Gelation kinetics of scleraldehyde-chitosan co-gels

The gelation kinetics of modified scleroglucan carrying various fractions of reactive aldehyde groups in the side-chains, scleraldehyde, and chitosan was determined using oscillatory shear rheometry to study the dynamic viscoelastic properties as a function of reaction time. The experimentally determined initial increase in the storage modulus per unit time, Delta G'/Delta t, for various scleraldehyde concentrations, C-Sc, degree of aldehyde substitution of scleraldehyde, D-Ald, and chitosan concentrations, C-Chi, revealed the existence of a lower critical concentration for gelation for both the two polysaccharides, C-Sc,C-0 and C-Chi,C-0. The observed power law coefficients rn and n, Delta G'/Delta t similar to (D-Ald C-Sc)(m) C-Chi(n), were both significantly larger than one in the employed relative concentrations ranges C-Sc/C-Sc,C-0 is an element of (1-20) and C-Chi/C-Chi,C-0 is an element of (1-10), which was accounted for by the total wastage reactions and network statistics. The rate of gel formation showed a maximum near pH 7 due to a delicate balance among several competing effects: (1) The free-amine concentration of the chitosan increases in the pH range 4.5-6.5 (pK(a) of the amino group), (2) the Schiff-base junction formation is acid catalyzed, and (3) the solubility of chitosan decreases when the pH exceeds 7. The increased aggregation resulting from increasing the pH from 7 to 7.6 yielded an increasing fraction of reactive groups involved in wastage reactions, yielding an increase both in C-Chi,C-0 and the power law coefficient n from 1.7 to 2.6. The temperature dependence of Delta G'/Delta t could be accounted for by incorporating an apparent activation energy E-a = 43 kJ mol(-1). The temperature dependence of G' of the fully cured scleraldehyde-chitosan co-gels was found to give rise to a shift from Delta G'/Delta T > 0 Pa/min to Delta G'/Delta T < 0 Pa/min, when the crosslink density increased. This suggests a change from entropy dominated elasticity at low crosslink density, to an enthalpy dominated elasticity when this density increases, possibly due to decreasing length of the elastically active chains. (C) 1998 Elsevier Science Ltd. All rights reserved.