Mechanical behavior, texture evolution and constitutive modeling of alpha and beta crystalline isotactic polypropylene

An experimental study of the finite strain response and morphological evolution of annealed α and β crystalline isotactic polypropylene (iPP) was conducted over a range of strain rates (−0.001, −0.01 and −0.5/s) and temperatures (25, 75, 110 and 135°C) using uniaxial compression tests. 69% crystalline α-iPP specimens and 74% β-iPP specimens were prepared by melt crystallization under identical heat treatment histories. Morphological evolution with inelastic deformation was examined using Differential Scanning Calorimetry (DSC) and Wide Angle X-ray Scattering (WAXS) analysis after unloading from increasing amounts of inelastic deformation. DSC and WAXS investigations of β-iPP revealed a continuous transformation of β crystals to α crystals with inelastic deformation at room temperature. This transformation was facilitated at higher temperatures. Uniaxial compression results indicate nearly identical macroscopic stress vs. strain behavior for α-iPP and for β-iPP to true strains in excess of −1.1 at room temperature despite the different initial morphologies and the β→α transformation. At larger compressive strains (>1.2), β-iPP shows more rapid strain hardening. The orientation of crystalline planes during straining differs at room temperature from that at high temperature, indicating a change of slip mechanisms as temperature increases. Also strain-induced crystallization occurred at the highest temperature examined in α-iPP. The β→α transformation occurred with no evidence of a mesomorphic phase nor melting. A solid-to-solid mechanism for the β→α transformation is proposed based on the propagation of conformational defects and a shear transformation of the crystal lattice during deformation. Scanning Electron Microscopy (SEM) was used to study the spherulitic morphology of both α and β crystals and found to support the solid-to-solid transformation mechanism. A modified Taylor model from the literature has been used to simulate the finite strain and texture evolution of α-iPP and β-iPP polycrystalline aggregates. The model captures the rate dependent yielding and the texturing of (040) and (110) planes of α-iPP at high temperatures. This model has been applied to β-iPP considering the β→α phase transformation and can qualitatively predict the increase in strain hardening of β-iPP over α-iPP.