Dynamo-mechanical and rheological characterization of guar gum hydrogels

Polymeric systems gained great interest in the last decades as specialties in the field of drug delivery, being very attractive for their peculiar physico-chemical characteristics. In this paper we report the characterization of a chemical network based on guar gum (GG) cross-linked with glutaraldehyde (Ga). The system was studied at three different temperatures, 7, 25 and 37 °C and at different ageing times in order to evaluate the influence of these two parameters on its stability. The experiments were carried out with a Texture Analyzer thus hardness, cohesion and Young modulus of the hydrogel were evaluated in the different conditions. Furthermore, relaxation experiments were performed and their interpretation, according to the Maxwell generalized model, allowed to describe the mechanical behaviour in terms of materials viscoelasticity theory. The kinetics of the chemical cross-linking was followed at 37 °C by means of rheological measurements, i.e. recording the mechanical spectra of the gelling system. The storage and loss moduli followed a power law, expression of the mechanical and structural self-similarity which evolves during the cross-linking reaction. The critical exponent at the gel point was evaluated, together with the critical time corresponding to the infinity connectivity of the system and the phase angle between stress and strain. Furthermore, also the fractal dimension was evaluated assuming a complete screening of the excluded volume of the chains at the critical point. Information about the release behaviour of the hydrogel, used as a matrix for modified drug delivery of model molecules with different steric hindrance, is also given.