The kinetics of membrane formation via thermally induced liquid-liquid phase separation

The kinetics of microcellular membrane formation via the Thermally Induced Phase Separation (TIPS) process has been investigated. The membranes were formed by inducing liquid-liquid phase separation (through a drop in temperature) in an initially homogeneous polymer solution. Liquid-liquid phase separation resulted in a diluent-rich phase being dispersed within the polymer-rich matrix. The diluent-rich phase upon extraction leaves behind empty spaces that form the cells and pores of the resulting microporous membrane. When a sample undergoing liquid-liquid phase separation is held at the phase separating temperature for an extended period of time the droplets of the diluent-rich phase grow in size and decrease in number. This phenomena, known as coarsening may be used to control the cell size of a microcellular structure formed via liquid-liquid TIPS. A thorough understanding of the kinetics of coarsening enables better control of cell size and size distribution. In the present study existing domain growth models have been evaluated for a polymer-diluent system and a new model based on a unique mechanism of domain growth is presented.;After establishing the equilibrium phase diagram for the model system isotactic polypropylene (iPP)-diphenyl ether (DPE), kinetic experiments were carried out at different temperatures and polymer concentrations. The growth of the diluent-rich phase droplets was monitored through a microscope, and analyzed using digital image analysis. The domain growth rate was dependent on the initial polymer concentration and temperature of phase separation. It has been demonstrated that volume fraction of the droplet phase has a profound influence on the growth rate exponent. It is quantitatively shown that existing models do not adequately describe domain growth in the late stages of liquid-liquid phase separation in a polymer-diluent system.;A unique mechanism of domain growth, which considers the influence of the coalescence of two drops on a neighboring drop has been proposed. An approximate model based on this mechanism has been derived to describe the kinetics of domain growth in the late stages of liquid-liquid phase separation. The new model provides reasonable agreement with experimental data.;The influence of the relative rates of liquid-liquid phase separation and polymer crystallization, the effect of spherulitic growth, and a two step quench in temperature on the final morphology of the membrane was evaluated using scanning electron microscopy.