Dynamics at the Interface of Structured Block Co-Polymer Thin Films with Polar Solvents: Molecular Dynamics Simulations Insights*

Structured ionic co-polymers consist of multiple blocks whose dynamics facilitate their uses. Here, the dynamics at the interface of an ABCBA pentablock co-polymer with polar solvents is studied by molecular dynamics simulations. The ionic block C is a sulfonated polystyrene (sPS) with sulfonation fraction f=0.55, B is polyethylene-propylene (PEP), and A is poly(t-butyl styrene) (tBS). Simulations are performed using LAMMPS with the polymer and propanol modeled with OPLS-AA force field. The interface evolution with exposure time to propanol is followed and the results are compared with those for the polymer-water interface. The air interface of the neat polymer is dominated by the PEP block. With exposure to propanol, more ionic groups migrate to the interface, similar to the polymer-water interface. However, in contrast to water exposed films, the interface remains dominated by the PEP blocks. Remarkably, the interfacial width of the membrane-propanol system grows with time while the width of the membrane-water interface decreases. Further, water molecules associate predominantly with the ionic blocks and break the ionic clusters while propanol associates with both the ionic and non-ionic polymer segments.