Supramolecular polymer networks have promising potential to serve as self-healing soft materials. To benefit from this ability, quantitative understanding of the underlying mechanisms of macromolecular dynamics and transient association must be achieved. A key to obtaining such understanding is to understand the dynamics and relaxation of supramolecular polymer networks, which is often complexed by inhomogeneous polymer-network structures. To overcome this limitation, we use a set of regular star-shaped poly(ethylene glycol) polymers end-capped with terpyridine groups that can transiently coordinate to different metal ions, thereby forming transient supramolecular polymer networks with determined homogeneous architectures and determined binding strength. We study these networks in view of their mechanics, dynamics, and relaxation from both macroscopic and microscopic perspectives through the use of rheology, dynamic light scattering, and fluorescence recovery after photobleaching. These studies reveal that whereas in a long-term average the networks exhibit percolated connectivity, temporal detachment of one of the arms of the star-polymer building blocks allows for their relocation within the networks, entailing relaxation and flow on long time scales.

The dynamics of associating bonds in transient polymer networks exerts a paramount influence on their relaxation and timedependent mechanical properties. In particular, diffusive motion of polymers mediated by the dissociation and association equilibrium of reversible junctions can affect the materials' structural stability, dynamic mechanical properties, and a broad spectrum of functionality that arises from the constant motion of polymer chains. In this work, forced Rayleigh scattering is used to measure the diffusion of terpyridine end-functionalized four-arm poly(ethylene glycol) polymers transiently interlinked by zinc ions in organic solvent across a wide range of length and time scales. Phenomenological superdiffusion, where the scaling of the squared length dimension vs time has a power-law exponent larger than one, is demonstrated as an intrinsic feature of these networks due to the interplay of chain dissociation and diffusion. Outside the superdiffusive regime, normal Fickian diffusion is recovered on both large and small length scales. The data are quantitatively described with a previously developed two-state model of one fast and one slow diffusing species that are allowed to interconvert. The extracted diffusivities show concentration-dependent scaling in good agreement with the sticky Rouse model. Diffusion of the same polymers but with only three associating arms through the same transient networks is also investigated, which exhibits faster chain diffusivities compared to the case of the polymers with four associating arms. These experimental results quantitatively show the effect of sticker density and valency on chain diffusion in transient polymer networks.

Thermoresponsive polymer gels exhibit pronounced swelling and deswelling upon changes in temperature, making them attractive for applications in sensing and actuation. This volume phase transition can be discussed in terms of mean‐field theoretical pictures to assess at which conditions it occurs continuously or discontinuously. However, this treatment disregards static nano‐ and micrometer‐scale inhomogeneities in gel polymer networks, which are a common feature of these materials. To check for the impact of such structural complexity, droplet‐based microfluidics are used to fabricate sub‐millimeter‐sized gel particles that exhibit critical compositions at the border between continuous to discontinuous volume phase transitions, along with determined static spatial polymer‐network heterogeneity on the nano­meter and micrometer length scales, which is characterized by low‐field NMR. These different microgels are then used to study their swelling and deswelling volume phase transitions from a sub‐millimeter perspective. In this investigation, microgel particles with similar content of crosslinker exhibit similar swelling and deswelling, independent of their extent of static polymer‐network inhomogeneity, in agreement with mean‐field theoretical predictions.

The energy consumption for a novel desalination approach using charged hydrogels under externally applied pressure is experimentally measured and calculated. The salt separation is based on a partial rejection of mobile salt ions caused by the fixed charges inside the polyelectrolyte network. Self‐synthesized and commercial poly(acrylic acid) hydrogels are used to study the desalination performance in reference to sodium chloride solutions within the concentration range of 0.1–35 g L−1. The influence of various synthetic parameters, such as the degree of crosslinking (DC) and the size and shape of the particles, is investigated. Furthermore, the effect of process parameters including the amount of the feed solution, the applied pressure profile, and the swelling time of the hydrogel is discussed. The best energy estimation found so far, is 8.9 kWh m−3 fresh water if a poly(acrylic acid) with a DC of 5 mol% is used in an infinite large salt bath.

No abstract