Morphology of the gamma form of isotactic polypropylene at 200 MPa

This study is the first attempt to examine the morphology of melt crystallized iPP at 200 MPa with a complete analysis using WAXD, OM, SEM, and AFM. On the crystal lattice level, results of the WAXD experiment indicate that pure γ-form is generated above the crystallization temperature of 176 °C. On the spherulitic level, the γ-form exhibits three distinct types of spherulites classified typically on the basis of the sign of the birefringence. A negative birefringence type is developed in the temperature range of 187-198 °C, whereas, a positive type is generated above 199° and below 184 °C. The third type is named `mixed birefringence' and produced in the temperature range of either 182-188 °C or 196-200°. It is well established that the change of birefringence in α-spherulites with crystallization temperature is directly related to the amount of branching. However, in the case of γ-spherulites the change of birefringence is due to different lamellar growth mechanisms. SEM and AFM studies of the lamellar morphology reveal a unique feather-like structure which can be generated at different crystallization temperatures. Furthermore, the results indicate that the feather-like structure is solely developed by self-epitaxial growth of γ-lamellae. This structure is believed to be the source of positive type spherulites. On the other hand the radial type accounts for the negative birefringence. A mixture of these two types, radial and feather-like, develops the mixed type birefringence, which is a transition region between negative and positive birefringence.