John Meadows

Control of the properties of xanthan/glucomannan mixed gels by varying xanthan fine structure.

The interaction of native xanthan gum, deacetylated xanthan gum and depyruvated xanthan gum with konjac glucomannan has been studied using DSC and controlled stress rheometry. In the absence of electrolyte the DSC cooling curves for native xanthan and deacetylated xanthan showed a single peak and there was a corresponding sharp increase in the storage modulus indicating gel formation. It is apparent that on cooling, association of the konjac glucomannan with the native xanthan molecules is triggered by the xanthan coil-helix transition. In the presence of electrolyte, there were two DSC peaks observed. The higher temperature DSC peak was attributed to the xanthan coil-helix transition while the lower temperature DSC peak was attributed to konjac glucomannan-xanthan association as noted by an increase in the storage modulus. The gels formed were much weaker than those in the absence of electrolyte. The DSC cooling curves for depyruvated xanthan in the absence of electrolyte showed two peaks. The higher temperature peak was attributed to the coil-helix transition while the lower temperature peak corresponded to gelation as noted by an increase in the storage modulus. The gels were very much weaker than for native xanthan gum and deacetylated xanthan gum.

Shear and extensional viscosity characteristics of a series of hydrophobically associating polyelectrolytes

The shear and extensional viscosity characteristics of aqueous solutions of a series of hydrophobically associating polyelectrolytes have been investigated and compared. The polymers studied consisted of a methacrylic acid/ethyl acrylate copolymer backbone containing varying levels of incorporation (0–2 mol%) of a pendant C18 hydrophobe. The extensional viscosities of all solutions were found to be higher than their corresponding shear viscosities, with the calculated Trouton ratio tending towards the predicted value of three at low strain rates but increasing with increasing extensional strain rate thereafter. In both flow modes, the viscosities of the solutions increased with increasing concentration and hydrophobe content of the polymers. In contrast to the purely shear-thinning characteristics of the polymer solutions observed in shear flow, the extensional viscosity–strain rate profiles of a number of systems were found to contain an initial region of strain-thickening, although strain-thinning behaviour was observed at higher strain rates. For any given system, the strain rate at which the evident maxima in extensional viscosity occurred decreased with increasing polymer concentration. In the presence of additives such as excess surfactant or a miscible alcoholic non-solvent, which are known to disrupt regions of intermolecular hydrophobic associations, the extensional viscosity of a typical solution of an associative polyelectrolyte was found to be significantly reduced and no longer to exhibit a maximum in the viscosity–strain rate profile.

Competitive adsorption of polymers and surfactants at the solid/liquid interface

The competitive adsorption of three cationic polymers (JR polymers) and a cationic surfactant (CTAB) onto a silica surface from a low ionic strength medium has been investigated. Competition between polymers showed that smaller molecules were adsorbed initially, preventing the subsequent adsorption of larger polymers. Adsorption from combined polymer and surfactant systems showed that the smallest polymer (JR 125) was excluded from the surface, the intermediate molecular mass polymer (JR 400) was almost unaffected by the surfactant and the largest molecular mass polymer (JR 30M) was partially excluded from the surface. These experiments showed the importance of having sufficient bonding groups on the polymer and the relative rates of diffusion to the surface.

Small strain deformation measurements of konjac glucomannan solutions and the influence of borate cross-linking.

The dynamic rheology of aqueous solutions of konjac glucomannan has been evaluated over a range of concentrations up to 2.35%, and the effect of borate cross-linking of such solutions evaluated in the range 0.02-40 mM borate. In preliminary work, conventional parallel plate geometries were employed and in situ cross-linking was investigated. For borate cross-linked samples a superior method, however, was found to be measurement of pre-formed cores of cross-linked polymer into which a four-bladed vane geometry was introduced. In order to compare with other associating polymer systems, rheological data were analysed by defining plateau moduli, corresponding relaxation times and zero shear viscosities and the scaling behaviour of these parameters with polymer and cross-linker concentrations was established. Maxwell fits and time-concentration superposition procedures were investigated. The rheological properties of the cross-linked polymer were shown to be the result of both increased network connectivity and retarded network dynamics.

Small deformation measurements of transient gels using vane rheometry

Measurement of a model transient gel system, i.e. pre-gelled borate cross-linked konjac glucomannan (KGM) has been facilitated by the use of a controlled stress rheometer in conjunction with a stainless steel vane geometry. Calibration of the vane was undertaken in both shear flow and oscillation and enabled reproduction of data recorded using traditional cone and plate and couette fixtures. The transient nature of the borate cross-links coupled with the sharp blades of teh vane facilitated ‘healing’ of the gel around the geometry, resulting in minimal disturbance to the gel. Alternative techniques employed to measure these gels include in-situ gelation in a parallel plate geometry. Using this method difficulties in achieving sample homogeneity imposed serious restrictions upon the range of polysaccharide and borate concentrations that could be studied, and long in-situ gelation times were unavoidable. The microstructure of borate cross-linked KGM gels has been investigated by small strain oscillation measurements using the vane and the trends in the network relaxation times with variation in both polysaccharide concentration and borate concentration established.