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Polymer membranes offer a promising method for creating highly effective water filtration devices, as well as life-saving medical drugs. Most polymer membranes are made through immersion precipitation which involves submerging a polymer solution into a nonsolvent bath. This process induces phase separation creating a polymer rich phase which becomes the membrane, and a polymer poor phase which becomes the membrane pores. It is still largely unknown why certain unique membrane microstructures form, as there are many physical interactions that take place during immersion precipitation. To better understand membrane microstructure formation, we used a multi-fluid model for ternary polymer solutions to alter the glass transition concentration in simulated membrane formation. The glass transition concentration determines at what polymer concentration a membrane vitrifies. We simulated the immersion precipitation process for many different glass transition concentrations. The resultant membranes’ structures were characterized by their porosity at different time steps in the membrane formation process. Our results showed that not all glass transition concentrations affect membrane formation the same and allowed us to determine that certain glass transition concentrations stop phase separation from occurring. In addition, we found that the pore sizes become asymmetric as the glass transition concentration is moved closer to the initial film composition. However, when the glass transition concentration is very close to the initial film composition, pore size asymmetry is not present. Because of this, we speculate that pore size asymmetry is influenced by the glass transition concentration being close to the binodal. These results allow us to better understand how the glass transition concentration affects the microstructure of polymer membranes.