The robustness of a thermoset of a main-chain type polybenzoxazine precursor prepared through a strategy of A-A and B-B polycondensation

Article abstract of DOI:10.1039/c5ra25619d

A polybenzoxazine precursor, P(BF-bapp)-1, was prepared through a strategy of A-A and B-B polycondensation. The A-A monomer is 5,5′-methylenebis(2-hydroxybenzaldehyde), and the B-B monomer is 2,2-bis[4-(4-aminophenoxy)phenyl] propane (BAPP). The inherent

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ARTICLE
The robustness of thermoset of main-chain type polybenzoxazine
precursor prepared through a strategy of A-A and B-B
polycondensation
Received 00th January 20xx,
Accepted 00th January 20xx
Meng Wei Wang,a Ru Jong Jeng,a† Ching Hsuan Linb†
DOI: 10.1039/x0xx00000x
A polybenzoxazine precursor, P(BF-bapp)-1, was prepared through a strategy of A-A and B-B polycondensation. The A-A
monomer is 5,5’-methylenebis(2-hydroxybenzaldehyde), and the B-B monomer is 2,2-bis [4-(4-aminophenoxy)phenyl]
propane (BAPP). The inherent viscosity (at 0.5 g/dL in NMP) of P(BF-bapp)-1 is as high as 0.34 dL/g, which is higher than that
(0.23 dL/g) of a structurally similar polybenzoxazine precursor, P(BF-bapp)-2, that was prepared through traditional Mannich
condensation of BAPP/bisphenol F/paraformaldehyde. A third structurally similar polybenzoxazine precursor, P(BF-bapp)-3,
with an inherent viscosity of 0.09 dL/g, and a phenol and BAPP-based benzoxazine monomer, P-bapp, were prepared for
properties comparison. After thermal curing of the benzoxazines, we found that a benzoxazine thermoset based on a high
molecular weight precursor has advantages in higher Tg, higher char yield, lower refractive index (low dielectric constant
according to Maxwell’s equation), and better mechanical properties. The results demonstrate the robustness of a
polybenzoxazine precursor prepared through a strategy of A-A and B-B polycondensation, which has never been reported
in the benzoxazine field.
www.rsc.org/
P(BF-bapp)-1 and systematically discuss the effect of the MW of benzoxazine
on the properties of a benzoxazine thermoset, we also prepared another
three structurally similar benzoxazines with different MW. The second
Introduction
Monomer type benzoxazines are generally prepared by Mannich
polybenzoxazine precursor, P(BF-bapp)-2, with
prepared through a traditional Mannich condensation of BAPP, bisphenol F,
and paraformaldehyde using toluene/ethanol as solvent. The third
a moderate MW, was
condensation of bisphenol/mono amine/formaldehyde, and can be
polymerized to thermosets by means of thermally activated ring-opening
polymerization.1-6 Benzoxazine thermosets exhibit many advantages, such as
good thermal properties4, 7-11, low surface energy12-14 and good dielectric
properties10, 15-17. However, benzoxazine thermosets based on monomer type
benzoxazines have some shortcomings such as poor film quality. To obtain
flexible polybenzoxazines with superior thermal properties and mechanical
properties, main-chain type polybenzoxazine precursors have been
developed.18, 19 For example, Endo et al. 20 Yagci et al.21-23 Liu et al. 24, Ishida et
al.25, and Takeichi et al.26 prepared main-chain type polybenzoxazine
precursors using strategies other than Mannich condensation. Takeichi et
al.27-29 Yagci et al.30, and Ishida et al.31 prepared main-chain type
polybenzoxazine precursors via Mannich condensation. The thermosets
based on main-chain type polybenzoxazine precursors exhibited better
thermal properties than thermosets based on monomer type benzoxazines.
