Enhanced Nucleation and Crystallization of Poly(l-lactic acid) by Immiscible Blending with Poly(vinylidene fluoride)

Abstract

Effects of poly(vinylidene fluoride) (PVDF) on the non-isothermal and isothermal crystallization kinetics, crystalline structure, and spherulitic morphology of poly(l-lactic acid) (PLLA) in their immiscible blends were investigated. Both the cold and melt crystallizations of PLLA are accelerated after blending with PVDF. The crystallization rate of PLLA further increases as the PVDF content increases. PLLA does not crystallize upon cooling (10 °C/min) from the melt, while it can almost crystallize sufficiently under the same conditions after blending with 10% PVDF. PLLA crystallizes after the crystallization of PVDF in the melt crystallization process. The isothermal crystallization kinetics of PLLA in the blends was analyzed by the Avrami model. Crystallization half-time decreases and crystallization rate constant of PLLA increases after blending with PVDF. With the presence of PVDF, the spherulitic size of PLLA decreases and spherulitic number increases. Nucleation density of PLLA increases by 2–3 orders of magnitude by blending with 10 or 25% PVDF. It is found that PLLA spherulites can epitaxially grow on the PVDF crystalline domains, forming a transcrystalline structure in the PLLA/PVDF interface. It is proposed that the PVDF-promoted crystallization of PLLA in their immiscible blends is ascribed to the heterogeneously epitaxial and interface-assisted nucleation mechanism.