Elasticity rubbery materials

Oils, very viscous liquids, to elastic rubbery materials, depending on X, which may vary up to 10,00021-22... 

Another type of geometric arrangement arises with polymers that have a double bond between carbon atoms. Double bonds restrict the rotation of the carbon atoms about the backbone axis. These polymers are sometimes referred to as geometric isomers. The X-groups may be on the same side (cis-) or on opposite sides (trans-) of the chain as schematically shown for polybutadiene in Fig. 1.12. The arrangement in a cis-1,4-polybutadiene results in a very elastic rubbery material, whereas the structure of the trans-1,4-polybutadiene results in a leathery and tough material. Branching of the polymer chains also influences the final structure, crystallinity and properties of the polymeric material. 

Kawahara, S., Yamamoto, Y, Fujii, S., Isono, Y, Niihara, K., Jinnai, H., Nishioka, H., and Takaoka, A. (2008). FIB-SEM and TEMT observation of highly elastic rubbery material with... 

In reality the ideal elastic rubber does not exist. Real rubbery materials do have a small element of viscosity about their mechanical behaviour, even though their behaviour is dominated by the elastic element. Even so, real rubbers only demonstrate essentially elastic behaviour, i.e. instantaneous strain proportional to the applied stress, at small strains. 

The large deformability as shown in Figure 21.2, one of the main features of rubber, can be discussed in the category of continuum mechanics, which itself is complete theoretical framework. However, in the textbooks on rubber, we have to explain this feature with molecular theory. This would be the statistical mechanics of network structure where we encounter another serious pitfall and this is what we are concerned with in this chapter the assumption of affine deformation. The assumption is the core idea that appeared both in Gaussian network that treats infinitesimal deformation and in Mooney-Rivlin equation that treats large deformation. The microscopic deformation of a single polymer chain must be proportional to the macroscopic rubber deformation. However, the assumption is merely hypothesis and there is no experimental support. In summary, the theory of rubbery materials is built like a two-storied house of cards, without any experimental evidence on a single polymer chain entropic elasticity and affine deformation. 

Waldo Semon was responsible for bringing many of the PVC products to market. As a young scientist at BF Goodrich, he worked on ways to synthesize rubber and to bind the rubber to metal. In his spare time, he discovered that PVC, when mixed with certain liquids, gave an elastic-like, pliable material that was rainproof, fire resistant, and did not conduct electricity. Under the trade name Koroseal, the rubbery material came into the marketplace, beginning around 1926, as shower curtains, raincoats, and umbrellas. During World War II, PVC became the material of choice to protest electrical wires for the Air Force and Navy. Another of his inventions was the SR patented under the name Ameripol that was dubbed liberty rubber since it replaced NR in the production of tires, gas masks, and other military equipment. Ameripol was a butadiene-type material. 

A good operational definition of rubber-like elasticity is high deformability with essentially complete recoverability.81 84 The high deformability can be remarkably high, with some rubbery materials extending up to 15 times their original lengths. 

Linear amorphous polymers can behave as either Hookian elastic (glassy) materials, or highly elastic (rubbery) substances or as viscous melts according to prevailing temperature and time scale of experiments. The different transitions as shown schematically in Figure 5.1 are manifestations of viscoelastic deformations, which are time dependent [1]. 

An external pressure (stress) that is exerted on a material will cause its thickness to decrease. A shear stress is applied parallel to the surface of a material, and may cause the sliding of atomic layers over one another. The resultant deformation in the size/shape of the material is referred to as strain, related to the bonding scheme of the atoms comprising the solid. For example, a rubbery material will exhibit a greater strain than a covalently bound solid such as diamond. Since steels contain similar atoms, most will behave similarly as a result of an applied stress. If a stress causes a material to bend, the resultant flex is referred to as shear strain. For small shear stresses, steel deforms elastically, involving no permanent displacement of atoms. The deformation vanishes when shear stress is removed. However, for a large shear stress, steel will deform plastically, involving the permanent displacement of atoms, known as slip. 

Because the materials most commonly mixed in kneaders are very viscous, often elastic or rubbery materials, a large amount of energy must be applied to the mixer blades. All that energy is converted to heat within the material. Often the material begins as a semisolid mass, with liquid or powder additives, and the blending process both combines the materials and heats them to create uniform bulk properties. 

The aforementioned analyses were essentially elastic in nature. However, Huang and Kinloch (7,8) developed a two-dimensional, plane-strain model to analyze the stress fields around the dispersed rubbery particles in multiphase, rubber-modified epoxy polymers. The epoxy matrix was modeled as either an elastic or elastic-plastic material. Their work revealed that the plane-strain model predicted higher stress concentrations within the glassy polymeric matrix than the axisymmetric model. Furthermore, they successfully applied their... 

Texture. A hard biscuit has a crisp or brittle texture. This implies that it deforms in a fully elastic manner upon application of a force, until it breaks (snaps) at a relatively small deformation. Breakage goes along with a snapping sound. It appears from empirical observations that a crisp material has an apparent viscosity of at least 1013 or 1014 Pa s. The water content or temperature above which crispness is lost closely corresponds to Tg. Sensory evaluation shows that an increase in water content by 2 or 3 percentage units, or in temperature by 10 or 20 K, can be sufficient to change a crisp food into a soft (rubbery) material. 

The elastic response of elastomers has been the subject of a great deal of study by many investigators because of its very great technological importance as well as its intrinsic scientific interest. Starting from one material, namely natural rubber, the development of polymerization techniques has resulted in a host of substances that may properly be called rubbers, and a giant synthetic-rubber industry has developed to exploit them commercially. The term "elastomer" has become the generic scientific name for a rubbery material. 

The response of rubbery materials to mechanical stress is a slight deviation from ideal elastic behavior. They show non-Hookean elastic behavior. This means that although rubbers are elastic, their elasticity is such that stress and strain are not necessarily proportional (Figure 14.3). 

We turn now to some features of the elastic response of rubbery materials that are still not fully understood. 

---