Effect of crystallinity on charge storage in polypropylene and polyethylene

Charge storage is greatly improved by annealing, different cooling rates, and stretching in polypropylene (PP) and polyethylene (PE). In annealed PP and PE storage increases linearly with both crystallinity and crystallite size. The half-value charge decay temperature can be used as a measure of the latter parameters. Annealing and cooling rate affect the rate of crystalline growth. Changes occur in the physical and/or chemical nature and concentration of defects in the crystalline region, and traps at chain-fold-amorphous interfaces. Annealing 4:1 stretched polypropylene film at 140°C gave a half-value charge decay temperature of 152°C, some 70°C higher than the unannealed, unstretched film. Stretching increases the amorphous content, thus creating new boundaries, decreases crystallite size, and creates defect traps