Plastic deformation and solid‐phase transformation in β‐phase polypropylene

Tensile specimens of isotactic polypropylene, initially crystallized in the β‐form with high purity, were tested at various temperatures. It was observed by WAXD that the most important phenomenon occurring in the course of a tensile test is the β‐smectic transformation for specimens drawn at lower temperature and β‐α transformation for specimens drawn at higher temperature, which takes place in the crystalline lamellae and is induced by plastic deformation. c‐Axis oriented β‐crystals have never been found in the drawn specimens. It was also shown that the β‐smectic transformation propagates inside the spherulites from the equatorial regions, in which the chain axes are nearly parallel to the drawing direction, towards the diagonal and polar regions. Finally, it was noted that microvoids are formed simultaneously during plastic deformation, which may be due to the volume contraction induced by the β‐α or β‐smectic transformation.