Low-rate shear viscosity

There have been many studies of hydrophobic crosslinking. For example, Flynn40 produced a series of poly (acrylamides) (PAM) and recorded the low shear rate viscosity as a function of the chain overlap parameter. This was performed for a range of molecular weights and concentrations. This procedure was then repeated with the same polymer backbone but with the addition of differing concentrations of alkyl side chains which give rise to hydrophobic association (HPAM). A comparison between hydrophobe and non-hydrophobe polymers is shown in Figure 5.30. 

The status of current theories of the low shear-rate viscosities (rj0) of polymer melts (and concentrated solutions) was reviewed by Berry and Fox in 1968 (52), since when there has been little development. The viscosity of linear polymers of low MW at constant temperature (or more precisely constant free volume is proportional to Mw, but at high MW it is proportional to a higher power M, where x is empirically about 3.4-3.5 the change of slope of a ogt]0/logMw plot is fairly abrupt The high exponent 3.4—3.5 is attributed to the effects of chain... 

At present, it cannot be said that there is a satisfactory theory of even the low shear-rate viscosities of branched polymers, since no existing theory accounts for the observed enhancement of melt viscosity in the cases referred to. Treatments that do not even predict the sign of an effect reliably can hardly be expected to predict its magnitude. 

The experimental evidence concerning the effects of LCB on the non-Newtonian behaviour of polyethylene melts is not as extensive as might be wished. Guillet and co-workers (167) studied fractions of both linear and branched polyethylenes and found that, for a given low shear-rate viscosity. 

Mendelson (169) studied the effect of LCB on the flow properties of polyethylene melts, using two LDPE samples of closely similar M and Mw plus two blends of these. Both zero-shear viscosity and melt elasticity (elastic storage modulus and recoverable shear strain) decreased with increasing LCB, in this series. Non-Newtonian behaviour was studied and the shear rate at which the viscosity falls to 95% of the zero shear-rate value is given this increases with LCB from 0.3 sec"1 for the least branched to 20 sec"1 for the most branched (the text says that shear sensitivity increases with branching, but the numerical data show that it is this shear rate that increases). This comparison, unlike that made by Guillet, is at constant Mw, not at constant low shear-rate viscosity. 

Graessley and co-workers have studied the rheological properties of solutions of branched PVAc in diethyl phthalate (178, 188), using polymer concentrations of 0.17, 0.225, and 0.35 g ml-1. At the lowest concentration, the low shear-rate viscosity was simply related to [17], so that it was lower for branched polymers the equation ... 

These equations are solved numerically under the assumptions of velocity, shear stress, and temperature continuity at all interfaces. They use the Sabia 4-parameter viscosity model (69), because of its ability to include the Newtonian plateau viscosity, which is important for multilayer extrusion, because of the existence of low shear-rate viscosities at the interfaces. 

The effective enlargement of the diameter of charged spheres leads to enhancement of the low-shear-rate viscosity. According to Russel (1978 Russel et al. 1989), the zero-shear viscosity out to second order in 0 of a disordered suspension of charged spheres is... 

The use of thickeners such as H EC and xanthan gum this will increase the low shear rate viscosity of the medium and hence slow down the diffusion of the small particles, preventing their entry into the oil droplets. These thickeners can produce gels in the continuous phase that is viscoelastic, and this can prevent particle diffusion. 

FIG. 5.5 Measured flow curves for aqueous solutions of two different HASE polymers, illustrating the effect of hydrophobe size (see legend) on the low shear rate viscosity. 

Figure 7. Low shear rate viscosity characteristics of formulations I and II from Table II (Brookfield LVF viscometer)...

Viscosity Maxima. The low-shear-rate viscosities of both commercial and model associative thickeners below their c /, values will increase with the addition of conventional low molecular weight surfactants or coalescing aid (22). With HEUR polymers, solution viscosities are observed to increase, achieve a maximum value, and then decrease with continued increase in surfactant concentration (23). This type of behavior is illustrated (Figure 5) for four commercial HEURs with a nonionic surfactant (typical of that used in coating formulations). A similar behavior has been observed (24) with a classical anionic surfactant and hydrophobically modified (hydroxy-ethyl)cellulose (HMHEC) and is reviewed in Chapter 18. Intermicellar networks, formed by the participation of one or more hydrophobes from different polymers in the micelles of conventional surfactants, were again recently suggested (25) to affect viscous solutions. 

In HEC-thickened formulations, low-shear-rate viscosities increase with decreasing latex particle size. This effect has been a major limitation in formulating small-particle latices. The phenomenon appears to arise from electro viscous, hydration, or flocculation effects, not a depletion layer mechanism. Associative thickeners achieve efficient viscosity in coating formulations via participation in synthesis and formulation surfactant micelles to form pseudo macromolecules and via an ion-dipole interaction between the cations of surface carboxylate groups on the latex and the ether linkages of the associative thickener. Generally, an excess of synthesis surfactant is found in the production of small-particle latices. The achievement of lower viscosities in small-particle ( 100 nm) latex coatings thickened with associative thickener appears to occur by extensive disruption of the polymer hydrophobe s participation in intermicellar networks. 

As the shear stress was increased, the viscosity of an ionomer decreased in common with high molecular weight polymers. There was a dramatic increase in the low shear rate viscosity, but the effect was much less at higher shear rates. This suggested that there was a breakdown in some flow unit in the ionomer at high shear rate when it behaved like the parent polymer, Exxpro elastomer. Similar results were reported by other workers.(22-2i)... 

The lower, low-shear-rate viscosities and the absence of a high extensional viscosity in POE/SGPS aqueous solution blends suggest complexation. Such complexation appears to effectively disrupt the three... 

The low shear rate viscosity increase results from the competition between the orienting influences of the wall and of the flow field. Tseng et al [11] also calculated the orientation distribution in capillary flow their calculations at two flow rates is shown in Figure 5. 

Figure 5.15 shows the good correlation of the Quemada model with 4>ma — 0-631 to the experimental results of low shear rate viscosity of Jones et al. (1991) for sihca hard sphere suspensions. 

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