Surface crystalline structure of thin poly(l-lactide) films determined by the long-range substrate effect

Abstract

Due to the stepwise crystallization behavior of polymorphous poly(l-lactide) (PLLA) films, the surface crystalline structures of thin PLLA films supported on Si/SiOx substrates with various thicknesses of both the film and adsorbed layer were investigated separately by grazing incidence wide-angle X-ray diffraction (GIWAXD). An unusual highly packed α-form crystal was observed at the surface at a crystallization temperature of 346 K, which transformed to a loosely packed δ-form (or called α′-form) crystal with decreasing film thickness or increasing adsorbed layer thickness. The mobility of the surface chains of PLLA films was scaled by the surface crystallization rate to elucidate the substrate effect on the corresponding surface crystalline structure. It was found that if the surface mobility is suppressed by the long-range substrate effect from much faster to slower than that in the bulk, then the surface crystalline structure changes from α-crystal to δ-crystal. A phase diagram was ultimately constructed to predict the surface crystalline structure at 346 K in thin PLLA films with various thicknesses of both the film and adsorbed layer.