Relationships between microstructure, fracture-surface morphology, and mechanical properties in ethylene and propylene polymers and copolymers

The fracturing of four different polyolefin materials (a polypropylene homopolymer, a propylene–ethylene copolymer, a polyethylene homopolymer, and an ethylene–hexene copolymer) was studied with the objective of developing a better understanding of the relationships between the morphology of semicrystalline polymers, the morphology and growth kinetics of their fracture surfaces, and their mechanical properties. A scanning electron microscope and an optical microscope were used to obtain images of the fracture surfaces. The samples were injection-molded or hot-pressed to generate different microstructures. Fracture experiments were performed at 23, 0, and −20°C to generate fracture surfaces with different morphologies from the same supermolecular structure. It appears that the fracture propagates through the spherulites in a brittle manner. The macroscopic aspect of the fracture surfaces is temperature-independent and changes are visible only at the microscopic scale. Over the range of temperatures studied, the rms roughness [root mean square roughness decreased by only about 20%, while the fracture energy of all but one of the materials (a high-density ethylene–hexene copolymer) decreased by about 60% as the temperature was reduced.