Degradative and morphological characterization of POSS modified nanohybrid polyurethane elastomers

Reported here is the synthesis and thermal characterization of a series of polyhedral oligomeric silsesquioxane (POSS) modified polyurethane elastomers. A novel polyurethane architecture has been synthesized which incorporates a partially opened POSS-diol cage (disilanolisobutyl POSS) directly into the methylene di-isocyanate/poly(tetramethylene) glycol urethane network as a substitute chain extender moiety without the need for an alkyl tether. The effects of the inclusion of a sterically hindered and rigid silsesquioxane cage structure on both the non-oxidative thermal stability and micro-phase segregated morphology of the resultant polyurethane elastomer have been studied extensively over a range of POSS inclusion levels by means of pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) and differential scanning calorimetry (DSC). The results of analytical pyrolysis assays of the polyurethane systems clearly demonstrate that low levels of POSS substitution (<10 wt. %) lead to a significant increase in both the onset temperature of thermal de-polymerization and a reduction in the yield of volatile degradation products. A characterization of the products of degradation demonstrate that the POSS modified elastomers show some subtle differences in thermal degradation mechanism, yielding increased levels of propenal and decreased levels of ethane when compared with an unmodified control. POSS inclusion enhances the hard-block crystallinity at low levels and DSC analysis demonstrates that the peak thermal stability of the systems corresponds with a maximum in hard-block crystallinity (at a level of 4-6 wt. % POSS). At higher mass fractions we observe a breakdown in the phase separation of the systems and a decline in hard-block crystallinity, which correlates with an observed decrease in the primary onset degradation temperature. The major mechanistic pathways of degradation (urethane bond depolymerisation followed by secondary radical degradation of the monomers) have been shown to be insensitive to the presence of POSS in the matrix. Rather, it is physical changes in the morphology of the elastomer systems, as a consequence of POSS inclusion that are responsible for the observed improvements in the thermal stability of these materials.