Viscoelastic putty

B. For the hyaluronate system in the viscoelastic putty state buffered at pH = pK, we have ... 

Deformation is the relative displacement of points of a body. It can be divided into two types flow and elasticity. Flow is irreversible deformation when the stress is removed, the material does not revert to its original form. This means that work is converted to heat. Elasticity is reversible deformation the deformed body recovers its original shape, and the appHed work is largely recoverable. Viscoelastic materials show both flow and elasticity. A good example is SiEy Putty, which bounces like a mbber ball when dropped, but slowly flows when allowed to stand. Viscoelastic materials provide special challenges in terms of modeling behavior and devising measurement techniques. 

Finally, some fluids that undergo viscosity changes on shearing are elastic as well. These are termed viscoelastic fluids. These materials have properties of both a liquid and a solid. An excellent example is silicone putty (e.g., Silly Putty), which shows three different types of behavior depending upon the shear rate. If a piece of this material is suspended (gravity, a low shear force), it will slowly flow downward like a very viscous fluid. If it is sheared fasten it has rubbery behavior. You can observe this by rolling some of it into a ball... 

Viscoelastic—The property describing a material that behaves either like an elastic solid or a viscous liquid, depending upon the force placed upon it (e.g., Silly Putty). 

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. 

One very impressive example of a viscoelastic material is Silly Putty ( bouncy putty ), which is a silicon-based polymer. If dropped onto a surface, it boimces back higher than a rubber ball, yet under light pressure, the material can be flattened with ease and remains in its new shape. If placed on the edge of, for example, a table, the material will slowly creep over the edge like liquid due to the influence of the force of gravity. Most working materials, however, do not behave in these extremes. 

To cause a polymer to deform or flow requires the application of a force. If a force is applied and then withdrawn quickly, the polymer molecules tend to revert to their previous undeformed configuration, a process called relaxation. In other words, the polymer melt exhibits a certain elastic quality. This elasticity comes about because the molecules were disturbed from what was a thermodynamically favorable arrangement. If, however, the force is applied gradually and consistently, the molecules begin to flow irreversibly. (Silly putty, a siloxane polymer, is ideal for demonstrating this effect. If dropped, it bounces but it can be shaped by the slow application of pressure.) Because of entanglement of the polymer chains and frictional effects, the flowing liquid will be very viscous. This combination of properties, namely elasticity and viscous flow, is why polymers are referred to as viscoelastic materials. 

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. 

Peculiar combination of viscous and elastic properties can be also see in the following siphon effect (Figure C12.2 g). Place a sample of viscoelastic polymer (such as properly shaped silly putty ) in a tilted glass container (A) such that a sufficient part of the material extends beyond the container... 

Measurement of C requires more sophisticated and expensive rheometers and more involved experimental procedures. It must be remembered that experiments have to he carried out below the critical strain value (see Sec II), or in [he region of linear viscoelastic behavior. This region is determined by measuring the complex modulus G as a function of the applied strain at a constant oscillation frequency (usually 1 Hz). Up to 7, G does not vary with the strain above Yr, G tends to drop. The evaluation of oscillatory parameters is more often restricted to product formulation studies and research. However, a controlled-fall penetrometer may be used to compare the degree of elasticity between different samples. Creep compliance and creep relaxation experiments may be obtained by means of this type of device. In fact, a penetrometer may be the only way to assess viscoeIa.sticity when the sample does not adhere to solid surfaces, or adheres too well, or cures to become a solid or semisolid. This is the case of many dental products such as fillings, impression putties, sealants, and cements. 

The extensional thickening of polymer solutions is one form of viscoelastic behavior. This ability to support a tensile stress can also be demonstrated in a tubeless syphon with dilute aqueous solutions of polsrmers such as polyacrylamide or polyethylene oxide. If you suck up solution with a medicine dropper attached to a water aspirator and then lift the dropper out of the solution, the solution will still be sucked up. In shear, viscoelastic fluids develop normal stresses, which causes rod climbing on a rotating shaft, as opposed to the vortex and depressed surfaces that form with Newtonian liquids. Polsrmer solutions and semiliquid poljnners exhibit other viscoelastic behaviors, where, on short time scales, they behave as elastic solids. Silly putty, a childrens toy, can be formed into a ball and will slowly turn into a puddle if left on a flat surface. But if dropped to the floor it boimces. 

Usually this criterion allows us to unambiguously classify a phase as either a solid or a fluid. Over a sufficiently long time period, however, detectable flow occurs in any material under shear stress of any magnitude. Thus, the distinction between solid and fluid actually depends on the time scale of observation. This fact is obvious when we observe the behavior of certain materials (such as Silly Putty, or a paste of water and cornstarch) that exhibit solidlike behavior over a short time period and fluid-like behavior over a longer period. Such materials, that resist deformation by a suddenly-applied shear stress but undergo flow over a longer time period, are called viscoelastic solids. 

Viscoelastic materials, such as polymers, exhibit behavior that is intermediate between that of an ideal solid and that of an ideal liquid, showing characteristics of both. A classic example is silly putty, which can be rolled into a baU that will bounce elastically like a solid, but if placed on a table will slowly flow over a period of several hours into a puddle like a liquid. When it is stretched slowly, it elongates continuously, but when stretched quickly, it fractures Uke a brittle solid. The timescale and temperature of observation are critical to the relative degree of solid- and liquidlike behavior exhibited by viscoelastic materials. Generally, they will behave more solidUke at lower temperatures or over shorter timescales, but more liquidlike at higher temperatures or longer timescales. 

One manifestation of the time dependent character of polymers is that they exhibit characteristics of both an elastic solid and that of a viscous fluid as with the example of silly putty above. For this reason, materials such as polymers that exhibit such properties are often said to be viscoelastic. Sometimes the term viscoelastic is used primarily for solid polymers... 

Other examples of viscoelastic materials are synovial fluid, molten polymers (with thread-forming properties used in fiber spinning or film blowing), and "bouncing putty" or "nutty putty," which will flow if stretched slowly, but bounces if struck hard against a hard surface. 

Non-Newtonian fluids have both viscous and elastic properties, and they are called viscoelastic fluids. An example is so-called "silly putty," which is made from poly-dimethyl-siloxane (silicone). It flows like a liquid out of the container, but when it forms a ball, it behaves as elastic, i.e., it bounces back. The crucial factor determining the viscous and elastic behavior is the time period of the force applied short force pulse leads to elastic response, whereas long-lasting force causes flow. The viscoelasticity in polymers is due to shear-induced entanglements and nonlinear behavior of tire chains, coils. A well-known natural viscoelastic material is for, example, the egg white, which springs back when a shear force is released. A polymer resembles both liquid and solids. 

The degree to which a viscoelastic material behaves as an elastic solid or a viscous liquid depends largely on the time scale and pattern of the applied stress and the response time required by the system. For example, a ball of Silly Putty dropped to the floor will bounce elastically as if it were made of rubber. Here there is a very brief applied compressive stress as the ball hits the floor. If that same ball is allowed to rest on the floor for a sufficient length of time, it will slowly start to flow and end up as a puddle, like a viscous liquid. 

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