Rubbery materials deformation

Several aspects of the deformation and fracture of rubbery materials are reviewed in this chapter. They reveal some important gaps in our present understanding of the behavior of simple mbbery solids. 

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. 

Much slower spreading occurs with the rubbery material, with approximately 30 minutes being necessary to achieve equilibrium. This is attributed to local deformation of the substrate leading to viscoelastic braking of the spreading of TCP. The hypothesis is corroborated by line a of Figure 7. This line has been obtained by plotting the difference be-... 

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. 

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. 

From the organic point of view, reversible contraction/dilation phenomena should be observed in photochromic networks below their glass transition temperatures, i.e. in the rubbery state where segment mobility is important. Considering that isomerization occurs in the rubbery state at a rate similar to that in solution, it should be expected that the use of isomerization reactions for photocontractile behavior should be optimal with easily deformable networks, i.e. swollen gels and rubbery materials. 

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. 

A thermoplastic elastomer (TPE) is a rubbery material with properties and functional performance very similar to those of a conventional thermoset rubber, yet it can be fabricated in the molten state as a thermoplastic. ASTM D 1566 defines TPEs as a diverse family of rubber-like materials that, unlike conventional vulcanized rubbers, can be processed and recycled like thermoplastic materials. Many TPEs meet the standard ASTM definition of a rubber, since they recover quickly and forcibly from large deformations, they can be elongated by more than 100 percent, their tension set is less than 50 percent, and they are sometimes insoluble in boiling organic solvents. Figure 4.35 indicates hardness ranges for various types of TPEs and conventional elastomers. 

The two-phase structure is critical to impact modification. Impact energy is absorbed during fracture when a propagating crack meets a rubbery domain and fracture energy is dispersed by deformation of the rubbery material (28). As a rule of thumb, an average particle size of about 1 pm must be achieved to provide low-temperature impact. 

The beads are designed to be deformable to effect the seal and a rubbery material is thus required. 

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