Morphology and phase transition of high melt temperature crystallized poly(vinylidene fluoride)

When PVDF is crystallized at temperatures above 155 degrees C it presents a multiform morphology composed of ringed, non ringed and mixed spherulites. Infrared spectroscopy showed that the ringed spherulites are formed exclusively by the alpha phase when crystallization takes place at temperatures below 155 degrees C. Higher temperatures induce a solid-state alpha --> gamma phase transformation in these structures, increasing the amount of gamma phase with crystallization time. The rate at which this transformation takes place increases with crystallization temperature. The non ringed spherulites, only formed at crystallization temperatures above 155 degrees C, consist predominantly of the gamma phase, crystallized from the melt, with small alpha phase inclusions. The melt process of the different spherulites, observed by optical microscopy and calorimetric measurements (DSC) showed that the melt temperature of the gamma phase, originated from the phase transition, is 8 degrees C higher than that crystallized directly from the melt. Optical micrographs of samples heated up to 186 degrees C and quickly cooled allowed visualization of the ringed spherulite regions which underwent the alpha --> gamma phase transformation at different crystallization times and temperatures. (C) 2000 Kluwer Academic Publishers.