Highly heat resistant and thermo-oxidatively stable borosilane alkynyl hybrid polymers

Abstract

A type of boron–silicon containing hybrid polymer with C≡C units (HBS) was prepared, and the effects of three different substituents on the properties of the polymers were studied. The polymers were synthesized with ethynylmagnesium bromide, dichlorosilane and boron fluoride etherate by using a Grignard reagent method. The structures of HBS were characterized by Fourier-transform infrared spectra, proton-NMR (¹H-NMR), ¹³C-NMR, ²⁹Si-NMR, and gel permeation chromatography. Thermal and oxidative stabilities were studied using differential scanning calorimetry and thermogravimetry analysis, and the crosslinking reaction mechanisms of the HBS are discussed. All the polymers exhibit excellent thermal and oxidation resistance, particularly, HBS-1 with Si–H bonds which was highly heat resistant and showed good thermo-oxidative stability. The temperatures of 5% weight loss (T_d5) were 624 °C and 607 °C in nitrogen and air, respectively, and the residues at 1000 °C were 86.8% and 77.5%, respectively. The thermal and oxidative stabilities of the polymers were attributed to the synergistic effect of boron and silicon elements and the cross linking during hydrosilylation and Diels–Alder reactions.

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Introduction

Inorganic–organic hybrid polymers, especially silicon containing polymers, have been studied for several years, because of their unique properties of being hard, and highly heat resistant when used as ceramics and being moldable and soluble when used as plastics.^1,2 The silicon-based polymers have many applications in different fields, such as ceramic precursors, coating materials, electronic materials, matrix composites and components in space vehicles.^3–6 Itoh et al.^7–10 have focused on researching silicon containing polymers for years. They synthesized poly(phenylsilylene)ethynylene-1,3-phenyleneethynylene (MSP), which was prepared using a dehydrogenative coupling polymerization reaction between phenylsilane and m-diethynylbenzene. The results showed that the temperature at which a 5% weight loss had occurred (T_d5) of the polymer was greater than 800 °C, and the residue at 1000 °C were 90% or 25.5% under inert or air atmospheres, respectively. Homrighausen and Keller¹¹ synthesized a linear silarylene–siloxane–diacetylene polymer which was prepared via polycondensation of 1,4-bis(dimethylaminodimethylsilyl)butadiyne with 1,4-bis(hydroxydimethylsilyl)benzene, and the results showed that the polymer exhibited long-term thermo-oxidative stability up to 350 °C in air. Although the silicon containing polymers have high heat resistance, the thermal oxidation performance needs to be improved.

In recent years, the incorporation of inorganic elements such as boron into organic polymers also has been researched. Yajima et al.¹² synthesized poly(borodiphenylsiloxane) using the reaction of boric acid and diphenyldichlorosilane or diphenylsilanediol and observed the process of its thermal decomposition. Sundar and Keller¹³ synthesized linear boron–silicon–diacetylene copolymers that had weight residues above 50% at 1000 °C in air, and the hybrid systems showed outstanding thermal oxidative properties compared to commercial organic systems. Deepa et al.¹⁴ synthesized a type of boron siloxane oligomer through the condensation of boric acid with phenyltrimethoxysilane and phenyltriethoxysilane, and the results showed that the oligomers gave ceramic residues in the range of 64–75% (at 900 °C in an inert atmosphere).

In our preliminary study, some novel linear silicon containing hybrid polymers were reported with C≡C units which showed excellent thermal and oxidative stabilities.^15–19 Phenyl acetylene terminated poly(carborane-silanec) (PACS) was synthesized using the coupling reaction of methyl dichlorosilane with 1,7-dilithio-m-carborane and lithium phenylacetylide, which exhibits extremely thermal and oxidative properties.²⁰ Nevertheless, the relative high cost of carborane limited the utilization and development of the polymers.

Based on these factors, further research on the thermal oxidation enhancement of silicon containing polymers is required. An effective approach for the performance enhancement is by introducing another inorganic element, and the hybrid of boron has become increasingly attractive for this. In this paper, three kinds of borosilane alkynyl hybrid polymers (HBS) with different substituent groups were synthesized. The characterization of the three polymers, thermoset mechanism and thermal oxidation properties are discussed.

