| This work has focused on the rheological properties of chitosan solutions and chitosan-based hydrogels. Experimental results and mathematical modeling are presented and discussed in terms of molecular properties.; Chapter 1 states the scientific objectives of the work and the reasons underlying these objectives. Chapters 2 and 3 present an exhaustive study of chitosan in solutions. The experimental parameters investigated have been: temperature (25, 30, 37, and 45°C), chitosan concentration (0.5, 1.0, 1.5, 2.0% w/w), acid present in the aqueous chitosan solution (acetic, lactic, and hydrochloric), as well as the effect of ionic strength via salt addition (0.2M NaCl solution).; Rheological measurements in solution have been performed using two rheometers: a controlled-deformation rheometer with a coaxial cylinder geometry (Contraves Low Shear) and a controlled-stress rheometer with cone-and-plate geometry (Bohlin CVO). Viscosity and normal forces have been determined as well as the dynamic moduli G′ and G″. The results are presented and discussed with the help of master curves using classical models (Cross model for steady-shear tests and Maxwell model for oscillatory-shear tests) and empirical rules (Cox-Merz and Laun rules) for viscosity, normal forces, and oscillatory modules.; For the experimental conditions studied, chitosan solutions behave as non-Newtonian shear-thinning fluids with normal stresses: they show a low-shear rate (or frequency) Newtonian region and a high-shear rate (or frequency) non-Newtonian region. As expected, all material functions decrease with an increase in temperature and a decrease in chitosan concentration (tending to a Newtonian behavior).; Chapters 4 and 5 present experimental results and rheological modelling for chitosan/xanthan hydrogels in aqueous media. The experimental parameters investigated have been: hydrogel concentration (5, 6, 7, 8, 9, and 10% w/w), range of particle size of the dry hydrogel (125–75 μm and 75–55 μm), solvent used for the gel dispersion (distilled water, 0.05M NaCl solution, and phosphate buffer solution at pH = 7.0), the “waiting time” (time between the gel preparation and the start of the rheological test), as well as the use of two chitosan samples with different molecular weights and degrees of acetylation.; It has been found that chitosan/xanthan hydrogels behave like weak gels where their moduli are almost linear in the frequency range studied; they show an increase of the storage moduli with an increase in hydrogel concentration. (Abstract shortened by UMI.)... |
