The effect of annealing on the viscoplastic response of semicrystalline polymers at finite strains

A constitutive model is developed for the viscoplastic behavior of a semicrystalline polymer at finite strains. A solid polymer is treated as an equivalent heterogeneous network of chains bridged by permanent junctions (physical crosslinks, entanglements and lamellar blocks). The network is thought of as an ensemble of meso-regions linked with each other. In the sub-yield region of deformations, junctions between chains in meso-domains slide with respect to their reference positions (which reflects sliding of nodes in the amorphous phase and fine slip of lamellar blocks). Above the yield point, this sliding process is accompanied by displacements of meso-domains in the ensemble with respect to each other (which reflects coarse slip and disintegration of lamellar blocks). To account for the orientation of lamellar blocks in the direction of maximal stresses and formation of micro-fibrils in the post-yield region of deformations (which is observed as strain-hardening of specimens) elastic moduli are assumed to depend on the principal invariants of the right Cauchy-Green tensor for the viscoplastic flow. Stress-strain relations for a semicrystalline polymer are derived by using the laws of thermodynamics. The constitutive equations are determined by six adjustable parameters that are found by matching observations in uniaxial tensile tests on injection-molded isotactic polypropylene at elongations up to 80%. Prior to testing, the specimens were annealed at various temperatures ranging from 110 to 163 °C. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. The effect of annealing temperature on the material parameters is studied in detail.