Effect of thermal history on the evolution of crystal and amorphous fractions of poly[(R)-3-hydroxybutyrate] upon storage at ambient temperature

The manuscript details an investigation on the kinetics of vitrification of the rigid amorphous fraction in poly[(R)-3-hydroxybutyrate] (PHB) in dependence of thermal history. PHB was cooled from the melt at various rates, then the combined evolution of crystal and amorphous fractions was monitored during storage at 25 °C. The permanence at room temperature induces progressive growth of PHB crystals and concurrent vitrification of rigid amorphous fraction. During the initial stages of the crystal growth, the amount of rigid amorphous fraction that vitrifies depends on the previous thermal history. After prolonged storage at 25 °C, the increase of the crystal fraction corresponds to a reduction of both mobile amorphous and rigid amorphous content. The decreased coupling between the amorphous and crystal segments, reported in the literature for a few other semicrystalline polymers upon annealing at high temperatures, is shown to occur also in PHB, but during crystallization at low temperature. This finding provides one step forward a thorough understanding of the relation between crystal formation and reduced mobility of the coupled amorphous portions.