Processing–structure–properties relationship of multilayer films. 1. Structure characterization

Low density polyethylene (LDPE)/poly(ethylene terephthalate) (PET) multilayer films with and without a reactive tie layer were prepared by blowing process. The layers contents were fixed and the parameters evaluated were blow-up ratio (BUR), draw-down ratio (DDR) and frost-line height (FL). Their effect on the structure was studied. No crystallinity for the PET layer was induced by the blowing process. The orientation of the multilayer films was characterized by birefringence and infrared spectroscopy (FTIR). The measured birefringence showed a negligible orientation for PET layer, and the LDPE layer oriented mainly along the machine direction. FTIR characterization also showed a negligible orientation for PET layer, and for LDPE layer, the crystalline a-axis tended toward machine direction, crystalline b-axis located in transverse–normal plane with no orientation for the amorphous phase. The lamellar arrangement of LDPE layer with respect to machine direction was obtained by dissolving the amorphous phase in solution of potassium permanganate in a mixture of equal volumes of orthophosphoric and sulfuric acid. The crystallization of oriented LDPE melts produced a morphology consisting of rows of lamellar crystals aligned parallel to the machine direction at higher draw-down ratio. It was not always of the row-nucleated type, the exact morphology patterns greatly depended on the distribution of the crystalline a-axis and b-axis orientation, which in turn was dependent on DDR, BUR and frost-line height.