Shear rate hydrophobically associating polymer

A further complication to understanding the rheology of hydrophobically associating polymers is their unique response to shear rate and solvent quality (e.g., salt content). As shown in Figures 5 and 6, the viscosity can be independent (Newtonian), decrease (pseudoplastic), or even increase (dilatant)... 

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 two associative thickeners examined in the remainder of this text whose synthesis has not been discussed are hydrophobe-modified alkali-swellable emulsions (HASE) discussed in Chapters 25, 27, and 28, and hydrophobe-modified (hydroxyethyl)cellulose (HMHEC, discussed in Chapters 17, 18, and 27). HASE thickeners, by far the lowest cost hydrophobe-modified thickeners produced, should have achieved the largest market share on the basis of cost of production, but this situation does not appear to be the case (discussed in Chapter 28) in large part because of the poor properties observed with the lowest cost latex, vinyl acetate, used to form the continuous film. The applied-film properties 46) of vinyl acetate can be substantially improved through the use of HEUR polymers. HMHEC, synthesized by a matured (30-year-old) commercial slurry process (47) has achieved commercial acceptance, in large part because of linear high shear rate viscosities achieved in blends with HEUR thickeners (Chapter 27). 

Solutions of HMHEC with specific ranges of hydrophobic modification exhibit non-Newtonian behavior at low shear rates. This behavior is interesting in that solutions of HEC under similar conditions are Newtonian. Because the enhanced viscosity arises from the intermolecular association of polymers, the application of shear results in a disruption of the aggregates, and viscosity drops quickly. Figure 11 compares the viscosity profiles of HMHEC and HEC of equivalent molecular weight under different shear rates. 

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. 

The flow curves of polymer will change because of hydrophobic association. Figure 43 shows the flow curves of 0.75 mol% N octylacrylamide/acrylamide copolymer. At polymer concentrations greater than 3,000 ppm the apparent viscosity is constant at low shear rate, then increases with shear rate (shear thickening) up to a maximum, and finally decreases with increasing shear rate (shear thinning). This unique and complex behavior is due to shifting the relative amount of inter and intramolecular association with shear rate [89]. One possible explanation for... 

Itmay be expected that aqueous solutions of such hydrophobically "crosslinked" polymers show a shearaqueous solution of the 5 mol % Chol-Cs-MA polymer at 30 C is independent of Ae shear rate up to 250 s. This observation suggests that hydrophobic association between cMesterol pendants is tight enough not to be dissociated by mechanical shear. 

Additional evidence for intermolecular hydrophobic association is provided by the effects on the Brookfield viscosity by addition of NaCLThus, the viscosity of the copolymer solution increases with increasing NaCl concentration, especially at lower shear rate (0.4 s ) (Fig. 7.6). At polymer concentrations above 2000 ppm, the viscosity increases are especially pronounced between 1.0 and 5.0... 

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