Crystallization of polypropylene : experiments and modeling

In this study, the quiescent crystallization of several polypropylenes (PPs) with different molecular characteristics was first examined using Differential Scanning Calorimetry (DSC) and Polarized Optical Microscopy (POM). The Avrami/Nakamura equation was employed to fit and predict crystallization kinetics under isothermal and non-isothermal conditions. The Avrami/Nakamura model was found to predict the non-isothermal crystallization data of the various PPs very well over a range of cooling rates. POM was used in line with a rotational rheometer to further examine the behaviour under quiescent condition at different temperatures and/or cooling rates. The growth rate of crystals was impeded exponentially with increase of temperature. To study the effect of flow on crystallization behaviour of PPs, the Anton Paar MCR-502 rotational rheometer equipped with various fixtures including parallel-plate and POM to induce shear flow was used. Generally, an increase in strain and strain rate or decrease of temperature is found to decrease the thermodynamic barrier for crystal formation and thus enhancing crystallization kinetics at temperatures between the melting and crystallization points. Popular models based on suspension theory, which are often used to relate the degree of crystallinity to normalized rheological functions are validated experimentally. It is found that the constant(s) of various suspension models should be dependent on the flow parameters in order for the suspension models to describe the effect of shear on FIC, particularly at higher shear rates. Finally, a capillary rheometer was used to investigate flow-induced crystallization (FIC) of various resins at high shear rates relevant to polymer processing. It is found that the crystallization kinetics are enhanced with increasing molecular weight indicating the importance of high-end tail of MWD on FIC. Various dies with different physics were used to investigate the effect of flow on FIC. The Cogswell analysis was applied on the capillary data to obtain the apparent extensional strain rate and strain as well as the apparent extensional viscosity. FIC was found to depend strongly on the L/D ratio of the capillary die. Finally, temperature impacted the FIC behaviour extensively since it alters the activation energy needed for the formation of macroscopic structures.