Carboxymethylation-induced changes in Sesbania Gum: From microstructure to shear, tribological, and extensional rheology.

Sesbania gum, as a member of the galactomannan family, exhibits good thickening properties. However, its slow dispersion limits its applications. To address this limitation, carboxymethylation was performed to obtain carboxymethyl sesbania gum (CMSG) with different degrees of substitution (DS). After modification, both the bulk density and zeta potential increased with DS, and all CMSG samples exhibited markedly improved dissolution rate, up to a tenfold increase in dissolution rate compared with SG. Unlike the random coil conformation of native SG in water, CMSG exhibited a more extended and less coiled chain conformation with increasing DS. SEM observations further revealed that CMSG solutions formed more sheet-like morphologies. All changes in the microstructure are also reflected in the rheological behavior. The reduced molecular weight, combined with the rigid conformation and more extended chain structure of CMSG, resulted in lower viscosity and weaker shear-thinning behavior in steady shear experiments, and more pronounced viscous behavior in oscillatory shear, compared to SG. Moreover, the rigid conformation contributed to a lower friction coefficient in tribology experiments, while restricted chain mobility prolonged the extensional filament breakup time and increased extensional viscosity in capillary breakup extensional viscometry.