Experimental

Materials

All the reactions were performed under inert conditions with dry nitrogen because the starting materials were sensitive to oxygen and moisture. Tetrahydrofuran (THF) was distilled from benzophenone/sodium before use. Ethynylmagnesium bromide (0.5 mol L⁻¹ in THF) was purchased from the Sinopharm Chemical Reagent Co. (Shanghai, China) and titrated before use. Dichloromethylsilane, dimethyldichlorosilane, diphenyldichlorosilane and boron fluoride etherate were obtained from the Aldrich Chemical Co. (Shanghai, China). All other chemicals were reagent grade and used as required.

Synthesis of HBS

A flame dried 500 mL round three-necked flask, connected to an argon source, was equipped with a stirrer. Magnesium metal ribbon (1.8 g, 0.075 mol), a small piece of iodine and THF (20 mL) were placed in the flask. A solution of ethyl bromide (7.63 g, 0.07 mol) in THF (50 mL) was introduced via a dropping funnel over a 30 min period, with stirring. After completing the addition, the reaction mixture was refluxed for 2 h to produce ethylmagnesium bromide. Then ethynylmagnesium bromide solution (130 mL, 0.065 mol) was added to the reaction system over 30 min, and the reaction was refluxed for 2 h to give a product which was white mixed organic magnesium reagents (1).

Polymer HBS-1. A solution of dichloromethylsilane (0.03 mol) in THF (10 mL) was introduced dropwise to the reaction system (1) over 30 min with stirring. White precipitates of the organic magnesium reagent immediately disappeared before the completion of the addition, and the solution became almost clear. The reaction mixture was refluxed for 3 h to obtain silicon-based acetylene polymers (2). Then boron fluoride etherate (0.02 mol) in THF (10 mL) was added. After this addition, the reaction mixture was further reacted for 5 h. Then an aqueous solution of hydrochloric acid was slowly added through the dropping funnel (over 30 min). The resulting oil phase was extracted with ethyl acetate, then separated using a separating funnel and washed with deionized water until the pH became neutral. Then the oil phase was dried by adding sodium sulfate. After evaporation of the solvent, a yellowish viscous product with yields of 73% was obtained.

Polymer HBS-2 and HBS-3. A solution of dimethyldichlorosilane (0.03 mol) (for the synthesis of HBS-2) or diphenyldichlorosilane (0.03 mol) (for the synthesis of HBS-3) in THF (10 mL) was added dropwise to the reaction system (1). Other steps were the same as the synthesis of HBS-1, and yellowish-brown viscous products with yields of 75–78% were obtained.

Measurements

Fourier transform infrared (FTIR) spectra were measured on a Nicolet 6700 spectrometer using potassium bromide (KBr) pellets for solid samples and films were deposited on KBr plates for liquid samples. Proton-nuclear magnetic resonance (¹H-NMR), ¹³C-NMR and ²⁹Si-NMR spectra were recorded on a Bruker Avance 500 spectrometer (at 500 MHz for ¹H-NMR, 125.77 MHz for ¹³C-NMR and 500 MHz for ²⁹Si-NMR), with tetramethylsilane as the internal reference. Molecular weights were determined using gel permeation chromatography (Waters 1515 Isocratic HPLC pump), using THF as eluent at a flow rate of 1.0 mL min⁻¹ and polystyrene as the standard. The viscosities were recorded on Brookfield Engineering CAP 2000+ instrument at 25 °C. Differential scanning calorimetry (DSC) measurements were performed on Netzsch DSC 200 F3 instrument at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere. Rotational rheometer measurements were performed using a Thermo Haake RheoStress 600 with shearing rates of 0.01 s⁻¹. Thermogravimetric analyses (TGA) were performed on a PerkinElmer Pyris Diamond from 30 °C to 1000 °C using heating rates of 10 °C min⁻¹ under a nitrogen atmosphere with flow rates of 20 mL min⁻¹. The morphologies of the curing polymers were observed using an Hitachi S-3400N scanning electron microscope (SEM).

Results and discussion

Synthesis and characterization of HBS

The polymers were generated via polycondensation reactions using a Grignard reagent method and the synthetic routes are summarized in Scheme 1. The structures of HBS were confirmed using FTIR, ¹H-NMR, ¹³C-NMR and ²⁹Si-NMR spectra and the relevant data are shown in Table 1, which proves that the polymers obtained are consistent with the designed structures.

Scheme 1 The synthesis routes of HBS.

