Simulations of Polymeric Membrane Formation by Immersion Precipitation: Liquid-liquid Demixing
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Simulations of Polymeric Membrane Formation by Immersion Precipitation: Liquid-liquid Demixing Bo Zhou and Adam Powell Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. ABSTRACT The immersion precipitation process makes most commercial microporous membranes, which enjoy widespread use in filtration and purification. This process begins with liquid-liquid demixing of a nonsolvent/solvent/polymer ternary system into polymer-rich and polymer-lean phases. This demixing step determines much of the final morphology. In this work, a ternary Cahn-Hilliard formulation incorporating a Flory-Huggins homogeneous free energy function is used to simulate phase separation during demixing. Then the formulation is coupled with constant-viscosity interface-driven fluid flow. Simulations begin with uniform initial conditions with small random fluctuations, and then with two-layer polymer-solvent/nonsolvent initial conditions to simulate actual membrane fabrication conditions. The results are presented in 2-D and 3-D, which demonstrate the effects of mp (degree of polymerization), Ki j (gradient penalty coefficients) and χi j (Flory-Huggins interaction parameters) on phase separation behavior. INTRODUCTION Polymeric membranes have been developed for a variety of industrial applications, including microfiltration, ultrafiltration and reverse osmosis [1]. Each application imposes specific requirements on the membrane material and its membrane structure. The final morphology of the membranes will vary greatly, depending on the properties of the materials and the process conditions. Most commercial membranes are prepared by immersion precipitation process. In this process, a homogeneous polymer solution is cast on a suitable support and immersed in a coagulation bath containing a nonsolvent. The nonsolvent begins to diffuse into the polymer solution and the solvent begins to diffuse into coagulation bath, bringing the composition of the polymer solution into the miscibility gap of the ternary phase diagram. Hence, the polymer solution is decomposed into two phases: a polymer-rich phase and a polymer-poor phase. At a certain stage during phase demixing, the polymer-rich phase is solidified into a solid matrix by crystallization or vitrification, while the polymer-poor phase develops into the pores. The performance of this membrane depends largely on the morphology formed during phase separation and solidification. The thermodynamic basis of the immersion precipitation method, which is the phase diagram of the nonsolvent/solvent/polymer system, has been well developed [2- 4]. Some mass transfer models in 1D have been done to study the kinetics of the immersion precipitation process before phase [5-9]. A small number of studies have looked at the onset of phase demixing [10,11]. A better understanding of the kinetics of the immersion precipitation process is needed. In this present work, a ternary Cahn-Hilliard formulation incorporating a FloryHuggins homogeneous free
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