EFFECT OF CHITOSAN MOLECULAR WEIGHT ON RHEOLOGICAL BEHAVIOUR OF CHITOSAN MODIFIED NANOCLAY IN HIGHLY HYDRATED STATES

Effect of chitosan molecular weight (M-cs) on the rheological properties of chitosan-modified clay (CMCs) at highly hydrated state was investigated. With special emphasis on its effect on the thixotropy of CMCs, the structure recovery at rest after underwent a pre-shearing process was further performed by the oscillatory shear measurements in the linear viscoelastic region. It was observed that both stress-strain behaviour and moduli of CMCs are closely correlated to M-cs. An interesting slope transition from negative to positive in the tan delta versus Ig omega plot is observed at omega = 2.5 rad/s. The value of tan delta < 0.5 over the entire range of frequency suggests a strong association in CMCs and a dominant elastic response of CMCs. Small peak-like transition of the shear viscosity, which denotes the yield region of the CMCs network, shifts to high shear rate with the increase of M-cs. Remarkably different thixotropic behaviour of these considered CMCs are observed relating to M-cs.G'(After) of CMCs with high M-cs shows a linearly monotonic increase with the increase of the testing time, which well matches the power law of G'(After) similar to t(n). G'(After) of those CMCs with low M-cs, however, is distinctly deviated from the power law and possesses an exponential increase. The deviation of G'(After) becomes more marked with the decrease of M-cs. Furthermore, the effect of the pre-shearing history on the thixotropic properties of CMCs is performed by employing a large range of the pre-shear rate. Analysis of the empirical Cox Merz rule indicates a failure of the rule in CMCs system. CMCs prepared from low M-cs chitosan are easier to be orientated at low shear rate.