Table 1 Structure, spectral data and basic properties of HBS

Sample	Mn (g mol^−1) and molecular weight distribution	Viscosity (mPa s^−1)	Spectral data
			FTIR (cm^−1)	^1H-NMR (ppm)	^13C-NMR (ppm)	^29Si-NMR (ppm)
	1786, 1.55	2560	2955 (–CH3), 2170 (Si–H, B–C≡C), 2035 (Si–C≡C), 1411 (B–C), 1252 (Si–C)	4.34 (Si–H), 0.45 (Si–CH3)	83.43 (C≡C), −2.61 (–CH3)	−62.6 ((CH3)SiH)
	1509, 1.25	2880	2954, 2861 (–CH3), 2155 (B–C≡C), 2038 (Si–C≡C), 1415 (B–C), 1256 (Si–C)	0.36–0.53 (Si–CH3)	85.82 (C≡C), −2.35 (–CH3)	−41.8 (Si(CH3)2)
	997, 1.12	3435	2956–3069 (Ph–H), 2154 (B–C≡C), 2040 (Si–C≡C), 1482–1663 (Ph), 1429 (B–C), 1262 (Si–C)	7.32–7.76 (Ph–H)	123–136 (Ph), 89.10 (C≡C)	−52.3 (Si(Ph)2)

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The FTIR spectra of HBS are shown in Fig. 4, curve (a), (c) and (e) shows spectra of HBS-1, HBS-2 and HBS-3, respectively. The characteristic stretching and deformation bands of all expected motifs are shown in Table 1. By comparing the three cures, a sharper and stronger absorption for HBS-1 than HBS-2 and HBS-3 at 2170 cm⁻¹ is observed, which is attributed to the overlap of Si–H group and B–C≡C group. Without the Si–H group, HBS-2 and HBS-3 have a weak absorption peak at 2150 cm⁻¹ of B–C≡C group. In addition, all three polymers have a strong absorption peak at 2040 cm⁻¹, which is a characteristic peak of the Si–C≡C group. However, all polymers show absorptions around 1410 cm⁻¹ (B–C) and 1250 cm⁻¹ (Si–C) region which reflect the presence of boron and silicon.

The NMR spectra of HBS-1 are depicted in Fig. 1. In the ¹H-NMR spectrum, HBS-1 shows a broad signal centred at 0.45 ppm which is attributed to the Si–CH₃ groups. A resonance assigned to the Si–H proton is observed at 4.34 ppm. The spectrum of ¹³C-NMR shows a broad signal at −2.61 ppm which is assigned to Si–CH₃ sites, and the C≡C bond is identified in 83.43 ppm region. The introduction of elemental Si is shown by signals at −62.6 ppm in the ²⁹Si-NMR spectrum. The NMR spectral data of HBS-2 and HBS-3 are summarized in Table 1.

Fig. 1 ¹H-NMR, ¹³C-NMR and ²⁹Si-NMR spectra of HBS-1.

Pale yellow to brown viscous liquid polymers were obtained from HBS-1, HBS-2 and HBS-3. The polymers were highly soluble in many general organic solvents such as THF, toluene, acetone and chloroform. The molar mass (number average molar mass M_n), molecular weight distribution (M_W/M_n) and viscosity of HBS are also shown in Table 1. The molecular weight of HBS-1 is the highest among the three polymers, which is attributed to the small units of Si–H in the backbone that are easier to rotate and move to form a long molecular chain, and then results in an increase of molar mass. Meanwhile, Si–Ph units are rigid groups and have a large space steric hindrance, and the flexibility of Si–CH₃ units is between those of Si–H units and Si–Ph units,²¹ leading to the change in viscosity and molar masses of HBS.

Curing behavior of HBS

The curing behaviors of the three polymers were examined by DSC and rheometry. From the DSC exothermic curves shown in Fig. 2, all the polymers (HBS-1, HBS-2, HBS-3) show a strong exotherm during 250–320 °C, which is a characteristic of the acetylene crosslinking reaction.⁶ This means that these exothermic transformations are related to a chemical process, i.e., a thermally induced cross-polymerization through the acetylene groups which will modify irreversibly the backbone of the polymers. In investigating the thermal polymerization in HBS-1, it was of utmost interest to consider the thermosetting mechanism.

Fig. 2 DSC exothermic curves of HBS (curve (a): HBS-1; curve (b): HBS-2; curve (c): HBS-3).

