Microscopic lamellar organization in high-density polyethylene banded spherulites studied by scanning probe microscopy

Surface topography and lamellar aggregation structure of high-density polyethylene (HDPE) banded spherulites were investigated by scanning probe microscopy. HDPE films were prepared by isothermal crystallization at various crystallization temperatures from the melt, Polarizing near-field scanning optical microscopic (NSOM) observations for the HDPE films revealed submicron-scale correlation between surface topography and birefringence of banded spherulites. The height profile of the film surface along the spherulitic radius periodically changed corresponding to the intensity profile of transmitted light along the radius of the extinction ring. This correlation was more clearly observed in the topographic and NSOM images of permanganic etched PE films. Therefore, it was apparently suggested that the crystallographic c-axis of the orthorhombic unit cell was parallel and perpendicular to the film surface at the peak and the valley in the surface corrugation of the banded spherulite, respectively. The band spacing obtained by polarizing NSOM and atomic force microscopy (AFM) was comparable to that determined by polarizing far-field optical microscopic observation under crossed nicols. The band spacing and the peak-to-valley height difference in the corrugation increased with an increase in isothermal crystallization temperature. AFM observations directly indicated local lamellar orientation and stacking manner.