Viscosity viscoelastic effects

Coalescence of mesophase is often said to be determined by the mesophase viscosity. This aspect requires much further investigation. However, it is clear that, amongst other factors, the rheological behaviour (including viscoelastic effects) of each phase is important in mesophase growth and coalescence. Diffusion of molecular species through the isotropic pitch to the mesophase spheres is likely to be related to the viscosity of the isotropic med i urn. 

Applications in which dynamic uniaxial elongational viscosities (DUEVs) have been important to the performance of water-borne formulations are discussed. The achievement of high viscosities at low shear rates with minimum mechanical degradation of the water-soluble polymer and with minimum viscoelastic effects in applications that involved converging flows are discussed. These two factors serve as driving forces for the acceptance of hydrophobically modified water-soluble polymers. Complexities arising from the combined contribution of shear and elongational deformations in applications and in their measurement are discussed in the final section. 

In many materials, the mechanical response can show both elastic and viscous types of behavior the combination is known as viscoelasticity. In elastic solids, the strain and stress are considered to occur simultaneously, whereas viscosity leads to time-dependent strain effects. Viscoelastic effects are exhibited in many different forms and for a variety of structural reasons. For example, the thermoelastic effect was shown earlier to give rise to a delayed strain, though recovery of the strain was complete on unloading. This delayed elasticity is termed anelastic-ity and can result from various time-dependent mechanisms (internal friction). Figure 5.9 shows an example of the behavior that occurs for a material that has a combination of elastic and anelastic behavior. The material is subjected to a constant stress for a time, t. The elastic strain occurs instantaneously but, then, an additional time-dependent strain appears. On unloading, the elastic strain is recovered immediately but the anelastic strain takes some time before it disappears. Viscoelasticity is also important in creep but, in this case, the time-dependent strain becomes permanent (Fig. 5.10). In other cases, a strain can be applied to a material and a viscous flow process allows stress relaxation (Fig. 5.11). 

It is useful to calibrate the EXJCM, i.e., to determine Cf, e.g., by electrodeposition and electrodissolution of silver. The rigidity layer behavior can be tested by depositing films of different thicknesses. Usually relatively thin films (10 nm - some hundreds nm) show rigid layer behavior. The deviation from the linearity regarding the Am vs. Q function is related to the appearance of the viscoelastic effect. By the help of impedance measurements the viscoelastic characteristics of the surface film can also be tested [4, 5, 6, 7, 10]. In the absence of any deposition the change of the density and viscosity in the double layer or in the diffusion layer may cause 0.1-10 Hz frequency change. It may interfere with the effect caused by the deposition of monolayers or submonolayers. In some cases other effects, e.g., stress, porosity, pressure, and temperature, should also be considered. 

If a viscoelastic material is forced to flow from a large reservoir through a circular tube, the diameter of the extrudate is found to be larger than the tube diameter. Many researchers [61-65] have argued that that causes the die swell. They developed the following three points of view for the cause of the die swell polymer chain orientation within the capillary caused by the high shear field recovery of the elastic deformation and viscose heat effects. The most important concept is the recovery of the elastic deformation imposed in the capillary. 

QCM is extremely sensitive and accurate. Data can be collected in real time to follow the kinetics of adsorption, reaction, and film growth. In addition to measuring the mass, the technique can also measure the mechanical properties, eg, changes in the stiffness and viscosity of the material, in real time. However, these viscoelastic effects can complicate interpretation of the data. Attachment of cells and the formation of biofilms can also be monitored. Drawbacks are that the measurement can be time-consuming. Automation using robots may be necessary to increase the otherwise low throughput. 

The same group has also observed enhanced impact and toughness behaviour of the blends due to the secondary phase separation effect on the same rubber-modified system. The effect is originated due to the combined effect of hydrodynamics, viscoelastic effects of the rubber phase, diffusion, surface tension, polymerisation reaction and phase separation. In a dynamic asymmetric system, the diffusion of the fast dynamic phase is prevented by the slow dynamic phase, and hence the growth of the fast dynamic phase is retarded by the slow dynamic phase. In the case of low viscosity blends the... 

Indeed, to exploit effectively the viscoelasticity of fluids for chaotic flow instability, and thus mixing, sharper and smaller geometries should be employed. Stress singularities developed at such corners have been the source of elastic instabilities in many macroscale experiments [3], while rounded comers tend to suppress elastic behavior. From a practical standpoint, it is necessary to understand the rheological nature of such flow in order to optimize the use of viscoelastic effects in microfluidic mixing applications. The complex interplay that arises between the elasticity and viscosity of the fluids, and the ratio of contraction of the channel is the key to efficient mixing of fluid streams in microfluidic channels. 

Most concentrated structured liquids shown strong viscoelastic effects at small deformations, and their measurement is very useful as a physical probe of the microstructure. However at large deformations such as steady-state flow, the manifestation of viscoelastic effects—even from those systems that show a large linear effects—can be quite different. Polymer melts show strong non-linear viscoelastic effects (see chap. 14), as do concentrated polymer solutions of linear coils, but other liquids ranging from a highly branched polymer such as Carbopol, through to flocculated suspensions, show no overt elastic effects such as normal forces, extrudate swell or an increase in extensional viscosity with extension rate [1]. 

When a low-viscosity viscoelastic liquid is tested at high rotational speed in a cone-and-plate geometry, there is an inertial force acting outwards that tends to pull the plates together, thus counteracting the normal-force effects which tend to push the plates apart. When this effect is just measurable, the overall force is then given by... 

The shear thinning behavior, as generally observed with polymer systems, is a typical nonlinear viscoelastic effect, so that by combining the Carreau-Yasuda and the Arrhenius equations a general model for the shear viscosity function can be written as follows ... 

The degree of polymerization and, therefore, the viscosity, shearing effects, and viscoelastic properties are controlled by the ratio of monofunctional UPy added to the solution, which end-caps the growing chains. By protecting the UPy end-groups or the monofunctional UPy with the photolabile group o-nitrobenzyl, the ability of UPy to dimerize is prevented. 

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