Self-Assembled Morphologies of Polystyrene-block-poly(ethylene oxide)/1-Ethyl-3-methylimidazolium Thiocyanate Membranes by Mesoscopic Dynamics Simulation

The stability of gas separation membranes cast from diblockcopolymer/ionicliquid (IL) blends depends on the resulting self-assembled morphologiesof the cast films. Therefore, controlling the copolymer/IL morphologyby tuning parameters such as IL loading and copolymer block size ratiois essential to prevent the membrane from leaching out the IL at hightransmembrane pressures. In the present study, we used the dynamicmean-field density functional method to investigate the self-assemblyof polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymers in 1-ethyl-3-methylimidazolium thiocyanate([EMIM][SCN]) at different PS:PEO block size ratios and IL loadings(10-90 vol %) at room temperature (298 K). The IL was observedto be either confined by the hydrophilic PEO phase (designated asIL-PEO) due to strong IL-PEO interactions or yield a separate partiallyor fully encapsulated microphase (IL-Micro). The copolymer morphologiesobserved herein were lamellar (L), cylindrical (C), body-centeredcubic (BCC), and spherical micelle (S). The dominant copolymer/ILmorphology on the ternary phase diagram was L/IL-PEO, which formedat medium loadings of the three components (40 vol % < PS <80 vol %, PEO < 90 vol %, and IL < 50 vol %). The IL-Micro morphologiesappeared as transition phases to the IL-PEO phases, typically at lowIL loadings and PS:PEO block size ratios. We also investigated themorphology evolution of select copolymer/IL compositions. Overall,the G, L, and C copolymer morphologies were observed at low to mediumIL loadings, while the BCC and S copolymer morphologies appeared athigher IL loadings. The potential applications of these self-assembledmorphologies could be further explored by investigating the role ofelectrostatic interactions and varying the types and loadings of ILs,as well as the type of the diblock copolymers, to discover new membranesystems with unique properties for gas separations.