Evolution of crystal and amorphous fractions of poly[(R)-3-hydroxybutyrate] upon storage

Quantitative thermal analysis of the evolution of crystal and amorphous fractions of poly[(R)-3-hydroxybutyrate] (PHB) upon storage at room temperature is detailed in this contribution. Conventional and temperature-modulated calorimetry were used to quantify the crystallinity, as well as the mobile and rigid amorphous fractions, of an initially partially crystallized PHB, subsequently maintained at 25 °C for various times. PHB undergoes progressive crystallization during storage, with an increase in crystal fraction (w C) from the initial w C = 0.35 up to w C = 0.71 attained after 1 year of storage. Crystallization is accompanied by vitrification of rigid amorphous segments, which leads to a noteworthy increase of the overall fraction of the material that is solid at room temperature, leaving only a mobile amorphous fraction w A = 0.04 after 1 year at 25 °C. The quantitative thermal analysis allowed to clarify the kinetics of evolution of both the ordered and unordered fractions of PHB upon storage, which leads to a severe deterioration of material's properties.