Surface melting of crystallized poly(vinylidene fluoride) under carbon dioxide

We investigated the melting behavior of the crystallized poly(vinylidene fluoride) (PVDF) under pressurized CO2 by in-situ Hv light scattering measurement. In the heating process, the Hv light scattering intensity started to decrease at low temperature far below the melting temperature while the ordering of the crystallites, estimated by the broadening of the light scattering profile, did not change up to the vicinity of the melting temperature. The decrease of the scattering intensity was steeper with increasing CO2 pressure and was thermally reversible. These results suggest that surface melting occurs in the crystallized PVDF under pressurized CO2. The surface melting under CO2 might be attributed to the dragging of the crystalline chains by the amorphous chains in which the molecular motion is enhanced by the plasticization effect of CO2. Owing to the decrease of the lamellar thickness by the surface melting, the melting temperature depression occurred under pressurized CO2.