Greatly enhanced thermo-oxidative stability of polybenzoxazine thermoset by incorporation of m-carborane

Article abstract of DOI:10.1002/pola.27525

Novel polybenzoxazine precursor containing m-carborane unit in the main-chain has been firstly synthesized through click reaction of diazidomethyl m-carborane (DAMC) and diacetylene bisbenzoxazine (DABB). Meanwhlie, the traditional polybenzoxazine precursor was also prepared through click reaction of diazidomethyl p-benzene (DAPB) and DABB as a control. 1H NMR was used to confirm the structures of the monomers and the resulting polymers. FT-IR and differential scanning calorimetry (DSC) were used to study the curing behavior of carborane-containing benzoxazine polymer (CCBP). Dynamic mechanical analysis (DMA) study demonstrated that the cured CCBP had high storage moduli and high Tg. Thermogravimetric analysis (TGA) and ablation test showed that the cured CCBP had outstanding thermo-oxidative stability. During thermal ablation of cured CCBP, organic material was degraded, and a passivation layer with oxidized m-carboranes was formed, which prevented the underlying polymer from further degradation.

Full text of DOI:10.1002/pola.27525

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Greatly Enhanced Thermo-oxidative Stability of Polybenzoxazine
Thermoset by Incorporation of m-Carborane
Xin Huang, Qiuhong Zhang, Zheng Meng, Jiangjiang Gu, Xudong Jia, Kai Xi
Department of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory
of Microstructures, Nanjing University, Nanjing 210093, China
Correspondence to: X. Jia (E-mail: jiaxd@nju.edu.cn)
Received 28 September 2014; accepted 25 December 2014; published online 00 Month 2015
DOI: 10.1002/pola.27525
ABSTRACT: Novel polybenzoxazine precursor containing m-car-
borane unit in the main-chain has been firstly synthesized
through click reaction of diazidomethyl m-carborane (DAMC)
and diacetylene bisbenzoxazine (DABB). Meanwhlie, the tradi-
tional polybenzoxazine precursor was also prepared through
click reaction of diazidomethyl p-benzene (DAPB) and DABB as
a control. 1H NMR was used to confirm the structures of the
monomers and the resulting polymers. FT-IR and differential
scanning calorimetry (DSC) were used to study the curing
behavior of carborane-containing benzoxazine polymer (CCBP).
Dynamic mechanical analysis (DMA) study demonstrated that
the cured CCBP had high storage moduli and high Tg. Ther-
mogravimetric analysis (TGA) and ablation test showed that
the cured CCBP had outstanding thermo-oxidative stability.
During thermal ablation of cured CCBP, organic material was
degraded, and a passivation layer with oxidized m-carboranes
was formed, which prevented the underlying polymer from fur-
ther degradation. V
2015 Wiley Periodicals, Inc. J. Polym. Sci.,
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Part A: Polym. Chem. 2015, 00, 000–000
KEYWORDS: click reaction; m-carborane; polybenzoxazine;
thermo-oxidative stability; thermoset
2
0,21
INTRODUCTION High-temperature resistant polymers have
been developed with the purpose of being used as high-
performance matrices in electronics and aerospace indus-
contents.
The thermo-oxidative stability of the polymers
could be greatly improved by chemical incorporation of ico-
sahedral caboranes. Carborane-containing polymers, such as
polyetherketones, polysilanes and polysiloxanes have been
1
tries. Polybenzoxazines are a class of novel phenolic resins
formed by thermal ring-opening polymerization of the corre-
sponding 1,3-benzoxazine. They have many fascinating char-
prepared as advanced materials with unique thermostabil-
2
22–34
ities.
However, to the best of our knowledge, no study
acteristics
such
as
near-zero
shrinkage
during
of polybenzoxazines based on icosahedral caboranes has
been reported. The incorporation of icosahedral carboranes
is expected to improve the thermo-oxidative stability of poly-
benzoxazine. Among the icosahedral carboranes, m-carborane
and p-carborane are usually modified to synthesize different
polymers. In contrast to m-carborane, p-carborane is very
expensive and has not been readily available in larger quan-
tities. Therefore, m-carborane may be the better candidate
for the synthesis of carborane-containing polymer.
polymerization, low water absorption, high thermal stability
and wide flexibility concerning the design of molecular struc-
3
–8
ture.
native
bismaleimides and even polyimides are used.