However, preparing a main-chain type polybenzoxazine precursor through
Mannich condensation of diamine, bisphenol and paraformaldehyde led to
polybenzoxazine precursor with moderate-to-low molecular weight (MW)
due to side reactions.13, 27-31 Therefore, to prepare polybenzoxazine precursor
with moderate-to-high MW, a strategy other than Mannich condensation
should be applied. In this work, we prepared a polybenzoxazine precursor,
P(BF-bapp)-1, with a moderate-to-high MW, through an A-A and B-B type
a
polybenzoxazine precursor, P(BF-bapp)-3, with a moderate-to-low MW, was
prepared from the Mannich condensation of BAPP, bisphenol F, phenol, and
paraformaldehyde, in which phenol was used as a chain terminator. The
fourth benzoxazine, a monomer type benzoxazine, P-bapp, was prepared
from the Mannich condensation of BAPP, phenol, and formaldehyde. Through
study of these four structurally similar benzoxazines, we demonstrate the
benefit of a polybenzoxazine precursor prepared through a strategy of A-A
and B-B polycondensation in thermal, mechanical, and dielectric properties
of the resulting benzoxazine thermoset.
Experimental
Materials. 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) was
purchased from Chriskev. 2-Hydroxybenzaldehyde and phenol were
purchased from Showa. s-Trioxane and sodium borohydride (NaBH4) were
purchased from Acros. Paraformaldehyde and bisphenol F were purchased
from TCI. N,N-Dimethylacetamide (DMAc) was purchased from Tedia and
purified by distillation under reduced pressure over calcium hydride (from
Acros) and stored over molecular sieves. The other solvents used were
commercial products (HPLC grade) and were used without any further
purification.
polycondensation.
The
A-A monomer
is
5,5’-methylenebis(2-
hydroxybenzaldehyde), and the B-B monomer is 2,2-bis [4-(4-
aminophenoxy)phenyl] propane (BAPP). To demonstrate the robustness of
Characterization. Differential scanning calorimetry (DSC) scans were
obtained using a Perkin-Elmer DSC 7 in a nitrogen atmosphere at a heating
rate of 10 oC/min. NMR measurements were performed using a Varian Inova
400 NMR in CDCl3 or DMSO-d6, and the chemical shift was calibrated by
setting the chemical shift of CDCl3 (DMSO-d6) as 7.24 (2.49) ppm. Thermal
gravimetric analysis (TGA) was performed with a Perkin-Elmer Pyris1 at a
heating rate of 20 oC/min in an atmosphere of nitrogen or air.
Thermomechanical analysis (TMA) was performed by an SII TMA/SS6100 at a
heating rate of 5 °C/min. The coefficient of thermal expansion (CTE) in the
a.Institute of Polymer Science and Engineering, National Taiwan University, Taipei,
Taiwan.
b.Department of Chemical Engineering, National Chung Hsing University, Taichung,
Taiwan.
†
Corresponding: Ching Hsuan Lin (linch@nchu.edu.tw); Ru Jong Jeng
(rujong@ntu.edu.tw).
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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o
temperature range of 50-150 oC was recorded from the second-run data. 80 C for 24 h. After that, the mixture was dried by evaporation of the
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Dynamic mechanical analysis (DMA) was performed with a Perkin-Elmer Pyris solvent under vacuum. A light yellow powder 1.5 g (95% yield) was
DOI: 10.1039/C5RA25619D
Diamond DMA with a sample size of 5.0 cm x 1.0 cm x 0.2 cm. The storage obtained.
