Understanding Membrane Formation in Nonsolvent-Induced Phase Separation

Polymer membranes are essential to water security: they purify our drinking water, desalinate seawater into potable water, and treat wastewater before its release to the environment. The separation performance of these membranes is largely determined by their microstructure. An asymmetric microstructure is advantageous: the smaller pores on the feed-side of the membrane enable separation while larger pores deeper into the membrane provide mechanical support without hindering transport of the permeate across the membrane. Nonsolvent-induced phase separation (NIPS) is a typical way to make these asymmetric membranes. In NIPS, a polymer solution film is immersed in a nonsolvent bath, inducing phase separation of the film into a polymer-rich phase that becomes the membrane matrix and a polymer-poor phase that becomes the membrane pores. Due to our limited understanding of NIPS, membrane manufacturers continue to rely on experimental heuristics to link NIPS process parameters to the resulting membrane morphologies.In this dissertation, we demonstrate the mechanisms of asymmetric membrane formation using phase-field simulations. We show that mass-transfer-induced spinodal decomposition, thermal fluctuations, and structural arrest are essential and sufficient to the formation of asymmetric morphologies. Specifically, we show that the competition between the propagation of the phase-separation and glass-transition fronts determines the degree of pore-size asymmetry of the membrane. We also explore how these formation mechanisms change with the glass-transition concentration and dope composition, two important parameters in the formulation of NIPS systems.To complement our study of membrane formation, we also examine coarsening dynamics in the bulk near a glass transition, implemented as a mobility and viscosity contrast between the polymer-rich and polymer-poor phases. In the case of polymer-poor clusters in a polymer-rich matrix, the glass transition imposes structural arrest. In the opposite case, the glass transition changes the transient concentration of the polymer-rich phase, thus leading to a change in shape of the discrete domains. This effect introduces several complexities to the coarsening process, including inversion of the polymer-rich and polymer-poor phases---a phenomenon normally attributed to viscoelastic phase separation