Evidence for the Soft and Hard Epitaxies of Poly(l-lactic acid) on an Oriented Polyethylene Substrate and Their Dependence on the Crystallization Temperature

The epitaxial cold crystallization of poly(l-lactic acid) (PLLA) on an oriented polyethylene (PE) film was investigated by atomic force microscopy. Two populations of oriented PLLA lamellae in its α-form have been observed. One of them is oriented with molecular chains being perpendicular to the PE chain axis, i.e., the draw direction of the PE film, which is associated with soft heteroepitaxy based on the groove made of alternatively aligned PE crystalline and amorphous regions. The other one is oriented ∼64° counterclockwise away from the PE molecular chain direction in the top surface of PLLA. The morphology in the contact plane of PLLA is different from the top surface. There are still PLLA lamellae with molecular chains aligned perpendicular to the PE chain direction as viewed in the top surface. The other population is, however, aligned 64° clockwise with respect to the PE chain direction in the contact plane of PLLA. The mirror inclination geometry of these lamellae in the top surface and contact plane confirms unambiguously that the inclined PLLA lamellae are caused by homoepitaxy of PLLA based on parallelism of the helical path. It is further found that the morphology of PLLA in the contact plane depends strongly on the crystallization temperature. At lower crystallization temperature, e.g., 100 °C, mainly PLLA lamellae with molecular chains aligned perpendicular to the chain direction of PE are observed in the contact plane, demonstrating a higher heterogeneous nucleation ability of PE toward PLLA. With increasing crystallization temperature, the accelerated homoepitaxial crystallization of PLLA results in the formation of inclined lamellae even in the contact plane.