Charge Trapping Sites in Spherulitic Polypropylene

Abstract

Charged areas of spherulitic polypropylene (PP) samples were visualized by a simple technique of attaching oppositely charged fine dye particles. A combination of this visualization technique and thermally stimulated current (TSC) spectroscopy was applied to identify the regions where shallow and deep charge traps exist densely in positively corona-charged spherulitic PP. The shallow and deep traps in the PP sample are directly related to its two TSC bands, B L and B H , appearing at temperatures around 60°C and above 100°C, respectively. From visualized charge patterns of the PP samples with and without thermal cleaning of the B L band, it was found that the shallow traps mainly exist in the boundaries and the peripheral regions of spherulites while the deep traps are located in the central parts. Furthermore, the relative numbers of the shallow and the deep traps in the PP spherulites were estimated and found to depend on spherulite size: the ratio of shallow to deep traps decreased from 60/40 for 130–190 µm spherulites to 43/57 for 320–400 µm spherulites.