Hydrophilic microporous membranes were produced via the TIPS process using two hydrophilic ethylene-acrylic acid copolymers (zinc salt) with different co-unit contents. These co-polymers were confirmed to be hydrophilic by contact angle measurements. Low density polyethylene homopolymer was also used in this work as a reference to investigate the effect of polymer properties on the membrane structures. First, dynamic phase diagrams for these three polymer systems were determined. The cloud point curves shifted to higher temperatures and the crystallization temperature curves shifted to lower temperatures as the acrylic acid content of the co-polymer increased. The membrane structures were investigated and related to the initial polymer concentration and cooling rate. Increasing either the polymer concentration or the cooling rate decreased the pore size. Furthermore, structures with smaller pores and a skin layer at the top surface were obtained by introducing an evaporation process before the cooling to generate a polymer concentration gradient in the melted polymer solution.