The influence of rigid amorphous fraction on crystallization kinetics of poly[(R)-3-hydroxybutyrate] and its role on properties deterioration upon storage

.The influence of mobility of the amorphous chains on crystallization of the naturally occurring polyester poly(3-hydroxybutyrate) (PHB) was investigated by conventional and temperature-modulated calorimetry. Thermal analysis of initially amorphous PHB shows two distinct exotherms, indicating that cold crystallization of PHB proceeds via a two-stage process. The experimental data were interpreted taking into account the role of vitrification/devitrification of the rigid amorphous fraction (RAF) on crystallization kinetics of PHB. The rigid amorphous structure, which is established simultaneously with the crystals growth during the first stage of cold crystallization, slows down crystallization before completion. The coupling between the crystalline and amorphous segments hinders further crystallization by the creation of an immobilized amorphous layer that surrounds the crystals and leads to metastability of the system before completion of ordering. Further increase of the temperature results in mobilization of the RAF. Once the amorphous layer coupled with the crystals has gained sufficient mobility, chain rearrangements needed to accommodate the PHB segments into ordered structures can take place, and crystallization can proceed further. Moreover, the role of the three-phase structure on embrrittlement of PHB upon storage is discussed. Soon after molding, PHB is ductile, but when the molded product is stored at room temperature, a detrimental ageing process makes the material very brittle. The change in mechanical behavior upon storage is caused by a variation of the three-phase structure, including progressive crystallization and vitrification of the rigid amorphous fraction.