Synthesis and thermal properties of an acetylenic monomer containing boron and silicon

Article abstract of DOI:10.1039/c6ra19410a

To improve the thermo-oxidative stability of acetylenic aromatic compounds, 1,2-bis(4-trimethylsilylethynylphenyl)-carborane (CBTMS) was designed, synthesized and characterized by FT-IR, ¹H-NMR, ¹³C-NMR and mass spectrometry. The analysis of the DSC results showed that the acetylenic monomer had a melting point at 195.5 °C. The cross-linking process of CBTMS included a Diels-Alder cycloaddition reaction confirmed by FT-IR spectroscopy. Nonisothermal DSC studies showed CBTMS has an activation energy similar to that of the phenylethynyl-terminated compound. The thermoset and ceramic derived from the acetylenic monomer exhibited extremely thermo-oxidatively stable properties studied using thermogravimetric analysis (TGA). The thermoset showed a weight gain in air at elevated temperature and char yield of 98.8% at 1000 °C in air, and the ceramic residue had almost no weight loss up to 1000 °C in air. We demonstrated that trimethylsilylethynyl could be used as a crosslinking group for thermosetting polymers.

Full text of DOI:10.1039/c6ra19410a

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Synthesis and thermal properties of an acetylenic
monomer containing boron and silicon†
Cite this: RSC Adv., 2016, 6, 88403
Shengli Cheng,a Lishuai Zong,a Kuanyu Yuan,a Jianhua Han,a Xigao Jianb
b
*
and Jinyan Wang
To improve the thermo-oxidative stability of acetylenic aromatic compounds, 1,2-bis(4-
trimethylsilylethynylphenyl)-carborane (CBTMS) was designed, synthesized and characterized by FT-IR,
1H-NMR, 13C-NMR and mass spectrometry. The analysis of the DSC results showed that the acetylenic
monomer had a melting point at 195.5 ꢀC. The cross-linking process of CBTMS included a Diels–Alder
cycloaddition reaction confirmed by FT-IR spectroscopy. Nonisothermal DSC studies showed CBTMS
has an activation energy similar to that of the phenylethynyl-terminated compound. The thermoset and
ceramic derived from the acetylenic monomer exhibited extremely thermo-oxidatively stable properties
studied using thermogravimetric analysis (TGA). The thermoset showed a weight gain in air at elevated
temperature and char yield of 98.8% at 1000 ꢀC in air, and the ceramic residue had almost no weight
loss up to 1000 ꢀC in air. We demonstrated that trimethylsilylethynyl could be used as a crosslinking
group for thermosetting polymers.
Received 2nd August 2016
Accepted 27th August 2016
DOI: 10.1039/c6ra19410a
www.rsc.org/advances
pyrolyzed and exposed to an oxidative environment at elevated
temperatures.15 Polymers containing the carborane unit within
Introduction
the backbone exhibit appreciable enhancement in their oxida-
tive stability at elevated temperatures.16–23 Upon exposure to air
at elevated temperatures, boron is oxidized to B2O3 that is re-
ported to ll and seal micro-cracks that develop from thermal
cycling at elevated temperatures.15 Compounds with trime-
thylsilylethynyl groups have better solubility compared with
phenylethynyl groups. In addition, trimethylsilyl groups have
excellent thermal stability.24,25 Reactive end groups such as
ethynyl, phenylethynyl have been used as thermally reactive end
groups for polymer cross-linking.4,26–29 However, to the best of
our knowledge, there has no report about using trimethylsily-
lethynyl as crosslinking groups for thermosetting polymers in
previous literatures. Here we report our studies on acetylenic
monomer containing carborane and trimethylsilylethynyl. This
monomer has been well characterized, and its curing behaviors
and thermal and thermo-oxidative stability were studied.
High-temperature resistant polymers have been developed with
the purpose of being used as high-performance matrices in the
electronics and aerospace industries.1,2 Resins with terminal
acetylenic groups have received a great deal of attention in the
past four decades for the fact that these resins cure via an
addition reaction a consequence of which is that no volatile
byproducts are evolved during the processing stage.3 Besides
maintaining easy processing, they afford cured products that
have high thermal stability and good mechanical properties.