modulus E' and tan δ were determined as the sample was subjected to the
temperature scan mode at a programmed heating rate of 5 oC/min at a Curing of polybenzoxazine precursors. Polybenzoxazine precursors,
frequency of 1 Hz. The test was performed by a tension mode with an P(BF-bapp)-1, P(BF-bapp)-2, and P(BF-bapp)-3, were dissolved in NMP,
amplitude of 25 μm. Tensile testing was performed by EZ-SX with sample with a concentration of 10 wt%, 25 wt%, and 30 wt%, respectively. The
dimensions of 20 mm × 5 mm × 50 μm at 25 °C. The refractive index (n) at solution was then cast onto glass by an automatic film applicator. The
o
various wavelengths was obtained by an Ellipsometry GES-5E. The dielectric resulting films were transparent and colorless. They were dried at 80 C
constant (ε) was calculated from the Maxwell’s equation with ε =1.1n2, in overnight and cured at 120, 160, 200, 240 oC for 1h each in an air-
which n is the refractive index at 633 nm. Size exclusion chromatography (SEC) circulating oven. After that, brown transparent films (about 60 um)
was carried out on a Perkin-Elmer series 200a using tetrahydrofuran as the were obtained by soaking the coated glass in water. The sample ID of
eluent at 40 oC with a flow rate of 1.0 mL/min. Size exclusion chromatography the cured polybenzoxazine films are named C-P(BF-bapp)-1, C-P(BF-
(SEC) was also carried out on
a Hitachi LaChrom Elite using N- bapp)-2, and C-P(BF-bapp)-3, respectively. The letter C in the sample ID
methylpyrrolidinone (NMP) as the eluent at 60 oC with a flow rate of 0.6 corresponds to the word cured.
mL/min. The data were calibrated with polystyrene standards. The inherent
viscosities were determined at 0.5
g
dL-1 concentration in N- Curing of P-bapp. P-bapp was melted, stirred and transferred to an aluminum
methylpyrrolidinone (NMP) using a Prema B801-2 viscometer at 30 oC.
mold and then cured at 180, 200, 220, 240 oC for 2 h each in an air-circulating
oven. Thereafter, sample was allowed to cool slowly to room temperature to
Synthesis of 5,5'-methylenebis(2-hydroxybenzaldehyde) (1).32 s-Trioxane prevent cracking. The sample ID of the cured P-bapp is named C-P-bapp.
10.0 g (0.111 mol), 2-hydroxybenzaldehyde 89.46 g (0.732 mol), acetic acid
100 g, and sulfuric acid 3.09 g were introduced into a 250-mL glass flask
equipped with a nitrogen inlet and a magnetic stirrer. The mixture was
Results and Discussion
reacted at 85 oC for 22 h. After the reaction solution was cooled to room
temperature, the precipitate was filtered, dissolved in methanol, and poured
into water. The precipitate was filtrated and then dried in a vacuum oven at
70 oC.
Synthesis of benzoxazines. To prepare a polybenzoxazine precursor with a
high MW, we applied a strategy other than Mannich condensation. The
benzoxazine, P(BF-bapp)-1, was prepared by a A-A and B-B type, three-step
polycondensation procedure (Scheme 1). The starting material, 5,5’-
methylenebis(2-hydroxybenzaldehyde) (1) was prepared from 2-
hydroxybenzodehyde and s-trioxane in the presence of sulfuric acid according
to a reported procedure.32 In the first step, a polymer with imine linkages was
produced by the polycondensation of BAPP and (1). Sodium borohydride was
then employed to reduce the imine linkage in the second step, yielding pre-
P(BF-bapp)-1. The structure of pre-P(BF-bapp)-1 was confirmed by a 1H-NMR
spectrum (Figure S1). In the third step, paraformaldehyde was added to
induce the ring closure of pre-P(BF-bapp)-1, yielding P(BF-bapp)-1. To
demonstrate the robustness of P(BF-bapp)-1 and systematically discuss the
effect of the MW of benzoxazine on the properties of the resulting thermoset,
we prepared another three structurally similar benzoxazines. The second
