Gas Transport in Interacting Planar Brushes

Recent experiments on melts of spherical nanoparticles (NPs) densely grafted with polymer chains show enhanced gas transport relative to the neat polymer (without NPs). Simulations on such systems do not reproduce these experimental trends. As a means of understanding this unexpected behavior, here we consider the simpler case of two interacting planar brushes, under conditions representing a polymer melt far below its critical point (i.e., where the free volume or holes act akin to a poor solvent). Computer simulations illustrate, in agreement with mean-field ideas, that the density profile far away from the walls is flat but with a value that is marginally larger than that of the corresponding polymer melt under identical state conditions. We find that tracer particles, which represent the gas of interest, segregate preferentially to the grafting surface, with this result being relatively insensitive to the nature of polymer-surface interactions. These brush layers therefore correspond to heterogeneous transport media: gas molecules near the grafting surface have accelerated dynamics (presumably parallel to the wall) relative to the corresponding polymer melt, but they have slower dynamics in the central region of the brush. We therefore find that gas molecules perform hop-like motions.they spend a significant part of their time in the regions of fast transport, separated by motions where they hop from one surface to the other. These phenomena in combination lead to an overall speedup in gas dynamics in these brush layers relative to a polymer melt, in good agreement with the experimental data.

Automated summary

By studying the simpler case of two interacting planar brushes, it is found that gas molecules have accelerated dynamics near the grafting surface but slower dynamics in the central region of the brush. Gas molecules exhibit hop-like motions in the brush layers, spending most of their time in regions of fast transport separated by motions where they hop between surfaces. These phenomena in combination lead to an overall speedup in gas dynamics in the brush layers compared to a polymer melt, in good agreement with experimental data.