Multiscale Modeling of Crystallization Morphologies in High Speed Fiber Spinning of Semicrystalline Polymers

We apply here a systematic multiscale procedure to account flow-induced effects in the analysis of the chain conformations and in the evaluation of effective (nonequilibrium) thermodynamic potentials for the amorphous region of various semicrystalline morphologies of a dense polymer system. The basis for this approach is efficient lattice-based Monte Carlo simulations at the microscopic (segment) nanoscale level. The approach followed here is a hierarchical one using a continuum macroscopic model for flow-induced deformation in the bulk amorphous phase and a lattice-based Monte Carlo stochastic description of the amorphous regions within given microscopic semicrystalline morphologies. Three different microscopic morphologies, bulk amorphous, lamellar semicrystalline and fibrillar were investigated assuming a macroscopically imposed uniaxial extensional flow. Analysis of the chain conformations showed a significant nonlinearity in the interactions between the flow-induced deformation and orientation changes and those induced by the adjacency to a crystal-amorphous interface. This justifies the need of nonequilibrium microscopic simulations under flow. A comparison of the extended free energies of the lamellar and the fibrillar structures showed that at low extensional rates the lamellar morphology is the thermodynamically most favored one. However, at high extensional rates the situation changes, and the fibrillar morphology is the one that has the minimum free energy. This is consistent with available experimental observations in literature. It is the first time that a model has been constructed that can a-priori determine quantitatively the conditions (such as Weissenberg number) defining the transition from one semicrystalline morphology to another.