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 350400 nm and 18002000 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 Tc and thickness confinement are two factors for the unconventional concentric bands.