Effect of initial polypropylene structure on its deformation via crazing mechanism in a liquid medium

Abstract

Atomic force microscopy has been employed to study the structure of isotactic polypropylene (PP) deformed in a physically active liquid medium (PALM) by the crazing mechanism. The investigations have been performed directly in the liquid, in which deformation is carried out, and under conditions excluding PP contraction. In order to study the effect of the initial PP structure on the crazing mechanism, the polymer structure was varied by annealing. The crazing mechanism, as well as the parameters and morphology of the polymer, has been investigated at different tensile strain values as depending on the initial PP structure. It has been shown that the development of deformation via the classical or intercrystallite crazing mechanisms is predetermined by the degree of crystallinity and the thickness of lamellae in initial PP. Before annealing, PP is deformed via the classical mechanism, i.e. as a uniform one-phase material, with the formation of crazes and zones of the bulk polymer located between them. Because the lamellae are «weak», the lamellar structure is transformed into a fibrillar structure, with porosity developing in crazes between fibrillar strands. After annealing of PP, the thickness of lamellae, degree of crystallinity, and, accordingly, the drag of crystallites increase, cavitations arise in the amorphous phase, and the fibrillar-porous structure develops in the space between crystallites moving apart from each other.