Diffusion through Reversibly Associating Polymer Networks

Many natural macromolecules, like proteins and DNA, are equipped with site-specific, non-covalent molecular interactions. These interactions lead to intricate secondary structures such as the double helix, self-assembled phospholipid membrane bilayers, and precisely folded protein structures that are vital to life and rely on non-covalent interactions to “guide” molecular organization. Mankind has begun to borrow such concepts from nature to engineer responsive materials. In recent years, for example, the use of strong, highly directional hydrogen bonds has enabled polymers with temperature-tunable architectures to be engineered. Sitespecific hydrogen bonding and ionic interactions in solution can lead to aggregation, gelation, or sudden viscosity changes that are triggered by slight changes in polymer concentration, pH, or temperature. In the melt, rigid and elastic polymer networks can be reversibly transformed into a low viscosity polymer melt simply by heating. This new materials concept is playing an important role in the development of recyclable (thermoplastic) elastomers. The quest to fully understand structure-property relationships of polymers decorated with hydrogen bonding groups has opened a new field at the interface of polymers and supramolecular chemistry. Our laboratory has synthesized novel polymer networks which contain both covalent crosslinks and non-covalent crosslinks (Figure 1). Non-covalent crosslinks arise from the presence of ureidopyrimidone (UPy) sidegroups, which are well known to undergo strong, yet reversible, H-bond association. At low temperatures, the rate of H-bond dissociation is slow, and the material behaves as if it is highly crosslinked—like a rigid solid. At higher temperatures, H-bond dissociation is fast, and the material behaves as if it only contains covalent crosslinks—like a soft elastomer. Consequently, mechanical properties show unusual temperature dependence. Shape-memory responses of these and similar networks have been carefully studied. While a great deal of research has focused on controlling polymer rheological properties, to our knowledge no studies have examined how reversible association affects molecular transport. The primary goal of our research is to determine how the rate of molecular transport across dynamic hydrogen bonded networks depends on temperature. We hypothesize that the presence of long-lived hydrogen-bonds will decrease the rate of molecular transport at lower temperatures. A secondary question we raise is how the molecular size of the penetrants influences molecular diffusion. To address this issue, molecular transport of ethanol across polymer networks is compared to that of a much larger organic dye.