Abstract
Polycondensation of boric acid (BA) with mixtures of phenyltriethoxysilane (PTEOS) and vinyltriethoxysilane (VTEOS) and with mixtures of phenyltrimethoxysilane (PTMOS) and vinyltriethoxysilane (VTEOS) in diglyme at 83–87 °C using HCl as catalyst resulted in vinyl-functionalized borosiloxane oligomers. Effect of variation of PTEOS:VTEOS and PTMOS:VTEOS ratio (keeping BA:alkoxysilanes ratio constant) and the effect of variation of BA:alkoxysilane (BA:PTEOS:VTEOS and BA:PTMOS:VTEOS) ratio on the solubility, thermal stability and ceramic residue were studied. The oligomers obtained were characterized by FTIR, GPC, pyrolysis GC and TGA. For PTEOS + VTEOS system all the oligomers were soluble in the reaction medium, but after removal of alcohol (byproduct) and diglyme (solvent), the oligomers obtained in the solid form were insoluble in common organic solvents. Unlike the PTEOS-based system, for PTMOS-based system the oligomers synthesized from monomer feed ratios (BA:PTMOS:VTEOS) 1:1:1 and 1:1.67:0.33 even after the removal of ethanol and diglyme are soluble in tetrahydrofuran, dioxane and diglyme. These two soluble oligomers show bimodal molecular weight distribution with $$\overline{{\text{M}}}_{{_{{\text{W}}} }}$$ of 3,650 and $${\overline{\text{M}}}_{{\text{n}}}$$ of 1860 for 1:1:1 mol ratio, and $$\overline{{\text{M}}}_{{_{{\text{W}}} }}$$ of 2,540 and $${\overline{\text{M}}}_{{\text{n}}}$$ of 1700 for 1:1.67:0.33 mol ratio. 29Si-NMR spectra of the soluble oligomers show peaks at − 69 and − 78 ppm which are attributed to T2 and T3 structures respectively. Thermogravimetric analysis of vinyl-functionalized borosiloxane oligomers from PTEOS and PTMOS indicates that the ceramic residue of the oligomers in argon atmosphere at 900 °C varies from 68 to 89% depending on the monomer feed ratio of BA to organoalkoxysilane ratio and phenyltrialkoxysilane to VTEOS ratio. With the increase in concentration of VTEOS in the monomer feed the thermal stability as well as the ceramic residue of the oligomers increase and a reverse trend is observed for variation of BA concentration. Pyrolysis of the oligomers produce C2, C3 hydrocarbons and benzene as the main pyrolysis products. Ceramic conversion of a typical oligomer was carried out at 900 °C, 1500 °C and 1650 °C in argon atmosphere. The ceramics obtained at 900 °C and 1500 °C are amorphous SiBOC which transform to β-SiC at 1650 °C.
Published Version
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