2. Preparation of Polymeric Membranes

This chapter mainly describes the principles of membrane formation process for polymeric membranes. With a brief introduction of relevant background information such as various membranes and membrane processes, a comprehensive list of polymer materials, which are suitable for making membranes, has been given. The most common technique used to prepare polymeric membranes – phase inversion process, including thermally induced phase separation (TIPS) and diffusion induced phase separation (DIPS), is discussed in detail. The thermodynamic behavior of the casting polymer solution, the process of membrane formation, and the fabrication of hollow fiber and flat sheet membranes are involved.

The thermodynamic description of the polymer solution is based on the concepts of spinodal, binodal, vitrification boundary, gelation boundary et al. in the phase diagram. The linearized cloud point curve correlation is presented. In addition, two important parameters, the approaching ratio of the polymer solution and the approaching coagulant ratio, are discussed in association with membrane formation. For the membrane formation process, the delay time and gelation time are two macroscopic time scales, which influence the membrane morphologies simultaneously. The formation of nascent porous membranes, the vitrification of the membrane morphology and the membrane surface formation are described. The macrovoid formation is related to the viscous fingering phenomenon. In the fabrication of hollow fiber membranes, the shear flow of the polymer solution inside the spinneret and the elongation flow in the air gap strongly influence the performance of resultant membranes. Thus, the shear flow and elongation flow are discussed in great detail. The hydrodynamics of the polymer solution at the casting window in the process of preparing flat sheet membranes is also concerned.