Polymeric items

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. 

Why would a manufacturer of polymeric items be interested in the rate of crystallization within a semicrystalline polymer ... 

Antioxidants are species that accept the reactive byproducts of oxidation reactions. They are typically hindered amines or phenols that accept radicals, inactivating them and preventing further effects of oxidation. The level of antioxidant used in a polymeric item depends on the expected lifetime of the final part, the environment in which the part will be used, and the susceptibility of the polymer to oxidation. Figure 9.7 shows two common antioxidants used in polyolefins. 

Most polymers are applied either as elastomers or as solids. Here, their mechanical properties are the predominant characteristics quantities like the elasticity modulus (Young modulus) E, the shear modulus G, and the temperature-and frequency dependences thereof are of special interest when a material is selected for an application. The mechanical properties of polymers sometimes follow rules which are quite different from those of non-polymeric materials. For example, most polymers do not follow a sudden mechanical load immediately but rather yield slowly, i.e., the deformation increases with time ( retardation ). If the shape of a polymeric item is changed suddenly, the initially high internal stress decreases slowly ( relaxation ). Finally, when an external force (an enforced deformation) is applied to a polymeric material which changes over time with constant (sinus-like) frequency, a phase shift is observed between the force (deformation) and the deformation (internal stress). Therefore, mechanic modules of polymers have to be expressed as complex quantities (see Sect. 2.3.5). 

It is possible to draw the conclusion that the development of forming tools for the production of polymeric items in which application of ultrasound is possible is an important new direction for research. 

The profile section and the components of the horn are produced separately for supply of high fidelity and control of the inner surfaces. The extrusion heads for fabrication profile - per unit length of polymeric items with the application of ultrasonic oscillation... 

Figure 5.2 Cross sections of polymeric items investigated...

Use of the extrusion head with application of ultrasound allows the hardness of an item to be increased and expand the opportunities to use new kinds of polymeric items with complex configurations. 

The mechanical loading on a polymeric item not only changes its shape and sizes but also affect substantially its supramolecular structure. The mechanical loading on the amorphous-crystalline polymer (polyolefins) substantially influences first of all on the amorphous phase of the polymer. The stretching strain results in the conformational transitions the number of gauche conformations decreases and the number of trans conformations increases (polyethylene, poly(ethylene terephthalate)). Under strain chains of macromolecules are additionally oriented and the rotation of the rad-... 

---