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

Abstract

When PVDF is crystallized at temperatures above 155°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 α phase when crystallization takes place at temperatures below 155°C. Higher temperatures induce a solid-state α → γ phase transformation in these structures, increasing the amount of γ 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°C, consist predominantly of the γ phase, crystallized from the melt, with small α phase inclusions. The melt process of the different spherulites, observed by optical microscopy and calorimetric measurements (DSC) showed that the melt temperature of the γ phase, originated from the phase transition, is 8°C higher than that crystallized directly from the melt. Optical micrographs of samples heated up to 186°C and quickly cooled allowed visualization of the ringed spherulite regions which underwent the α → γ phase transformation at different crystallization times and temperatures.