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.