Impact of Annealing on the Melt Recrystallization of a-PDLA/ α -PLLA Double-layered Films

Poly(lactic acid) (PLA) as a bio-based polymer with biodegradability and biocompatibility has attracted much attention. To manipulate its properties for different applications, regulation of crystal structure and crystalline morphology becomes an attractive research topic. In this work, the structure evolution of layered samples containing an amorphous poly(D-lactide) (PDLA) layer and a crystalline poly(L-lactide) (PLLA) layer with highly oriented edge-on α lamellar crystals after annealing at 150 °C or/and after melt-recrystallization has been studied by AFM, FTIR, and TEM combined with electron diffraction. The results demonstrate that melt recrystallization of the as-prepared sample leads to the formation of abundant randomly oriented PLA stereo-complex (PLA SC) crystals. Annealing at 150 °C results in the formation of a small amount of oriented PLA SC crystals at the interface. These PLA SC crystals show great impact on the recrystallization behavior of sample after melting at 190 °C and then crystallizing at 90 °C. First, they impede the mutual diffusion of the overlying PDLA and underlying PLLA, and thus reduce their stereocomplexation ability as manifested by the decreased amount of PLA SC crystals. Second, they act as substrate to initiate the epitaxial crystallization of the overlying PDLA and underlying PLLA, which ensures the production of a highly oriented structure of PDLA and PLLA after melt recrystallization again.