Associating polymer shear rate effects

The value for n is often given as 2/3, but polymer melts have shown a wide range of values. The constant d is associated with mpture of the linkages in the stmcture of the fluid. The effect of different values of a, ie, at the same values of T q and, is shown in Figure 4. As d increases, breakdown occurs at lower and lower shear rates. 

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. 

One of the limitations of this study was the lower frequency limit of ca. 10 Hz. Our recent measurements of noise effects associated with the flow of polymer solutions at frequencies down to 5 mHz have shown that this low frequency region is especially interesting. For instance, thermal noise increased with the shear rate, exhibiting a l// frequency distribution for the solutions discussed. 

The elastic effects in polymer melts are associated with the molecular coil deformation shown in Fig. 3.9. The effects include die swell, a diameter increase when the melt exits from a die and flow instabilities such as melt fracture (causing a rough surface). One measure of the elastic effects is the tensile stress difference — a-yy that occurs in shear flow in the xy axes. There can be a tensile stress in the direction of flow, or a compressive stress (Tyy on the channel walls, or a combination of the two. Figure 5.7 shows that, as the shear rate increases, the value of m... 

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. 

Some special problems exist in normal stress measiurement of filled polymer systems. Abnormal effects at low rates are observed due to the interactions between the measuring equipment and the yield stress property associated with the filled systems. As a result, a proper zero point for measurement cannot be obtained. Furthermore, the uncertainty is proportional to the level of the peld stress [19]. It is thus difficult to measure normal stresses accmately especially at low shear rates [27]. 

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... 

Slip/stick occurs when the shear rate at the die wall exceeds the adhesive force of the melt to the surface. When this occurs, the melt jerks forward as a plug, relieving the pressure behind it and allowing the oriented chain segments to recoil somewhat. Once the pressure is relieved the rate of movement of the polymer slows and it re-adheres to the die wall. Shear flow resumes until once again the shear rate exceeds the critical value [91]. The effect is also known as spurting due to the erratic polymer output associated with it. During slip/stick flow the pressure within the die fluctuates and the polymer output is unsteady, both of which may vary periodically or erratically. The effects of stick/slip are closely related to those of melt fracture. 

In both polymers, creep of compression-molded specimens is caused mainly by crazing, with shear processes accounting for less than 20% of the total time-dependent deformation. Crazing is associated with an increasing creep rate and a substantial drop in modulus. The effects of stress upon creep rates are described by the Eyring equation, which also offers an explanation for the effects of rubber content upon creep kinetics. Hot-drawing reduces creep rates parallel to the draw direction and increases the relative importance of shear mechanisms. 

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