Effects of amorphous poly(vinyl acetate) on crystalline morphology of poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid)

Abstract

The amorphous and crystalline phase behavior, spherulite morphology, and interactions between amorphous poly(vinyl acetate) (PVAc) and poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) were examined using differential scanning calorimetry, polarized-light optical and scanning electron, atomic-force microscopy (DSC, POM, SEM, AFM), and small-angle X-ray scattering (SAXS). The PHBV/PVAc blend was found to be miscible with an almost linear T _g-composition relationship, indicating perfect homogeneity. Interaction parameter by melting point depression is a negative value of χ = −0.32, suggesting quite favorable interaction strength. With the intimate interaction between the amorphous PVAc and crystalline PHBV polymers, effects of PVAc on the spherulitic morphology of PHBV are quite significant. Owing to the higher T _g of PVAc (than that of PHBV), the spherulite growth rate of PHBV was depressed by increasing PVAc content in blends. Neat PHBV exhibits ring-banded spherulites when crystallized at ${T_{\rm{c}}} = {6}0\sim {11}0^\circ {\hbox{C}}$; however, with increasing PVAc content in the blends, the temperature range at which the PHBV/PVAc blends exhibit ring-banded spherulites remains similar but the regularity increases, and the inter-ring spacing significantly decreases. In addition, the spherulite size and ring-band patterns therein are strongly dependent on T _max (190 vs. 220 °C, respectively, for erasing prior nuclei), from which the blends were quenched to a T _c (60–110 °C) for crystallization. For PHBV/PVAc blends crystallized at the same T _c from different T _max, higher T _max tends to erase nuclei, leading to larger spherulites. However, such larger spherulites owing to higher T _max are not necessarily packed with thicker lamellae.