Elastic deformation elastomers

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

A rubber-like solid is unique in that its physical properties resemble those of solids, liquids, and gases in various respects. It is solidlike in that it maintains dimensional stability, and its elastic response at small strains (<5%) is essentially Hookean. It behaves like a liquid because its coefficient of thermal expansion and isothermal compressibility are of the same order of magnitude as those of liquids. The implication of this is that the intermolecular forces in an elastomer are similar to those in liquids. It resembles gases in the sense that the stress in a deformed elastomer increases with increasing temperature, much as the pressure in a compressed gas increases with increasing temperature. This gas-like behavior was, in fact, what first provided the hint that rubbery stresses are entropic in origin. 

Equation (6-94) has been found to be valid for a number of filled systems up to a value of f of about 0.3, whereas (6-95) and (6-96c) can be used at somewhat higher concentrations. These equations were first used to describe the viscosity of liquids with suspended solid particles. In fact equation (6-94) was derived using basic hydrodynamic principles. Equations of this type have been "borrowed" to be used for the elasticity of filled elastomers, based on the analogy between steady viscous flow and elastic deformation as described in equations (3-4) and (2-14), respectively. Certainly an additional justification... 

Elastomers constitute a third class of polymers. Similarly to thermosets, elastomers have a network structure formed by crosslinking between the polymer chains. However, the number of links is less than in the case of thermosets which gives these materials elastic properties. Elastomers can be deformed by the application of external forces. When these forces are suppressed, the polymer recovers its original form. From a commercial point... 

Figure 4. Elastic deformation of Figure 5. Carbon disulfide-insoluble amorphous sulfur along a surface sulfur elastomer in the cross section...

Ektar Performance Plastic s trade name for its family of copolyester thermoplastic elastomer, elastic constant See modulus of elasticity, elastic deformation See deformation, elastic en-ergy, plastic work rheology, elastic fracture See melt fracture, elastic hysteresis See hysteresis, elastic, elasticity A property that causes plastic to return to its original size and shape after removal of a force causing deformation. See coefficient of elasticity deformation and toughness extruder-web stretching and... 

Ogden, R.W., 1997. Nonlinear Elastic Deformations, Dover Publications, New York, p. 499. Oppermann, W., Rennar, N., 1987. Model PDMS elastomers. Prog. Coll. Polym. Sci. 75, 49. Osada, Y, Khokhlov, A.R. (Eds.), 2002. Polymer Gels and Networks. Marcel Dekker, New York. Perkins, T.T. et al., 1994a. Relaxation of a single DNA molecule observed by optical microscopy. Science 264, 822-826. 

The width of the molecular network mesh sequence length) plays a central role in copolymerization (thermoplastics and elastomers/duroplastics) and determines material properties during elastic deformation. 

The remaining class of polymeric materials to be considered are the rubbers or elastomers. These are also oriented by stretching, but in this case the orientation is reversible, together with the elastic deformation. 

In summary, polymeric materials exhibit rubber elasticity if they satisfy three requirements (a) the polymer must be composed of long-chain molecules, (b) the secondary bond forces between molecules must be weak, and (c) there must be some occasional interlocking of the molecules along the chain lengths to form three-dimensional networks. Should the interlocking arrangements be absent, then elastomers lack memory, or have a fading memory and are not capable of completely reversible elastic deformations. 

Finally, the use of elastomer bitumen increases Marshall stability, but it may also increase the Marshall flow above the limiting value. This increase cannot be taken as a drawback, since the nature of the test is such that it does not take into account the recovered (elastic) deformation during unloading and in effect does not simulate dynamic loading. Regarding the optimum binder content of the asphalt with or without modified bitumen, it was found that, when asphalt concrete mixtures were tested, the optimum binder content as determined by the Marshall procedure was not significantly different (Nikolaides and Tsochos 1992). 

---