Soft metastable materials are of fundamental importance to biological systems. Recent measurements of the dynamics in soft biopolymer networks have shown a long- time relaxation similar to that of other soft glassy systems, such as colloidal gels. Currently there is little explanation for the physical processes facilitating this long-time relaxation. To help elucidate this process we have modified the anionic polysaccharide pectin to produce a range of polymer fine structures with which to produce gels manifesting a range of junction-zone lengths, and hence different sensitivities to thermal fluctuations.
We measured the structural properties of the networks using small-angle x-ray scattering (SAXS), which gave access to structual information on length scales from 600nm to 0.6nm. The dynamics were measured using a recently developed photon-correlation-spectroscopy-based imaging technique to obtain both spatial and temporal information about the dynamics.
Results from SAXS measurements show that polymers modified to have longer binding regions indeed formed longer junctions, with a length of 66 nm, while those with shorter binding regions had correspondingly short junctions of 8nm. The radius of junctions stayed constant at ~1 nm, corresponding to the radius of four pectin chains. The dynamics of gels formed with longer junctions were considerably slower in comparison to gels with shorter junctions. These results suggest that the long-time relaxation is through the slippage of junctions, as is indicated by the faster relaxation in gels with weaker and shorter junctions.