Unconventional non-birefringent or birefringent concentric ring-banded spherulites in poly(L -lactic acid) thin films

Unconventional concentric up-and-down bands, differing from the conventional bright- extinction or blue/orange bands in other polymers, in poly(L-lactic acid) are found via controlled thin films. Non-birefringent and birefringent concentric banded spherulites are obtained from 350-400 nm and 1800-2000 nm film thicknesses, respectively. Non-birefringent concentric ring-banded spherulites are arranged only by flat-on lamellae; in contrast, complex combinations of flat-on, edge-on, and round-shaped lamellae in four transitional zones are present within a single band in the thicker films of birefringent, concentric, ring-banded spherulites. Interband is an optically extinction crevice that segregates two neighboring bands. Unique lamellar arrangements at specific T_c and thickness confinement are two factors for the unconventional concentric bands. The film thickness and different lamellar arrangements are proven to induce the formation of non-birefringent or birefringent concentric ring-banded spherulites in thin poly(L-lactic acid) (PLLA) spin-cast films (about 350-400 nm or 1800-2000 nm).