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

Abstract

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 45–55nm under the catalysis of dibutyltindilaurate (DBTDL), but show bigger polysilsesquioxanes nano domain around 50–150nm 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.