Rheological Results

The effect of glycerol on planar La-phase has been discussed in detail in our previous paper. For nonclosed planar structures, the bilayers have a well-defined spacing that is the result of equilibrium between attractive van der Waals forces and repulsive undulation forces. Upon substitution of the solvent water gradually by glycerol-water mixed solvent, the attractive van der Waals forces approach zero. 

This is because the refractive index of the solvent and that of the bilayers becomes closer, so that the Hamaker constant in the expression of van der Waals forces gets close to zero  

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Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

In this paper we report some rheological studies of aqueous concentrated polystyrene latex dispersions, in the presence of physically adsorbed poly(vinyl alcohol). This system has been chosen in view of its relevance to many practical systems and since many of the parameters needed for interpretation of the rheological results are available (15-18). The viscoelastic properties of a 20% w/w latex dispersion were investigated as a function of polymer coverage, using creep measurements. 

Two models may be used to interpret the rheological results of the present flocculating system. Both models have been introduced by Hunter and his coworkers (13,14,29-32). ... 

For the intepretation of the rheological results, using the elastic floe model, it is necessary to have a model for the flocculated structure. For the present case, flocculation probably takes place by interpenetration of PVA tails under worse than 9- conditions for the chain. A typical floe may be assumed to consist of strings of particles linked together in a more-or-less three-dimensional network. The compactness of the floe (as measured by Cpp) is related to its strength by the number of chains, n, which pass through unit cross sectional area of the floe (29,31). n can be calculated from the total number of bonds per floe (36), i.e. 

Another fractal structure of interest is considered by Adler (1986). A three-dimensional fractal suspension may be constructed from a modified Menger sponge, as shown in Fig. 7(b). A scaling argument permitted calculating the effective viscosity of such a suspension however, this viscosity should be compared with numerical results for the solution of Stokes equations in such a geometry before this rheological result is accepted unequivocally. 

This is expected since the extrapolation procedure usually overestimates the yield value. The data of Figs. 3 and 4 show a rapid increase in yield value above a critical PEO concentration, 4. This concentration corresponds to the critical flocculation concentration of the free polymer. However, since the rise in Tg did not occur at a sharp value, the latter was taken as the intersection point at which the extrapolated hprizontal and vertical lines meet. This gave values of 4 of 0.02 0.002, O.OItO.001 and O.OOStO.OOl for PEO with of 20,000, 35,000 and 90,000 respectively. Moreover, the 4p values obtained from the two sets of rheological results were almost the scime within the error of locating p. 

Interpretation of rheological results The trends in the variation of Xg with are similar to those obtained recently (3) using a model polystyrene latex dispersion. The 1 values obtained in the present system are also close to tSose obtained with the model dispersion(0.017, 0.008 and 0.005 for PEO 20,000, 35,000 and 90,000 respectively). As mentioned before the sharp increase in x above + indicates that at the onset of flocculation the dispersions show marked viscoelasticity. The flocculation obtained at corresponds to the onset of the "semidilute" region, p, i.e., where the polymer coils in solution begin to arremge themselves in some... 

The above theories may be used for interpretation of the rheological results, in particular the increase in the Binghoun yield value, Tg, with increase in As mentioned... 

In this paper, we report some rheological results on the effect of addition of sodium bentonite, a commonly-used antisettling system, to a pesticide suspension concentrate. Steady state shear stress-shear rate measurements were carried out to obtain the yield value and viscosity as a function of shear rate. These e]q>eriments were supplemented by low deformation measurements to investigate the viscoelastic... 

The rheological results obtained using both high and low deformation measurements are all consistent, showing a rapid increase in the rheological parameters, Tg, G, and G... 

The quantitative interpretation of rheological results with such complex systems, as those used in the present investigation, is at present not possible. However, Hunter and collaborators (18-21) introduced two models to interpret the rheological... 

In summary, the QCM-D technique has successfully demonstrated the adsorption of pectin on the BSA surface as well as determined the viscoelastic properties of the pectin layer. As pectin concentrations increase, the adsorbed mass of pectin estimated from the Voigt model show higher values than those estimated from the Sau-erbrey equation because the former takes into account the hydrated layer. But the similar increase of thickness of pectin suggests that the pectin chains form a multilayer structure. In agreement with our previous rheology results, the main elastic character of the pectin layer in terms of Q-tool software tells us the network structure of the pectin layer on the BSA surface. In summary, QCM-D cannot only help to better understand the polysaccharide/protein interactions at the interface, but also to gain information of the nanoscale structure of polysaccharide multilayers on protein surface. 

For xanthan gum (XG) in water, Rochefort and Middleman observed that DR effectiveness is significantly reduced at high salt concentration. Their rheological results indicated that as the ionic strength increased, XG molecules underwent a transition from highly extended, semiflexible rods with disordered backbone structure to collapsed, semirigid rods with ordered backbone structure. 

Care should be taken when analysing the rheological results as coalescence leads to an increase in droplet size that is usually followed by a reduction in the viscosity of the emulsion. This trend is only observed if the coalescence is not accompanied by flocculation of the emulsion droplets (which results in an increase in the viscosity). Ostwald ripening can also complicate the analysis of rheological data. 

The above experiments are carried out at various storage times (perhaps every two weeks) and temperatures. From the change in rj o) and ct with storage time and temperature, information may be obtained on the degree and the rate of flocculation of the system. Clearly, the interpretation of these rheological results requires an expert knowledge of rheology, as well as measurements of particle size distribution as a function of time. 

As mentioned in Part 7.1, for polymer blends the relation between the steady-state shear viscosity and concentration can be quite complex. In the following discussion, the constant stress (not the constant rate) viscosity, corrected for the yield and time effects, will be considered. To illustrate flexibility of Equation 7.125 to describe (and thus to facilitate interpretation of the rheological results) r vs. < > dependence examples of computations are shown in Figures 7.24-7.31. 

Contrary to the reports discussed above, Liu et al. (60) reported rheological results indicating that the LLDPE-B (45,700 M, 2.1 PDI, and 13.5 ethyl branches per 1000 backbone carbons) and LDPE (102,000 M , 6.8 PDI, and 27.5 branches per 1000 backbone carbons) blend is an immiscible system. 

Rheology results are shown in Figure 5 for combination of dispersants with a commercially available HASE (hydrophobically modified alkali swellable acrylic emulsion) thickener. 

The rheological results plotted in Figures 2a and b for polyethylene and for polypropylene, respeetively show... 

This work was supported by JNICT, INVOTAN and The British Council. The authors are thankful to Dr. J. A. Covas for valuable discussions about the rheological results. 

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