Liquid-like viscous mechanism

As the term implies, viscoelasticity is the response of a material to an applied stress that has both a viscous and an elastic component. In addition to a recoverable elastic response to an applied force, polymers can undergo permanent deformation at high strains, just as was the case for metals and some glasses, as described previously. The mechanism of permanent deformation is different in polymers, however, and can resemble liquid-like, or viscous flow, just like we described in Chapter 4. Let us first develop two important theoretical models to describe viscoelasticity, then describe how certain polymers exhibit this important property. 

Rheology details the behaviour of liquid-like materials under the influence of mechanical stresses and covers viscous and viscoelastic behaviour. 

Polymers are also unique in their viscoelastic nature, a behavior that is situated between that of a pure elastic solid and that of a pure viscous liquid-like material their mechanical properties present a strong dependence on time and temperature. Given all the factors that have to be taken into account to determine the mechanical properties of polymers, their measurement would appear to be very complex. However, there is a series of general principles that determine the different mechanical properties and that give a general idea of the expected results in different mechanical tests. These principles can be organized in a systematic manner to determine the interrelation of polymer structure and the observed mechanical properties, using equations and characteristic parameters of polymeric materials. 

Figure 5.11 presents creep curves, registered for a sample of polystyrene under shear-stress at various temperatures between —268°C and 296.5 °C. We observe a creep compliance which encompasses the enormously broad range of nine orders of magnitude. At the lowest temperatures, the mechanical properties are those of a glass. At the other limit, the high temperature end, the behavior is dominated by viscous flow as indicated by the characteristic linear increase of J with time. The transition from the solid-like to the liquid-like behavior occurs continuously, and most importantly, obviously in a system-... 

An amorphous polymer may behave like a glass at low temperatures, a rubbery solid at intermediate temperatures [above the glass transition temperature (Section 15.12)], and a viscous liquid as the temperature is further raised. For relatively small deformations, the mechanical behavior at low temperatures may be elastic—that is, in conformity to Hooke s law, a = Ee. At the highest temperatures, viscous or liquid-like behavior prevails. For intermediate temperatures, the polymer is a rubbery solid that exhibits the combined mechanical characteristics of these two extremes the condition is termed viscoelasticity. 

These tests show that CC -foam is not equally effective in all porous media, and that the relative reduction of mobility caused by foam is much greater in the higher permeability rock. It seems that in more permeable sections of a heterogeneous rock, C02-foam acts like a more viscous liquid than it does in the less permeable sections. Also, we presume that the reduction of relative mobility is caused by an increased population of lamellae in the porous medium. The exact mechanism of the foam flow cannot be discussed further at this point due to the limitation of the current experimental set-up. Although the quantitative exploration of this effect cannot be considered complete on the basis of these tests alone, they are sufficient to raise two important, practical points. One is the hope that by this mechanism, displacement in heterogeneous rocks can be rendered even more uniform than could be expected by the decrease in mobility ratio alone. The second point is that because the effect is very non-linear, the magnitude of the ratio of relative mobility in different rocks cannot be expected to remain the same at all conditions. Further experiments of this type are therefore especially important in order to define the numerical bounds of the effect. 

Almost every biological solution of low viscosity [but also viscous biopolymers like xanthane and dilute solutions of long-chain polymers, e.g., carbox-ymethyl-cellulose (CMC), polyacrylamide (PAA), polyacrylnitrile (PAN), etc.] displays not only viscous but also viscoelastic flow behavior. These liquids are capable of storing a part of the deformation energy elastically and reversibly. They evade mechanical stress by contracting like rubber bands. This behavior causes a secondary flow that often runs contrary to the flow produced by mass forces (e.g., the liquid climbs the shaft of a stirrer, the so-called Weissenberg effect ). 

From a technical standpoint, it is also important to note that colloids display a wide range of rheological behavior. Charged dispersions (even at very low volume fractions) and sterically stabilized colloids show elastic behavior like solids. When the interparticle interactions are not important, they behave like ordinary liquids (i.e., they flow easily when subjected to even small shear forces) this is known as viscous behavior. Very often, the behavior falls somewhere between these two extremes the dispersion is then said to be viscoelastic. Therefore, it becomes important to understand how the interaction forces and fluid mechanics of the dispersions affect the flow behavior of dispersions. 

At relatively high concentrations (>20%), poloxa-mers form thermoreversible gels however, they gel on heating rather than cooling The amphiphilic nature supports the gelling mechanism of poloxamers, where micelle-like junction zones form at or above room temperature. The junction zones consist of large populations of micelle-like structures, which apparently form a viscous, liquid crystalline phase. Poloxamers can also form gels in dilute hydroalcoholic solutions. 

Nevertheless, the microscopic mechanism that allows water diffusion in these glasses has remained an unsolved puzzle. We present here a computer simulation study of the microscopic mechanisms of water diffusion in carbohydrate from the viscous liquid state up to the glass. To understand the nature of water diffusion in glassy food-like systems, we employed molecular d)mamics (MD) simulations techniques to study in detail the structure and d)mamics of a concentrated glucose solution, a simple binary... 

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