Flow behaviour, linear viscoelasticity and surface properties of chitosan aqueous solutions

A study of rheological properties of chitosan solutions was carried out in terms of polymer concentration (0.05-2.5 wt%) and temperature (5-25 degrees C). The chitosan solutions studied exhibited Newtonian behaviour at concentrations lower than 0.25 wt%. Above this value, the rest of the systems showed shear-thinning behaviour. The viscoelastic properties of chitosan solutions above 0.25 wt% were characterized by oscillatory shear measurements under small-deformation conditions and the results showed fluid-like viscoelastic behaviour. Time-concentration and time-temperature superposition methods were carried out to obtain an overall master curve; where the rheological properties of chitosan solution at a specific temperature or concentration can be predicted over a broad time scale. The fact that the activation energy for steady shear flow was higher than for oscillatory shear promoted deviations from the Cox-Merz rule, when temperature was decreased at the highest chitosan concentration studied In addition, their dynamic surface tension behaviour and surface viscoelastic properties were explored. The time-dependent decrease in surface tension along with the limiting elasticity of the interface seems to indicate that commercial chitosan may play the role of an emulsifier as well as a stabiliser of O/W emulsions. (C) 2010 Elsevier Ltd. All rights reserved.