Diffusion kinetics and phase separation phenomena during asymmetric membrane formation

In this thesis, both mathematical modeling and experimental analysis of the dynamics of the phase inversion process for forming asymmetric membrane structure have been carried out. A comprehensive model combining the evaporation and quench steps of phase inversion has been developed by using an improved numerical algorithm based on a modification of the jump mass balance equations. This approach enables accurate calculations of precipitation times and concentration profiles on both the polymer film and coagulation bath sides before the onset of phase separation. Results clearly show that final membrane structure is quite sensitive to the evaporation time with significant membrane property changes occurring as a result of a 20 second evaporation. Through a combination of decreased initial film thickness and a forced-convective evaporation step, both selectivity and permeability of the resulting membrane can be improved. In addition, increases in film surface area and initial solvent vapor in the casting atmosphere should lead to membranes having lower selectivity and higher permeability.Mass transfer and phase separation processes taking place in polymer films during the quench period were monitored by using an experimental method based on dark ground optics, reflected light illumination, and film-bath interface visualization. Observations of the refractive index distribution near the diffusion front, gelation front, precipitation time, and phase separation mechanism taking place at the film-bath interface are reported. The general trends observed in film shrinkage and the effect of initial-polymer concentration on interfacial phase separation mechanism are found to be consistent with the predictions of the ternary diffusion model. A phenomenological model has been developed to describe the initial square root of time dependence found for the gel front motion, the fact that $Y\sb{gel}$ scales as L, and the effects of film/bath compositions on gelation kinetics.U of I OnlyETDs are only available to UIUC Users without author permissio