Effect of temperature and cross-linking density on rheology of chemical cross-linked guar gum at the gel point

The evolution of viscoelasticity within it solution of guar gum (GG) in the presence of in excess of cross-linker (glutaraldehyde, Ga) has been monitored, at different temperatures (7, 15, 25 and 37 degrees C), evaluating the sol-gel transition by means of dynamic mechanical experiments. Furthermore, at 25 degrees C, the samples were characterized as a function of the different amounts of cross-linker. The gelation time of the systems. evaluated with the loss tangent. decreased with increasing temperature and, at a fixed temperature (25 degrees C), decreased with increasing cross-linker concentration. At the gel point a power-law frequency dependence of the dynamic storage modulus (G approximate to omega(n)) and loss modulus (G(n) approximate to omega(n)) was observed. The value of n, evaluated according to three different methods (n(slope), considering the parallelism or the two moduli, n(tan) from the frequency-independence of tan delta, and from the crossing point between the apparent exponents of G' and G '', n(app)), decreases with increasing cross-linking densities and with increasing temperature. The fractal dimension (d(f)) was determined for the incipient GG/Ga hydrogels. The value of d(f) increases with increasing cross-linker concentration and decreases with increasing temperature. This trend of d(f) Suggests a more tight structure of the network at higher cross-linking densities and at lower temperatures. The activation energy (E-a) of the chemical reaction between GG and Ga was estimated: the experimental value was close to those obtained in the case of similar chemical reactions among, polymers and different kinds of cross-linkers. (C) 2008 Elsevier Ltd. All rights reserved.