Phenomenological behaviour of long and short chain polypropylenes under tension

Abstract

Long and short chain polypropylenes, designated PP-L (long chains with lower melt flow index) and PP-R (short chains with higher melt flow index), were subjected to tensile deformation at strain rates in the range 10-5-10-1 s-1 to examine microstructural evolution and tensile flow behaviour as a function of strain and strain rate. Scanning electron microscopy was used to characterise surface deformation processes and fracture surfaces of the polypropylenes. The surface of tensile deformed PP-L was characterised by closely spaced crazes. The array of crazes multiplied with an increase in strain and strain rate and grew inwards. The final separation occurred by tearing along the individual bundles of crazes growing from opposite faces, resulting in crazing–tearing at the edge and a brittle mode of fracture in the centre. In PP-R, the predominant mode of deformation at all strain rates was wedge cracking. Additionally, ductile ploughing was observed at low strain rates and less pronounced crazing at all strain rates investigated. Final fracture at various strain rates occurred through a combination of wedge cracking and brittle fracture, connected by plastic flow, around the wedge cracks. The modes of deformation are summarised and depicted in terms of rate of mechanical deformation–strain diagrams, providing a broad perspective of deformation processes operating in different deformation rate–strain regimes. The modes and domains of deformation processes varied depending on the physical and mechanical characteristics of the material (long chain and short chain). Regarding the tensile flow behaviour, the yield stress increased linearly with an increase in strain rate, and followed the thermal activation concept, with an activation volume consistent with the thickness of the lamellar crystallite. True stress–true strain plots indicated an increase in flow stress with increase in strain rate. Depending on the melt flow conditions, the flow behaviour of the two polypropylenes exhibited differences in strain rate sensitivity.