(624a) Understanding the Role of Crosslinks, Dynamic Bonds, and Specific Interactions on Mass and Ion Transport in Polymer Networks

Understanding the role of crosslinks, dynamic bonds, and specific interactions on mass and ion transport in polymer networks

Mass and ion transport are key aspects of a sustainable future including low energy separations and energy storage. Polymer networks provide a model platform for tuning the modulus, glass transition, and mesh size of the material. When dynamic bonds are present, the viscosity can also be controlled through bond selection. The competition between all of these factors is critical to understanding the design and implementation of new materials for controlling transport.

We first probed mass transport in permanent ionic networks with precisely 11 carbons between crosslinks and cationic sites at the junctions. Using bulky, ionic-liquid-like charges, the membranes showed high selectivity for the separation of toluene from heptane. Both are seven carbon molecules, and the difference is attributed to specific interactions with the network and mesh confinement effects. The distance between crosslink points is comparable to the occupied volume of the penetrants, and molecular shape effects were probed using a series of alkanes spanning from seven to 14 carbons. Next, permanent neutral networks with a butyl acrylate backbone were studied. Here, the crosslinking is statistical and the average degree of polymerization between crosslinks spanned from ~100 to 1. A fluorescent dye was investigated using Fluorescence Recovery after Photobleaching to monitor translational diffusion, and a power law relationship was found when diffusion was plotted against ratio of dye length to mesh size. With increasing temperature, diffusion increases for a given network. However, a larger diffusion coefficient is observed at a fixed value of T_g/T₀ for a series of experiments performed at different measurement temperatures (T₀) on networks with varying glass transition temperatures (T_g). The competition of mesh size with slowing segmental dynamics will be discussed.

Ion transport in dynamic networks is a promising route towards self-healing and recyclable electrolytes. We prepared a series of ethylene oxide (EO) networks with dynamic covalent bonds (boronic esters) and a precise number of EO repeat units between crosslinks. Adding salt increased conductivity, primarily through changes in Tg, while the interactions of anions with boron led to a large decrease in modulus and viscosity, in contrast to linear PEO electrolytes. Analysis of the molar conductivity versus fluidity showed superionic behavior at low salt concentrations, but subionic behavior with added salt. Changing the dynamic bond type to vinylogous urethanes leads to fundamentally different behavior, as the cation of the salt coordinates. A tradeoff between conductivity and network relaxation is observed, pointing to the important of dynamic bond selection on ion transport through polymers.