In this regard, polybenzoxazines are promising alter-
materials
for applications
where
epoxies,
Although
9
–11
the stability of polybenzoxazines is rather excellent under
inert atmospheres, the thermal degradation of polybenzoxa-
zines under oxidizing atmospheres, like epoxies and polyi-
1
2–14
mides, is still serious.
The thermal degradation of
An effective approach to introduce covalent linkages is via
click chemistry, which describes a number of simple and
highly specific chemical reactions that feature high yields
under mild conditions. The most prominent click reaction is
the well-known copper(I)-catalyzed 1,3-dipolar cycloaddition
traditional polybenzoxazines under oxidizing atmospheres
limits the application under special conditions, hence novel
polybenzoxazines which have high thermo-oxidative stability
is highly desirable.
3
5–37
Icosahedral caboranes are carbon-containing polyhedral
of alkynes and azides to produce 1,2,3-triazole linkages.
boron-cluster compounds including: o-carborane, m-carbor-
Through highly efficient click polycondensation, the benzoxa-
zine ring was successfully introduced into the polymer sys-
tem without any side reaction.
1
5–19
ane and p-carborane.
They have special thermal stability
3
8–43
because of their stable cage structures and rich boron
Additional Supporting Information may be found in the online version of this article.
2015 Wiley Periodicals, Inc.
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In this study, the functional monomer of diazidomethyl m-
carborane (DAMC) was prepared by chemical modification of
m-carborane, and then reacted with diacetylene bisbenzoxa-
zine (DABB) to obtain a novel polybenzoxazine precursor.
Followed by curing, the carborane-containing benzoxazine
polymer (CCBP) with high modulus and prominent thermo-
oxidative stability under high temperature was finally
obtained.
The mixture was stirred for 3 h at room temperature. Then,
paraformaldehyde (4.95 g, 165 mmol) was added all at once,
and the mixture was stirred for 3 h at 0 C and 24 h at
ꢀ
room temperature, respectively. Then, the reaction was
ꢀ
quenched with 20 mL of 10% HCl aqueous solution at 0 C,
and the mixture was extracted with diethyl ether. The
organic layer was washed with brine and dried over anhy-
drous Na SO , followed by vacuum evaporation. The yellow
2
4
crude product was finally passed through silica gel column
chromatography (solution of PtCl was used as chromogenic
2
EXPERIMENTAL
agent for derivatives of m-carborane, the process of purifica-
tion was described in Supporting Information) using n-hex-
ane/AcOEt [5/1 (v/v)] as an eluent to give 1 as a white
Materials
m-carborane was purchased from KATCHEM. Paraformalde-
hyde, 4-dimethylaminopyridine, copper(I) acetate (99%), p-
xylylene dibromine (97%), and trifluoromethanesulfonic
anhydride (99%) were purchased from Sigma–Aldrich. Prop-
argylamine and n-butyllithium (2.5 M in n-hexane) were pur-
chased from Acros Organics. Diethyl ether, tetrahydrofuran,
pyridine, ethyl acetate, n-hexane, acetone, dichloromethane,
chloroform, N-methyl-2-pyrrolidone (NMP), dimethyl sulfox-
ide, hydrochloric acid and sodium azide were obtained from
Nanjing Reagent Corporation, China. Paraformaldehyde and
m-carborane was dried under vacuum. Diethyl ether and tet-
rahydrofuran were dried over sodium and distilled under
1
powder (yield: 88%). H NMR (DMSO; d, ppm): 1.50–3.20
1
3
(BH, 10H, br m), 3.60 (CH , 4H, s), 5.81 (OH, 2H, br s).
2
C
NMR (DMSO; d, ppm): 63.8 (CH ), 78.7 (C ).
2
Cb
Synthesis of Compound 2
Compound 2 was synthesized according to the previous liter-
4
7
ature.
