Diffusion of probe polymer in gellan gum solutions during gelation process studied by gradient NMR

The diffusion coefficients of pullulan added in gellan gum solutions as a probe polymer were measured using pulsed-field-gradient stimulated-spin-echo (PFG-Ste) 1H NMR in order to investigate the gelation mechanism and gel structure. The echo intensity of gellan was steeply decreased at around the gelling temperature Tgel indicating stiffening of the gellan chains upon aggregation and formation of the network. The diffusion coefficient of pullulan Dpull increased with decreasing temperature below Tgel. This result suggests that the decrease in concentration of solute gellan in the interspaces of the network below Tgel, as evidenced by the decrease in its echo intensity, leads to an overall decrease of hydrodynamic interactions between gellan and pullulan. The characteristic hydrodynamic shielding length ξ was calculated from the relation Dpull/Dpull,0 = exp(−Rh/ξ), where Dpull,0 is Dpull in dilute solution and Rh is the hydrodynamic radius of pullulan. The temperature dependence of ξ, which was investigated for varying concentrations of several cations, was found to follow closely the gelling temperature, and in particular, showing a very similar thermal hysteresis.