Structure–property relationships in microwave-assisted carboxymethylated tara gum: balancing substitution, molecular degradation, and rheology

Carboxymethylated tara gum (CMTG) is an anionic galactomannan whose macroscopic behavior depends on the interplay between degree of substitution (DS), molecular degradation, and charge density. In this work, a rapid microwave-assisted carboxymethylation strategy was developed and optimized using response surface methodology and compared with a conventional reflux etherification method. Microwave irradiation enabled reproducible substitution levels close to unity (DSCM ≈ 1.0) within 20 min, compared with 3 h under reflux conditions, but simultaneously promoted significant alkaline-induced chain fragmentation. Spectroscopic analyses (FTIR, ¹H and ¹³C NMR) confirmed successful etherification, whereas SEC analysis revealed multimodal molecular weight distributions with substantial low-molecular-weight fractions generated under microwave processing. Rheological analysis demonstrated that carboxymethylation markedly reduced the zero-shear viscosity (η₀) from 6.55 Pa·s for native tara gum to 0.59–1.99 Pa·s for CMTG derivatives, together with a reduction in relaxation time (λ) from 0.62 to 0.18 s, indicating decreased chain entanglement and faster structural relaxation. Despite this molecular fragmentation, highly substituted microwave-synthesized CMTG exhibited pronounced shear-thinning behavior, improved water solubility, and enhanced viscoelastic responsiveness associated with electrostatic chain expansion and charge-induced intermolecular interactions. Overall, microwave-assisted carboxymethylation provides a rapid and tunable route to engineer galactomannans with controllable substitution, molecular architecture, and rheological response for potential thickening, stabilization, and delivery applications.