Studies on the β‐crystalline form of isotactic polypropylene

A study has been made of the mechanical, thermal, and morpholigical characteristics of melt‐crystallized isotactic polypropylene containing high levels of the β or pseudohexagonal crystalline form. Different levels of β‐form crystallinity were produced in the polymer by blending in low levels of quinacridone dye nucleating agent. Microscopical studies of the crystallization process revealed that both α‐form (monoclinic), and β‐form spherulites nucleated on the dye particles, with α‐spherulite growth commencing at a higher temperature. These observations were able to qualitatively explain the dependence of β‐form level on both the nucleant concentration and its state of dispersion in the polymer. Improving the dispersion of the nucleant was found to reduce the level of β‐form crystallinity if the nucleant concentration exceeded an optimum level. A new procedure for quantifying the volume fraction of β spherulites in a sample was developed which utilized the technique of selective solvent extraction. From volume‐fraction, x‐ray, and density data, the pure α and β crystal densities were obtained. Dynamic mechanical measurements‐obtained on unoriented specimens containing varying levels of β‐form crystallinity showed an increase in the magnitude of the damping in the post‐Tg region with increasing β content. High levels of the β form lead to lower values of the modulus and yield stress, and higher values of the elongation at break and impact strength.