Hence, much effort has been focused on using these resins as
matrices for high-performance, ber-reinforced composites.4–8
In addition, their high thermal stability also qualies them as
excellent candidates for carbon matrices in carbon/carbon
composites.7,9
However, acetylenic aromatic monomers have the same
disadvantages as carbon based materials, which are susceptible
to decomposition at elevated temperatures in oxidative envi-
ronments.10–14 Incorporating inorganic elements, such as
silicon and boron, into a polymeric material improves the
thermo-oxidative stability relative to carbon.5,6 Protective inor-
ganic oxides are formed acting as a barrier to protect the inte-
rior from oxidation and degradation when these materials are
Experimental
Materials
B10H12(CH3CN)2 was synthesized according to literature proce-
dures.30 Decaborane was purchased from Zhengzhou Sigma
Chemical Co., Ltd. (China). Trimethylsilylacetylene, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), copper(I) iodide (CuI),
palladium(II)bis(triphenylphosphine) dichloride (PdCl2(Ph3)2),
1-bromo-4-iodobenzene were purchased from J&K Chemical Co
and used without further purication. Acetonitrile, toluene and
triethylamine were purchased from Tianjin Fuyu Fine Chemical
aPolymer Science & Materials, Chemical Engineering College, Dalian University of
Technology, Dalian, 116024, China
bState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian,
116024, China. E-mail: wangjinyan@dlut.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra19410a
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Co., Ltd. (China). Acetonitrile was distilled from phosphorus 3.90%. Anal. calcd for C14H18B10Br2: C, 37.02%; H, 3.99%. FT-IR
pentoxide prior to use. Triethylamine was distilled from CaH2 (KBr, cmꢁ1): 3068.7 (Ar–H), 2592.7 (B–H), 2569.1 (B–H), 2554.4
prior to use. Toluene was distilled from sodium and benzo- (B–H), 1586.7 (C]C), 1490.2 (C]C), 1397.1, 1064.9, 1013.6,
phenone prior to use.
829.7. 1H NMR (500 MHz, CDCl3, Me4Si): d 7.32–7.30 (4H, d, Ph),
7.29–7.27 (4H, d, Ph), 3.5–1.5 (10H, br s, B–H). 13C NMR (500
MHz, CDCl3, Me4Si): d 132.05, 131.73, 129.56, 125.38, 83.10.
HRMS (EI-TOF) m/z: M+ calculated for C14H18B10Br2 454.0758;
found 454.0792.
Measurements
All NMR spectra were measured on an Bruker AVANCE III 500
spectrometer (500 MHz). High Performance Liquid Chroma-
tography (HPLC) analyses were performed on a Alliance2695-
2696 instrument. Elemental analyses were performed on an
elemental analysis Vario EL series. The FT-IR spectra were ob-
tained using a Thermo Nicolet Nexus 470 Fourier transform
infrared (FT-IR) spectrometer. The wide-angle X-ray diffraction
(WAXD) measurements were undertaken on a D/Max 2400.
Thermogravimetric analyses (TGA) of the polymers were per-
formed on a Mettler TGA/SDTA851 thermogravimetric analysis
instrument in a nitrogen atmosphere at a heating rate of 20 ꢀC
minꢁ1 from 30 to 800 ꢀC. The differential scanning calorimetric
(DSC) was measured on Mettler DSC822 DSC under a nitrogꢁe1n
Synthesis of compound 3 (CBTMS)
A 250 mL pear-shaped ask equipped with a magnetic stirrer
and a reux condensing tube was charged with 0.336 g (0.48
mmol) of PdCl2(Ph3)2, 0.304 g (1.60 mmol) of CuI and 10.9 g (24
mmol) of compound 2. And then the ask was sealed with
a septum, evacuated and relled with nitrogen three times. 11.3
mL (80 mmol) of trimethylsilylacetylene, 24 mL (173 mmol) of
trimethylamine and 80 mL of toluene were added to the ask
under the nitrogen purge. Aer addition, the reaction mixture
ꢀ
was stirred for 24 h at 80 C. Then, the reaction was quenched
ow (50 mL minꢁ1) aꢁt 1various heating rate of ꢁ51 ꢀC min
,
with 100 mL of 10% HCl aqueous solution, and the mixture was
extracted with diethyl ether. The organic layer was washed with
brine and dried over anhydrous Na2SO4, followed by vacuum
evaporation. The yellow crude product was nally passed
through silica gel column chromatography using petroleum
ether as eluent to give the pure compound as a white powder
(yield: 92.3%). Purity: 99.04% (HPLC). Found: C, 58.79%; H,
7.41%. Anal. calcd for C24H36B10Si2: C, 58.97%; H, 7.42%. FT-IR
(KBr, cmꢁ1): 2963.5 (C–H), 2592.4 (B–H), 2565.0 (B–H), 2157.9
(C^C), 1602.9 (C]C), 1502.0 (C]C), 1405.7, 1249.9 (Si–C),
863.8, 842.7. 1H NMR (500 MHz, CDCl3, Me4Si): d 7.34–7.33 (4H,
d, Ph), 7.22–7.21 (4H, d, Ph), 0.22 (9H, S, Si–CH3). 13C NMR (500
MHz, CDCl3, Me4Si): d 131.90, 130.55, 125.54, 103.44, 97.71,
84.91, 0.00. MS (TOF-LD): m/z 489.2 Mꢁ.
