Effect of crystallinity on charge storage in polypropylene and polyethylene

The improvement of charge storage with annealing, the use of different cooling rates, and stretching in polypropylene (PP) and polyethylene (PE) is discussed. 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, the concentration of defects in the crystalline region, and the concentration of traps at chain fold-amorphous interfaces. Annealing 4:1 stretched PP film at 140°C gives a half-value charge decay temperature of 152°C, ≈70°C higher than that for unannealed, unstretched film. Stretching increases amorphous content creating new boundaries, decreases crystallite size, and creates defect traps