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. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2370–2382, 2000