10 ꢀC minꢁ1, 15 ꢀC min , 20 ꢀC minꢁ1, 30 ꢀC min from 25 ꢀC
to 450 ꢀC.
Synthesis of compound 1
To 500 mL ask, equipped with a magnetic stirrer and a rubber
septum, was charged 1.26 g (1.8 mmol) PdCl2(PPh3)2, 1.14 g
(5.85 mmol) CuI, 50.9 g (0.18 mol) 4-iododiphenyl ether. The
ask was purged with dry nitrogen. DBU (72 mL), thrime-
thylsilyethynylene (13.77 mL), distilled water (1.30 mL), aceto-
nitrile (250 mL) was then added by syringe. The reaction ask
was covered in aluminum foil and was stirred at 50 ꢀC for 48 h.
Aer cooling, the reaction mixture was poured into 500 mL
distilled water. The precipitate was ltered and washed with
distilled water. The pure product was obtained by recrystalli-
zation from chloroform in a yield of 85% (25.7 g). Purity: 99.13%
Thermal cure of compound 3
ꢀ
CBTMS-280 was prepared by curing of CBTMS in air at 280 ꢀC/2
h. CBTMS-350 was prepared by curing of CBTMS in air at 280
(HPLC). Mp: 185 C (from chloroform). Found: C, 49.88%; H,
2.22%. Anal. calcd for C14H8Br2: C, 50.04%; H, 2.40%. FT-IR
(KBr, cmꢁ1): 1905.5, 1488.2 (C]C), 1070.9, 1002.7, 822.7,
509.1 (C–Br). 1H NMR (500 MHz, CDCl3, Me4Si): d 7.49–7.48 (4H,
d, Ph), 7.39–7.37 (4H, d, Ph). 13C NMR (500 MHz, CDCl3, Me4Si):
d 132.93, 131.60, 122.70, 121.78, 89.29.
ꢀ
ꢀC/2 h and 350 C/2 ꢀh. CBTMS-400 was prepared by curing of
CBTMS in air at 280 C/2 h, 350 ꢀC/2 h and 400 ꢀC/2 h.
Preparation of ceramic
Ceramic residue (CBTMS-1000) was obtained by heating
CBTMS-400 to 1000 ꢀC in air at a heating rate of 5 ꢀC minꢁ1
(yield: 95%).
Synthesis of compound 2
20.16 g (60 mmol) of compound 1, 14.56 g (72 mmol) of
B10H12(CH3CN)2 and 400 mL toluene were added to a 1000 mL
three-necked round bottom ask equipped with a magnetic
stirrer, a nitrogen inlet and a reux condensing tube. The
reaction mixture was magnetically stirred under nitrogen for 2 h
Results and discussion
Preparation and characterization of monomers
at 100 ꢀC and then stirred for 12 h at 115 ꢀC. When the reaction The synthesis routes for the desired compound 3 (CBTMS) are
was completed, the reaction solution was cooled to room illustrated in Scheme 1. Compound 1 was synthesized using
temperature and 50 mL methanol was added to resolve Sonogashira coupling reaction according to a reported litera-
unreacted B10H12(CH3CN)2. The toluene was evaporated to ture.31 Compound 2 was synthesized by combining compound 1
afford crude product and the crude product was puried by with B10H12(CH3CN)2 and reuxing in toluene. Compound 3
silica gel column chromatography using petroleum ether as was synthesized through a similar synthetic procedure as
eluent to obtain the pure compound 2 as a white powder (13.1 g synthesis of compound 1, except for using trimethylamine
yield: 48%). Purity: 99.90% (HPLC). Found: C, 37.17%; H, rather than DBU as base. Chemical structure of compound 3
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Scheme 1 Synthetic routes of acetylenic monomer CBTMS.
was conrmed by FT-IR, 1H-NMR, 13C-NMR and mass
spectrometry.
The FT-IR Spectra of CBTMS is showed in Fig. 3. It is obvious
that CBTMS exhibits an absorption peak at 2592 cmꢁ1, which is
ascribed to the stretching vibration of B–H on carborane cage
according to a literature.32 The absorptions observed at 2153
cmꢁ1 and 1249 cmꢁ1 were attributed to the stretching of C^C
and Si–CH3 groups respectively.20,33
Fig. 1 shows the 1H-NMR and 13C-NMR results of CBTMS in
CDCl3. All of the protons and carbons in CBTMS were detected
as expected, conrming that acetylenic monomer was synthe-
sized successfully. The 1H-NMR showed a singlet at 0.2 ppm
corresponding to the hydrogen atoms of CH3–Si. The broad
peak at d 1.5–3.5 was assigned to the protons of B–H in o-car-
borane cage.1
Fig. 1 1H-NMR and 13C-NMR spectra of CBTMS in CDCl3.
