Microstructure evolution in dry-wet cast polysulfone membranes by cryo-SEM: A hypothesis on macrovoid formation

Time-sectioning cryogenic scanning electron microscopy (cryo-SEM) was introduced in an earlier publication as a technique to directly visualize wet microstructures. In this work, the evolution of an asymmetric membrane microstructure of dry-wet cast (phase inversion) polysulfone solution coatings is methodically captured using cryo-SEM. The images show that the as-deposited, homogeneous coating (predecessor of the membrane) partially phase separates into a dispersion of droplets during the brief initial drying step, and then, on immersion in a coagulant, skins at the free surface and undergoes complete phase separation below, first by nucleation and growth, rapidly followed by partial coalescence into bicontinuous, open-celled structures. The phase-separated region is two-tiered consisting of an intermediate fine-scaled layer lying above a thicker and coarser layer. The membranes also display disproportionately large voids called macrovoids. Cryo-micrographs suggest that macrovoids in phase-separating coatings form due to a unique network instability triggered by successive tensile ruptures of the gelled polymer-rich network. In this wet cast process, a hypothesis is developed showing how (i) build-up of compressive pressure in pores and tensile stress in the network from overall swelling and local syneresis, (ii) vertical rupture (normal to substrate), (iii) stress localization, (iv) post-rupture relaxation and (v) plausible horizontal ruptures may cause this network instability and drive convective flows from adjacent pores into the growing void. Mathematical analysis of stress development and supporting cryo-micrographs of a dry cast process are also included in appendices.