Steady shear, small amplitude oscillatory shear and capillary break-up extensional rheology measurements of rod-like polymers

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Steady shear, small amplitude oscillatory shear (SAOS) and capillary break-up extensional measurements have been conducted to investigate the rheological properties of two different “rod-like” polymers (the polysaccharides, xanthan gum (XG) and scleroglucan (SGN)) both in aqueous solution, over a wide range of concentrations (0.01% -0.75% w/w). Detailed shear viscosity measurements show that both polymers are strongly shear-thinning. Above a critical concentration (c*),  0.067% for XG and > 0.01% for SGN, the zero-shear-rate viscosity scales with concentration as approximately c5.18 for XG and c3.18 for SGN. SAOS measurements, only possible for relatively concentrated solutions (c/c* >> 1), show that the longest relaxation time also scales in a power-law like way. A capillary break up extensional rheometer (CaBER), in conjunction with a high speed camera recording at 2000 frames per second, was used to probe the extensional properties of the solutions, again these measurements were only possible for relatively high concentrations. In these CaBER tests the filament diameter was observed to thin exponentially over a significant period of the break-up process in agreement with the simple one-dimensional theory of Entov and Hinch (1997) for “ideal” elastic liquids. Relaxation times, or more correctly, characteristic times for uniaxial extensional stress growth together with an estimation of the steady uniaxial extensional viscosity were obtained from this data. Such relaxation times were found to scale approximately linearly with concentration, as  c1.3, for both polymers, although the steady uniaxial extensional viscosity was found to be slightly higher for the SGN solutions.