Polymer concentration dependence of the gel point for chemically modified biopolymer networks using small amplitude oscillatory rheometry

Solutions of xanthan gum form a strong gel upon the addition of aluminium ions (Al(III)). We have previously reported on the stoichiometric (ratio of crosslinker to polymer present) dependence of the thermal gel point and final gel strength. The results presented here expand on previous work, presenting the thermal gelation characteristics for a range of xanthan concentrations and different stoichiometries. The limiting stoichiometric ratio, below which gelation does not occur, is found to be independent of polymer concentration. The relaxation exponent is shown to decrease with increasing polymer concentration, whilst the temperature of gelation Tg, is shown to increase. The ‘gel strength’ measured at the complete extent of gelation is shown to increase with increasing polymer concentration and the previously defined effective stoichiometry, re, becomes less pronounced. For systems of constant stoichiometry and varying polymer concentrations, the storage modulus is shown to follow a power law relationship with an exponent of approximately two. All observed trends support the previously proposed mechanism of gelation for xanthan-Al(III) systems and provide new insight into the gelation of xanthan in the presence of metal ions.