Introduction

1. SCLEROGLUCAN XANTHAN GUM f= hf / h0 hf 8.8 mm 0.04 s 0.02 s 0 s 0.06 s 0 s 0.06 s 0.04 s DMID (t) 0.02 s D = 4 mm h0 = 2 mm t =- 20 ms t > 0 0.12 s 0.10 s 0.08 s 0.14 s 0.12 s 0.10 s 0.08 s 0.14 s XANTHAN GUM SCLEROGLUCAN XANTHAN GUM SCLEROGLUCAN Steady shear, small amplitude oscillatory shear and capillary break-up extensional rheology measurements of rod-like polymers Azuraien Japper@Jaafar, Robert J. Poole Department of Engineering, University of Liverpool, Brownlow Street, Liverpool, L69 3GH United Kingdom • Steady and oscillatory shear measurements • Plotting o versus concentration provides a convenient way of estimating c*, the so-called critical overlap concentration, for both polymers. Below c*, which for XG is approximately 0.067% (670 ppm) and for SG, 0.019% (190 ppm), both solutions are dilute and o scales approximately as c1.44, above the critical overlap concentration, interactions between the molecules occur and o increases much more rapidly with concentration. Introduction The rheology of two different semi-rigid “rod-like” polysaccharides in aqueous solutions, Xanthan gum (XG, Ketrol TF from Kelco) and Scleroglucan (SG, Tinocare GL from Ciba) were measuredover a wide range of concentration (0.01% - 0.75% w/w). Xanthan gum is a polyelectrolyte produced using the bacterium Xanthomonas campestris. Scleroglucan is a non-ionic polysaccharide produced by the fungi of genus Sclerotium. The molecular weights of the polymers are reported by the suppliers to be in excess of 106 g/mol. • Methods • Steady and oscillatory shear measurements • TA Instrument Rheolyst AR 1000N controlled stress rheometer • Small amplitude oscillatory shear (SAOS) measurements only possible for higher concentration solutions • All SAOS measurements were conducted in linear viscoelastic region • Capillary break-up measurements • ThermoHaake capillary break-up extensional rheometer (CaBER) with laser micrometer (resolution~10m) • High-speed digital imaging using Dantec Dynamics Nano Sense MKIII high-speed camera at 2000 frames per second with Nikon 60mm f/2.8 lens Capillary break-up measurements • In this simple technique a cylindrical liquid bridge of the ‘test’ liquid is formed between two circular plates 4 mm in diameter. An axial step strain is then applied (i.e. the end plates are rapidly pulled apart to a fixed separation) which results in the formation of an elongated liquid thread. The thread diameter reduces due to surface tension and information about the extensional properties of the liquid can be deduced from the evolution of the filament midpoint diameter which is monitored using a laser micrometer. • A simple one-dimensional analysis, neglecting axial curvature and assuming that the filament is axially uniform, shows that the filament can be characterised simply by its midpoint diameter: • Alternatively you may calculate a Hencky strain at the midpoint, the strain rate and estimate an apparent ‘extensional viscosity’: • Trouton ratio is calculated from: • Newtonian-like linear thinning of the filament was observed sometime later after the initial step strain • Steady uniaxial extensional viscosity increases with concentration • The magnitude of the Trouton ratio confirms the non-Newtonian behaviour of both polymers as Tr >>3. However, the Trouton ratio exhibits a decrease in magnitude with increasing concentration 0.5% Xanthan gum at 20C (i,f=0.5, 2.2) 0.5% Scleroglucan at 20C (i,f=0.5, 2.2) • Conclusions • G is greater than G until the crossover frequency, which increases as the concentration decreases indicating that the behaviour corresponds to semi-rigid polymer chains and some entanglements still exist, as suggested by Lee (2001). • Due to the semi-rigid nature of the molecules, Newtonian-like linear filament thinning behaviour was observed in capillary break-up experiments. • Steady uniaxial extensional viscosity increases almost linearly with concentration. These results suggest that as concentration increases, the polymer exhibits a more ordered molecular structure resulting in increase molecular contact between molecules which subsequently leads to stronger molecular interactions and hence greater extensional behaviour. • The magnitude of the Trouton ratio (>>3) confirms the non-Newtonian behaviour of these polymer even though Newtonian-like linear thinning was observed in the capillary break-up experiments.