the triple bond disappeared aer curing at 400 ꢀC for 2 h. So the
curing processes were determined as 280 ꢀC for 2 h, at 350 ꢀC for
ꢀ
2 h and at 400 C for 2 h. As shown in Scheme 2, Kuroki et al.
reported from 13C-NMR and the semiempirical MO method
(MOPAC93/PM3) that the naphthalene ring forms by a Diels–
Alder reaction between the Ph–C^C group and the C^C group
in [–Si(Ph)H–C^C–C6H4–C^C–]n because the hydrogen trans-
fer reaction occurs easily.36 The activation energy of the
Curing behavior of CBTMS
The curing behavior of the CBTMS was investigated by DSC
analysis, and the result is shown in Fig. 2. In the DSC thermo-
gram of CBTMS, one endothermic peak centered at 195.5 ꢀC
and one strong exothermic peak centered at 385.4 ꢀC were
observed. The endothermic peak corresponds to the melting
point of CBTMS, while the exothermic peak that starts at 303.0
ꢀC and ends at 416.8 ꢀC with an enthalpy of 197.7 J gꢁ1 is
attributed to thermal cross-linking reactions of the reactive
C^C linkages of CBTMS. The results indicate that trime-
thylsilylethynyl could be cured without any incorporation of
cross-linking agents. Due to the steric hindrance of terminal
trimethylsilyl groups, the activity of the ethynyl groups in
CBTMS is lower than acetylene-terminated compounds re-
ported in the literature, as indicated by a broader exothermic
peak at higher temperature and a low exothermic energy.34,35
The chemical structures of CBTMS before and aer thermal
curing at different temperature were determined by FT-IR. The
results are shown in Fig. 3. The crosslinking reaction of the
trimethylsilylethynyl end group was determined by the disap-
pearance of acetylene group absorption band at 2153 cmꢁ1. The
absorption band of the acetylene group could still be observed
ꢁ1
ꢀ
aer curing at 350 C for 2 h, whereas the absorption band of Fig. 2 DSC curve for CBTMS at heating rate of 10 ꢀC min
.
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reaction. Accordingly, we concluded that the intermolecular
cross-linking reactions of CBTMS due to the Diels–Alder cyclo-
addition reaction between the Ph–C^C group and the C^C
group were the mode of the cross linking reaction and they were
self-crosslinking reactions. Scheme 3 shows the thermosetting
mechanism of CBTMS. These results suggested that trime-
thylsilylethynyl could be used as crosslinking groups for ther-
mosetting polymers. The absorptions at 2593 cmꢁ1 due to
stretching of B–H remained unchanged during the curing,
which suggested the invariance of carborane cages.
Non-isothermal cure kinetics of CBTMS
In order to better understand the thermal curing reaction and
cure kinetics of CBTMS, non-isothermal DSC tests at different
heating rates were conducted. The experimental results are
shown in Table 1.
Fig. 3 FT-IR spectra of CBTMS before and after curing at different
temperature: (a) uncured, (b) curing at 280 ꢀC for 2 h, (c) curing at 280
DSC curves of CBTMS at different heating rates of 5, 10, 15,
ꢀC for 2 h and at 350 ꢀC for 2 h, (d) curing at 280 ꢀC for 2 h, at 350 ꢀC 20, 30 ꢀC minꢁ1 respectively were shown in Fig. 4. It can be seen
for 2 h and at 400 ꢀC for 2 h.
that the curing peaks become sharper and the cure character-
istic temperatures (Ti, Tp and Tt) shi to higher values with
increasing heating rates. This can be attributed to the enhanced
thermal effect and larger temperature difference at higher
heating rates.32 As a result, the endothermic peak shis to
a higher temperature.
The characteristic temperatures (Ti, Tp and Tt) of CBTMS
were plotted as the function of heating rates (shown in Fig. 5.).
It is obvious that all the characteristic temperatures increase
linearly with increasing heating rates. The characteristic
temperatures at a heating rate of 0 ꢀC minꢁ1, corresponding to
the pre-curing temperatu re (Ti0), constant-curing temperature
(Tp0) and post-curing temperature (Tt0) respectively, were ob-
tained by extrapolation of the plots of characteristic tempera-
tures as a function of heating rates (b). According to this
Scheme 2 Thermosetting mechanism of poly[phenylsilylene]ethyny-
method, the characteristic temperatures at a heating rate of 0 ꢀC
lene-1,3-phenyleneethynylene] (MSP) based on solid-state NMR
minꢁ1 were obtained and they were 281.4 ꢀC (Ti0), 367.2 ꢀC (Tp0
)
results: hydrosilylation reaction between Si–H and C^C; Diels–Alder
reaction between Ph–C^C and C^C.36
and 400 ꢀC (Tp0) respectively. These parameters are very
important for optimizing the thermal curing process of CBTMS.
