Controlled Mesoporosity in SiOC via Chemically Bonded Polymeric "Spacers"

Silicon oxycarbides with controlled porosity in the mesopore range have been obtained through high-temperature pyrolysis of newly developed reactive siloxane formulations. The starting gels have been synthesized via Pt catalyzed hydrosilylation reaction between polyhydromethylsiloxane (PHMS) and vinyl-terminated polydimethylsiloxane (PDMS) of different molecular weights in the presence of tetravinyltetramethylcyclotetrasiloxane as a crosslinking enhancer. In our approach, the PDMS serves the double purpose of size-controlling templating agent as well as solvent at the early stages of the synthesis. During the curing step, the vinyl-terminated PDMS is chemically bonded to the preceramic network through the extremely efficient hydrosilylation reaction and solidify. Accordingly, its removal during pyrolysis occurs through decomposition of a solid phase with retention of the formed porosity. The structural and morphological evolution of the preceramic gels containing the molecular spacers have been investigated as a function of the thermal treatment temperature by N2 physisorption measurements, thermogravimetry, and SEM analyses. The results show that the pore size distribution of the resulting SiOCs depends on the molecular weight of the PDMS and is directly related to the molecular volume assumimg that the PDMS chains are entangled into spheroidal shapes. The total pore volume is related to the initial amount of templating PDMS assuming its complete decomposition during pyrolysis.