benzoxazine, P(BF-bapp)-2, was prepared from the Mannich condensation of
BAPP, bisphenol F, and paraformaldehyde (Scheme 2). Gelation occurred for
this synthesis in a homo-solvent such as chloroform, dioxane, and toluene
(Table S1). However, the gelation phenomenon could be significant improved
using toluene/ethanol as a reaction solvent, as suggested in our previous
work13. The third benzoxazine, P(BF-bapp)-3 was prepared from the Mannich
condensation of BAPP, bisphenol F, phenol, and paraformaldehyde (Scheme
2), in which phenol was used as a chain terminator, suggested by Ishida et
al.33. The fourth benzoxazine, P-bapp, was prepared from Mannich
Synthesis of P(BF-bapp)-1. P(BF-bapp)-1 was prepared by a three-step, two-
pot procedure. In the first pot, 5,5’-methylenebis(2-hydroxybenzaldehyde)
5.0 g (0.020 mole), BAPP 8.01 g (0.02 mole), and dry DMAc 42 mL were
introduced into a round-bottom 100 mL glass flask equipped with a nitrogen
inlet, and a magnetic stirrer (Scheme 1). The reaction mixture was stirred at
room temperature for 24 h. After that, the viscous solution was diluted with
DMAc 10 mL, and NaBH4 1.77 g (0.048 mole) was added into the solution. The
reaction mixture was further stirred at room temperature for 24 h. The
solution was then poured into water. The fiber-like precipitate was filtered
and dried in a vacuum oven at 60 oC. A white precipitate of 8.67 g (70% yield)
was obtained. The sample ID of the product is named pre-P(BF-bapp)-1. In the
second pot, pre-P(BF-bapp)-1 1.0 g (0.002 mole), paraformaldehyde 0.11 g
(0.004 mole), and chloroform 30 mL were introduced into a round-bottom
100 mL glass flask equipped with a nitrogen inlet, a condenser and a magnetic
stirrer. The reaction mixture was stirred at reflux temperature for 12 h. After
that, the mixture was poured into hexane. The precipitate was filtered and
dried in a vacuum oven at 60 oC. A light yellow powder of P(BF-bapp)-1 0.88
g (85% yield) was obtained.
Synthesis of P(BF-bapp)-2. BAPP 1.0 g (0.002 mole), bisphenol F 0.49 g (0.002
mole), paraformaldehyde 0.29 g (0.010 mole), and 9 mL of a mixed solvent of condensation of BAPP, phenol, and paraformaldehyde (Scheme 2).
toluene/ethanol (2/1, v/v) were introduced into a round-bottom 100 mL glass
flask equipped with a nitrogen inlet, a condenser and a magnetic stirrer
(Scheme 2). The reaction mixture was stirred at 80 oC for 15 h. After that, the
mixture was poured into methanol. The precipitate was filtered and dried in
a vacuum oven at 60 oC. A light yellow powder 1.28 g (80% yield) was obtained.
Synthesis of P(BF-bapp)-3. BAPP 3.0 g (0.007 mole), bisphenol F 1.02 g
(0.005 mole), phenol 0.41 g (0.004 mole), paraformaldehyde 0.88 g
(0.029 mole), and 30 mL of a mixed solvent of toluene/ethanol (2/1, v/v)
were introduced into a round-bottom 100 mL glass flask equipped with
a nitrogen inlet, a condenser and a magnetic stirrer (Scheme 2). The
reaction mixture was stirred at 80 oC for 15 h. After that, the mixture
was poured into hexane. The precipitate was filtered and dried in a
vacuum oven at 60 oC. A light yellow powder 3.92 g (82% yield) was
obtained.
Synthesis of P-bapp. BAPP 1.0 g (0.002 mole), phenol 0.46 g (0.005
mole), paraformaldehyde 0.29 g (0.010 mole), and 15 mL of a mixed
solvent of toluene/ethanol (2/1, v/v) were introduced into a round-
Scheme 1. Syntheis of (1) and P(BF-bapp)-1.
bottom 100 mL glass flask equipped with a nitrogen inlet, a condenser
and a magnetic stirrer (Scheme 2). The reaction mixture was stirred at
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DOI: 10.1039/C5RA25619D
Table 1. The ratios of crosslinking sites to benzoxazine units for the benzoxazines.
