Understanding Probe Diffusion in Dense Polymer Networks and the Roles of Mesh Size, Tg, and Segmental Dynamics*

The effect of permanent crosslinks on probe diffusion was investigated in dense poly(butyl acrylate), varying the number of repeat units between crosslink junctions from 2-100 repeat units. Fluorescence Recovery After Photobleaching (FRAP) was used to determine the translational diffusion as a function of Tg-normalized temperature (Tg/T) and the size ratio between the dye and average distance between crosslink junctions (d/l). Dielectric spectroscopy was used to determine segmental relaxation times and help understand how probe diffusion was coupled to segmental mobility, as Tg and Tg-breadth significantly increased with decreasing repeat units between crosslink junctions. Probe diffusion exhibited a single exponential decay dependence on both Tg/T and d/l, seeing a two order of magnitude reduction in the diffusion coefficient over the range of crosslink density investigated. Effective diffusion time scales showed a weaker dependence on Tg/T and d/l than predicted from segmental relaxation times, indicating probe diffusion partially decoupled from the segmental mobility and that the increasing breadth may play a critical role in this decoupling.