(403e) Coarse-Grained Molecular Dynamics Simulation of Phase Behavior in Poly(styrene)-Block-Poly(ethylene glycol)/1-Ethyl-3-Methylimidazolium Thiocyanate Mixtures

Isotropic, dense polymer membranes have traditionally been used for gas separation applications. Recently, diblock hydrophobic-hydrophilic copolymers in ionic liquids (ILs)¹ have been shown to exhibit high separation efficiencies for different gas mixtures, such as CO₂/CH₄. ILs possess high electrical and thermal stability, excellent ion conductivity, and low vapor pressure. Moreover, they are considered ideal media for the self-assembly of polymers. Developing efficient copolymer/IL gas separation membranes relies upon a thorough understanding of the equilibrium phase behavior of the membrane material. Although recent literature has increasingly targeted the morphology of copolymer/ILs from an experimental perspective,² theoretical reports of this kind are relatively scarce. In this work, we investigated the phase behavior of a poly(styrene)-block-poly(ethylene glycol) (PS-b-PEG) in 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) using molecular dynamics (MD) simulation. For this purpose, we first constructed coarse-grained, mesoscale models of the PS-b-PEG/[EMIM][SCN] mixtures with two different IL loadings (10 and 40 wt.%) and copolymer block size ratios (20 and 50% IL-philic PEG) using the dissipative particle dynamics (DPD) approach. We mapped the PS and PEG as separate bead types and [EMIM][SCN] as three bead types, representing a charged imidazole ring, a side CH₃ chain, and an [SCN] anion. DPD calculations derive interactions between the different beads from linking the Flory-Huggins coarse-grained and DPD parameters via solubility parameters. For the IL, which is the charge-carrying component of the copolymer/IL mixture, we first calculated the Hansen solubility parameters (precise for systems having polar forces) using MD simulation. Next, we used these parameters to set the correct DPD interaction parameters between the IL and copolymer beads. We used a Slater-type distribution to define the electrostatic interactions, where the charges were spread out over a finite volume to prevent the collapse of the beads. Next, we equilibrated the models at 298 K and 1 atm using the LAMMPS software package. We are currently analyzing the phase behavior of the self-assembled copolymer/IL mixtures in terms of ternary phase diagrams and density distribution plots and will present our preliminary data. The outcomes of this research provide molecular insights into the development of next-generation copolymer/IL gas separation membranes with enhanced separation efficiencies.

References:

(1) Sasikumar, B.; Arthanareeswaran, G.; Ismail, A. F. Recent Progress in Ionic Liquid Membranes for Gas Separation. J. Mol. Liq. 2018, 266, 330–341.

(2) Bennett, T. M.; Chambers, L. C.; Thurecht, K. J.; Jack, K. S.; Blakey, I. Dependence of Block Copolymer Domain Spacing and Morphology on the Cation Structure of Ionic Liquid Additives. Macromolecules 2018, 51 (21), 8979–8986.