Optimized preparation of porous polyvinylphenylsiloxane monoliths for thermal evolution to lightweight SiOC

This work presents the optimized process to prepare porous polyvinylphenylsiloxane monoliths and an investigation on pyrolysis to lightweight silicon oxycarbide (SiOC). Gelation and phase separation during reaction were manipulated to adjust texture structure and mechanical property. Ratio of vinyl to phenyl and concentration of gelation catalyst played key role in obtaining proper texture and mechanical property to resist ambient-pressure drying shrinkage. The typical polyvinylphenylsiloxane monolith had density of 0.282 g‧cm⁻³ and compression strength of 3.01 ± 0.52 MPa. It was pyrolyzed into porous SiOC with 87 % ceramic yield at 1000 °C in nitrogen atmosphere. Pyrolysis shrinkage and damage were confined due to low weight loss and high strength of polysiloxane monolith. 1000 °C-derived SiOC had density of 0.386 g‧cm⁻³, and room-temperature thermal conductivity of 0.108 W‧(m‧K)⁻¹. SEM, TEM, and XRD analyses showed morphology and amorphous structure of SiOC. Nitrogen adsorption-desorption and mercury porosimetry results revealed pore structure evolution during pyrolysis. ²⁹Si NMR, FTIR and TG-MS analyses investigated the molecule structure transformation of polyvinylphenylsiloxane to SiOC by elimination, exchange and recombination reactions under different pyrolysis temperatures.