Significantly improving the thermal stability and dispersion morphology of polyhedral oligomeric silsesquioxane/polysiloxane composites by in-situ grafting reaction

In this work, two different polyhedral oligomeric silsesquioxanes (POSS) - octaisobutyl-POSS (OIBS) and allylisobutyl-POSS (AIBS) - were incorporated into poly(methylvinylsiloxane) (VMQ) by melt mixing and then the corresponding blends were chemically crosslinked by peroxide. NMR analysis revealed that AIBS molecules containing double bonds were successfully grafted onto VMQ macromolecular chains during the crosslinking of VMQ. Morphology and thermal stability studies by various methods discovered that the in-situ formation of chemical linking between POSS molecules and VMQ chains could significantly improve the dispersion of POSS particles and the thermal stability of VMQ. For the VMQ with 5 wt% POSS, AIBS exhibited nano-dispersion in VMQ matrix and improved the thermal stability of VMQ, but inert OIBS molecules aggregated extensively to form crystals and adversely affected the thermal stability, although AIBS and OIBS have similar solubility parameters and thermal stabilities. Further studies on the thermal stability of VMQ with different contents of AIBS showed that the thermal stability first increased with increasing content of AIBS but then began to decrease as the content of AIBS exceeded 7.5 wt%. X-ray diffraction and extraction experiments showed that extensive ungrafted AIBS molecules and their aggregates started to appear in the VMQ matrix as the content of AIBS exceeded 7.5 wt%. The study revealed that the grafted POSS hindered the thermal degradation of silicone rubber to cyclic oligomers but the ungrafted POSS favored the thermal degradation.