Banded spherulitic structures of poly(ethylene adipate), poly(butylene succinate) and in their blends

Banded spherulites of PEA, PBS and their PBS-PEA blends were studied by optical and atomic force microscopies. It was found that PEA forms a regular banded structure within the temperature range 23 degrees C to 36 degrees C, otherwise only Maltese-cross spherulites are observed. The banded PEA spherulites exhibit a double ring feature with band spacing between two equivalent birefringent rings increasing with temperature. The ring extinction in PEA banded spherulites is caused by a lamellar twist. There exist two equivalent positions in one twisting period, which show no birefringence. The PBS can also grow in ring-banded spherulites in a wide temperature window with the high temperature threshold for forming banded structures that are not sharply delineated, as in the case of PEA. Also the double ring feature is not so pronounced for the spherulites of pure PBS. Blending PBS with PEA is found to favor the formation of PBS banded structures. During the crystallization process, the early growing PBS spherulites at 75 degrees C show weak birefringence with an evident ring-banded structure. The crystallization of PEA at 0 degrees C makes a great contribution to the birefringence increment of the initially-birefringent rings. This is caused by the analogic crystal orientation of PBS and PEA based on interlamellar phase separation as revealed by AFM observation. The band spacing of PBS spherulites is found to be increase with both increasing temperature and increasing PEA content. This stems from an increment in chain mobility of PBS both with temperature and addition of the PEA component.