Tracer Diffusion in Heterogeneous Polymer Networks

The diffusion of linear polymer chains in swollen polymer networks is studied with different extents of spatial heterogeneity of their crosslinking density. Structural heterogeneity is imparted to these gels by photo-crosslinking of polymer precursor chains in semidilute solution, which comprise either just one batch of precursors, yielding gels with just a little spatial heterogeneity, or mixtures of two different batches of precursors, one with high and one with low degree of functionalization with crosslinkable moieties, yielding gels with more pronounced heterogeneity. In addition, heterogeneous gels are also prepared from crosslinkable precursor chains and defined fractions of pre-crosslinked microgel particles that serve as doped-in nanometer-scale domains of high local crosslinking density. The resulting gels are probed by static light scattering and shear rheology, and the self-diffusion of fluorescent linear tracer chains within the gels is studied by fluorescence recovery after photobleaching. The outcome of these investigations is that spatial heterogeneity of a given crosslinking density in swollen polymer networks decreases their ability to store mechanical deformation energy, but has no marked impact on the above-micrometer-scale diffusivity of linear tracer chains.