Effect of molecular structures and mobility on the thermal and dynamical mechanical properties of thermally cured epoxy-bridged polyorganosiloxanes

The effects of molecular structures and mobility on the thermal properties of epoxy-bridged polyorganosiloxanes have been investigated by solid-state 29Si and 13C solid state NMR in this study. Epoxy-bridged alkoxysilanes precursors with mono-(APDES), di-(APMDS), tri-(APTES) functional ethoxysilane terminal groups have been synthesized and thermally cured with or without the addition of catalysts to obtain epoxy-bridged polyorganosiloxanes. Three kinds of catalysts including acidic, basic, and organometalllic compounds have been used as the curing catalyst for the direct thermal curing of epoxy-bridged polyorganosiloxane precursors. The structures of epoxy-bridged polyorganosiloxanes with respect to the catalysts are quantitatively investigated. Acidic BF3·MEA shows the best catalytic effects on the formation of T3 and D2 structures in the epoxy-bridged polyorganosiloxanes from tri-functional epoxy-APTES and di-functional epoxy-APMDS precursors, but basic NBu4·OH has better enhancement on the formation of M1 structure in the epoxy-bridged polyorganosiloxanes from mono-functional epoxy-APDES precursor. TEM spectra show that the epoxy-bridged polysilsesquioxanes of epoxy-APTES precursors exhibit polysilsesquioxanes nano domain around 45-55 nm under the catalysis of dibutyltindilaurate (DBTDL), but show bigger polysilsesquioxanes nano domain around 50-150 nm under the catalysis of basic tetrabutylammonium hydroxide (NBu4·OH) in epoxy matrix after direct thermal curing process.The coefficient of thermal expansion of the epoxy-bridged polyorganosiloxanes are affected by the functionality of terminated alkoxysilanes and the species of catalyst used during curing process. The epoxy-bridged polysilsesquioxanes of epoxy-APTES precursor possesses the lowest coefficient of thermal expansion compared with the other two epoxy-bridged polyorganosiloxanes from mono-, and di-functional epoxy-bridged polyorganosiloxanes precursors. There is no obvious Tg observed in the epoxy-bridged polysilsesquioxanes of epoxy-APTES precursor from the analysis of TMA and DMA. 13C solid state NMR has been used to investigate the molecular motion behaviors of epoxy-bridged polyorganosiloxanes structures with respect to the changes in Tg and CTE. The T1ρH (the relaxation time of 13C after the spin lock process) of the epoxy-bridged polysilsesquioxanes of epoxy-APTES precursor is longer than that of epoxy-APMDS precursor, which indicates that the molecular mobility of epoxy-bridged polysilsesquioxanes of epoxy-APTES is highly restricted due to the strong intermolecular interaction of nano hybrid network.