Thermal characterization of epoxy nanocomposites containing polyhedral oligomeric silsesquioxane: Glass transition temperature and chemical conversion

Abstract

An epoxy resin (diglycidyl ether of bisphenol-A) was blended with different loadings of a glycidyl-polyhedral oligomeric silsesquioxane (POSS) and isothermally cured with an amine hardener at varying temperatures and times. The glass transition temperature (Tg) of the samples was measured at different chemical conversions (α) using differential scanning calorimetry (DSC). Time-temperature shifts were made for Tgvs. ln(time) data to be superposed at an arbitrary reference temperature in the kinetically controlled reaction regime, and these shift factors were used to obtain an Arrhenius activation energy. The influence of POSS on different reaction systems was investigated in terms of the Tg-α relationship, which was fitted with two models; DiBenedetto and Venditti/Gillham equations. It was found that POSS molecules played different roles at different stages of the curing process. At lower conversions, the inorganic cage of the incorporated POSS (up to 20 wt%) reduced the mobility of the molecular segments, giving rise to an increase in Tg. However, above the 20 wt% POSS, there was a depression of Tg, which may be associated with a plasticizing effect of organic substituents of the POSS molecules. Moreover, the effect of POSS on Tg became less pronounced when the conversion reached 0.8.