Polypropylene composite materials oriented by solid-state drawing: low-temperature impact behaviour

Isotactic polypropylene samples, both neat and containing short glass fibres, with two different interfacial adhesions, were oriented by solid state drawing with typical neck propagation. A microscopic examination showed that the glass fibres turned to the orientation direction rather suddenly in the narrow region at the neck shoulder. Such extensive movement of fibres up to 1 mm long indicates high mobility of the polypropylene matrix in this region. Thus, these results seem to support the hypothesis of stress-induced melting of semicrystalline polymers in the propagating neck. The resulting oriented materials showed high resistance to crack propagation at cryogenic temperatures. Inspection of the crack surfaces after impact testing revealed that macroscopic failure was accompanied by multiple fractures and splitting of the specimens along the orientation direction. Such a mechanism effectively blunts the crack tip and dissipates mechanical energy even at very low temperatures.