Abstract
The rheological properties of aqueous xanthan solutions have been studied in 0·1 m NaCl at 25°C at concentrations up to 0·8 g dl −1. Two critical concentrations are found from viscosity measurements: C∗ = 0·03 g dl −1 , the limit of the dilute domain, and C∗∗ = 0·11 g dl −1 beyond which η sp 0 varies as C 4·2. The viscoelastic properties also show evidence of a critical concentration close to C∗∗. On each side of C∗∗, a different master curve is obtained using a time-concentration superposition principle. Beyond C∗∗, the density of junction zones of the elastic network does not seem to increase. A mechanism of parallel packing of chain segments could explain these results. This mechanism is compatible with the rigid structure of the xanthan chain and is in agreement with the hypothesis of side-by-side association proposed by several authors.
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