Synthesis and thermal properties of modified room temperature vulcanized (RTV) silicone rubber using polyhedral oligomeric silsesquioxane (POSS) as a cross linking agent

Abstract

Incompletely condensed tetra-silanol-phenyl-polyhedral oligomeric silsesquioxane (TOPO) was synthesized first and then copolymerized with hydroxy terminated polydimethylsiloxane (HPDMS) as a cross linking agent to prepare room temperature vulcanized (RTV) silicone rubber (TOPO–PDMS). The structure of TOPO was characterized by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) and nuclear magnetic resonance (NMR). The Fourier transform infrared spectroscopy (FT-IR) spectra suggested successful bonding of TOPO silanols and HPDMS. Scanning electron microscopy (SEM) and wide angle X-ray diffraction (WAXD) analysis showed that POSS could dissolve in silicone rubber at the molecular level. Thermal stability of TOPO–PDMS was investigated by thermogravimetric analysis (TGA) and the results demonstrated that the chemical incorporation of POSS into polydimethylsiloxane (PDMS) networks significantly enhanced the thermal stability of the modified RTV silicon rubber. The degradation mechanism(s) was further monitored by TGA coupled with FTIR. The results suggest that the remarkable improvement in thermal stability can be attributed to (a) the consumption of OH in the condensation reaction between TOPO and HPDMS which decreases the ‘back biting’ reaction and (b) the nanoreinforcement effect of the POSS cage that retards polymer chain motion. Additionally, the characteristic temperatures of 5%, 20% and the maximal rate of weight loss exhibit different trends with content increase of the cross-linker TOPO and the cause has been discussed in detail.