Finite strain response, microstructural evolution and β→α phase transformation of crystalline isotactic polypropylene

An experimental study of the finite strain response of annealed α and β crystalline isotactic polypropylene (iPP) was conducted over a range of temperatures (25, 75, 110 and 135 °C) using uniaxial compression tests. 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. 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. In addition, strain-induced crystallization occurred at the highest temperature examined in α-iPP. A continuous transformation of β crystals to α crystals with inelastic deformation at room temperature was observed and it was facilitated at higher deformation temperatures. Scanning electron microscopy (SEM) observations of deformed β-iPP provide strong evidence that the transformation is achieved via a solid-to-solid mechanism despite the different helical hands in α and β crystal structures. Molecular simulations were used to investigate a conformational defect in the 31 helical chains of β-iPP, characterized by a 120° helical jump. The propagation of this conformational defect along molecular chains provides the reversal of helical hand required by the solid-to-solid transformation. The β→α phase transformation in iPP is proposed to be accomplished via a solid transformation that includes slip along β(110) and β(120) planes during shear of the crystal lattice.