Effects of Soaking Solvents by an Isothermal Crystallization Process on Physical Properties and Structures in One-Step-Drawing Films of Biodegradable P(3HB) Copolymers

In this work, to investigate the role of soaking solvent in the microcrystalline nucleation stretching method, poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] (PHBV) and poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBH) films were prepared by stretching after constant isothermal crystallization (IC) in different cold soaking solvents near the glass-transition temperature (T-g). Using ice-water, hexane, methanol, and air (without contact with solvent) as different soaking conditions to analyze the interaction on amorphous films in the IC process, we found that the drawn films in ice-water or hexane for IC showed white turbidity, whereas the drawn films in air for IC showed transparent and necking morphology. Interestingly, small and dense microcrystalline nucleus was observed in undrawn films with ice-water or hexane for IC, but no spherulites were observed in methanol from the polarization microscope results. Drawn films in ice-water or hexane showed oriented porous structure from scanning electron microscopy results, which showed a significant increment in tensile strength up to 99 MPa, about two times, compared to the drawn films in air. The coagulation effect of ice-water and hexane on films promoted the formation of microcrystalline nucleus during IC, while the contact between methanol and amorphous film inhibits the mobility of molecular chains and decreases stretchability. The case of air maintained the amorphous state of the molecular chains during the IC process. This work shows a novel results in the different interaction between polymer and solvent, offering a significant reference for controlling their structures.