Additives and Relaxation Fluidize Molecular Transport Within Covalent Adaptable Networks

Incorporating dynamic covalent bonds into polymer networks could offer stimuli-responsive control over molecular transport, often requiring additives to facilitate bond rearrangement. Here super-resolution localization microscopy was employed to examine fluorescent probe transport in unhydrated, elastic networks containing thiol-thioester dynamic covalent chemistry (DCC). In static, DCC-free networks, displacement distributions revealed frequent trapping and slow, anomalous, and heterogeneous diffusion. Introducing a dynamic catalyst additive markedly increased mobile fraction and produced diffusivity that was more fluid-like. These effects were observed with several classes of fluorescent probes, two distinct additives, and for networks with or without DCC. Experiments with a photo-releasable catalyst (NPPOC-TMG) separated additive effects from those of activated exchange. Catalyst release conferred additional fluidization of the network. Importantly, these changes were not accompanied by macroscopic plasticization, suggesting that small additive concentrations can strongly affect transport in confining environments. Spatial analysis further indicated that transport enhancement was primarily due to restructuring of an initially heterogeneous network rather than local bond exchange near the diffusing probes. Additive incorporation and post-cure exchange likely reduced defects and homogenized the crosslink density. In sum, these results demonstrate how DCC modulates transport, emphasize the importance of the activation method, and highlight its potential in separation and controlled-release systems.