Relationship between the structure and mechanical properties of polypropylene: Effects of the molecular weight and shear-induced structure

A series of homopolymer polypropylenes (PPs), within a weight-average molecular weight (Mw) range of 100-1600 kg/mol, were manufactured as dumbbell microspecimens. The effects of the molecular weight and shear-induced crystallization on the mechanical properties and morphology were studied to gain a better understanding of the structure-property relationship. The results showed that the crystallinity decreased from 50 to 41% and the lamellar thickness increased as Mw increased. Tensile tests demonstrated that the stiffness and especially the tensile strength rose to extremely high values (Young's modulus = 2400 N/mm2, stress at 30% strain = 120 N/mm2). Furthermore, the strain hardening effect was strongly affected by the lamellar thickness and highly oriented superstructures. Dynamic mechanical analysis demonstrated that the mobility of the molecular chains depended on Mw and on the lamellar thickness. In addition, the viscoelastic properties of unannealed and annealed samples indicated further the existence of shish-kebab structures caused by shear-induced crystallization during injection molding.