Trifluoromethanesulfonic anhydride (10.0 g, 35
mmol) in dry dichloromethane (30 mL) and pyridine (2.6 g,
33 mmol) in dry dichloromethane (30 mL) were added to a
solution of compound 1 (3.3 g, 16 mmol) in dry dichlorome-
ꢀ
thane (50 mL) at 0 C under argon, and the mixture was
ꢀ
nitrogen. CH
nitrogen.
2
Cl
2
was dried over CaH
2
and distilled under
stirred for 5 h at 0 C. Then the reaction mixture was
poured into distilled water and extracted with dichlorome-
thane. The combined organic extracts were washed with
brine and dried over anhydrous Na SO , followed by vacuum
Measurements
NMR spectra were obtained at ambient temperature on
2
4
evaporation. The yellowish crude product was finally passed
through silica gel column chromatography using n-hexane/
1
13
Bruker DRX-500 spectrometer ( H NMR, 500 MHz; C NMR,
25 MHz) with tetramethylsilane (TMS) as the standard.
1
dichloromethane [8/1 (v/v)] as an eluent to give 2 as a col-
1
1
Chemical shifts were reported relative to TMS (d H 5 0.00
orless liquid (yield: 85%). H NMR (CDCl ; d, ppm): 1.50–
3
13
1
3
ppm, d C 5 0.00 ppm). The IR spectra were recorded on a
3.20 (BH, 10H, br m), 4.54 (CH , 4H, s). C NMR (CDCl ; d,
2
3
Bruker Vector 22 spectrophotometer with KBr pellets in the
ppm): 70.3 (C ), 74.1 (CH ), 112.1 (CF ), 116.3 (CF ), 120.5
Cb 2 3 3
2
1
4
(
000–400 cm
DSC) was measured on METTLER TOPEM TM DSC under
nitrogen flow (50 mL/min). Dynamic mechanical analysis
region. Differential scanning calorimetry
(CF ), 124.7 (CF ).
3
3
Synthesis of Compound 3 (DAMC)
Compound 3 was synthesized according to the literature pro-
(
(
DMA) was carried out on a Dynamic Mechanical Analyzer
4
8
METRAVIB DMA 1 450, France). The viscoelastic properties
cedure. Sodium azide (10.0 g, 154.0 mmol) was gently
added to a solution of compound 2 (7.2 g, 15.4 mmol) in dry
DMSO (50 mL) at room temperature under argon. The mix-
ꢀ
ꢀ
were measured at a heating rate of 2 C/min from 50 C to
3
ꢀ
00 C, and at a frequency of 2 Hz. Thermogravimetric anal-
ꢀ
ysis (TGA) was performed on NETZSCH STA 449 F3 TG/DSC
ture was allowed to react with stirring at 45 C for 12 h.
ꢀ
with a heating rate of 10 C/min under nitrogen or air flow
Then, the mixture was poured into distilled water and
extracted with diethyl ether. The combined organic extracts
were washed with brine and then dried over anhydrous
Na SO , followed by vacuum evaporation. The yellow crude
(
50 mL/min). The boron element of the sample surface was
investigated through X-ray photoelectron spectroscopy (XPS)
using a PHI 5000 Versa Probe electron spectrometer and an
AlKR line excitation source with the reference of at 284.60
eV (C 1s). Scanning electron microscopy (SEM) was per-
formed using a Hitachi S-4800. Accelerating the voltage to
2
4
product was finally passed through silica gel column chro-
matography using n-hexane/diethyl ether [10/1 (v/v)] as an
1
eluent to give 3 as a colorless oil (yield: 88%). H NMR
1
0 kV with Au coating of the sample was used to image the
(CDCl ; d, ppm): 1.50–3.20 (BH, 10H, br m), 3.55 (CH , 4H,
3
2
1
3
surface morphology.
s). C NMR (CDCl ; d, ppm): 54.7 (CH ), 74.0 (C ).
3
2
Cb
Synthesis of Compound 1
Synthesis of Compound 4 (DAPB)
Compound 1 was prepared according to the previous
Compound 4 was synthesized according to the literature pro-
cedure. Sodium azide (9.75 g, 150 mmol) was gently added
4
4–46
49
works.