The analysis of curing kinetic for CBTMS was performed with
Kissinger's and Ozawa's methods.
hydrogen transfer reaction calculated by Kuroki et al. was
smaller than the activation energy of the Diels–Alder reaction.
However, CBTMS has no Si–H group for hydrogen transfer
reaction. Therefore, we had reason to believe that Diels–Alder
cycloaddition reaction is one possible mode of the cross linking
reaction of CBTMS. In addition, Keller et al. reported the ther-
mosetting mechanism study of poly(carborane-siloxane-
arylacetylene) by solid-state NMR spectroscopy, which has
similar crosslinking groups as CBTMS.37 According to Keller's
study, Diels–Alder cycloaddition reaction was the pathway for
the thermal crosslinking process of [–Si(CH3)2–O–Si(CH3)2–(m-
carborane)–Si(CH3)2–O–Si(CH3)2–C^C–Ph–C^C–].
FT-IR
spectra of CBTMS and cured CBTMS possess several charac-
teristic absorption bands (Fig. 3), the bands at 2963 cmꢁ1 and
1250 cmꢁ1 correspond to –CH3 and Si–CH3 respectively were all
still remaining aer crosslinking, while the acetylene group
absorption band at 2153 cmꢁ1 became invisible aer curing
process further conrming the occurrence of the Diels–Alder
Scheme
3 Thermosetting mechanism of CBTMS by Diels–Alder
reaction between the Ph–C^C group and C^C group.
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ꢀ
ꢃ
Table 1 DSC data of CBTMS at different heating ratesa
ꢁ .
ꢂ
2
d ln b Tp
Ea
R
ꢄ ꢅ
ꢆ
Heating
¼ ꢁ
(1)
d 1 Tp
rate
Characteristic temperatures
where b is the heating rate, Tp is the exothermic peak temper-
ature, Ea is the apparent activation energy and R is the gas
constant. Plot of ln(b/Tp2) against 1/Tp was shown in Fig. 6. The
Pearson's linear correlation coefficient was calculated and given
near the line. The activation energy Ea of CBTMS was calculated
to be 146.73 kJ molꢁ1 according to Kissinger's method, which
was similar to that of cure reaction of phenylethynyl-terminated
compound DPADPE (Table 2).35 The higher activation energy of
CBTMS compared with DADPE means that its cure reaction
takes place more difficultly, implying that the steric factors from
trimethylsilylethynyl moieties linked to the acetylenic groups
depress the reactivity of triple bonds.
Sample
CBTMS
b (ꢀC minꢁ1
)
Ti (ꢀC)
Tp (ꢀC)
Tt (ꢀC)
5
285.6
303.0
313.3
323.1
334.7
370.3
385.4
394.6
404.1
412.0
406.1
416.8
430.0
440.5
449.9
10
15
20
30
a
Ti: initial curing tempetature, Tp: peak temperature, Tt: terminal
temperature, b: heating rate.
Ozawa's method assumes that the degree of conversion at
peak temperatures for different heating rates is constant.
According to Ozawa's equation:
Ea
lnb ¼ ꢁ1:052
þ C
(2)
RTp
where C is constant, plot of ln b against 1/Tp is prepared and
shown as a straight line in Fig. 7. The activation energy of the
curing reaction of CBTMS was calculated to be 150.05 kJ molꢁ1
.
Very close Ea values were obtained through Kissinger's and
Ozawa's methods, which veried the nonisothermal DSC
studies of the curing kinetics of CBTMS.
Thermal stability of cured CBTMS
The thermal stability of cured CBTMS was characterized by TGA
Fig. 4 DSC curves of CBTMS at different heating rates.
ꢀ
under nitrogen and air atmosphere at a heating rate of 20 C
minꢁ1, and the thermograms were shown in Fig. 8. The weight
loss temperatures of 5% and 10% (T5 and T10) for cured CBTMS
ꢀ
and the char yield at 1000 C were summarized in Table 3. As
shown in Fig. 8, thermoset exhibited excellent heat resistant
property in nitrogen. When heating rate was 20 ꢀC minꢁ1, there
ꢀ
was no weight loss up to 510 C. Then the weight loss process
Fig. 5 Linear relationship between the heating rates and characteristic
temperatures of CBTMS.
According to Kissinger equation (eqn (1)), the activation
2
energy E can be obtained from the slope of the plots of ln(b/Tp
versus 1/Tp.
)
Fig. 6 Plots of ln(b/Tp2) as a function of 1/Tp for CBTMS to calculate Ea
according to Kissinger's method.
