decreased. This is due to the fact that
t-BF-sa-a monomer has flexible long chain alkyl. The flexibility
of polymer chain segment is enhanced, which leads to the
decrease of E' and the improvement of thermomechanical
properties of poly(P-a/t-BF-sa-a). This also is mainly due to the
formation of aliphatic chains after the addition polymerization
of allyl, which increases the flexibility of molecular chains.
It can be seen from the figure that when t-BF-sa-a monomer is
added into BA-a monomer, the Tg of poly(BA-a/t-BF-sa-a) also
increases obviously, and the E′ decreases, which is the same as
that of P-a monomer modified by t-BF-sa-a monomer. The Tg
of poly(BA-a/t-BF-sa-a) is increased 34.3 °C compared with
that of poly(BA-a).This is related to the structure of BA-a
monomer. The structure of bifunctional BA-a monomer is close
to that of t-BF-sa-a monomer, so they can be copolymerized
well, which makes the cross-linking network of poly(BA-a/t-BF-
sa-a) more compact.
(B)
The crosslinking density was calculated by using the derived
equation of statistical theory of rubber elasticity by Nielsen[53]
.
′
(
)
(
)
푙표푔10 퐸푒 = 7 + 293 × 휌푥
Table2-Summary of the thermal stability parameters of neat and copolymers
Sample
T5 (°C)
T10 (°C)
Yc (%, 800°C )
Poly(P-a)
Poly(P-a/t-BF-sa-a)
Poly(BA-a)
280
338
316
331
335
370
336
361
36.7
44.7
28.1
40.8
Figure 12-SEM micrographs of the neat and copolymer resins, poly(P-a)
(A),poly(P-a/t-BF-sa-a)(B), poly(BA-a) (C), and poly(BA-a/t-BF-sa-a) (D)
Poly(BA-a/t-BF-sa-a)
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Conclusions
View Article Online
DOI: 10.1039/D0NJ04262E
’
Where Ee in (dyn·cm-2) is the equilibrium storage modulus
measured at (Tg + 40°C) and ρx is the cross linking density
(mol·cm-3).The crosslinking densities of both copolymers were
increased due to the copolymerization reaction between two
monomers. The crosslinking of oxazine rings and allyl groups in
two monomers improved the crosslinking densities of the
copolymers from 0.00415 mol·cm-3 [poly(P-a)] and 0.00479
mol·cm-3 [poly(BA-a)] to 0.00566 mol·cm-3 [Poly(P-a/t-BF-sa-a)]
and 0.00699 mol·cm-3 [Poly(BA-a/t-BF-sa-a)], respectively,
which confirms the enhancement in the macromolecule
The curing reaction of t-BF-sa-a follows the thermal ring-
opening polymerization of 1,3-benzoxazine. The introduction
of fluorene ring in the polymer network structure improves the
rigidity of the molecular chain and limits the thermal
movement and internal rotation of the polymer chain
segment. With the increase of the number of oxazine ring and
polymerizable group, the crosslinking density of the polymer is
increased and the network structure becomes more compact.
Therefore, the poly(t-BF-sa-a) displays a higher Tg and thermal
stability than those of the corresponding bifunctional fluorene-
based polybenzoxazines. In the meantime, t-BF-sa-a monomer
shows low melting point and good processing property. DSC,
DMA and TGA are used to study the polymerization behavior,
thermal properties and thermomechanical properties of
poly(P-a/t-BF-sa-a) and poly(BA-a/t-BF-sa-a), and their cross-
section morphologies are observed by SEM. The results
indicate that t-BF-sa-a can copolymerize with P-a and BA-a.
Under the catalysis of t-BF-sa-a monomer, the initial
exothermic temperature and peak temperature of BA-a/t-BF-
sa-a blends are significantly reduced, show a significant
catalytic activity. The Tgs and thermal stabilities of the
copolymers are improved obviously due to the introduction of
fluorene ring and allyl group and the increase of network
structure crosslinking degree. In addition, due to t-BF-sa-a
monomer contains a long alkyl chain, the E' values of the
copolymers are decreased significantly. The cross sections of
the copolymers show uniform and disordered dimples or
treelike fold, and show obvious ductile fracture characteristics,
which indicates that the introduction of allyl group into the
copolymer network structure has obvious toughening effect.
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segments formation in the copolymer [54]
3.4.3 Thermal stability of copolymers
.
The TGA test results of poly(P-a), poly(BA-a), poly(P-a/t-BF-sa-a),
and poly(BA-a/t-BF-sa-a) are displayed in Figure 11, and extracted
data are summarized in Table 2.
We can easily observe that the initial thermal decomposition
temperatures (T5 and T10) of poly(P-a/t-BF-sa-a) are 338 °C and
370 °C, respectively, and the carbon residue rate (Yc) is 44.7% at
800 °C, which is 58 °C, 35 °C, and 8% higher than that of poly(P-a),
even higher than that of bisphenol fluorene-aniline-based
polybenzoxazine. This is mainly due to the addition of t-BF-sa-a can
improve the crosslinking density and the thermal stability of the
molecular chain of poly(P-a/t-BF-sa-a), and prevent the
volatilization of unstable components in the network structure.
The T5, T10, and Yc for poly(BA-a/t-BF-sa-a) are recorded as 331 °C,
361 °C, and 40.8%, respectively, which are also higher than that of
poly(BA-a). This is due to the introduction of rigid fluorene ring and
polymerizable group in t-BF-sa-a monomer, which results in the
increase of the crosslinking density and molecular chain rigidity of
the copolymer. And it is easier to form carbon deposition, so it has
excellent thermal properties.
3.4.4 Morphology of copolymers
The fracture morphologies of pristine poly(P-a) and poly(BA-a),
poly(P-a/t-BF-sa-a) and poly(BA-a/t-BF-sa-a) copolymers are
depicted in Figure 12.
The fracture surface of poly(P-a) is smooth and flat, and shows low
toughness. However, when t-BF-sa-a monomer was added, the
Conflicts of interest
There are no conflicts to declare.
cross-section structure of poly(P-a/t-BF-sa-a) was rough. This is Acknowledgements
because the crosslinking degree of the copolymer increases with
Funding: This work was supported by the National Natural
the addition of t-BF-sa-a monomer, and the high crosslinking
network structure can prevent the sample from fracture under
external force, so the fracture occurs after a lot of deformation. The
fracture surface of poly(P-a/t-BF-sa-a) presents uniform dendritic
folds, and the number of creases is larger, showing obvious ductile
fracture characteristics [52], which indicates that the toughness of
the copolymer has been significantly improved. The addition
polymerization of the allyl group in t-BF-sa-a monomer results in
the formation of aliphatic chains, which can increase the flexibility
of molecular chains and show tough fracture. For poly(P-a/t-BF-sa-
a), the impact energy from the outside can not only be absorbed or
buffered, but also the direction of stress propagation can be
prevented or changed due to the increase of flexible polymer
segments so that the mechanical properties can be improved
Similarly, the poly(BA-a) and its copolymer showed a similar cross-
sectional morphology and homogeneous structure with no obvious
defects.
Science Foundation of China [grant number 51773048]; the
Natural Science Foundation of Heilongjiang Province [grant
number E2017022]; and the Fundamental Research Funds for
the Central Universities [grant number HEUCFP201724].
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