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

Solutions of xanthan gum form a strong gel upon the addition of aluminum io ns (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 th e presence of AI(III). Xanthan-Al(III) solutions are shown theologically to undergo a sol-gel transition upon heating, the extent of gelation is contr olled by the temperature at which the system equilibrates. The rapid therma l equilibration of xanthan-Al(III) provides an ideal system to test the app licability of two established gelation criteria [1,2]. At the gel point, co ngruency in G' and G " is observed over 2-3 decades of frequency demonstrat ing the applicability of the method of Winter and Chambon [1]. The difficul ties 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. Th e relaxation exponent (n) is determined at the limiting stoichiometric rati o to be n = 0.22. A value of 0.22 for n is not in agreement with previous w ork for systems at a limiting stoichiometric ratio, The relaxation exponent is found to vary between 0.22 and 0.16 for a 0.5 wt% xanthan and a range o f Al(III) concentrations. The temperature at which the gel point is observe d, T-g, follows an asymptotic relationship with stoichiometric ratio sugges ting the importance of the order-disorder transition of xanthan molecules o n the gelation process of xanthan-Al(III) gels. (C) 2000 Elsevier Science L td. All rights reserved.