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

Butyl acrylate polymer networks were synthesized to understand the effect of permanent cross-links on large, anisotropic dye diffusion. The average degree of polymerization between cross-links (Nx) was decreased from 92 to 2, leading to a 2 order of magnitude decrease in the probe translational diffusion coefficient (D). The mesh size (ax) was determined from Young's modulus, and D showed a weakening single exponential decay dependence on the ratio of the probe long axis (dlong) to ax with increasing experimental temperature (T0). Cross-linking led to a significant increase in the glass transition temperature (Tg) of the networks and its breadth determined by both calorimetry and dielectric spectroscopy. Segmental relaxation times and inverse diffusion coefficients exhibited weakening single exponential relationships with dlong/ax and Tg/T0 at lower measurement temperatures. These results provide new insights regarding the effect of covalent cross-links on probe diffusion necessary for the design and development of separation membranes.