A distribution kinetics approach for polymer crystallization and phase separation

Abstract

The mechanism of polymer crystallization is extensively studied and still far away from consensus. This research adopted cluster size distribution kinetics approach to explore the kinetics of polymer crystallization and phase separation within spinodal region. The kinetics of polymer crystallization is studied by integrating nucleation, crystal growth and ripening. Population balance equations based on crystal size distribution and concentration of amorphous polymer segments are solved numerically and the related differential moment equations are also solved. The model accounts for nucleation and crystal growth. Different mass dependences of growth and dissociation rate coefficients are proposed to investigate the fundamental features of crystal growth. The effect of temperature is also investigated for isothermal and nonisothermal polymer crystallization. Incorporating temperature effects of nucleation and crystal growth rate, the model presents time dependencies of polymer concentration, number and size of crystals, and crystallinity for different temperatures and cooling rates. The effect of denucleation is investigated by comparing moment and numerical solutions of the population balance equations. Incubation periods introduced in nonisothermal crystallization are studied under different cooling rates and different initial temperatures. The distribution kinetics approach is also extended to the investigation of crystallization of polymer blends. Blending effects from polymer-polymer interactions are incorporated into the diffusion coefficient. The melting temperature, activation energy of diffusion, and phase transition enthalpy also depend on polymer blends composition, and lead to characteristic kinetic behavior of crystallization. The influence of different composition is presented through the time dependence of polymer concentration, number and size of crystals, and crystallinity. Another extended application of distribution kinetics approach is the study of the kinetics of spinodal decomposition. Spinodal decomposition occurs under conditions of large supersaturation and/or small ratio of interfacial to thermal energy when the energy barrier for nucleation is negligible. A cluster distribution kinetics model without nucleation is established to describe the unique kinetics of new phase domain growth. Population balance equations show how clusters aggregate and rapidly lead to phase separation.