Nanoscale Order and Crystallization in POSS-PCL Shape Memory Molecular Networks

The angstrom- and nanometer-scale crystallization and long-range ordering behavior of polyhedral oligosilsesquioxane-poly(epsilon-caprolactone) (POSS-PCL) shape memory nanocomposites under thermomechanical shape memory cycles and uniaxial stretching were studied by simultaneous wide-angle and small-angle X-ray scattering (WAXS/SAXS). POSS/PCL cross-linked molecular networks featuring a single POSS moiety centered between two PCL network chains, previously reported [Alvarado-Tenorio et al. Macromolecules 2011, 44, 5682-5692], with molecular weight of 2600 g/mol and exhibiting shape memory behavior, were synthesized with variation of cross-linking molar ratio (POSS-PCL diacrylate/tetrathiol cross-linker, 2:1, 2:1.5, and 2:2). The photocured networks exhibited a morphology consisting of POSS crystals embedded in an amorphous PCL matrix, and the POSS crystals were ordered in a cubic nanostructure. However, under tensile stress afforded by a shape memory cycle, the networks yielded a double-induced orientation (90 degrees and 180 degrees) of the POSS crystals, as indicated by the 101 reflection. Moreover, we detected stretch-induced crystallization of the otherwise amorphous PCL chains. Investigation of nanometer-scale structure by SAXS revealed long periods along the meridional and equatorial axes corresponding to a lamellar nanostructure of PCL chains coexisting with the cubic POSS superstructure. We conclude that the shape memory cycles promoted crystallization and highly ordered nanostructures.