A novel thermally-activated crosslinking agent for chitosan in aqueous solution: a rheological investigation

The use of 2,5-dimethoxy-2,5-dihydrofuran (DHF) as a temperature-controlled gelation agent for chitosan under acidic conditions has been examined by dynamic oscillatory and viscometry techniques. In particular, the rate and extent of gelation have been examined over a range of different temperatures (40–98 °C), DHF concentrations (10–100 mM) and pH conditions (0.9–2.1). The gelation time, tG, decreases, and the maximum gelation rate increases substantially as a function of rising temperature. When fit with a simple Arrhenius function, the tG data yield an activation energy for gelation of 55±8 kJ mol⁻¹. Gelation is found to occur on the shortest time-scale, and the strongest gels result, at the highest DHF concentrations investigated. Similarly, the gelation rate and gel strength are highest for the most acidic solution conditions examined. Experimental findings are interpreted in terms of a competition between the crosslinking reaction (which drives gel formation, and is initially dominant) and protolytic decomposition of chitosan (which disrupts the gel structure, and becomes increasingly important as time progresses). Syneresis phenomena additionally impact results obtained at DHF concentrations ≥50 mM