A solution of 2.5 M n-BuLi in n-hexane (44.0 mL,
110 mmol) was dropwise added into a solution of m-carbor-
to a solution of p-xylylene dibromine (13.2 g, 50 mmol) in
ane (7.2 g, 50 mmol) in 200 mL of dry tetrahydrofuran at
dry DMF (50 mL) at room temperature under argon. The
mixture was slowly heated to 80 C and allowed to react for
ꢀ
ꢀ
2
78 C under argon by using standard Schlenk techniques.
2
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SCHEME 1 Synthetic route of DAMC. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.
com.]
a period for 12 h. Then, the mixture was poured into dis-
tilled water and extracted with diethyl ether. The combined
organic extracts were washed with brine and then dried
over anhydrous Na SO , followed by vacuum evaporation.
anhydrous dichloromethane. 12 mg (0.1 mmol) of copper(I)
acetate and 24 mg (0.2 mmol) of DMAP were added to the
above mixture solution. Then the mixture was stirred at 40
ꢀ
C for 24 h. After the reaction was completed, the crude
2
4
The yellow crude product was finally passed through silica
gel column chromatography using n-hexane/diethyl ether
product was washed with saturated aqueous solution of eth-
ylenediaminetetraacetic acid disodium salt. The organic layer
[
8
(
(
10/1 (v/v)] as an eluent to give 4 as a colorless oil (yield:
was dried over anhydrous Na SO4 and evaporated under
2
1
2%). H NMR (DMSO; d, ppm): 4.44 (CH , 4H, s), 7.38
reduced pressure to a concentrated solution (about 10 mL).
The concentrated solution was dropped into 1000 mL of
2
1
3
CArH, 4H, s). C NMR (DMSO; d, ppm): 53.4 (CH ), 128.8
2
1
C ), 135.6 (C ).
diethyl ether to give a white powder (yield: 80%). H NMR
Ar
Ar
(
CDCl ; d, ppm): 1.58 (CH , 6H, s), 1.50–3.20 (BH, 10H, br
3
3
Synthesis of Compound 5 (DABB)
m), 3.58 (CArACH
(CCbACH AN, 4H, s), 4.81(OACH AN, 4H, s), 6.67 (ArH, 2H,
2 2
d), 6.80 (ArH, 2H, s), 6.95 (ArH, 2H, d),7.49 (@CHAN, 2H, s).
2
2
AN, 4H, s), 4.04 (ArACH AN, 4H, s), 4.70
Compound 5 was synthesized according to the literature pro-
5
0
cedure. 3.00 g (100.0 mmol) of paraformaldehyde, 2.75 g
25.1 mmol) of propargylamine and 5.68 g (12.5 mmol) of
,4’-isopropylidenediphenol were dissolved in 150 mL of
(
4
Synthesis of Polymer 2 (BCBP)
Polymer 2 was synthesized from compound 5 (DABB) and
chloroform at room temperture, and then the mixture was
stirred at 75 C for 24 h. After the reaction was completed,
the solution was washed with 10% Na CO aqueous solution
2 3
ꢀ
compound 4 (DAPB) in a similar manner to polymer 1.
1
(
yield: 82%). H NMR (CDCl
3
; d, ppm): 1.58 (CH
3
, 6H, s),
3
.95 (CtrizoleACH AN, 4H, s), 4.05 (C ACH AN, 4H, s), 4.82
and brine. The chloroform solution was dried with anhy-
drous Na SO and followed by vacuum evaporation. The yel-
2 Ar 2
(
OACH AN, 4H, s), 5.48 (C ACH AN, 4H, s), 6.67 (ArH, 2H,
2 Ar 2
2
4
lowish crude product was finally passed through silica
gel column chromatography using chloroform/acetone [40/1
(
(
(
v/v)] as an eluent to give 5 as a colorless viscous liquid
1
yield:78%). H NMR (CDCl ; d, ppm): 1.58 (CH , 6H, s), 2.29
3
3
BCAH, 2H, s), 3.95 (CACH
2 2
AN, 4H, s), 4.04 (ArACH AN,
4
H, s), 4.81 (OACH AN, 4H, s), 6.67 (ArH, 2H, d), 6.82 (ArH,
2
1
3
2H, s), 6.94 (ArH, 2H, d). C NMR (CDCl ; d, ppm): 31.0
3
(
CH ), 41.0 (CAC@CAH), 41.7 [C(CH ) ], 49.9 (ArACH AN),
3 3 2 2
7
2.7 (CBCAH), 79.8 (CBCAH), 81.3 (OACH AN), 115.9
2
(C ), 118.4 (C ), 125.4 (C ), 126.3 (C ), 143.2 (C ), 151.4
Ar Ar Ar Ar Ar
(C
Ar).
