Understanding penetrant transport in dense, dynamic polymer networks

Polymer networks are routinely used in applications involving mass and ion transport. In this talk, dense polymer networks crosslinked by dynamic covalent bonds where the mesh size can approach 1 nm are investigated with a range of fluorescent dyes up to 2.8 nm in length. With increased crosslink density, the glass transition (Tg) increases and dye diffusion rapidly slows. Large dyes which are effectively slowed in permanent networks show a large enhancement of diffusion coefficients when the networks contain fast exchanging dynamic bonds. Deconvolution of mesh size effects and segmental dynamics is pursued through a combined experimental-theory-simulation approach and the role of the mesh confinement is a much weaker effect than the slowing segmental dynamics. Penetrants which can reversibly bind with the network are also analyzed, and when the reaction is slow relative to the diffusive hopping time a massive slowing of diffusion is observed. Finally, the role of network topology will be discussed comparing networks with regular dynamic bond distributions versus clustered bonds.