Viscoelastic state viscous

Colloidal State. The principal outcome of many of the composition studies has been the delineation of the asphalt system as a colloidal system at ambient or normal service conditions. This particular concept was proposed in 1924 and described the system as an oil medium in which the asphaltene fraction was dispersed. The transition from a coUoid to a Newtonian Hquid is dependent on temperature, hardness, shear rate, chemical nature, etc. At normal service temperatures asphalt is viscoelastic, and viscous at higher temperatures. The disperse phase is a micelle composed of the molecular species that make up the asphaltenes and the higher molecular weight aromatic components of the petrolenes or the maltenes (ie, the nonasphaltene components). Complete peptization of the micelle seems probable if the system contains sufficient aromatic constituents, in relation to the concentration of asphaltenes, to allow the asphaltenes to remain in the dispersed phase. 

Steady state, fuUy developed laminar flows of viscoelastic fluids in straight, constant-diameter pipes show no effects of viscoelasticity. The viscous component of the constitutive equation may be used to develop the flow rate-pressure drop relations, which apply downstream of the entrance region after viscoelastic effects have disappeared. A similar situation exists for time-dependent fluids. 

Calendering takes place at a transient temperature between the polymer viscoelastic and viscous-flow states. Thermoplastic materials displaying a wide range of the flow temperatures and sufficient melt viscosity (e.g. PVC)... 

Electret films can be produced without application of any outer source of electric or magnetic fields. The extruded poljTOer film blank in a viscous flow is subjected to stretching and brought into contact with shaping parts made of short-circuited unlike metals. After cooling to a viscoelastic state, the film... 

In many cases, a material may exhibit the characteristics of both a liquid and a solid, and neither of the hunting laws will adequately describe its behavior. The system is then said to be in a viscoelastic state. A particularly good illustration of a viscoelastic material is provided by a silicone polymer known as bouncing putty. If a sample is rolled into the shape of a sphere, it can be bounced like a rubber ball, i.e., the rapid apphcation and removal of a stress causes the material to behave like an elastic body. If, on the other hand, a stress is applied slowly over a longer period the material flows like a viscous liquid, and the spherical shape is soon lost if left to stand for some time. Pitch behaves in a similar, if less spectacular, manner. 

Figure 11-12. Schematic representation of the compliance C as a function of time, tr is the orientation time. I, Glassy state II, viscoelastic state III, entropy-elastic state IV, viscous flow.

Figure 9. Tj minima when in viscous or viscoelastic state...

Another advantage of acquiring data in a viscous or viscoelastic state is presence of short Ti times. On the chart of molecular correlation time versus Ti time, depicted pictorially in Figure 9, dilute solutions and crystalline domains have long Ti times at the two ends of the correlation spectrum. Concentrated solutions and amorphous domains have shorter T i times, but the viscous state is close to the Ti minima and has the shortest T i time. This was reported a number of years ago by Farrar and Becker 10. ... 

In polymer rheology, and are important because at these points drastic change occurs in the hydrodynamic nature of the states, that is, glassy, viscoelastic, and viscous fluid. Transitions can be dealt with using thermodynamics and kinetics. 

At very short times this simplifies to elastic behaviour. Then at t = r, the stress is 1/e of its value at steady state, where it is cr= 77/, i.e. purely steady-state viscous behaviour. The start-up of real viscoelastic liquids may need to be modelled using a number of Maxwell elements. However, for most realistic experiments using this kind of test, the response quickly enters the non-linear region since the strain is continually increasing. 

Polymers can form materials that are purely elastic, viscoelastic, or viscous, and an entangled polymer network is a good example of this phenomenon. Different relaxation times result from the different kinds of mechanical deformation that can take place in the material. On very small length scales, the bonds between atoms in a polymer chain can be stretched. This deformation relaxes back to its equilibrium state quickly. In the same material on a larger, molecular level length scale, the elastic network itself can be deformed but in this case will relax back to equilibrium slowly as polymer chains must move around each other in the network. 

The four-parameter model provides at least a qualitative representation of all the phenomena generally observed in the creep of viscoelastic materials instantaneous elastic strain, retarded elastic strain, steady-state viscous flow, instantaneous elastic recovery, retarded elastic recovery, and permanent set. It also describes at least qualitatively the behavior of viscoelastic materials in other types of deformation. Of equal importance is the fact that the model parameters can be identified with the various molecular response mechanisms in polymers, and can, therefore, be used to predict the influences that changes in molecular structure will have on mechanical response. The following analogies may be drawn. 

