Reassessment of the epitaxial mechanism of Poly(l-lactic acid) on polyethylene substrate

Epitaxial crystallization of poly (l-lactic acid) (PLLA) on oriented polyethylene (PE) films with fold chain lamellar structure has been confirmed to produce cross-hatched edge-on lamellar structure with majority of PLLA lamellae aligned along the PE chain direction while minor PLLA lamellae inclined by 64° to the PE chain direction. The origin of the major PLLA lamellae has been correlated to a soft epitaxy, also referred to as graphoepitaxy, based on the alignment of PLLA chains along the grooves made of alternatively arranged PE crystalline and amorphous regions. Here in this study, oriented PE films with fibril structure have been prepared and crystallization of PLLA on them has been studied in detail by atomic force microscopy and transmission electron microscopy combined with electron diffraction. It has been found that PLLA can also grow epitaxially on these PE films, and the epitaxial crystallization results in essentially the same lamellar structure. This is clearly against the previous explanation of soft epitaxy, since grooves in PE film with fibril crystals are along its molecular chain direction. It has, therefore, been correlated to the heteroepitaxy based on an excellent matching between (hk0) lattice planes of PE and (00l) lattice planes of PLLA, especially the one between (110) and (007) lattice planes of PE and PLLA with a discrepancy of only 0.3 %. Although the mismatching between d₍₁₀₀₎ of PE and d₍₀₀₄₎ of PLLA, ca. 2.5 %, looks somewhat inferior, it is actually more feasible to realize the epitaxy from the view of molecular topology.