“…At higher concentrations of methanol, phase separation is approached, and this seems to promote the formation of “softer” gels. The finding that the dynamic moduli for incipient gels are nearly frequency independent has previously been reported ,,− for other polysaccharide systems that have not been exposed to shear-induced gelation. The gel strength parameter S raises with increasing methanol content in the mixture, and this feature probably reflects that both the cross-linking density and the chain rigidity increase.…”
Turbidity, swelling, and rheological features of semidilute systems of pectin in methanol-water media of different composition have been investigated. By increasing the percentage of methanol in the mixture, the thermodynamic properties of the pectin/methanol/water system become poorer, as shown by increasing turbidity and decreasing swelling. Effects of oscillatory and steady shear flows on intermolecular associations and gelation of pectin in methanol/water mixtures are reported. The effects of methanol concentration on the growth and structure of shear-induced gels, stabilized through hydrogen bonds, are analyzed. Steady shear measurements on these systems reveal shear thickening at low shear rates and disruption of intermolecular associative junctions at high shear rates.
“…At higher concentrations of methanol, phase separation is approached, and this seems to promote the formation of “softer” gels. The finding that the dynamic moduli for incipient gels are nearly frequency independent has previously been reported ,,− for other polysaccharide systems that have not been exposed to shear-induced gelation. The gel strength parameter S raises with increasing methanol content in the mixture, and this feature probably reflects that both the cross-linking density and the chain rigidity increase.…”
Turbidity, swelling, and rheological features of semidilute systems of pectin in methanol-water media of different composition have been investigated. By increasing the percentage of methanol in the mixture, the thermodynamic properties of the pectin/methanol/water system become poorer, as shown by increasing turbidity and decreasing swelling. Effects of oscillatory and steady shear flows on intermolecular associations and gelation of pectin in methanol/water mixtures are reported. The effects of methanol concentration on the growth and structure of shear-induced gels, stabilized through hydrogen bonds, are analyzed. Steady shear measurements on these systems reveal shear thickening at low shear rates and disruption of intermolecular associative junctions at high shear rates.
“…the discussion below). The finding that the dynamic moduli around the gel point are nearly frequency independent has previously been reported for other polysaccharide systems. − At temperatures below the gel temperature, G ‘ becomes much larger than G ‘ ‘, which is characteristic of the solidlike state that evolves in the postgel region. 5 Frequency dependences of the storage modulus and the loss modulus at different stages of the gel-forming process for the pectin−chitosan mixture ratios indicated.…”
The synergistic interaction between pectin and chitosan in aqueous acid solution and in the gel phase has been studied by oscillatory shear measurements. Mixtures of pectin and chitosan form thermoreversible gels over a broad composition range by lowering the temperature. The value of the gelation temperature depends on the composition of the mixture, with low values for mixtures with low pectin contents. For incipient gels, a power law can describe the frequency dependence of the complex viscosity, with power law exponents close to -1. The gel evolution of pectin-chitosan mixtures upon a temperature quench below the gel point has been studied. Evidence is provided for a relation between gelation and phase separation in the process of temperature-induced gelation of pectin-chitosan mixtures. A simple model is proposed to rationalize the gelation process in these systems.
“…At the time of gelation, the G ‘(ω) and G ‘ ‘(ω) curves become parallel and power laws in frequency are observed (see the lower inset of Figure ) with power law exponents of 0.09. The finding that the dynamic moduli for incipient gels are nearly frequency-independent has previously been reported for other polysaccharide systems. − 2 Illustration of methods for the determination of the gel point for the pectin (1.5 wt %)−water system. Viscoelastic loss tangent as a function of time for the angular frequencies is indicated.…”
We report effects of oscillatory and steady shear flows on intermolecular associations and gelation in a semidilute aqueous solution of pectin in the absence and presence of a hydrogen bond breaking agent (urea). The effect of urea on the growth of intermolecular complexes stabilized through hydrogen bonds is analyzed. It is shown that when the systems are exposed to high shear rates, prominent intermolecular associative junctions emerge upon return to low shear rates.
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