Processing-structure-property relationships during uniaxial rolling and subsequent tensile drawing of polyolefin cast films and composites

Extreme plastic deformation in semi-crystalline polyolefins is facilitated by chain mobility in the crystalline fraction. The temperature at which such motion is activated – the alpha-transition temperature (T_α) – and the nature of such motion (e.g. torsional chain-slip) depends heavily on chain conformations within lamella and near crystalline interfaces. Accordingly, processing history can have cascading effects on structural progression during orientation of extruded films. Here, we examine multi-stage structure-property relationships during the solid-state orientation of polyethylene films. The thickness of the films is first reduced by uniaxial rolling at temperatures below the melting point. Next, the rolled films are tensile drawn at elevated temperature. In the first stage, deformation primarily occurs through modification of the as-extruded spherulitic structure. DMTA measurements indicate that the intermediate structures resulting from rolling at different temperature have varying T_α. In the tensile drawing stage, the total orientation and mechanical properties are largely dependent on the proximity of the drawing temperature to T_α; far below T_α deformation is substantially elastic, while far above T_α deformation is localized due to poor stress transmission. Independent of rolling temperature, the most effective orientation occurs when the drawing temperatures are near the onset of the alpha-transition as identified by linear thermal expansion measurements.