Multiprotocol Rheological Characterization of the Order–Disorder Thermal Transition in a Xanthan Gum Solution

Xanthan gum (XG) forms weak-gel aqueous dispersions whose solid-like elasticity is progressively lost on heating through a helix-to-coil (order–disorder) transition. How far that transition reaches into the nonlinear and time-dependent mechanical response remains not fully characterized. A 0.5% (w/v) XG solution was characterized from 10 to 60 °C by steady shear, strain-amplitude and frequency sweeps, step-strain relaxation, creep–recovery, and large-amplitude oscillatory shear (LAOS), and by 1H NMR spectroscopy. A common broken-line criterion applied to 33 descriptors across seven data sets, 28 of which were resolvable, gave an average breakpoint temperature of 32.4 ± 1.7 °C. LAOS revealed strain-dependent weakening of the intracycle nonlinearities (strain stiffening/softening, shear thickening/thinning) upon heating. The dissipation ratio ϕ increased with temperature within the linear regime, but the dependence reversed and was much weaker at large strain amplitudes. The weak Type III G″ overshoot attenuated roughly 8-fold on heating, so the amplitude-sweep response approached Type I strain thinning.