Deformation and nano-void formation of β-phase isotactic polypropylene during uniaxial stretching

T. Kawai and S. Kuroda

Graduate School of Science and Engineering, Gunma University, Ota, Gunma 373-0057, Japan

Pseudo-hexagonal β-form is known to transform into thermodynamically stable monoclinic α -form during elongation. It is also reported that the nano-sized void is formed during deformation. Since the crystal deformation/void formation mechanism of β-iPP is not fully understood, we aim in this study to clarify the deformation behavior of β-iPP in both terms of crystal transformation (angstrom scale) and the void formation (nanometer scale). The film of β-iPP was prepared by melt crystallization of PP with 0.2% DCNDCA as a nucleating agent (kβ = 0.94). The samples were drawn uniaxially at 100ºC with fixed strain rate of 0.66 min-1. Synchrotron radiation WAXD/SAXS measurements were performed at BL40B2 in SPring-8, Japan. Deformation of β -iPP proceeded as follows; (i) at the yielding point of ε = 0.1 β-form started to decrease followed by increase in amorphous fraction. (ii) at ε = 0.4, α-form crystal with the chain orientation parallel to the stretching direction was formed. Importantly, as soon as α -form crystallized, formation of nano-sized void was initiated. Above findings strongly suggest that the β -form transforms to amorphous and/or mesomorphic state before recrystallization into α-form crystal. A detailed analysis on void structure by means of SAXS streak scattering is also to be discussed based on lamellar deformation during elongation.