Flow-enhanced Crystallization Kinetics of iPP during Cooling at Elevated Pressure: Characterization, Validation, and Development

Using dilatometry combined with shear flow at conditions comparable to realistic processing conditions, the flow-induced crystallization of polymers is modelled. The model describes the kinetics of quiescent nucleation, flow-enhanced point nucleation, fibrillar growth, and the time evolution of the dimensions of the resulting crystalline structures. The growth rate of nuclei is coupled to the backbone stretch of a mode with a relaxation time representative of the average of the molecular weight distribution. The eXtended Pom-Pom (XPP) model is used to calculate the backbone stretches from flow conditions. Three important model parameters are determined over a broad range of temperatures, pressures, and shear rates for a fixed shear time; a prefactor to the creation rate of flow-induced nucleation, a prefactor to the shish growth rate, and the critical molecular stretch defining the transition between flow-enhanced nucleation and flow-induced crystallization of oriented fibrilar structures. Excellent agreement is obtained between calculated and experimentally determined crystallization kinetics of iPP. Moreover, the extended experimental dataset leads to an important adaption of the model, i.e., a new criterion for the initiation of shish growth.