Crystallization and melting of polymers .24. Formation of beta-modification during the late stage of crystallization of isotactic polypropylene

In the late stage of the crystallization of isotactic polypropylene (IPP) melt inclusions are encapsulated by the crystallized phase. The contraction caused by the proceeding crystallization within the inclusion leads to a reduced pressure resulting in the formation of vacuum bubbles. In the surface layer of the melt surrounding the vacuum bubbles, the polymer chains are subjected to tensile load leading to the formation of alpha-row nuclei. As it has been observed during the shear-induced crystallization of IPP, the row-nuclei induce formation of the beta-modification. It was demonstrated that occasionally a alpha to beta transition took place on the surface of growing a-spherulites which was explained by the inductive effect of a local stress caused by contraction. Based on experimental results, suggestions were made for the origin of the strongly birefringent phase observed during the crystallization of blends of IPP with IPP grafted with maleic anhydride.