Ratio of reactive sites to
Sample
Structure
benzoxazine
P(BF-bapp)-1
P(BF-bapp)-2
6/2 = 3
(6n+8)/(2n+2)
8/2 = 4
P(BF-bapp)-3
P-bapp
weight (Mw) of P(BF-bapp)-2 and P(BF-bapp)-3 is 79000 and 7000 g/mol,
respectively. However, no SEC data of P(BF-bapp)-1 is available since it is
insoluble in THF for SEC measurement. Therefore, SEC analysis were also
performed in NMP, and the data were listed in Table 2. P(BF-bapp)-1 shows
the highest Mn (4.2 × 104) and Mw (1.7 × 105). Through the viscosity and SEC
data, the order of MW of the benzoxazines is P(BF-bapp)-1> P(BF-bapp)-2>
P(BF-bapp)-3> P-bapp.
Scheme 2. Synthesis of three benzoxazines for comparison.
Table 2. The inherent viscosity and SEC data of P(BF-bapp)-1, P(BF-bapp)-2,
and P(BF-bapp)-3.
viscosity
(dL/g)a
0.34
0.23
0.07
Mw/Mn
(in THF)
Mw/Mn
(in NMP)
Sample
1.7 × 105/4.2 × 104
1.1 × 105/2.0 × 104
9.3 × 103/4.4 × 103
P(BF-bapp)-1
P(BF-bapp)-2
P(BF-bapp)-3
-/-b
7.9 × 104/1.9 × 104
7.0 × 103/2.7 × 103
a Inherent viscosity, measured at 0.5 g/dL in NMP at 30 oC.
b Insoluble in THF.
Figure
1
shows the 1H-NMR spectra of the four benzoxazines. The
characteristic peaks of oxazine at 4.5 and 5.3 were clearly observed for P(BF-
bapp)-1 and P(BF-bapp)-2. The characteristic peaks of oxazine at 4.6 and 5.4
were also clearly observed for P-bapp. For P(BF-bapp)-3, the characteristic
peaks of oxazine were split due to the different environment of benzoxazine
in the repeating unit and terminal units. . Table 2 shows the inherent viscosity
(at 0.5 g/dL in NMP) of P(BF-bapp)-1, P(BF-bapp)-2, and P(BF-bapp)-3. The
value is 0.34, 0.23, and 0.07 dL/g, respectively. No inherent viscosity data is
available for P-bapp since it is only a monomer. SEC data were measured in
THF. The number-average molecular weight (Mn) of P(BF-bapp)-2 and P(BF-
bapp)-3 is 19000 and 2700 g/mol, respectively. The weight-average molecular
Figure 1. 1H-NMR spectra of the benzoxazines.
DSC analysis. Figure 2 shows the DSC thermograms of the benzoxazines. The
exothermic data such as peak temperature and enthalpy are listed in Figure
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2. P(BF-bapp)-1, with the highest MW, display the highest exothermic peak
temperature. Tsai et al. studied DSC thermograms of polyacrylonitrile
precursors with different MWs.34 They found that the polyacrylonitrile
precursor with the highest MW displayed the highest onset exothermic
temperature and peak temperature. The higher MW hinders the mobility of
polymer chain to access each other is thought to be the reason for higher ring-
opening polymerization temperature of P(BF-bapp)-1. Figure 2 also shows
the order of exothermic enthalpy is P-bapp> P(BF-bapp)-3 >P(BF-bapp)-2≒
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DOI: 10.1039/C5RA25619D
1.5
1010
109
108
107
1.0
0.5
P(BF-bapp)-1. According to previous researches on ring-opening
4, 35
polymerization mechanism of benzoxazine,1,
the free ortho and para
C-P(BF-bapp)-1
C-P(BF-bapp)-2
C-P(BF-bapp)-3
C-P-bapp
positions to the O or N of oxazine were reactive sites. The reactive sites of
each benzoxazine are marked as red arrows in Table 1. The ratio of reactive
sites to benzoxazine is 4 for P-bapp, (6n+8)/(2n+2) for P(BF-bapp)-3, and 3 for
both P(BF-bapp)-1 and P(BF-bapp)-2. The value of (6n+8)/(2n+2) is 3.09 if
n=10, and is 3.03 if n=30. Therefore, the order of the ratio of reactive sites to
benzoxazine is P-bapp> P(BF-bapp)-3 >P(BF-bapp)-2 ≒ P(BF-bapp)-1. The
trend is consistent with that of exothermic enthalpy.
