Initiation of Cavitation of Polypropylene during Tensile Drawing

The effect of stabilizers, additives, and low molecular weight fractions on cavitation during tensile drawing was studied in polypropylene. The additives were extracted from compression molded samples by critical CO2 and also by a mixture of nonsolvents. The extract was an oily liquid composed of antioxidant, processing stabilizer, and a spectrum of low molecular weight fractions of polypropylene. Purified polypropylene exhibited surprisingly more intense cavitation than pristine polypropylene as it was determined by small-angle X-ray scattering and volume strain measurements. Intensification of the cavitation process in the purified samples was explained by the changes in the amorphous phase, namely, the changes in free volume by eliminating low fractions and soluble additives. An increase in free volume was probed with positron annihilation lifetime spectroscopy. Intense formation of cavitation pores in purified polypropylene proves that initiation of cavitation in polypropylene has a homogeneous nature.