Effect of high-temperature annealing on the elastoplastic response of isotactic polypropylene in loading-unloading tests

A series of uniaxial tensile loading-unloading tests is performed on isotactic polypropylene at room temperature. Prior to mechanical testing, injection-molded specimens are annealed for 24 h at temperatures T = 145, 150, 155, 158, 160, 163, and 165°C, which cover the entire region of high-temperature annealing temperatures. A constitutive model is developed for the elastoplastic behavior of a semicrystalline polymer at small strains. The stress-strain relations are determined by six adjustable parameters that are found by matching observations in cyclic tests. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. It is shown that all material constants are affected by the annealing temperature, which is explained by changes in the crystalline morphology driven by thermal treatment. Some of the adjustable parameters experience finite jumps in the vicinity of the critical temperature T_c = 159°C. These jumps are attributed to the α₂ → α₂′ phase transformation.