Multiscale Quantitative Rheological Analysis of Composition−Temperature Relationships in Borate-Guar Hydrogels

Borate-cross-linked guar gum gels exhibit complex viscoelasticity driven by reversible covalent interactions between borate ions and cis-diol groups. Despite their widespread industrial use, limited knowledge of their thermorheological behavior makes it difficult to predict their performance at high temperatures. Here, 0.50 wt % guar gum dispersions with borax-to-guar ratios ranging from 1:1 to 1:8 (i.e., 0.5000-0.0625 wt % borax) were characterized from 25 to 140 °C using oscillatory rheometry. At 25 °C, gels displayed predominantly elastic behavior (<i>G</i>' ≫ <i>G</i>″); above 120 °C, viscous behavior dominated due to thermally induced cross-link dissociation. Time-temperature superposition was valid up to 120 °C, and Arrhenius analysis yielded activation energies of 72-85 kJ mol^-1 for junction relaxation and -10 to -21 kJ mol^-1 for modulus decay. A borax threshold near 0.1250 wt % delineated weak from strong gel regimes. Steady shear measurements revealed a three-region flow curve, including shear-thickening and shear-thinning regions that depended on temperature and cross-link density. All formulations deviated from the Cox-Merz rule at high shear. These findings support predictive design of thermoresilient borate-guar gels for energy and high-temperature applications.