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ꢀ
Table 2 Apparent activation energy Ea for the cure reaction of various
above 573 C under air atmosphere, indicating that decompo-
sition reaction was predominant at elevated temperature. The
char obtained from CBTMS-400 showed weight retention of
98.8% at 1000 ꢀC in air. However, most acetylene-substituted
aromatic thermosets are usually observed to undergo cata-
strophic weight losses in the 500–600 ꢀC temperature range
upon exposure to air. The outstanding thermal and oxidative
stability are attributed to the incorporation of both boron and
silicon units into the thermoset.
compounds
Structure
Ea (kJ molꢁ1
152.0
)
108.6
XRD studies
146.7
CBTMS-400 was hard solid with amber color. The WAXD pattern
of CBTMS-400 was performed in the region of 2q ¼ 10–80ꢀ at
room temperature. As shown in Fig. 9, WAXD pattern for
CBTMS-400 consists of a broad peak, which reveals its non-
crystalline character.
Preparation and characterization of ceramic
Ceramic residue (CBTMS-1000) was obtained by heatiꢁn1g
ꢀ
ꢀ
CBTMS-400 to 1000 C in air at a heating rate of 5 C min
.
Aer pyrolysis, the thermo-oxidative stability of CBTMS-1000 in
air and nitrogen were determined by TGA (Fig. S4†). We found
that the ceramic possessed excellent oxidative stability. When
ꢀ
heated in air to 1000 C, there was only 0.33% weight loss was
observed. In nitrogen, however, CBTMS-1000 showed a weight
loss from 540 ꢀC and char yield was 96.2%.
To further understand the chemical structure of the ceramic,
FT-IR (Fig. S5†) and XRD (Fig. 9) were employed to analyze the
chemical structure of the ceramic. According to previous
Fig. 7 Plots of ln b as a function of 1/Tp for CBTMS to calculate Ea
according to Ozawa's method.
Table 3 TGA data of CBTMS-400a
Td5
(ꢀC)
Td10
(ꢀC)
Char yield
CBTMS-400
at 1000 ꢀ
C
In N2
In air
669.5
>1000
>1000
>1000
92.0
98.8
a
Td5: temperature for 5% weight loss; Td10: temperature for 10% weight
loss.
Fig. 8 TGA curves of CBTMS-400 under nitrogen and air.
ꢀ
was very slow with resulting char yields of 92.0% at 1000 C in
nitrogen. In contrast to the TGA curve under nitrogen atmo-
sphere, there was a weight gain from 400 ꢀC and peak at 573 ꢀC
in the TGA curve under air atmosphere, which was attributed to
oxidation of boron to B2O3 and of silicon to SiO2 upon exposure
to oxidizing atmospheres at elevated temperatures.38 The
CBTMS-400 began to lose weight when the temperature was
Fig. 9 Wide angle X-ray diffraction patterns for (a) CBTMS-400 and (b)
CBTMS-1000.
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studies, a carbon-based polymer with protective layer composed 98.8% at 1000 ꢀC, which was attributed to the incorporation of
of boron and silicon could provide oxidation protection to 1000 inorganic elements boron and silicon. In summary, trime-
ꢀC.39 The outermost carbon burns away under high-temperature thylsilylethynyl could be used as crosslinking groups for ther-
oxidizing environments, while silicon and boron formed SiO2 mosetting polymers and thermosets derived from it exhibit
and B2O3, respectively. Liquid B2O3 melts above 450 ꢀC40 and excellent thermal stability. We believe CBTMS has potential
can ow into cracks in the SiO2 overlayer, sealing them and utility of as matrix materials for advanced composites and
hindering oxygen access to the active oxidation sites on carbon. precursors to shaped ceramics due to the oxidative stability,
Therefore, combination silicon and boron oxide layers provide high ceramic yield.
oxidation protection to 1000 ꢀC.39 Here we believe that the
structures of B–H in carborane and Si–CH3 in silane units of
CBTMS-400 are completely oxidized into boron oxide and
Acknowledgements
silicon oxide in the surface of the material during the ceramic
We thank the National High Technology Research and Devel-
process during the ceramic process, respectively. While boron
opment Program (**863** Program) of China (No.
oxide was very hygroscopic,41 the presence of moisture in the
2015AA033802) and the National Science Foundation of China
residue resulted in the formation of B–OH bonds. So two small,
(21074017 and 51273029).
