Viscoelastic loses

The relaxation and creep experiments that were described in the preceding sections are known as transient experiments. They begin, run their course, and end. A different experimental approach, called a dynamic experiment, involves stresses and strains that vary periodically. Our concern will be with sinusoidal oscillations of frequency v in cycles per second (Hz) or co in radians per second. Remember that there are 2ir radians in a full cycle, so co = 2nv. The reciprocal of CO gives the period of the oscillation and defines the time scale of the experiment. In connection with the relaxation and creep experiments, we observed that the maximum viscoelastic effect was observed when the time scale of the experiment is close to r. At a fixed temperature and for a specific sample, r or the spectrum of r values is fixed. If it does not correspond to the time scale of a transient experiment, we will lose a considerable amount of information about the viscoelastic response of the system. In a dynamic experiment it may... 

Some particles can be viscoelastic or lose liquid under compression, for example, hydrogels and plant cells, and the force versus displacement data may therefore vary with the compression speed. 

Although the microscopic theory remains to be the real foundation of the theory of relaxation phenomena in polymer systems, the mesoscopic approach has and will not lose its value. It will help to understand the laws of diffusion and relaxation of polymers of various architecture. The information about the microstructure and microdynamics of the material can be incorporated in the form of constitutive relation, thus, allowing to relate different linear and non-linear effects of viscoelasticity to the composition and chemical structure of polymer liquid. 

According to Partridge [163], toughening is efficient when, by comparison to the neat homopolymer tested under the same conditions, the impact resistance is multiplied by a factor of 10, without losing more than 25% of stiffness. The upper temperature limit for the use of rubber-modified blends is controlled by the matrix melt temperature, Tm, their lower limit by the glass transition temperature, Tg, of the particles. As soon as the viscoelastic response of the latter is too slow to accommodate an external loading, the polymer assumes a glassy state and breaks in a brittle way. 

In Figure 5.8d an intermediate behavior, called viscoelastic, is depicted such a relation is often called a creep curve, and the time-dependent value of the strain over the stress applied is called creep compliance. On application of the stress, the material at first deforms elastically, i.e., instantaneously, but then it starts to deform with time. After some time the material thus exhibits flow for some materials, the strain can even linearly increase with time (as depicted). When the stress is released, the material instantaneously loses some of it deformation (which is called elastic recovery), and then the deformation decreases ever slower (delayed elasticity), until a constant value is obtained. Part of the deformation is thus permanent and viscous. The material has some memory of its original shape but tends to forget more of it as time passes. 

Poon [32] explained the film behavior of grease in terms of viscoelastic behavior as the lubricant passes through the conjunction zone. Kauzlarich and Greenwood [25] suggested that because of its gel structure grease heats up by shear faster than oil and loses the heat by conduction more slowly. In their estimation, a thermal rather than an isothermal treatment of the elastohydrodynamic problem is required. 

When component-one s molecular weight is below Me, the viscoelastic response of component one is described by the Rouse theory, and tj (proportional to tci if the component-one chain reptates) can be equated to zero as the component-one chains lose the reptational mechanism. Then Eq. (ll.A.l) is transformed to... 

In fact purely elastic solids do not exist. Solids like elastomers are viscoelastic they lose energy when subjected to a cycle of deformation, and particularly at a crack tip where stresses and strain rates are high. G - w is the crack extension force applied to the crack tip under this force the crack takes a limiting speed v, instead of continuously accelerate as for elastic solids, and one can write (6)... 

Gelation is defined as the point during polymerization when the polymer transforms from a hquid to a rubbery state [97]. At the molecular level, this correlates to the moment at which the molecular weight approaches infinity upon incipient formation of a cross-linked network. Macroscopically, gelation is defined as an abrupt increase in viscosity after which the polymer loses its ability to flow and develops viscoelastic properties. The macroscopic definition of gelation does not necessarily correlate to gelation at the molecular level, since hnear polymers... 

However, in order to describe the deformation of a viscoelastic fluid it is necessary to follow a given material element with time as it moves to define a suitable measure of deformation that always refers to the same material element as time varies. The reason is that when a material element undergoes a finite deformation the coordinate positions of the given material element (with respect to a fixed origin) will vary. Hence, any measure of deformation defined in terms of infinitesimal deformation of fixed coordinate positions loses its physical significance since it will not always be associated with the same material element. 

During the manufacture of bonded components with some adhesives it is necessary to apply them to the faying surfaces which are combined when the adhesive is substantially free of solvent, partially polymerized, or in some other intermediate state depending on the adhesive itself and on the process used. At this stage of combination, the adhesive is required to make an instantaneous bond which need not be of high strength but is required for placement purposes. The ability of the adhesive to make this bond arises from the property of tack which is a viscoelastic property. Pressure-sensitive adhesives used in tapes remain permanently tacky but structurally used adhesives lose... 

---