Evaluation of morphology and crystallinity of biaxially oriented polypropylene films

An investigation on the structural and morphological variations of oriented polypropylene (PP) films processed by two different methods, including biaxial oriented cast film processing (film tentering) and film blowing process (tubular film extrusion) was conducted. Thermal properties obtained by differential scanning calorimetry (DSC) showed no significant difference in melting behaviors of machine direction oriented (MDO) film and the unoriented cast film. However, after transverse drawing (TDO) a small second melting peak was appeared on the left-hand side of the DCS thermogram which was ascribed to presence of kebab lamellae, and the appeared sharp peak was attributed to shish structures in the biaxially oriented PP (BOPP) film. On the other hand, a crystalline structure characterized by wide X-ray diffraction (WXRD) manifested an intensified peak pertinent to the formation of another crystalline structure, i.e. β-modification throughout the blown PP film, contrary to the cast films in which solely α phase existed. The topography of the surface taken by atomic force microscopy (AFM) represented an extensive fibrillar texture in the BOPP film which was in accordance with their DSC results.