1
Synthesis of Polymer 1 (CCBP)
.68 g (0.01mol) of compound 5 (DABB) and 2.54 g
FIGURE 1 H NMR spectra of DAMC (in CDCl ). [Color figure
3
3
can be viewed in the online issue, which is available at
(0.01mol) of compound 3 (DAMC) were added to 30 mL of
wileyonlinelibrary.com.]
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SCHEME 2 Synthetic step of CCBP and BCBP. [Color figure can be viewed in the online issue, which is available at wileyonlineli-
brary.com.]
d), 6.80 (ArH, 2H, s), 6.95 (ArH, 2H, d), 7.28 (ArH, 4H, s),
.50 (@CHAN, 2H, s).
The reference polymer 2 was synthesized from diacelylene
bisbenzoxazine (DABB) and bis(azidomethyl)-p-bezene
7
(
DAPB) in a similar manner to CCBP (Scheme 2). Molecular
RESULTS AND DISCUSSION
weights of CCBP and BCBP were obtained by gel permea-
tion chromatography (GPC) analysis, and the results were
listed in Table 1. The crude products were recrystallized in
diethyl ether to remove small molecule impurities and
oligomers, which gave a high molecular weight and low
polydispersity.
Preparation and Characterization of Monomers and
Polymers
The copper-catalyzed click-coupling reaction of terminal alkynes
with azide groups represents a powerful way to prepare main-
chain polymers. The reaction temperature of click reaction is
ꢀ
taken at 40 C, which is beneficial to the stability of oxazine ring.
Click-coupling polycondensation enables us to introduce m-car-
borane structure into the main-chain of polybenzoxazine precur-
sors. In this work, DAMC (3) was synthesized from m-carborane
molecule followed by hydroxymethylation, esterification and
nucleophilic substitution (Scheme 1). As the purity of reactants
was very critical to the click reaction, DAMC was purified by
means of column chromatography on silica gel to improve the
efficiency of the reaction. The structure of DAMC was examined
Curing Behavior and Polymerization of the CCBP
The curing behavior of the CCBP was tested by DSC. DSC of
each cure cycle for the polymer was performed as shown in
Figure 3. DSC thermogram of CCBP showed an exotherm
1
by FT–IR and H NMR spectroscopy. The absorptions observed at
2
1
21
2
608 cm and 2108 cm were attributed to the stretching of
BAH and N groups. The protons of BAH in m-carborane cage
3
were presented at d 1.5–3.2 ppm, and those of CH
2
were pre-
sented at d 3.55 ppm (Fig. 1).
The synthetic step of main-chain benzoxazine precursor was
1
shown in Scheme 2. H NMR of the product showed reso-
nance at 7.49 ppm, which was assigned to @CHAN of tria-
zole ring, and the oxazine ring structure was confirmed by
the peaks at 4.04 and 4.81 ppm, respectively (Fig. 2).
1
It was pointed out that the benzoxazine group was stable
during the reaction and the ring closure degree of the CCBP
was above 99%.
