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

The strict definition of t_G has been considered in detail by Chambon and Winter [75, 76, 77], who defined it as the condition at which tan(G’’/G’) is independent of frequency. Their treatment requires accurate measurements of G’ and G’’ over a wide range of frequencies both before and after t_G and additionally, requires that G’ and G’’ do not vary appreciably during each frequency sweep. Unfortunately, in the present study, G’ and G’’ were difficult to accurately assess prior to t_G due to measurements being undertaken at the lower stress limit of the rheometer. In addition, during the gelation process, G’ and G’’ were often found to vary significantly over the duration of each frequency sweep. As a result, t_G could not be accurately assessed using the treatment of Chambon and Winter [75, 76, 77]. The G’-G’’ crossover method of Tung and Dynes [78] was instead used and, while not providing a strict measure of t_G, does allow a comparison of the crosslinking behaviour at a common stage of gelation. It is, however, worthy of note that given the change in δ from ca. 80° to ≤1° is rapid, particularly at the higher temperatures investigated (see Fig. 2), it is expected that the values of t_G calculated using the G’-G’’ crossover method are in reasonable agreement with the Chambon and Winter gel point definition.