Robust propylene-ethylene copolymer/polypropylene films: Extensional stress-induced orientation realized at low temperature processing

From the viewpoint of energy conservation, exploring low temperature processing of robust polypropylene-based films is meaningful. By blending polypropylene (PP) and propylene-ethylene copolymer (coPP), the processing temperature of oriented films is significantly reduced to 80 °C, compared to that of PP which is normally close to its melting point. Besides, the addition of PP remarkably enhances mechanical properties of the films after stretching. When PP content is 60 wt%, the increasing draw ratio leads to increase in tensile strength of up to 250 MPa, showing enhancement of 600% attributed to extensional stress-induced orientation structure. Intriguingly, it is found with significance that coPP/PP blends exhibit the PP spherulites formed in the range of tens of micrometers, while phase separates in the range of sub-micrometers within the spherulites. Additionally, the evolution of phase and crystalline structures of coPP/PP system during the deformation is studied for the first time, demonstrating that the structural evolution of PP phase always lags behind that of coPP phase. These results give a new insight into the relationship between structure and properties of PP multiphase system during stretching, offering helpful guidance to economically fabricate robust films based on polypropylene related polymers widely used in industry.