Understanding the Effect of Permanent Crosslinks in Dense Polymer Networks on Probe Diffusion

The effect of permanent crosslinks on organic probe diffusivity was investigated in networks spanning 1-50 mol% crosslink density to better understand how mesh induced confinement affects mass transport. Fluorescent dyes were placed in n-butyl acrylate networks and the translational diffusion coefficient was determined using Fluorescence Recovery After Photobleaching (FRAP) as function of the size ratio between the dye and average distance between crosslink junctions (d/l). In lightly crosslinked networks, an order of magnitude decrease in dye diffusivity is found that can be well described by a model where diffusion is slowed by segmental dynamics as the glass transition temperature increases. At larger d/l ratios, the diffusivity was observed to further decrease by an order of magnitude and was fit to a hopping model. In the densest networks, the dye diameter was larger than the distance between crosslink junction and dye diffusion, suggesting the network is hindering translation. Comparisons with scaling theories will also be discussed.