Unprecedented Thermally Induced Structural Changes in Aerogels of Poly(3-hydroxybutyrate) during Heating

Thermally induced crystal structure changes in semicrystalline polymers involve solid-to-solid transition or melting of the starting crystal form followed by the recrystallization into different crystal forms. Understanding such crystal-to-crystal transitions by controlling the chain mobility of amorphous chains is rarely studied. Herein, we chose poly(3-hydroxybutyrate) (PHB) to investigate the temperature-induced structural changes in the bulk and aerogel samples and present the role of amorphous chain mobility on structural reorganization during heating. Aerogels of PHB were prepared by freeze-drying the thermoreversible gels and the films were prepared by hot pressing the PHB pellets. Both aerogels and films crystallized into the α form. We observed a major structural reorganization in aerogels upon heating prior to the melting and such a transition was not observed in the melt-crystallized PHB α form. We speculate that the enhanced mobility of the tie chains that are present between the α lamellar crystals during the heating of the aerogel triggered the conformational change from T 2 G 2 helical chains to all-trans conformation (T′TT̅′T̅) within the crystal lattice resulted in the crystal-to-crystal (α to α + β) transition.