General Applications of Viscoelastics

As a rule, reuse of remaining viscoelastic substance or splitting up a unit for application in more than one operation should be avoided. 

Viscosity and elasticity are the general rheologic characteristics influencing the maintenance of the anterior chamber. Theoretically, the higher the viscosity, the better the maintenance of chamber depth at rest i.e., at zero shear rate. Based on an in-vitro study performed by Miyauchi and Iwata (1986) involving a model with different viscosity and elasticity, it was shown that the ability to maintain space in the anterior chamber is far more dependent upon the elasticity than the viscosity. 

HPMC products are not suitable to maintain anterior chamber depth with concomitant increased vitreous pressure due to their low elasticity. Higher molecular weight sodium hyaluronate was proven to be superior for this clinical application (Strobel, 1997). 

In the case of intraoperative soft eye, almost all viscoelastic substances would be adequate. In clinical application, all currently available viscoelastic substances are capable of maintaining chamber depth, to some extent, without much difficulty (Liesegang, 1990). 

Osher and co-workers (1996) described the deepening of the anterior chamber with Healon following cataract operation and showed that it be sufficient to fill the anterior chamber halfway, instead of completely. However, it was pointed out that precaution be taken to carefully monitor lOP values to avoid potentially excessive rises. 

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Polymeric materials are all viscoelastic. The face each polymer shows to the observer—elastic, viscous flow, a combination of both—depends on the rate and duration of force application as well as on the nature of the material and external conditions including the temperature T. We discuss the nature of viscoelasticity below and additionally in Section 5. In general, properties of viscoelastics depend on time, in contrast to metals and ceramics. 

Application of the weighted residual method to the solution of incompressible non-Newtonian equations of continuity and motion can be based on a variety of different schemes. Tn what follows general outlines and the formulation of the working equations of these schemes are explained. In these formulations Cauchy s equation of motion, which includes the extra stress derivatives (Equation (1.4)), is used to preseiwe the generality of the derivations. However, velocity and pressure are the only field unknowns which are obtainable from the solution of the equations of continuity and motion. The extra stress in Cauchy s equation of motion is either substituted in terms of velocity gradients or calculated via a viscoelastic constitutive equation in a separate step. 

Viscoelastic characteristics of composite materials usually result from a viscoelastic-matrix material such as epoxy resin. General stress analysis of viscoelastic composites was discussed by Schapery [6-54]. An important application to laminated plates was made by Sims [6-55]. 

The theories of elastic and viscoelastic materials can be obtained as particular cases of the theory of materials with memory. This theory enables the description of many important mechanical phenomena, such as elastic instability and phenomena accompanying wave propagation. The applicability of the methods of the third approach is, on the other hand, limited to linear problems. It does not seem likely that further generalization to nonlinear problems is possible within the framework of the assumptions of this approach. The results obtained concern problems of linear viscoelasticity. 

General Electric scientists and engineers could find no practical applications for it Academics, of course, loved the material, as it was a beautiful teaching tool with which to demonstrate the fundamentals of viscoelasticity. It wasn t until 1949 that it was marketed as a children s toy. And as they say, "the rest is history." Silly Putty is still being produced in Pennsylvania and remains a favorite plaything for children and adults alike. 

The general trend of the data reported in Fig. 8 is suggesting the applicability of an empirical time-temperarnre reduction approach that has been already successfully applied to interpret the viscoelastic namre of crack propagation in polymers [31-36]. Master curves for the yielding and the necking/tearing related parts of the specific essential work of fracmre, both referred to a temperature of 23 °C, are reported in Fig. 9 and 10, respectively. The master curves for the Wg y and Wg jit components, have been obtained by horizontally shifting the data of Fig. 8 to best superposition with respect to the data obtained at 23°C. 

In actual long term applications of polymers, however, it is well known that chemical reactions occur which actually change the viscoelastic properties of the material while it is in use. In addition, environmental factors such as exposure to solvents or even water, while not always chemically modifying a material, can have a profound influence on its viscoelastic properties in much the same way as a true chemical transformation. If predictions based exclusively on time-temperature correspondence were to be successful, the rates of all of these processes would have to vary with temperature in exactly the same m inner as does the viscoelastic spectrum. While this might be approximately true in certain special cases, it is usually not so. Thus, a more general theoretical framework is necessary to predict the properties of simultaneously chemically reacting and physically relaxing networks. 

Even though time crosslink density correspondence can be used very efficiently to generate viscoelastic response curves of some non-reacting polymer networks, our findings make it clear that this technique is not generally applicable to all systems even in the case of limited extents of chemical reaction. 

This latter model was employed by Rouse (27) and by Bueche (28) in the calculation of viscoelasticity and is sometimes called the Rouse model. It was used later by Zimm (29) in a more general calculation which may be regarded as an application of the Kirkwood theory. As illustrated in Fig. 2.1, the Rouse model is composed of N + 1 frictional elements represented by beads connected in a linear array with N elastic elements or springs, hence the bead-spring model designation. The frictional element is assumed to represent the translational friction... 

Description of material behavior is basic to all designing applications. Many of the problems that develop may be treated entirely within the framework of plastic s viscoelastic material response. While even these problems may become quite complex because of geometrical and loading conditions, linearity, reversibility, and rate independence generally applicable to elastic material description certainly eases the task of the analyst for dynamic and static loads that include conditions such as creep, fatigue, and impact. 

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