Membrane formation via liquid-liquid thermally induced phase separation

In forming microporous membranes via Thermally Induced Phase Separation (TIPS), phase separation can be initiated by solid-liquid, liquid-solid, or liquid-liquid phase separation. In this study, the liquid-liquid phase separation mechanism was used. Membranes were made from an initially homogeneous polymer-diluent mixture by inducing phase separation through a temperature drop. Upon phase separation, droplets rich in diluent formed within a continuous matrix phase rich in polymer. Through a process known as coarsening, these droplets grew in size and decreased in number in an effort to minimize the interfacial area of the phase separated mixture. At any point in time, if the temperature of the mixture were dropped further to solidify the polymer, then upon removal of the diluent, the drops in the phase separated mixture became the cells in the resulting membrane. The overall objective of this research was to understand the influence of the independent variables phase separation temperature and initial weight fraction of polymer on the phase separation kinetics of liquid-liquid TIPS in an effort to control the resulting cell sizes, pore sizes, and porosities of membranes made via liquid-liquid TIPS.; The model system chosen for study in this work consisted of isotactic poly(propylene) (iPP) dissolved in diphenyl ether (DPE). Once the equilibrium phase diagram for the model system was determined, kinetic experiments were carried out a variety of temperatures and mixture compositions within the liquid-liquid phase separation region. The droplet growth was monitored under an optical microscope and was measured using digital image analysis. It was found that the drop growth rate was strongly dependent upon the quench condition. In fact, it was shown that the droplet phase volume fraction has a profound influence upon the drop growth rate, and this influence is nearly independent of temperature. No existing kinetic model could describe the droplet growth kinetics measured in this study.; The droplet phase volume fraction was determined using a tie-line construction on the equilibrium phase diagram. The interfacial tension between the two phases was measured using a spinning drop tensiometer and was found to increase with decreasing temperature. The viscosity of the polymer-rich matrix phase was measured in a cup and bob viscometer and was found to increase with decreasing temperature and increasing polymer concentration. The van der Waals Hamaker constant was estimated and found to increase with decreasing temperature.; A new model was derived to account for the influence of two coalescing drops upon neighboring drops. Excellent qualitative and quantitative agreement was found to exist between the model predictions and the experimental observations.; A method was developed to measure the pore size distributions using a liquid permeation technique. It was found that the average pore size was approximately one-tenth the average cell size for the experimental conditions studied.