Dynamic Covalent Bond Exchange Enhances Penetrant Diffusion in Dense Vitrimers

Polymer membranes are commonly pursued for passive separations, which require less energy than distillation. Typically, glassy materials are chosen for gas separations due to extreme sensitivity to size differences in penetrants, whereas rubbers are used for liquid separations due to solubility differences. Vitrimers, dynamic polymer networks with associative bond exchange, are an emerging class of polymers that have gained much attention as self-healing and recyclable materials. Here, in a new direction for vitrimers, we demonstrate the utility of the dynamic bond for eliciting large differences in molecular transport through dense polymer networks. Specifically, permanent and dynamic polymer networks with boronic ester crosslinks are synthesized across a broad range of dynamic bond densities, and the diffusion of a large aromatic dye is measured using fluorescence recovery after photobleaching. When dynamic bond exchange is accelerated by the presence of neighboring nitrogen groups, penetrant transport is enhanced relative to permanent networks by a factor that increases with increasing dynamic bond density and can exceed 1 order of magnitude. Dynamic bonds without the neighboring group effect produce no enhancement of diffusion. These results are interpreted in terms of the ratio of the bond exchange time (inferred from small-molecule experiments) to the hopping time of the penetrant (extracted from the diffusion coefficients). Our results point to a general route for imparting selectivity into polymer membranes through dense crosslinking and dynamic covalent chemistry.