GELATION KINETICS OF SCLERALDEHYDE-CHITOSAN CO-GELS

The gelation kinetics of modified scleroglucan carrying various fracti ons of reactive aldehyde groups in the side-chains, scleraldehyde, and chitosan was determined using oscillatory shear rheometry to study th e 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 ch itosan concentrations, C-Chi, revealed the existence of a lower critic al 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 significantl y 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 net work statistics. The rate of gel formation showed a maximum near pH 7 due to a delicate balance among several competing effects: (1) The fre e-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 i s acid catalyzed, and (3) the solubility of chitosan decreases when th e pH exceeds 7. The increased aggregation resulting from increasing th e pH from 7 to 7.6 yielded an increasing fraction of reactive groups i nvolved in wastage reactions, yielding an increase both in C-Chi,C-0 a nd the power law coefficient n from 1.7 to 2.6. The temperature depend ence of Delta G'/Delta t could be accounted for by incorporating an ap parent activation energy E-a = 43 kJ mol(-1). The temperature dependen ce of G' of the fully cured scleraldehyde-chitosan co-gels was found t o give rise to a shift from Delta G'/Delta T > 0 Pa/min to Delta G'/De lta T < 0 Pa/min, when the crosslink density increased. This suggests a change from entropy dominated elasticity at low crosslink density, t o an enthalpy dominated elasticity when this density increases, possib ly due to decreasing length of the elastically active chains. (C) 1998 Elsevier Science Ltd. All rights reserved.