Surface morphology of polymer films imaged by atomic force microscopy

The surface morphology of commercial polymer films has been studied by contact mode and tapping mode atomic force microscopy. Flame-treated isotropic HDPE obtained by cast-film extrusion exhibited randomly oriented, 20-50-nm thick lamellar features of 200-400 nm in length, while uniaxially oriented films showed a shish-kebab-like morphology. The surface features of extrusion-blown films of blends of low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) appeared as stacks of lamellae arranged in a cauliflower-like pattern. Images of isotactic polypropylene (PP) films obtained by cast-film extrusion consisted of branched fibrillar features. Corona treatment of the PP films resulted in the formation of 400-500 nm large ''droplet-like'' features at the surface. During metallization, 20-40-nm diameter Al particles were deposited. The metal layer formed followed the topography of the surface. The surface of ethylene-vinyl alcohol (EVOH) copolymer skins on PP consisted of radiating lamellae of 9-11 nm thickness. Contact mode AFM imaging of solvent-cast atactic polystyrene (PS) and polyvinylchloride (PVC) films induced surface erosion. During the scanning of PS and PVC films, wavy features formed with a predominantly perpendicular orientation with respect to the scan direction. The height of these features increased with plasticizer content from ca. 10 nm (unplasticized films) to over 100 nm. The height did not show any measurable relaxation over 24 h. This plastic deformation of the surface of the glassy polymer films studied is discussed. It is assumed that the glass-transition temperature of the outermost thin layer at the surface of glassy polymers is significantly lower than the bulk value.