0.0
50
100
150
200
250
300
Temperature(oC)
Figure 3. DMA thermograms of the benzoxazine thermosets.
800
700
600
500
400
300
200
100
0
C-P(BF-bapp)-1
C-P(BF-bapp)-2
C-P(BF-bapp)-3
C-P-bapp
Figure 2. DSC thermograms of the benzoxazines.
50
100
150
200
250
300
Temperature (oC)
Thermal properties. Figure S2 shows photos of C-P(BF-bapp)-1 and C-P(BF-
bapp)-2.. All the cured films are void-free and bendable. Figure 3 shows the
DMA thermograms of the benzoxazine thermosets. The Tg values determined
from the peak of tan(delta) for C-P(BF-bapp)-1, C-P(BF-bapp)-2, C-P(BF-bapp)-
Figure 4. TMA thermograms of the benzoxazine thermosets.
o
3, and C-P-bapp are 305, 302, 284, and 194 C, respectively. Figure 4 shows
the TMA thermograms of the benzoxazine thermosets. The Tg values taken
from the onset temperature of TMA thermograms are 256, 258, 244, and 173
oC, respectively. TMA and DMA data show that the Tg value of a benzoxazine
thermoset increased initially with the MW of its precursor, but reach a
plateau at a higher MW. The trend for the Tg and MW is similar to
thermoplastic polymers,36-38 but has never been reported for benzoxazine
thermosets. The coefficients of thermal expansion (CTE) in the temperature
range of 50-150 oC are 50, 51, 51, and 77 ppm/oC, respectively. C-P-bapp,
which has the lowest Tg, shows the highest CTE. However, no difference in
CTE value was observed for other benzoxazine thermosets. Figure 5 shows
100
90
C-P(BF-bapp)-1
C-P(BF-bapp)-2
C-P(BF-bapp)-3
C-P-bapp
80
70
60
50
40
the TGA thermograms of the benzoxazine thermosets. The
5 wt%
decomposition temperatures (Td5%) in a nitrogen atmosphere for C-P(BF-
bapp)-1, C-P(BF-bapp)-2, C-P(BF-bapp)-3, and C-P-bapp are 422, 429, 424, and
o
o
424 C, respectively. The char yields at 800 C in a nitrogen atmosphere are
60, 57, 52, and 41 %, respectively. The thermal stability of benzoxazine
thermosets is independent of the MW of its precursor. However, the char
yield at 800 oC increased with the MW of its precursor. According to Takechi’s
research27, the extension of network increases the crosslinking density in the
thermoset, leading to the enhancement in the char yield.
100
200
300
400
500
600
700
800
Temperature (oC)
Figure 5. TGA thermograms of the benzoxazine thermosets in N2
atmosphere.
Mechanical properties. Figure 6 shows the stress-strain curves of C-P(BF-
bapp)-1, C-P(BF-bapp)-2, and C-P(BF-bapp)-3. Note that the self-supporting
film of C-P-bapp is too brittle, so no stress-strain curve was presented. The
tensile strengths (elongation at break) for C-P(BF-bapp)-1, C-P(BF-bapp)-2,
and C-P(BF-bapp)-3 are 75 MPa (3.1%), 57 MPa (1.9%), and 29 MPa (1.3%),
respectively. Both tensile strengths of the benzoxazine thermoset increased
with the MW of its precursor, demonstrating an advantage of a higher
precursor’s MW in mechanical properties. The trend for tensile strengths and
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MW is similar to thermoplastic polymers.39-41 However, a typically yield probably explains the fact that the dielectric constant ofViaewbAernticzloexOanzliinnee
behavior was not observed due to the brittleness of thermosets.
thermoset decreased with the MW of its precursor.
