Abstract
Epoxy hybrid networks containing up to 50 wt.% pendant polyhedral oligomeric silsesquioxane (POSS) dispersed at molecular level are prepared by polymerizing DGEBA-POSS precursors with 4,4′-diaminodiphenylmethane (DDM). A high molecular weight control precursor was prepared without pendant POSS cage to quantify the effect of pendant POSS cage on network free volume, number of crosslinks per unit volume and glass transition temperature. Differential scanning calorimetry (DSC) shows drastic decrease in Tg with increasing pendant POSS content. The Tg value of hybrid network containing 50wt.% POSS is 100 °C lower than the DGEBA-DDM network compare to 77 °C for epoxy network prepared by high molecular weight control precursor (without POSS cage). In order to establish structure-property correlations, these variations have been correlated with free volume, crosslinking density, and chain packing in the network. For the epoxy hybrid networks containing up to 50wt% pendant POSS, the storage modulus of the glassy state (−50 °C) are lower than that of the DGEBA-DDM network, indicating that incorporation of pendant POSS at molecular level increases the spacing and free volume between the chains, and provides them freedom to promote motions. Also, lower storage modulus values in the rubbery state suggest there is no nano-reinforcement effect of POSS cages even at a very high loading (50wt%). Thermogravimetric analysis (TGA) and cone calorimetry show the POSS containing epoxy nanocomposites display high ceramic yields, suggesting the flame retardant behavior of the materials is improved by the molecular level dispersion of pendant POSS. A total of 62% increase in char yield and 53% decrease in peak heat release rate (PHRR) during combustion were observed in hybrid network containing 50wt% POSS compared to DGEBA-DDM network. This study gives new insights into the effects of structural changes in POSS containing thermosets on their performance properties.
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