In this work, asymmetric flat-sheet membranes based on the commercial polyetherimide–siloxane copolymer Siltem, whose gas permeability is one order of magnitude higher than that of polyetherimide Ultem 1000, were first obtained. Based on the solubility analysis and Hansen solubility parameters, N‑methyl-2-pyrrolidone was selected as the principal solvent, as it provides the best thermodynamic compatibility with both polymers. It was shown that Siltem STM1700 is soluble in a significantly wider range of organic solvents, including tetrahydrofuran and N-butylpyrrolidone. Furthermore, solutions of Siltem in N‑methyl-2-pyrrolidone exhibit substantially lower viscosity compared to those of Ultem, which allows the preparation and processing of polymer solutions at concentrations up to 36 wt %. Asymmetric flat-sheet membranes were fabricated by the phase inversion method, wherein the polymer solution was brought into contact with a nonsolvent coagulation medium. The obtained membranes were characterized by scanning electron microscopy, and their mechanical properties were investigated. To develop an enhanced porous structure in the asymmetric flat-sheet membranes based on commercial polyetherimides, tetrahydrofuran was introduced into the casting solution. The addition of tetrahydrofuran to the casting solution resulted in improved mechanical characteristics of the membranes: the elastic modulus increased by a factor of 2 to 2.5, and the tensile strength also increased.
Grishkov et al. (Sun,) studied this question.