FIGURE 2 H NMR spectrum of CCBP (in CDCl ). [Color figure
3
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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TABLE 1 Molecular Weights of CCBP and BCBP
Sample
M
n
M
w
PDI
CCBP
BCBP
19,000
16,000
26,000
21,000
1.36
1.31
ꢀ
ꢀ
with an onset at 180 C and a maximum at 238 C. The
amount of the exotherm was 137 J/g, which was corre-
sponded to the ring-opening polymerization of benzoxazine
group. It decreased after each curing stage. The curing
degree could be calculated from the amount of exotherm,
ꢀ
51
giving a value of 95% after curing at 240 C for 1 h. The
cured CCBP was further postcured at 260 C for 1 h so that
ꢀ
the curing reaction could completely be proceeded.
The IR spectrum was used to further confirm the ring-
opening polymerization process of the CCBP (Fig. 4). It was
clearly shown that the characteristic absorption bands at
FIGURE 4 FT–IR spectra of CCBP after each cure stage. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
2
1
21
9
41 cm and 1493 cm due to the trisubstituted benzene
2
1
ring disappeared, meanwhile the bands at 1322 cm due to
Mechanical Properties of Cured Polymers
CH wagging gradually decreased.
2
DMA is a useful technique to study the viscoelastic proper-
ties of polymers. The glass transition temperature (Tg) can
21
In addition, a broad absorption appeared next to 1493 cm
00
52
be estimated from the maximum of the loss modulus (E ).
due to the tetrasubstituted benzene ring mode, indicating
the ring-opening of benzoxazine to afford polybenzoxazine.
The sample bars of cured CCBP and cured BCBP were pre-
pared by casting films from NMP solution and curing them
2
1
The absorption at 1234 cm
assigned to asymmetric
ꢀ
ꢀ
ꢀ
for 1 h at 160 C, 1 h at 200 C, 1 h at 240 C and 1 h at
stretching of CAOAC of oxazine ring was almost disappeared
ꢀ
ꢀ
260 C, respectively. The DMA was performed by multi-
by the end of curing at 240 C, suggesting that the site of
frequency strain test. The storage and loss modulus of cured
polymers were shown in Figure 5, which showed that the
incorporation of m-carborane led to a significant enhance-
ment in the mechanical properties of polybenzoxazine,
including storage modulus and Tg.
cleavage was CAOAC linkage in the oxazine ring. Further-
more, the relative intensity of broad absorption around
21
3
400 cm increased at elevated curing temperature, which
was assigned to the intermolecular hydrogen bonding. The
21
absorptions at 2608 cm
due to stretching of BAH
remained unchanged during the curing, which suggested the
invariance of m-carborane cages.
In the case of cured BCBP as a control, the storage modulus
only took on a small range drop, which was more than 50
Gpa up to 200 C with a Tg at 255 C. While, the cured
CCBP showed a drastic increase in the Tg, which was above
ꢀ
ꢀ
FIGURE 3 DSC profiles of CCBP after each cure stage. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 5 DMA curves of the cured polymers. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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TABLE 2 Thermal Stability of Cured Polymers
ꢀ
ꢀ
a
Sample
Atmosphere
T
5
( C)
T
10 ( C)
Char Yield
Cured CCBP
Cured BCBP
Cured CCBP
Cured BCBP
Nitrogen
Nitrogen
Air
398
344
404
360
479
374
480
390
75%
55%
76%
0%
Air
a
ꢀ
At 800 C.
and dissolved in NMP together. The co-polybenzoxazines
cured C-co-B BP) were prepared through the same curing
(
process. The TGA profiles of cured C-co-B BP under nitrogen
and air atmosphere were also shown in Figures 6 and Figure
7. The thermal stability of cured C-co-B BP under nitrogen
FIGURE 6 TGA curves of cured polymers under nitrogen.
was close to those of cured CCBP and cured BCBP. The cured
C-co-B BP also had good thermo-oxidative stability, which
was far better than cured BCBP. The T5 and T10 for cured
CCBP and copolymers were significantly enhanced due to the
incorporation of m-carborane. The char yield of cured CCBP
[
Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
ꢀ
3
00 C. This observation suggested that the present of m-
carborane cage, due to their steric hindrance and chemical
incorporation to the polybenzoxazine chains, effectively hin-
dered the motion of the network. The rate of descent of stor-
age moduli of cured CCBP was not significant in a wide
range up to 250 C, which was higher than that of the cured
BCBP.
