Formation of ring-banded spherulites of α and β modifications in Poly(butylene adipate)

The morphology and crystalline structure of banded spherulites of poly(butylene adipate) (PBA) were investigated by polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), atomic force microscopy (AFM) and scanning electronic microscopy (SEM). It was found that after purification and fractionation, the obtained PBA fractions with different molecular weight formed ring-banded spherulites at different temperature ranges. Pure α and β form of PBA can form regular ring-banded spherulites. AFM and SEM observations of the thin film revealed that the alternative ridges and valleys along the radial direction consisted of edge-on and flat-on lamellae, respectively, indicating the ring-bands in PBA spherulites are the consequence of lamellar twisting. In addition, sequential growth of α and β form in one PBA spherulite at the same temperature is reported and interpreted by competition of primary nucleation and radial growth of the two crystalline modifications. At a certain temperature range, the α form PBA has larger primary nucleation rate but lower radial growth rate than the β form PBA, leading to formation of spherulites consisted of α form at center and β form at the outer region. But finally, a layer of α form ringless region appears at the outside of the β form ring-banded region before impingement of the neighboring spherulites. These results suggest that besides crystallization temperature, molecular weight has considerable effect on the formation of crystal modification of PBA and the corresponding banded spherulites.