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

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The evolution of viscoelasticity within a solution of guar gum (GG) in the presence of an excess of cross-linker (glutaraldehyde, Ga) has been monitored, at different temperatures (7, 15, 25 and 37 °C), evaluating the sol–gel transition by means of dynamic mechanical experiments. Furthermore, at 25 °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 °C), decreased with increasing cross-linker concentration. At the gel point a power-law frequency dependence of the dynamic storage modulus ( G′≈ ω n ) and loss modulus ( G″≈ ω n ) was observed. The value of n, evaluated according to three different methods ( n slope, considering the parallelism of the two moduli, n tan, from the frequency-independence of tan δ, 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.