Structural development during deformation of polyurethane containing polyhedral oligomeric silsesquioxanes (POSS) molecules

A unique polyurethane (PU) elastomer containing inorganic polyhedral oligom eric silsesquioxane (POSS) molecules as molecular reinforcements in the har d segment was investigated by means of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and transmission electron microscopy ( TEM) techniques. The mechanical properties of POSS modified polyurethane (P OSS-PU) were also compared to those of polyurethane without POSS. The cryst al structures of two different POSS molecules were first determined by X-ra y powder diffraction analysis, yielding a rhombohedral cell with a = 11.57 Angstrom, alpha = 95.5 degrees for octacyclohexyl-POSS (1,3,5,7,9,11,13,15- octacyclohexylpentacyclo[9.5.1.13,9.15,13] octasiloxane). and a = 11.53 Ang strom, alpha = 95.3 degrees for hydrido-POSS (1-[hydridodimethylsiloxy]-3,5 ,7,9,11,13,15-heptacyclohexylpentacyclo [9.5.1.13,9.15,15.17,13] octasiloxa ne). WAXD results showed that reflection peaks distinct to POSS crystal dif fraction were seen in POSS-modified polyurethane, which suggests that POSS molecules formed nanoscale crystals in the hard domain. During deformation, the average size of POSS crystals in POSS-PU was found to decrease while e longation-induced crystallization of the soft segments was observed at stra ins greater than 100%. The SAXS results showed microphase structure typical of segmented polyurethanes, with an initial long spacing of 110 Angstrom b etween the domains. At high strains, the average length of strain-induced m icrofibrillar soft-segment crystals was estimated to be about 60 Angstrom b y SAXS. The TEM analysis of highly stretched samples showed a preferred ori entation of deformed hard domains perpendicular to the stretching direction , indicating the destruction of hard segment domains by strain. (C) 2000 Pu blished by Elsevier Science Ltd.