Gel point studies for chemically modified biopolymer networks using small amplitude oscillatory rheometry

Abstract

Solutions of xanthan gum form a strong gel upon the addition of aluminum ions (Al(III)). Previous studies have reported that the addition of chromium and ferric ions induces gelation in xanthan solutions; this study presents a characterisation of the transition from solution to gel for xanthan in the presence of Al(III). Xanthan-Al(III) solutions are shown rheologically to undergo a sol–gel transition upon heating, the extent of gelation is controlled by the temperature at which the system equilibrates. The rapid thermal equilibration of xanthan-Al(III) provides an ideal system to test the applicability of two established gelation criteria [1,2]. At the gel point, congruency in G′ and G″ is observed over 2–3 decades of frequency demonstrating the applicability of the method of Winter and Chambon [1]. The difficulties associated with determining an equilibrium relaxation modulus (E) as a criterion for gelation [2] are discussed.The gelation behaviour of xanthan-Al(III) gels is characterised for a range of polymer crosslinker combinations (stoichiometric ratios) and a limiting stoichiometric ratio below which gelation does not occur is determined. The relaxation exponent (n) is determined at the limiting stoichiometric ratio to be n=0.22. A value of 0.22 for n is not in agreement with previous work for systems at a limiting stoichiometric ratio. The relaxation exponent is found to vary between 0.22 and 0.16 for a 0.5wt% xanthan and a range of Al(III) concentrations. The temperature at which the gel point is observed, Tg, follows an asymptotic relationship with stoichiometric ratio suggesting the importance of the order–disorder transition of xanthan molecules on the gelation process of xanthan-Al(III) gels.