| A novel microstructural model for flow-induced crystallization (FIC) is developed based on polymer kinetic theory and nucleation theory principles. An untransformed (melt) phase is modeled as a concentrated suspension of flexible macromolecules with finite chain extensibility, and a semi-crystalline phase is approximated as a collection of rigid rod molecules that grow and orient in the flow. The crystallization rate is approximated by a non-isothermal Avrami equation with a frame-invariant enhancement factor, and the system relaxation times are coupled with both temperature and crystallinity. The model is tested in the simulation of melt spinning by coupling the evolution of the microstructure (chain extension, molecular orientation and crystallinity) with the macroscopic balance equations, employing the thin filament approximation in both a one-dimensional and a two-dimensional domain. The model includes the combined effects of FIC, viscoelasticity, filament cooling, radial thermal conduction, air drag, inertia, surface tension and gravity.; The one-dimensional model is robust over a wide range of processing conditions and model parameters and captures all of the observed physics under all spinning conditions, including the neck-like deformation and associated extensional softening at high-speed conditions, the freeze point predicted naturally at all conditions, tensile stress at the freeze point and microstructure development. The excellent fitting and predictive capabilities of the model for velocity, temperature, and flow birefringence experimental profiles along the spinline are extensively shown for various nylons and PET at both low- and high-speed conditions and material properties.; The two-dimensional model was applied to low- and intermediate-speed conditions for both nylon and PET. Apart from predicting the available one-dimensional velocity and temperature data very well, it predicts the radial variation of tensile stress and microstructure driven by the radial variation of the temperature, caused by low polymer thermal conductivity. The formation of a skin-core structure observed experimentally is predicted, where the molecular orientation, crystallinity and tensile stress are highest at the surface and lowest at the centerline.; We suggest that our model can be used as an optimization tool for melt spinning, enabling prediction of the final fiber properties through the radial variation of the tensile stress and microstructure, as well as other polymer processes involving FIC. |
