Impact of the Exposure Plane on the Homoepitaxy of Poly(L-lactic acid)

It is confirmed that cold crystallization of PLLA on melt-drawn polyethylene (PE) produces a majority of lamellae with long axes aligned along the PE chain direction and a minority of lamellae inclined by 64° to the major ones. These inclined PLLA lamellae are correlated to the homoepitaxy of PLLA on the early-formed major lamellae. There is, however, no experimental evidence for it. In this work, the homoepitaxy of PLLA with different optical purities on its single polymer substrates with varied orientations has been studied. Intriguing results have been found, which shed more light on the homoepitaxy of PLLA. First, the homoepitaxy of amorphous PLLA on different PLLA substrates takes place constantly regardless of optical purity and orientation status of the used substrates, leading to the formation of an ordered structure. The ordered structure is, at all events, optical purity- and substrate orientation status-dependent. It has been discovered that the homoepitaxy of PLLA with an optical purity of 95% (PLLA-95) on the uniaxially oriented PLLA-95 melt-drawn films generates an ordered lamellar structure, consistent with the used substrates in all cases. On the other hand, the homoepitaxy of PLLA-95 on PE-induced PLLA-95 oriented films with the (100) exposure plane produces abundant lamellae inclined by 64° to the lamellae in the substrate. Moreover, the homoepitaxy of PLLA with 89% optical purity (PLLA-89) cannot result in the formation of inclined lamellae on the PE-induced PLLA-89 substrate at all, while PLLA-95 can be less pronounced. This demonstrates the important role of optical purity of PLLAs for both the epitaxial layer and the substrate film in its homoepitaxy behavior.