Kunio Nakamura

Viscoelastic properties of aqueous gellan solutions: the effects of concentration on gelation

Abstract The effects of gellan concentration ( C p ) on gelation were viscoelastically investigated in the concentration range from 0.50 to 1.33%. The gelation temperature ( T s ) shifted to higher temperatures with increasing gellan concentration. Gelation curves observed as a function of time at T s were well approximated by first-order reaction kinetics for gels of concentrations >0.81 % and the rate constant increased with increasing C p . The first-order kinetics were not applicable to gelation curves for gels of concentration T s for gels of C p of >0.75% indicated the relaxation behavior of Zener type. A master curve for 0.5% Cp changed to the behavior of power law relaxation. The gel modulus at high frequencies varied with C p 2 and that at low frequencies with C p 3.9 .

Ultrasonic velocity and circular dichroism in aqueous gellan solutions

Abstract Circular dichroism spectra and ultrasonic velocities of aqueous gellan solutions with and without added salts were measured at various temperatures. At higher temperatures the molecular ellipticity at 204 nm [ θ ] 204 is almost independent of the temperature but decreases steeply at a given temperature ( T c ) with further lowering temperature. With decreasing temperature, the difference in velocities between aqueous gellan solution and solvent ( Δ V) increases in the same manner as that of most aqueous systems, followed by a slight maximum or shoulder at T c , and then increases again. These abrupt changes in sound velocity and circular dichroism at T c may be caused by a thermally induced conformational change of gellan molecule in aqueous solutions. The decrease in the ultrasonic velocity was explained in terms of the dehydration of the water molecules. Effects of addition of some monovalent cations on the ultrasonic velocities were also studied.

Dynamic mechanical properties of aqueous gellan solutions in the sol-gel transition region

Abstract Influence of the gelation of aqueous gellan solutions on the dynamic viscoelastic behavior was investigated. Dynamic shear moduli for aqueous gellan solutions (0.60-3.0wt%) were determined over the frequency range from 0.01 to 10Hz and the temperature range from 10 to 60 °C by the method of annular pumping between coaxial cylinders. Cole-Cole diagrams in the complex viscosity were constructed, considering temperature and frequency as variables. The complex viscosity was characterized by the circular arc in the viscoelastic gel region and by the skewed circular arc in the sol region. The change in the relaxation from circular arc to skewed circular arc caused by gelation was interpreted in terms of the change in the frequency dependency from the Cole-Cole type to the Williams-Watts type.

Ultrasonic velocities in aqueous gellan solutions

The velocity of ultrasonic waves propagating through aqueous gellan solution and gel with and without sodium chloride was measured over a temperature range from 15 to 81°C. On the whole, the difference in velocities between aqueous gellan solution and solvent, ΔV, increases with cooling in a manner similar to most aqueous systems. On cooling, however, ΔV values do not increase, but decrease in some cases, at a certain temperature (Tv). The gelling temperature of the solution was determined by a falling ball method. Tv is in the vicinity of the gelling temperature and almost independent of the gellan concentration. While the gelling temperature increases with gellan concentration, on heating ΔV values decrease steadily. A hysteresis loop can be observed in measuring the velocity in aqueous gellan solution with falling and rising temperatures. The behavior of velocities was tentatively interpreted in terms of random coil-to-helix conformational change of the gellan molecule followed by dehydration of the water molecule. Similar results were obtained for gellan solutions containing sodium chloride.

Viscosities, densities, and sound velocities of dilute aqueous gellan solutions

Abstract The viscosities, densities, and sound velocities for dilute aqueous solutions of soluble gellan were measured over the temperature range from 5 to 45°C. With decreasing temperature, the solution viscosities increase very steeply at a certain temperature. This characteristic temperature increases with increasing gellan concentration. It is supposed that the gellan solution constructs a ‘pre-gel’ network structure, though rather loose, at lower temperature. From the density and sound velocity data the apparent specific volumes and the apparent specific adiabatic compressions of gellan in water were estimated at various temperatures. No pronounced transition was found in these thermodynamic properties at a ‘pre-gel’ temperature. The pre-gel network structure may be caused by the disruption of the hydrogen bond between gellan and water and the reconstruction of hydrogen bond between gellan molecules. It is considered that this process is accompanied by no significant change in volume or compression.

Interactions between Solvent Molecules and Networks in Biogels —Viscoelasticity, Strength—

It has long been known that the rheological properties of gels are affected by the interactions between solvent molecules and networks. The nature and properties of these interactions have been studied extensively during the last 40 years, and in many types of gel systems the frameworks of gel can now be predicted reasonably. The objective of this report was to investigate the relationship between dynamic viscoelastic properties and gelation.