In these three polymers, the one exception is polymer HBS-1, which displays two exothermic maxima at 150 °C and 280 °C. Because of the high reactivity of the Si–H bonds, the crosslinking is likely to occur at a lower temperature. The first exotherm at approximately 150 °C could be caused by intermolecular crosslinking reactions between the Si–H and C≡C bonds. As a result, it has crosslinked to form network structures, which leads to stereo hindrance in the thermoset. So the second exotherm caused by acetylene polymerization is at a higher temperature at approximately 280 °C. HBS-2 and HBS-3 without Si–H bonds have only one exothermic peak each, which are caused by a Diels–Alder reaction between the C≡C bonds. In addition, the different substituent groups in the backbone also affect the thermosetting of the polymers. The steric hindrance of Si–CH₃ groups is smaller compared to the phenyl-substituted polymer, so the exothermic peak shifts to a higher temperature with an increase of Si–Ph units in the structure, and the polymer possessing methyl substituents on silicon visually appear to crosslinking at a lower temperature. Finally, the exothermic peak above 200 °C leads to an order of HBS-3 > HBS-1 > HBS-2, as shown in Fig. 2.

The rheological behavior upon heating was monitored by acquiring the viscosity at different temperatures, and the rheological curves of HBS are shown in Fig. 3. The three curves in Fig. 3 all have their respective inflection points, i.e., gel point. When the temperature reaches the gel point, the crosslinking reaction between the acetylene groups gets relatively intense and a network structure is gradually formed, which causes the graph of the viscosity of the polymer system to rise straight up. It is revealed that the three polymers, HBS-1, HBS-2, HBS-3 each have a gel point at 210 °C, 255 °C and 290 °C, respectively, which is between the initial temperature and the peak temperature corresponding to the DSC curves.

Fig. 3 Rheological curves of HBS (curve (a): HBS-1; curve (b): HBS-2; curve (c): HBS-3).

To elucidate the curing behavior, the FTIR spectra of polymers cured at 350 °C were measured. Fig. 4 shows the vibration absorption change before and after curing of the three polymers. By comparing curves (a) and (b), (c) and (d), (e) and (f), separately, it suggests that the characteristic vibration bands of Si–H (2170 cm⁻¹) and C≡C (2040 cm⁻¹, 2170 cm⁻¹) completely vanish after curing, indicating that the hydrosilylation and Diels–Alder reactions are complete. Curve (b) for HBS-1 has an absorption peak at 1637 cm⁻¹ which is a characteristic peak of C=C bond, demonstrating that the hydrosilylation reaction occurs, accompanied by the generation of a double bond. A strong band is present at 1250 cm⁻¹ (Si–C) together with a band of medium intensity at 2900 cm⁻¹ (aliphatic C–H stretching vibration) which indicate that Si–CH₃ for HBS-1 and HBS-2, and Si–Ph for HBS-3 are stable after curing.

Fig. 4 FTIR spectra of HBS ((a) HBS-1 before curing, (b) HBS-1 after curing; (c) HBS-2 before curing, (d) HBS-2 after curing; (e) HBS-3 before curing, (f) HBS-3 after curing).

As well as TGA, FTIR studies were carried out at a heating rate of 10 °C min⁻¹ in an air atmosphere from 30 °C to 420 °C. The typical stacked FTIR diagrams of HBS-1 are shown in Fig. 5. At a relatively low temperature (<100 °C), the absorption peaks change little. When the temperature increases (100–250 °C), the absorption peaks of Si–H and C≡C bonds gradually weaken, which indicates that the hydrosilylation reaction had occurred. As the temperature continues to rise (250–350 °C), these absorption peaks drastically weaken, which reflects that the Diels–Alder reaction had occurred. Eventually the absorption peaks disappear when the temperature is up to 400 °C which completely demonstrates the completion of the hydrosilylation and Diels–Alder reactions upon heating.

Fig. 5 Stacked FTIR diagrams of HBS-1.

Fig. 6 shows the SEM images of the three cured polymers HBS-1, HBS-2 and HBS-3. Glassy density states of the polymers were formed after curing. The surfaces of the three polymers were smooth and no obvious holes were observed, which means that there was little production of gaseous by-products or volatilization of lower molecular species. The addition reaction during curing process creates less pore formation and volatile species, which obviates the problems of extremely exothermicity and severe shrinkage.

Fig. 6 SEM images of (a) HBS-1, (b) HBS-2, (c) HBS-3.

