Rheological behaviours of guar gum derivatives with hydrophobic unsaturated long-chains

Three guar gum derivatives with hydrophobic unsaturated long-chains were successfully synthesized, including oleic guar gum (OGG), linoleic guar gum (LGG) and erucic guar gum (EGG). The guar gum derivatives were characterized by Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). Moreover, comparative studies on the rheological behaviours of guar gum (GG) and three guar gum derivatives were performed. Regardless of their hydrophobic side groups, all sample solutions showed the properties of non-Newtonian shear-thinning fluids, and their complex viscosities (eta*) increased significantly with increasing concentration. In contrast to GG, OGG or EGG, LGG exhibited the strongest shear-thinning property when the concentration is 1 wt%. It was found that the frequency-dependent viscous (G '') and elastic modulus (G ') curves of GG and its derivatives aqueous solutions cross at a specific frequency. This crossover frequency decreases with increasing sample concentration, indicating that the viscoelasticity is highly dependent on polymer concentration. In addition, when the molar substitution (MS) of the repeating sugar units in the guar gum derivatives is less than 0.08, the LGG solution showed more obvious solid-like rheological properties than the OGG and EGG solutions at the same concentration.