Finger-Like Structure

Finger-like structure is one of the typical membrane structures, as shown in Fig. 1. It is also called macrovoid structure. Figure 1 shows asymmetric membranes with a thin top skin layer supported by a finger-like sublayer. Formation of finger-like structure follows two processes: pore initiation and growth. As a polymer solution undergoes a liquid-liquid phase separation, both polymer-rich and polymer-lean phases are developed. The nuclei of the polymer-lean phases initiate the finger-like structure. The nuclei are mostly formed just beneath the top layer, and growth of the nuclei depends on the state of the solution in the frontier of the phase separation (Marcel 1996). The nuclei may grow if the polymer solution in the front is stable and could supply enough solvent (or solvent/nonsolvent) to sustain the growth. The growth ends when new nuclei are initiated in the frontier of phase separation or the concentration of the polymer-lean phases enters the glass transition point. At an...