The thermosetting mechanism of the polymers

From these results, it was concluded that the thermoset mechanism of the three polymers were different. The intermolecular crosslinking reactions of HBS-1 were because of the hydrosilylation reaction between Si–H and C≡C bonds, and the Diels–Alder reaction between C≡C proceeded at temperatures above 200 °C. The crosslinking reactions of HBS-2 and HBS-3 were attributed to the Diels–Alder reaction between C≡C bonds only (Scheme 2). The process of undergoing thermally induced cross-polymerization reactions could greatly influence the thermal behavior of the polymers.

Scheme 2 Possible thermosetting mechanisms of HBS (a) HBS-1; (b) HBS-2 and HBS-3.

Thermal and oxidative stabilities of HBS

The thermal and oxidative stabilities of HBS were investigated using TGA under a flow of nitrogen or air, as shown in Fig. 7 and 8, and the relevant data are summarized in Table 2. The TGA traces show that all three polymers exhibited excellent thermal and oxidation resistance both in nitrogen and air. The weight loss of the HBS occurred mainly between 460 °C and 650 °C, and the T_d5 values were above 480 °C and 460 °C in nitrogen and air, respectively. The char yields at 1000 °C were above 72% and 69% in nitrogen and air, respectively, which is much higher than for MSP¹⁰ (the residue at 1000 °C in air was 25.5%) and PMES (linear silicon-containing hybrid polymers with Si−C≡C units)¹⁹ (the residue at 1000 °C in air was 47%). Notably, the performance of HBS-1 with Si–H bonds is outstanding. The most obvious feature of HBS is that the residual ratios in the air have been greatly improved.

Fig. 7 TGA curves of the cured HBS in N₂ (curve (a): HBS-1; curve (b): HBS-2; curve (c): HBS-3).

Fig. 8 TGA curves of the cured HBS in air (curve (a): HBS-1; curve (b): HBS-2; curve (c): HBS-3).

Table 2 TGA Data of Cured HBS in different atmospheres

Sample	N2		Air
	Td5 (°C)	Residue at 1000 °C (%)	Td5 (°C)	Residue at 1000 °C (%)
HBS-1	624	86.8	607	77.5
HBS-2	485	72.3	467	69.2
HBS-3	595	82.6	543	71.8

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The exceptional thermal oxidative stabilities of HBS can be attributed to the barrier effect to oxygen exhibited by the boron–silicon alkynyl hybrid systems. First, the condensation reactions of Si–H and C≡C bonds allow the structures of the polymers to be converted into highly a three-dimensional network. The performance for HBS-1 is because of the suppression of the molecular mobility of polymer segments by the bulky crosslinked networks which were generated by a hydrosilylation reaction and a Diels–Alder reaction. For polymer HBS-3 with phenyl units, the thermal properties owning to the complex network structure by Diels–Alder reaction and the rigid cyclobenzene groups in the networks. Nevertheless, polymer HBS-2 with methyl units in the chain has a large number of free carbons which dominate the thermal properties and significantly decrease the amount residue formed. With an increase of Si–H units in the C≡C bond-containing structures, the thermal and oxidation resistance are stronger (HBS-1 > HBS-3 > HBS-2). Also, when the temperature is higher than 1000 °C, the silicon, and boron atoms in the polymers can form inorganic compounds such as silicon carbide, boron carbide, silicon dioxide and boron trioxide that are thermally stable and antioxidative.²² Based on the results given in the previous discussion, the introduction of silicon and boron elements in the structure as well as the incorporation of Si(H)–C≡C units into backbone can greatly improve the heat and oxidation resistance properties of the polymers.

Conclusion

A type of polymer (HBS) with three different substituent groups that contain boron, silicon elements and C≡C units was synthesized by condensation reactions using a Grignard reagent method in this study. The structures, curing mechanisms, thermal and oxidation resistances of the HBS were characterized. The results showed that the three polymers were highly heat resistant and had excellent oxidation stabilities. Remarkably, the performance of HBS-1 with Si–H units was outstanding compared to the other two polymers that were substituted with methyl and phenyl, which had T_d5 values above 600 °C and residues at 1000 °C above 75% in nitrogen and air. In addition, the detailed curing mechanisms were discussed. It was suggested that the Si(H)–C≡C unit would increase the crosslinking density and then assisting with the inorganic elements of silicon and boron could form the most thermally stable structure.

Acknowledgements

The research in this paper was supported by the National Natural Science Foundation of China (51073053), China Postdoctoral Science Foundation (2013M541486), The Shanghai Natural Science Foundation (15ZR1409800) and Shanghai Postdoctoral Scientific Program.

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