The term viscoelasticity combines viscous and elastic stress-strain flow characteristics. If materials behavior is dominated by viscous flow it is generally referred to as a fluid, whereas if the elastic properties dominate the mechanical properties of a material it is considered to be solid. Most adhesives are applied in a liquid or pasty condition to allow wetting and promote spreading and then are required to phase change into a solid. In the liquid state, rheology provides the methods to differentiate between elastic and viscous flow characteristics while, for example, dynamic mechanical analysis of cured adhesive polymers uses similar principles to access elastic and viscous parameters of the stress-strain response. 

Dyna.mic Viscometer. A dynamic viscometer is a special type of rotational viscometer used for characterising viscoelastic fluids. It measures elastic as weU as viscous behavior by determining the response to both steady-state and oscillatory shear. The geometry may be cone—plate, parallel plates, or concentric cylinders parallel plates have several advantages, as noted above. 

Viscoelasticity illustrates materials that exhibit both viscous and elastic characteristics. Viscous materials tike honey resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. Viscoelasticity is the result of the diffusion of atoms or molecules inside an amorphous material. Rubber is highly elastic, but yet a viscous material. This property can be defined by the term viscoelasticity. Viscoelasticity is a combination of two separate mechanisms occurring at the same time in mbber. A spring represents the elastic portion, and a dashpot represents the viscous component (Figure 28.7). 

In the molten state polymers are viscoelastic that is they exhibit properties that are a combination of viscous and elastic components. The viscoelastic properties of molten polymers are non-Newtonian, i.e., their measured properties change as a function of the rate at which they are probed. (We discussed the non-Newtonian behavior of molten polymers in Chapter 6.) Thus, if we wait long enough, a lump of molten polyethylene will spread out under its own weight, i.e., it behaves as a viscous liquid under conditions of slow flow. However, if we take the same lump of molten polymer and throw it against a solid surface it will bounce, i.e., it behaves as an elastic solid under conditions of high speed deformation. As a molten polymer cools, the thermal agitation of its molecules decreases, which reduces its free volume. The net result is an increase in its viscosity, while the elastic component of its behavior becomes more prominent. At some temperature it ceases to behave primarily as a viscous liquid and takes on the properties of a rubbery amorphous solid. There is no well defined demarcation between a polymer in its molten and rubbery amorphous states. 

Actually, some fluids and solids have both elastic (solid) properties and viscous (fluid) properties. These are said to be viscoelastic and are most notably materials composed of high polymers. The complete description of the rheological properties of these materials may involve a function relating the stress and strain as well as derivatives or integrals of these with respect to time. Because the elastic properties of these materials (both fluids and solids) impart memory to the material (as described previously), which results in a tendency to recover to a preferred state upon the removal of the force (stress), they are often termed memory materials and exhibit time-dependent properties. 

Majerus (61, 62) has approached the failure behavior of highly filled polymers by a thermodynamic treatment in which the ability to resist rupture is related to the propellant s ability to absorb and dissipate energy at a certain rate. An energy criterion which requires failure to be a function of both stress and strain was originally stated by Griffith (36) for brittle materials and later adapted to polymers by Rivlin and Thomas (80). Williams (115) has applied an energy criterion to viscoelastic materials such as solid propellants where appropriate terms are included for viscous energy dissipation. 

VISCOELASTICITY. Mechanical behavior of material which exhibits viscous and delayed clastic response to stress in addition to instantaneous elasticity. Such properries can be considered to be associated with rate effects—time derivatives of arbitrary order of both stress and strain appearing in the constitutive equation—or hereditary or memory influences which include the history of the stress and strain variation from the undisturbed state. See also Rheology. 

Viscoelastic materials are those which exhibit both viscous and elastic characterists. Viscoelasticity is also known as anelasticity, which is present in systems when undergoing deformation. Viscous materials, like honey, polymer melt etc, resist shear flow (shear flow is in a solid body, the gradient of a shear stress force through the body) and strain, i.e. the deformation of materials caused by stress, is linearly with time when a stress is applied [1-4]. Shear stress is a stress state where the stress is parallel or tangencial to a face of the material, as opposed to normal stress when the stress is perpendicular to the face. The variable used to denote shear stress is r which is defined as ... 

---