Origin of Epitaxial Cold Crystallization of Poly(l‑lactic acid) on Highly Oriented Polyethylene Substrate

Cold crystallization of poly(l-lactic acid) (PLLA) from amorphous state on highly oriented polyethylene (PE) substrate has been studied by electron diffraction and infrared spectroscopy. Both electron diffraction and infrared spectroscopy analyses show that the PLLA in the as-prepared PLLA/PE double layers is in the amorphous state. Annealing of the as-prepared PLLA/PE samples at 130 °C leads to the cold crystallization of PLLA. It is confirmed that the PLLA grown on the oriented PE surface exhibits high orientation with molecular chains aligned perpendicular the chain direction of PE, indicating the occurrence of heteroepitaxy of PLLA on the PE substrate. This provides a first example of perpendicular chain orientation of polymer epitaxy. Moreover, it demonstrates the capability of polymer epitaxy from amorphous state through annealing. Infrared spectroscopy analysis on the as-prepared PLLA/PE double layers shows that the orientation of PLLA molecules in contact with the PE substrate exists already in the amorphous state. It is these oriented PLLA chains in amorphous phase that transform into crystal nuclei during cold crystallization and initiate the oriented overgrowth of PLLA on the PE substrate.