Reversible deformation

Elasticity is another manifestation of non-Newtonian behavior. Elastic Hquids resist stress and deform reversibly provided that the strain is not too large. The elastic modulus is the ratio of the stress to the strain. Elasticity can be characterized usiag transient measurements such as recoil when a spinning bob stops rotating, or by steady-state measurements such as normal stress ia rotating plates. 

In order to understand the mechanical properties of polymers it is useful to think of them in terms of their viscoelastic nature. Conceptually we can consider a polymeric item as a collection of viscous and elastic sub-components. When a deforming force is applied, the elastic elements deform reversibly, while the viscous elements flow. The balance between the number and arrangement of the different components and their physical constants controls the overall properties. We can exploit these relationships to create materials with a broad array of mechanical properties, as illustrated briefly by the following examples. 

It can be deformed reversibly and repeatedly without permanent distortion or relaxation of features [70]. 

When we pour a solution, or stir it or shake it, we are applying a stress to the solution and are deforming it. For a Newtonian fluid, this deformation is irreversible. If we pour some olive oil from a bottle into a frying pan, the liquid flows across the bottom of the pan, assuming a new shape. On the other hand, if we stretch a rubber band and then release it, the rubber band returns to its original shape. This deformation is completely reversible, and is called elastic. These two types of deformation, reversible (elastic) and irre- /... 

An ideal elastic body (also called Hooke s body) is defined as a material that deforms reversibly and for which the strain is proportional to the stress, with recovery to the original volume and shape occurring immediately upon release of the stress. In a Hooke body, stress is directly proportional to strain, as illustrated in Fig. 3. The relationship is known as Hooke s law, and the behavior is referred to as Hookean behavior. 

PDMS emerged as the polymer of choice for micropatterned surfaces and microfluidic devices. Fabrication is particularly straightforward since PDMS can be cast against a suitable mold with high fidelity. The optical, thermal, interfacial, permeability, and reactivity properties of PDMS make possible numerous functionalities including optical detection, reversible deformation, reversible wetting, and management of cell proliferation. "... 

The brittleness and crystalline character of the films was emphasized by the deformation test. The linear viscoelastic regime (the deformation range at Fig. 6c which the films are deformed reversible) was limited to deformations of about 0.1%, which is an extremely low value. This points to the presence of energy elastic systems. As a compromise to the resolution of the measuring device, in the other test setups the applied strain was set to 0.1 or 0.2%. 

At solid body deformation the heat flow is formed, which is due to deformation. The thermodynamics first law establishes that the internal eneigy change in sample dU is equal to the sum of woik dW, carried out on a sample, and the heat flow dQ into sample (see the Eq. (4.31)). This relation is valid for any deformation, reversible or irreversible. There are two thermo-d5mamically irreversible cases, for which dQ = -dW, uniaxial deformation of Newtonian liquid and ideal elastoplastic deformation. For solid-phase polymers deformation has an essentially different character the ratio QIW is not equal to one and varies within the limits of 0.35 0.75, depending on testing conditions [37]. In other words, for these materials thermodynamically ideal plasticity is not realized. The cause of such effect is thermodynamically nonequilibrium nature or fractality of solid-phase polymers structure. Within the frameworks of fractal analysis it has been shown that this results to polymers yielding process realization not in the entire sample volume, but in its part only. 

Modulus - A material constant defined as the ratio between the applied stress and any resulting elastic deformation (reversible on removing the stress). There are several commonly used variants - depending on the loading method and the direction in which the deformation is measured relative to the direction of the applied stress. Young s modulus is that where both are co-axial. 

We normally perform flexural testing over a limited range of strain, generally only sufficient to determine the sample s flexural yield point. From an end use point of view, this is a reasonable place to halt the test. If we are using a polymer in an application where it is exposed to flexural loads, such as car body panels or bulk liquid storage tanks, it is generally considered to have failed at the point where it no longer deforms reversibly. We obtain a value of flexural modulus from the force versus deformation curve, much as we do for the tensik equivalent. 

Another type of experiment often done in conjunction with creep is creep recovery, the recoil of strain after the stress is removed, as illustrated in Figure 3.3.2. After the stress has been removed from a viscoelastic material, the deformation reverses itself. We can define a recoverable creep function... 

Elasticity is the property of materials that can deform reversibly under external forces, or stress. The relative deformation amount is called the strain. For example, Hooke s spring has linear elasticity with the force being proportional to the deformation, F = kx, where k is the spring constant. 

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