Multiphase-field modeling of non-isothermal polymer crystallization

This work presents a coarse-grained multiphase-field model for modeling polymer crystallization at the mesoscopic scale. The crystallinity evolution is governed by the Nakamura model, while a novel formulation for the crystallization rate constant is proposed to overcome limitations found in existing approaches. The free energy density is formulated as the sum of a structural contribution associated with the polymer’s degree of crystallinity and a caloric term related to the specific heat capacity. The latent heat contribution of the heat conduction equation accounts for thermal effects due to phase transformation, which is linked to the evolution of the order parameter. The model is evaluated through a series of numerical simulations using material parameters for polyamide 6. The effects of cooling rate, latent heat contribution, as well as spatial temperature and cooling rate gradients on crystallization kinetics and resulting microstructure are investigated. Including the latent heat contribution to the heat evolution equation leads to locally non-negligible temperature gradients driving the phase transitions and the formation of coarser microstructures. Simulations with imposed cooling rate gradients reveal directional grain growth patterns that are consistent with experimental observations. Finally, a 3D simulation mimicking a typical DSC sample demonstrates the model’s applicability to realistic conditions. The crystallization kinetics show good agreement with experimental data.