Dynamic Covalent Bond Exchange Enhances Penetrant Diffusion in Dense Vitrimers

Abstract

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.

Reversible Reactions, Mesh Size, and Segmental Dynamics Control Penetrant Diffusion in Ethylene Vitrimers

“…Networks with C 6 diol (50 mg, 2 equiv) and a fast boronic acid cross-linker (1 equiv) were prepared by the same procedure as discussed above, and a stoichiometric ratio of alcohol and acid groups was maintained. The difference in exchange kinetics is a factor of ∼10,000 as reported in prior work on small molecules and vitrimers. , The term precise only refers to the carbon spacing, not the network topology. Fourier transform infrared spectroscopy (FTIR, Figure S9) of the networks was performed at 80 °C to confirm the absence of the broad OH peak in the 3000 to 3500 cm –1 range corresponding to unreacted species of the diols and boric acid.…”

“…Recently, we demonstrated that vitrimers with fast exchanging dynamic bonds can greatly enhance the diffusion of a large aromatic dye relative to a permanent network with the same chemistry and volumetric cross-link density. 43 Model butyl acrylate networks with boronic ester cross-links were synthesized over a broad range of cross-link densities with either fast or slow bond exchange relative to the probe hopping time. If the characteristic exchange time is within an order of magnitude of the diffusive hopping time (determined from the experimentally measured diffusion coefficient), then the exchange process facilitates probe motion.…”

“…If the network mesh is conserved, the ability of a penetrant to move may be restricted relative to that of a dissociative dynamic network. Recently, we demonstrated that vitrimers with fast exchanging dynamic bonds can greatly enhance the diffusion of a large aromatic dye relative to a permanent network with the same chemistry and volumetric cross-link density . Model butyl acrylate networks with boronic ester cross-links were synthesized over a broad range of cross-link densities with either fast or slow bond exchange relative to the probe hopping time.…”

“…The difference in exchange kinetics is a factor of ∼10,000 as reported in prior work on small molecules and vitrimers. 43,44 The term precise only refers to the carbon spacing, not the network topology. Fourier transform infrared spectroscopy (FTIR, Figure S9) of the networks was performed at 80 °C to confirm the absence of the broad OH peak in the 3000 to 3500 cm −1 range corresponding to unreacted species of the diols and boric acid.…”

“…Analogous ethylene networks were prepared with a fast cross-linker reported in prior work 44 and investigated by us for probe diffusion in acrylic vitrimers. 43 The neighboring nitrogen groups accelerate the rate-limiting proton transfer step of the boronic transesterification and increase the exchange rate by a factor of ∼10,000. 52 The faster cross-linker has six carbons connecting the boronic acids, and thus we only prepare the C 6 network for these comparisons (Scheme 1).…”