A new method for achieving nanoscale reinforcement of biaxially oriented polypropylene film

Nanolayers of poly(ethylene oxide) (PEO) produced by layer-multiplying coextrusion crystallize as single, high aspect ratio lamellae that resemble large single crystals. The confined crystallization habit imparts two orders of magnitude reduction in the gas permeability. We now demonstrate how the highly oriented lamellar nanolayers can be obtained with biaxial stretching. For this purpose, we chose biaxially oriented polypropylene (BOPP) film for modification and incorporated PEO nanolayers under conditions that mimicked the typical fabrication process. Sheet that contained a center core with 33 alternating layers of polypropylene (PP) and PEO was coextruded and subsequently biaxially oriented at 145 °C. Biaxial stretching reduced the PEO layer thickness from the spherulitic microscale to nanolayers of highly oriented PEO single lamellae. The nanolayers improved the oxygen barrier by an order of magnitude without sacrificing the high clarity and good tear resistance of BOPP film.