sharp peaks at 2q ¼ 15ꢀ and 28ꢀ were observed in XRD pattern,
which was a typical feature of B(OH)3.42,43 In addition, absorp-
tions at 1463 cmꢁ1 and 1195 cmꢁ1 in FT-IR spectrum can be
attributed to B–O stretching and B–OH deformation vibrations,
References
respectively.43 Additionally, peak at 3240 cmꢁ1 assigned to –OH
stretching mode was recorded.44 Huang45 has reported several
carbon materials prepared by oxidative pyrolysis of carborane-
incorporated poly(dimethylsilylene-ethynylenephenyleneethy-
nylene) (CB-PSEPE), a similar polymer precursor as CBTMS,
were mainly composed of carbon. According to Huang's reports,
ceramic residues prepared by oxidative pyrolysis of CB-PSEPE at
1000 ꢀC for 1 h under static air were carbon materials with
silicon and boron elements conrmed by elemental analyses.45
In this work, we are inclined to believe that the bulk composi-
tion of CBTMS-1000 is carbon material with silicon and boron
elements too. In the XRD pattern of ceramic residues, two broad
diffraction reections located at 2q ¼ 25.8 and 42.3ꢀ (corre-
sponding to (002) and (001) planes of graphite structure)42,46,47
were presented. The peak at 797 cmꢁ1 was assigned to Si–C
deformation vibrations. In addition, there was no sign of
absorptions at 1120–1020 cmꢁ1 (n Si–O) and 460–400 cmꢁ1 (d Si–
O–Si)48 in the FT-IR spectrum conrming that CBTMS-1000 was
not composed of silicon oxide. The ceramic we obtained showed
absence of SiC or B4C crystals based oꢀn the analysis of FT-IR and
1 X. Huang, Q. Zhang, Z. Meng, J. Gu, X. Jia and K. Xi, J. Polym.
Sci., Part A: Polym. Chem., 2015, 53, 973–980.
2 Q. Zhou, Z. Mao, L. Ni and J. Chen, J. Appl. Polym. Sci., 2007,
104, 2498–2503.
3 S. Bourbigot and S. Duquesne, J. Mater. Chem., 2007, 17,
2283–2300.
4 S. B. Sastri, T. M. Keller, K. M. Jones and J. P. Armistead,
Macromolecules, 1993, 26, 6171–6174.
5 H. Katzman, J. Mallon and W. Barry, Polyarylacetylene-Matrix
Composites for Solid Rocket Motor Components, DTIC
Document, 1995.
6 X. Zhang, Y. Huang, T. Wang and L. Liu, Composites, Part A,
2007, 38, 936–944.
7 R. J. Zaldivar, G. S. Rellick and J.-M. Yang, Processing Effects
on the Mechanical Behavior of Polyarylacetylene-Derived
Carbon–Carbon Composites, DTIC Document, 1992.
8 H. Fu, Y. Huang and L. Liu, Mater. Sci. Technol., 2004, 20,
1655–1660.
9 R. J. Zaldivar, G. S. Rellick and J. Yang, J. Mater. Res., 1993, 8,
501–511.
XRD spectra, which suggested 1000 C may be insufficient for 10 D. Bucca and T. M. Keller, J. Polym. Sci., Part A: Polym. Chem.,
the formation of these crystalline structures.49
1999, 37, 4356–4359.
11 D. Bucca and T. M. Keller, J. Polym. Sci., Part A: Polym. Chem.,
1997, 35, 1033–1038.
12 M. Westwood, J. Webster, R. Day, F. Hayes and R. Taylor, J.
Mater. Sci., 1996, 31, 1389–1397.
Conclusion
A novel carborane-containing acetylenic monomer was synthe-
sized and conrmed by 1H-NMR, 13C-NMR, FT-IR spectroscopy 13 L. Snell, A. Nelson and P. Molian, Carbon, 2001, 39, 991–999.
and mass spectrometry. Because of the high steric hindrance of 14 Q.-G. Fu, H.-J. Li, X.-H. Shi, K.-Z. Li and G.-D. Sun, Scr.
trimethylsilyl groups, the endothermic peak for CBTMS
appeared at a higher temperature than traditional ethynyl- 15 P. E. Pehrsson, L. J. Henderson and T. M. Keller, Surf.
terminated compounds. Thermosetting mechanism of CBTMS Interface Anal., 1996, 24, 145–151.
was Diels–Alder reaction between the Ph–C^C group and the 16 H. M. Colquhoun, D. F. Lewis, J. A. Daniels, P. L. Herbertson,
Mater., 2005, 52, 923–927.
C^C group conrmed by FT-IR spectroscopy. The thermal
curing kinetics studies show that cure reaction of CBTMS has an
J. H. MacBride, I. R. Stephenson and K. Wade, Polymer, 1997,
38, 2447–2453.
activation energy similar to that of phenylethynyl-terminated 17 T. M. Keller and D. Y. Son, Synthesis and Characterization of
compounds. The excellent thermo-stability of thermoset Advanced Materials, 1998, vol. 681, pp. 248–258.
derived from CBTMS was conrmed by TGA under air with 5% 18 X. Zhang, L. Kong, L. Dai, X. Zhang, Q. Wang, Y. Tan and
weight loss temperature above 1000 ꢀC and a high char yield of
Z. Zhang, Polymer, 2011, 52, 4777–4784.
This journal is © The Royal Society of Chemistry 2016
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RSC Advances
Paper
19 A. Delman, J. Kelly and B. Simms, J. Polym. Sci., Part A-1: 35 F. Wang, J. Xu, J. Zhang, F. Huang, Y. Shen and L. Du, Polym.
