Snap-back

The various elastic and viscoelastic phenomena we discuss in this chapter will be developed in stages. We begin with the simplest the case of a sample that displays a purely elastic response when deformed by simple elongation. On the basis of Hooke s law, we expect that the force of deformation—the stress—and the distortion that results-the strain-will be directly proportional, at least for small deformations. In addition, the energy spent to produce the deformation is recoverable The material snaps back when the force is released. We are interested in the molecular origin of this property for polymeric materials but, before we can get to that, we need to define the variables more quantitatively. 

It is necessary to establish some conventions concerning signs before proceeding further. When the applied force is a tensile force and the distortion is one of stretching, F, dL, and dw are all defined to be positive quantities. Thus dw is positive when elastic work is done on the system. The work done by the sample when the elastomer snaps back to its original size is a negative quantity. 

In viw PAI and antithrombin are stabilized in their active forms by binding to vitronectin and heparin, respectively. These two serpins seem to have evolved what Max Perutz has called "a spring-loaded safety catch" mechanism that makes them revert to their latent, stable, inactive form unless the catch is kept in a loaded position by another molecule. Only when the safety catch is in the loaded position is the flexible loop of these serpins exposed and ready for action otherwise it snaps back and is buried inside the protein. This remarkable biological control mechanism is achieved by the flexibility that is inherent in protein structures. 

Figure 8.17. Shaping of sheet, (a) Application of vacuum, (b) and (c) Air prressure. (d) Mechanical pressure, (e) Combination of methods (vacuum snap-back). (After Estevez and Powell )...

An elastomer is a rubberlike material (natural or synthetic) that is generally identified as a material which at room temperature stretches under low stress to at least twice its length and snaps back to approximately its original length on release of the stress (pull) within a specified time period. The term elastomer is often used interchangeably with the term plastic or rubber (2,14). 

The rather low coordination in the (100) and (110) surfaces will clearly lead to some instability and it is perhaps not surprising that the ideal surface structures shown in Figure 1.2 are frequently found in a rather modified form in which the structure changes to increase the coordination number. Thus, the (100) surfaces of Ir, Pt and Au all show a topmost layer that is close-packed and buckled, as shown in Figure 1.3, and the (110) surfaces of these metals show a remarkable reconstruction in which one or more alternate rows in the <001 > direction are removed and the atoms used to build up small facets of the more stable (111) surface, as shown in Figure 1.4, These reconstructions have primarily been characterised on bare surfaces under high-vacuum conditions and it is of considerable interest and importance to note that chemisorption on such reconstructed surfaces can cause them to snap back to the unreconstructed form even at room temperature. Recently, it has also been shown that reconstructions at the liquid-solid interface also... 

A stretched rubber band of length L2 is allowed to snap back to length Li under adiabatic conditions and without doing any work. Will its temperature change Show clearly the reason for your conclusion. Show that you need to find the derivative dTjdL)u-... 

A type of material known as shape memory alloy (SMA) can perform this trick. SMAs are more complicated than electrorheological fluids and the other smart materials previously described in this chapter. An SMA does not only react or respond to environmental conditions, it also has a memory that enables it to return to a specific structure, or sometimes switch between two different structures. After the material has been set, it can recover from a deformation that would be permanent in other materials. When the temperature is raised by an amount that depends on the specific material, it snaps back into shape automatically. The memory is based on phase transitions, as described in the sidebar on page 120. 

There is a great need for strong materials such as alloys that can snap back into shape. Medical applications include prostheses— artificial limbs—and implanted devices such as heart valves. Most biological substances are smart, and the ability to replace lost or injured tissues and organs with smart materials would be a tremendous medical advance. 

In the rubbery region, which is just above (in terms of temperature) the leathery region, polymer chains have high mobility and may assume many different conformations, such as compact coils, by bond rotation and without much disentanglement. When these rubbery polymers are elongated rapidly, they snap back in a reversible process when the tension is removed. This elasticity can be preserved over long periods of time if occasional cross-links are present, as in vulcanized soft rubber, but the process is not reversible for linear polymers when the stress is applied over long periods of time. 

Stretched with little tendency to snap back to original form... 

Stretched with great tendency to snap back because of cross-links... 

A material which at room temperature stretches under low stress to at least twice its length and snaps back to the original length upon release of the stress. 

Alloy with Memory. In seeking a way to reduce the brittleness of titanium, U.S. Navy researchers serendipitously discovered a nickel-titanium alloy having an amazing memory. Previously cooled clamps made of the alloy (nitinol) are flexible and can be placed easily in position. When warmed to a given temperature, the alloy hardware then exerts tremendous pressure. Use of conventional clamps for holding bundles of wires or cables in a ship or aircraft structure requires special tools. For this and other applications in industry and medicine, nitinol has been in demand. The alloy, however, is not easy to produce because only minor variations in composition can affect the snap back" temperature by several degrees of temperature. 

Exercise 29-9 The material popularly known as Silly Putty" is a polymer having an —O—Si(R)2—O—Si(R)2—O— backbone. It is elastic in that it bounces and snaps back when given a quick jerk, but it rapidly loses any shape it is given when allowed to stand. Which of the polymers listed in Table 29-1 is likely to be the best candidate to have anything like similar properties Explain, What changes would you expect to take place in the properties of Silly Putty as a function of time if it were irradiated with x rays (see Exercise 29-7) ... 

Soldiers or no, I can see why anyone would want to steal it, she breathed, scarcely aware of her words. Quickly, she forced herself to leaf on through the photographs, snapping back into character with an effort. 

When rubber becomes cold, the molecules change from being flexible and stretchy to being stiff and rigid. When this happens, they cannot snap back into position. When a rubber ball gets very cold it will not bounce very high. 

An elastomer with a low modulus will often have far better erosive wear than material with a higher modulus. An abrasive wear test (such as the DIN abrader) will show a poor result for a soft elastomer. This is also shown in field applications such as tire wear. When the application is changed from abrasive to erosive wear, the softer elastomer will wear very well. The reason for this is that the low modulus of the soft elastomer allows the stresses from each impact to be dissipated more readily than for hard polyurethanes. The soft material will stretch further and then snap back before damage is done. Any microcracks formed will have a slower growth rate and hence less erosion will occur. 

One of the prominent features of polymeric liquids is the property to recover partially the pre-deformation state. Such behaviour is analogous to a rubber band snapping back when released after stretching. This is a consequence of the relaxation of macromolecular coils in the system every deformed macro-molecular coil tends to recover its pre-deformed equilibrium form. In the considered theory, the form and dimensions of the deformed macromolecular coil are connected with the internal variables which have to be considered when the tensor of recoverable strain is to be calculated. Further on, we shall consider the simplest case, when the form and dimensions of macromolecular coils are determined by the only internal tensor. In this case, the behaviour of the polymer liquid is considered to describe by one of the constitutive equations (9.48)-(9.49) or (9.58). 

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