ꢀ
was around 75% at 800 C both in nitrogen and air atmos-
phere, which also indicated its good thermal stability.
The superior thermo-oxidative stability of cured CCBP could
be attributed to the presence of passivation layer with oxi-
dized m-carboranes, which was formed during thermal abla-
ꢀ
5
4–56
tion.
Toward this end, the thermal ablation process of
Thermal Properties of Cured Polymers
the cured polymers was performed in muffle furnace, and
The thermal stability of the newly developed polybenzoxa-
zines was evaluated by TGA under nitrogen and air atmos-
phere, and the thermograms were shown in Figures 6 and 7.
The weight loss temperatures of 5% and 10% (T and T )
for the cured polymers were summarized in Table 2. The
cured CCBP exhibited highly thermally stable with T
4
benzoxazine resin. To further study the effect of m-carbor-
ane on the thermal stability of polybenzoxazine, CCBP and
BCBP were mixed in a certain mass ratio (75/25; 50/50)
ꢀ
the cured polymers were heated for 15 min at 500 C under
air atmosphere. As shown in Figure 8, the exterior of cured
CCBP remained basically unchanged while the cured BCBP
was mostly burned up. The surface of cured CCBP after ther-
mal ablation was further investigated through XPS and SEM.
5
10
5
around
ꢀ
00 C in nitrogen and air, which exceeded that of typical
5
3
FIGURE 7 TGA curves of cured polymers under air. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 8 Thermal ablation photos of cured polymers under
air. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
6
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of cured CCBP still looked relatively smooth except for the
presence of a certain amount of spotted deposition [Fig.
10(c)]. A passivation layer with oxidized m-carboranes was
formed on the surface, which possibly retarded further ero-
sion. For the cured BCBP, the lack of a protective layer
resulted in continuous oxidative degradation of the organic
portions, leading to dramatic loss and a rough surface.
CONCLUSIONS
In conclusion, we demonstrated that main-chain benzoxazine
with m-carborane moieties in the main-chain could be pre-
pared by click reaction of a DAMC and DABB in the presence
of Cu(OAc) catalyst and an amine base (DMAP). Traditional
main-chain benzoxazine was prepared in a similar way as a
control. The benzoxazine groups were shown to undergo
thermally activated ring-opening reaction and formed cross-
linked networks. Because of the high steric hindrance of m-
carborane and high cross-linking degree of polybenzoxazine,
FIGURE 9 XPS curves of cured CCBP after thermal ablation.
[
Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
ꢀ
the cured CCBP showed high Tg above 300 C and high stor-
The thermal oxidation of m-carborane on the surface was
evidenced from the XPS image (Fig. 9). Peak at 193.2 eV was
assigned to BAO, while the BAH peak around 187 eV disap-
ꢀ
age moduli more than 50 Gpa in a wide range up to 250 C,
which was higher than that of the cured BCBP. The excellent
thermo-oxidative stability was confirmed by TGA under air
5
4
ꢀ
peared. SEM images of cured CCBP and cured BCBP films
with 5% weight loss temperature above 400 C and a high
ꢀ
before and after thermal ablation were presented in Figure
char yield of 76% at 800 C, which is attributed to the incor-
1
0. Before thermal ablation, as shown in Figure 10(a,b), the
poration of m-carborane cage and the formation of protecting
layers with oxidized m-carboranes. The results demonstrate
great promise for carborane-containing polybenzoxazines as a
new generation of space-survivable materials.
surfaces of cured CCBP and cured BCBP were smooth. How-
ever, after thermal ablation, the surface of cured BCBP
became rough and coralliform [Fig. 10(d)], while the surface
FIGURE 10 SEM images of cured CCBP (a,c) and cured BCBP (b,d) films before and after thermal ablation.
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ACKNOWLEDGMENTS
29 T. M. Keller, D. Y. Son, Synth. Charact. Adv. Mater. 1998,
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81, 248–258.
The authors thank the Program for Changjiang Scholars and
Innovative Research Team in University and the Fundamental
Research Funds for the Central Universities (1104020505) for
financially supporting this research.
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