DOI: 10.1039/C5RA25619D
80
60
40
C-P(BF-bapp)-1
C-P(BF-bapp)-2
20
0
C-P(BF-bapp)-3
0
2
4
Figure 8. Illustration of crosslink network from benzoxazines with different
MWs.
Strain (%)
Figure 6. Stress-strain curves of C-P(BF-bapp)-1, C-P(BF-bapp)-2, and C-P(BF-
bapp)-3.
Conclusions
Refractive index and dielectric constant. Figure 7 shows the refractive index
of C-P(BF-bapp)-1, C-P(BF-bapp)-2 and C-P(BF-bapp)-3 at various wavelengths.
Note that the self-supporting film of C-P-bapp cannot be achieved, so no
refractive index-wavelength curve was presented. The refractive indexes for
C-P(BF-bapp)-1, C-P(BF-bapp)-2 and C-P(BF-bapp)-3 at 633 nm are 1.692,
1.709, and 1.712, respectively. The dielectric constant calculated by the
Maxwell’s equation ɛ= 1.1n2, in which n is the refractive index at 633 nm, are
3.14, 3.22, and 3.25, respectively. The dielectric constant of the benzoxazine
thermoset decreased with the MW of its precursor. The refractive index data
demonstrate the advantage of higher precursor’s MW in dielectric properties.
We report a strategy to prepare a polybenzoxazine precursor, P(BF-
bapp)-1, by an A-A and B-B type polycondensation. The P(BF-bapp)-1
exhibits much higher MW than the structurally similar polybenzoxazine
precursor, P(BF-bapp)-2, which was prepared by a traditional Mannich
condensation. To demonstrate the robustness of P(BF-bapp)-1 and
systematically discuss the effect of the MW of benzoxazine on the
properties of the resulting thermoset, we also prepared other
benzoxazines, P(BF-bapp)-3 and P-bapp. TMA and DMA data show that
the Tg of the benzoxazine thermoset initially increased with the MW of
its precursor, but reached a plateau at higher MW (Figures 3-4). TGA
data show that the thermal stability of benzoxazine thermosets is
independent of the MW of its precursor. However, the char yield
increased with the MW of its precursor (Figure 5). The tensile strength
and elongation at break of benzoxazine thermosets increased with the
MW of its precursor (Figure 6). The refractive index of the benzoxazine
thermoset decreased with the MW of its precursor (Figure 7). The
experimental data show that a benzoxazine thermoset based on a high
MW precursor has advantages in higher Tg, higher char yield, lower
refractive index (lower dielectric constant according to Maxwell’s
equation), and better mechanical properties. Through this study, we
demonstrate the benefit of a polybenzoxazine precursor prepared
through a strategy of A-A and B-B polycondensation in thermal,
mechanical, and dielectric properties of the resulting benzoxazine
thermoset.
1.85
C-P(BF-bapp)-1
1.80
1.75
1.70
1.65
C-P(BF-bapp)-2
C-P(BF-bapp)-3
400
500
600
700
800
900
1000
Acknowledgements
Wavelength (nm)
The authors thank the Ministry of Science and Technology of Taiwan for
financial support.
Figure 7. The refractive index of benzoxazine thermosets at various
wavelengths.
Takeichi et al. reported the enhancement in the thermal stability of the Notes and references
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Graphical Abstract
DOI: 10.1039/C5RA25619D
The robustness of thermoset of main-chain type polybenzoxazine precursor
prepared through a strategy of A-A and B-B polycondensation
Meng Wei Wang,a Ru Jong Jeng,a* Ching Hsuan Linb*
1