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 (I PP) melt inclusions are encapsulated by the crystallized phase. The co ntraction caused by the proceeding crystallization within the inclusio n leads to a reduced pressure resulting in the formation of vacuum bub bles. In the surface layer of the melt surrounding the vacuum bubbles, the polymer chains are subjected to tensile load leading to the forma tion of alpha-row nuclei. As it has been observed during the shear-ind uced crystallization of IPP, the row-nuclei induce formation of the be ta-modification. It was demonstrated that occasionally a alpha to beta transition took place on the surface of growing a-spherulites which w as explained by the inductive effect of a local stress caused by contr action. Based on experimental results, suggestions were made for the o rigin of the strongly birefringent phase observed during the crystalli zation of blends of IPP with IPP grafted with maleic anhydride.