(667g) Mesoscale Simulation of Nanoscale Segregation in Polyelectrolyte Membranes
Nanostructured polyelectrolyte membranes (PEM) are widely used as perm-selective diffusion barriers in fuel cell technologies and electrochemical processing. PEMs are also suitable for protection against chemical warfare agents. PEMs are made of nanostructured polymers that contain hydrophilic and hydrophobic fragments. For example, in Nafion polymer (Du Pont) hydrophilic sidechains terminated by sulfonate groups are attached to a hydrophobic perfluorocarbon backbone. Upon hydration, PEM segregates into hydrophobic and hydrophilic subphases. The former is made by the hydrophobic organic backbone, while the latter comprises water, counterions and ionic groups of the polymer, forming a dynamic network of channels in the hydrophobic organic matrix. With respect to transport and adsorption of chemicals, solvated PEM represents a nanoporous material. Water diffuses easily through the network of pores comprised of hydrophilic channels, while more hydrophobic chemical agents get trapped in the essentially immobile hydrophobic subphase.
Mesoscale simulations of nanoscale segregation in PEM provide a powerful tool for optimization of the polymer structure and membrane composition. In this work, we describe two different approaches to mesoscale modeling of PEM, using Nafion and sulfonated polystyrene-polyolefinepolystyrene triblock copolymers as examples. First, we developed a dissipative particle dynamics scheme suitable for PEM. The polymers were presented as sequences of beads, either linear (triblocks) or branched (Nafion) linked by harmonic bonds. Weak harmonic angles were introduced for rigidity. We applied two methods for the DPD parameter derivation from (1) radial distribution functions obtained in atomistic molecular dynamics simulations of smaller fragments and (2) thermodynamic properties of reference solutions. The second approach to mesoscale modeling of PEM is based on self-consistent field (SCF) theory, which we extended to complex systems that include a charged triblock copolymer, water, and polar chemical agents. SCF is parameterized from available experiments coupled with ab-initio and thermodynamic modeling of reference solutions. With these techniques we explored nanoscale segregarion in Nafion (DPD only) and sSEBS (DPD, SCF). We predicted the morphology of hydrated Nafion with equivalent weight between 1200 and 1800D and 4-17 wt % water content, observing a transition between separated hydrophilic clusters and a continuous 3D network of hydrophilic channels. No particular segregation morphology in Nafion was detected. In sSEBS, the morphology was more regular, transiting from reverse micelles at lower hydration to lamellae and even reverse micelles as water and DMMP content increased. We discuss the influence of polymer composition and hydration on the shape and connectivity of hydrophilic channels. Using this information on the membrane morphology, we perform lattice Monte Carlo simulations of water and chemical agent transport through PEMs and compare our results with the experiments.