Polym. Chem., 1970, 8, 111–123. Int., 2006, 55, 1063–1068.
20 M. Ichitani, K. Yonezawa, K. Okada and T. Sugimoto, Polym. 36 S. Kuroki, K. Okita, T. Kakigano, J. Ishikawa and M. Itoh,
J., 1999, 31, 908–912. Macromolecules, 1998, 31, 2804–2808.
21 X. Huang, Q. Zhang, Z. Meng, J. Gu, X. Jia and K. Xi, J. Polym. 37 M. K. Kolel-Veetil, D. D. Dominguez, C. A. Klug, K. P. Fears,
Sci., Part A: Polym. Chem., 2015, 53, 973–980.
22 T. Xing and K. Zhang, Polym. Int., 2015, 64, 1715–1721.
S. B. Qadri, D. Fragiadakis and T. M. Keller, J. Polym. Sci.,
Part A: Polym. Chem., 2013, 51, 2638–2650.
23 J. Yue, Y. Li, H. Li, Y. Zhao, C. Zhao and X. Wang, RSC Adv., 38 R. A. Sundar and T. M. Keller, J. Polym. Sci., Part A: Polym.
2015, 5, 98010–98019. Chem., 1997, 35, 2387–2394.
24 J. D. Lichtenhan, N. Q. Vu, J. A. Carter, J. W. Gilman and 39 P. E. Pehrsson, L. J. Henderson and T. M. Keller, Surf.
F. J. Feher, Macromolecules, 1993, 26, 2141–2142. Interface Anal., 1996, 24, 145–151.
25 H.-S. Kim, Y.-H. Kim, S.-K. Ahn and S.-K. Kwon, 40 D. W. McKee, C. L. Spiro and E. J. Lamby, Carbon, 1984, 22,
Macromolecules, 2003, 36, 2327–2332. 507–511.
26 S. A. Swanson, W. W. Fleming and D. C. Hofer, 41 D. Buc, I. Bello, M. A. Caplovicova, M. Mikula, J. Kovac,
Macromolecules, 1992, 25, 582–588.
27 W. Huang and S. Wunder, J. Appl. Polym. Sci., 1996, 59, 511–
520.
28 M. E. Wright, D. A. Schorzman and L. E. Pence,
Macromolecules, 2000, 33, 8611–8617.
29 X. Fang, D. F. Rogers, D. A. Scola and M. Stevens, J. Polym.
Sci., Part A: Polym. Chem., 1998, 36, 461–470.
30 R. Schaeffer, J. Am. Chem. Soc., 1957, 79, 1006–1007.
I. Hotovy, Y. M. Chong and G. G. Siu, Thin Solid Films,
2007, 515, 8723–8727.
42 C. F. Wang, F. R. Huang, Y. Jiang, Y. Zhou and L. Du, J. Am.
Ceram. Soc., 2012, 95, 71–74.
43 G. D. Soraru, N. Dallabona, C. Gervais and F. Babonneau,
Chem. Mater., 1999, 11, 910–919.
44 J. L. Parsons and M. E. Milberg, J. Am. Ceram. Soc., 1960, 43,
326–330.
31 M. J. Mio, L. C. Kopel, J. B. Braun, T. L. Gadzikwa, K. L. Hull, 45 C. F. Wang, F. R. Huang, Y. Jiang, J. F. Li, Y. Zhou and L. Du,
R. G. Brisbois, C. J. Markworth and P. A. Grieco, Org. Lett.,
2002, 4, 3199–3202.
32 J. Yue, Y. Li, H. Li, Y. Zhao, C. Zhao and X. Wang, RSC Adv.,
2015, 5, 98010–98019.
33 C. Wang, Y. Zhou, F. Huang and L. Du, React. Funct. Polym.,
2011, 71, 899–904.
34 W.-C. Tseng, Y. Chen and G.-W. Chang, Polym. Degrad. Stab.,
2009, 94, 2149–2156.
Ceram. Int., 2012, 38, 3081–3088.
46 R. J. P. Corriu, P. Gerbier, C. Guearin and B. Henner, J. Mater.
Chem., 2000, 10, 2173–2182.
47 X. L. Wei, P. J. Carroll and L. G. Sneddon, Chem. Mater., 2006,
18, 1113–1123.
48 L. Esquivias, M. C. Matos, L. M. Ilharco and R. M. Almeida, J.
Non-Cryst. Solids, 1992, 147, 232–237.
49 C. Wang, F. Huang, Y. Jiang, Y. Zhou and L. Du, J. Am. Ceram.
Soc., 2012, 95, 71–74.
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