Role of bond exchange on mass, ion, and thermal transport
Bond exchange in polymer networks has received much attention as a route to recycling and circularity of materials which are typically not reprocessable. In addition, the presence of dynamics bonds can lead to large impacts on the transport of guest molecules relevant for membrane separations, ion conducting electrolytes, and thermal transport.
In recent work, our group has investigated acrylic networks as a model platform for molecular separations. Increasing the crosslink density of a permanent network leads to a massive drop in the diffusion coefficient of a fluorescent probe molecule. Adding dynamic bonds which exchange on a similarly “fast” timescale to the dye hopping, this leads to a large enhancement of transport and provides a new approach to selective separations. If the added bonds are “slow”, they are effectively static to the dye and provide no benefit.
Dynamic bonds comprise a broad range of chemistries, and we have probed the role of bond chemistry on electrolyte performance. When boronic esters are chosen, the anion of an added salt coordinates with the network and leads to a large drop in viscosity and modulus, while not impacting the conductivity substantially. The ability to decouple mechanics from ion transport is an important goal in solid electrolytes. If a different dynamic bond (vinylogous urethane) is used, the cation coordinates and serves in a catalytic role for bond exchange. The viscosity can be decreased with minimal impact on modulus or conductivity. These unique relationships are enabled by the incorporation of the dynamic bonds.
Many polymer networks are amorphous, but using short and precise linkers allows for crystallization when dynamic bonds are present. We have observed that the melting temperature of ethylene dynamic networks can evolve over months, in stark contrast to linear polymers. The long term evolution is associated with dynamic bond exchange allowing the crystal to perfect itself. A key implication of this morphological development is that dynamic networks can have much higher thermal conductivities than a linear polymer of similar crystallinity.