Origin Of Epitaxial Cold Crystallization Of Poly(L-lactic Acid)on Highly Oriented Polyethylene Substrate

In this study, firstly highly oriented thin PE films were prepared by a melt-draw technique. The orientated structure of the used melt-drawn PE thin films was characterized by transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR) and atom force microscope (AFM). From the Bright-field electron micrograph and AFM phase micrograph, we can clearly see edge-on lamellae aligned perpendicular to the drawing direction, indicating an uniaxial orientation with the molecular chains aligned in the drawing direction.Secondly, cold crystallization of poly(L-lactic acid)(PLLA) from amorphous state on highly oriented polyethylene (PE) substrate has been studied by TEM and FTIR. 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 at130℃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 pre-orientation of PLLA molecules in contact with the PE substrate exists already in the amorphous state. It is these pre-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.