Influence of Crystallinity and Isotacticity on Charge Decay of Polypropylene Homopolymer Blends

The impact of crystallinity on the electret charge stability in binary blends of polypropylene (PP) homopolymers was been investigated with mechanical, rheological, calorimetric, and dielectric techniques. This showed that the incorporation of a low isotactic, high molecular weight PP into high isotactic PP (iPP) decreases the overall crystallinity according to the blend ratio, whereas the crystal melting temperature of the iPP phase is hardly affected as observed using differential scanning calorimetry (DSC). At the same time, the electret charge stability decreases considerably as revealed by thermally stimulated surface potential decay (TSSPD), despite a substantial fraction of the highly isotactic PP remaining crystalline. This suggests that, notwithstanding the unchanged iPP crystal melting temperature, the overall increase of the noncrystalline fraction reduces the charge stability by disrupting the polymer macrostructure and affecting the composition of the crystal-amorphous interphase layers. The increase in amorphous fraction was supported by a more pronounced glass transition in both dynamic mechanical analysis (DMA) and DSC. The blend rheology was analyzed by melt flow index (MFI) and parallel plate rheology. This gave further insights in the molecular weight distribution of the polymer samples and the flow behavior in shear rate regions commonly encountered within extrusion processes.