Elastomers stretched

Rubbers, or elastomers, stretch readily to elongate by a factor of 10 before breaking. Natural rubber, obtained from the sap of certain trees, can be hardened and toughened by addition of sulfur in the vulcanization process. Synthetic rubber is produced by addition copolymerization of butadiene and styrene. 

A molecular chain of PDES contains no mesogenic elements still two columnar mesophases Xi and X2 exist above the melting point of the crystalline state. In cross-linked PDES elastomers stretching can induce an amorphous-mesophase transition. With increasing stress the transition shifts to higher temperature. Kinetically this transition follows a 2D growth model. 

Fig. 21 (a) Molecular structure of the main-chain smectic system TR5 with pentaphenyl transverse rods, (b) X-ray picture of the smectic-A structure of the elastomer stretched at room temperature, (c) Structural model of the elastomer [160]... 

In an analogous strip of rubber, the corresponding situation would be the spontaneous elongation of the strip (a rubber band stretching itself). On a molecular level, such motions are spontaneous, but because of the low probabilities involved, are but momentary. Again, the phenomenon is possible but unlikely. Instead of a pressure P to hold the gas in the volume V, a stress <7 (force per unit area) will be required to keep the elastomer stretched from Lq to L (a=LILo). 

Ju W-E, Moon Y-J, Park C-H, Choi ST (2014) A flexible tactile-feedback touch screen using transparent ferroelectric polymer film vibrators. Smart Mater Stmct 23 074004-1-074004-10 Kawai H (1969) The piezoelectricity of poly (vinylidene fluoride). Jpn J Appl Phys 8 975-976 Kombluh RD, PeWne R, Prahlad H, Wong-Foy A, McCoy B, Kim S, Eckerle J, Low T (2012) Dielectric elastomers stretching the capabilities of energy harvesting. MRS Bnll 37 246-253 Lacour SP, Jones J, Wagner S, Li T, Suo Z (2005) Stretchable interconnects for elastic electronic surface. Proc IEEE 93 1459-1467... 

Elastomers. Elastomers is a generic name for polymers that exhibit rubberlike elasticity. Elastomers are soft yet sufficiently elastic that they can be stretched several hundred percent under tension. When the stretching force is removed, they retract rapidly and recover their original dimensions. 

It is necessary to establish some conventions concerning signs before proceeding further. When the applied force is a tensile force and the distortion is one of stretching, F, dL, and dw are all defined to be positive quantities. Thus dw is positive when elastic work is done on the system. The work done by the sample when the elastomer snaps back to its original size is a negative quantity. 

For large deformations or for networks with strong interactions—say, hydrogen bonds instead of London forces—the condition for an ideal elastomer may not be satisfied. There is certainly a heat effect associated with crystallization, so (3H/9L) t. would not apply if stretching induced crystal formation. The compounds and conditions we described in the last section correspond to the kind of system for which ideality is a reasonable approximation. 

A great many liquids have entropies of vaporization at the normal boiling point in the vicinity of this value (see benzene above), a generalization known as Trouton s rule. Our interest is clearly not in evaporation, but in the elongation of elastomers. In the next section we shall apply Eq. (3.21) to the stretching process for a statistical—and therefore molecular—picture of elasticity. 

The quantity in parentheses is always positive for a > 1, the case of elongation, making AS < 0 for stretching. Therefore AS is positive for the opposite process, showing that entropy alone is sufficient to explain the elastomer s snap. To get an idea of the magnitude of this entropy effect, consider the following example. 

Permanent Set. When an elastomer is stretched and then allowed to relax, it will not completely recover its original dimensions. This divergence from its original form is called its permanent set. It is principally affected by the affinity of the elastomer for the filler surface and is, therefore, primarily a function of the surface energy or wetting of the filler. 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

The class of mbbers that show the abiHty to crystalline when stretched represent a special class of mbbers. In the listing of the various principal elastomers in Table 1, this unique property is shown to increase the strength of mpture of these elastomers. 

The tendency of polychloroprene to crystallize enhances its value as an adhesive (97). The cured or uncured polymer can crystallize on stretching thereby increasing the strength of gum vulcaniza tes. Elastomers that caimot crystallize have poor gum vulcaniza te properties (98). 

Elastomer A material under ambient conditions that can be stretched and, upon release of the applied stress, returns with force to its approximate original size and shape. 

Elastomers are amorphous polymers that have the ability to stretch out and spring back to their original shapes. These polymers must have low Tg values and a small amount of cross-linking to prevent the chains from slipping over one another. In addition, the chains must have an irregular shape to prevent crystallite... 

Elastomer (Section 31.5) An amorphous polymer that has the ability to stretch out and spring back to its original shape. 

An elastomer is a rubberlike material (natural or synthetic) that is generally identified as a material which at room temperature stretches under low stress to at least twice its length and snaps back to approximately its original length on release of the stress (pull) within a specified time period. The term elastomer is often used interchangeably with the term plastic or rubber (2,14). 

Plastic elastomers are generally lower-modulus flexible materials that can be stretched repeatedly and will return to their approximate original length when the stresses are released. The rubber materials have been around for over a century. They will always be required to meet certain desired properties, but thermoplastic TPEs are replacing traditional TS natural and synthetic rubbers (elastomers). TPEs are also... 

Gough-Joule effect When an elastomer/ rubber is stretched adiabatically (without heat entering or leaving the system), heat is evolved, This effect was first reported discovered by Gough in 1805 and rediscovered by Joule in 1859. 

However, not all properties are improved by filler. One notable feature of the mechanical behaviour of filled elastomers is the phenomenon of stresssoftening. This manifests itself as a loss of stiffness when the composite material is stretched and then unloaded. In a regime of repeated loading and unloading, it is found that part of the second stress-strain curve falls below the original curve (see Figure 7.13). This is the direct opposite of what happens to metals, and the underlying reasons for it are not yet fully understood. 

Lower-density E-plastomers have found alternate use in cast film processes to make elastic film laminates with good breathability which contain laminates of liquid impermeable extensible polymeric films with extensible-thermoplastic-polymer-fiber nonwovens and nonwoven webs of polyethylene-elastomer fibers as the intermediate layers. The development relates to a breathable film including an E-plastomer and filler that contributes to pore formation after fabrication and distension of the film. The method and extent of distension is designed to produce a breathable film by stretching the film to form micropores by separation of the film of the E-plastomer from the particulate solids. This film is useful for manufacture of absorbent personal-care articles, such as disposable diapers and sanitary napkins and medical garments. In detail, these constructions comprise a liquid impermeable extensible film comprising polyolefins. The outer layer contains extensible-thermoplastic-polymer-fiber nonwovens, and an elastic intermediate layer contains nonwoven webs of fiber E-plastomers. The intermediate layer is bonded to the film layer and the outer... 

The main models are described in a review by Vrhovski and Weiss [8]. For ideal elastomers in the extended mode, all the energy resides on the backbone and can therefore be recovered upon relaxation [18]. Generally, it is believed that the mechanism of elasticity is entropy-driven, thus the stretching decreases the entropy of the system and the recoil is then induced by a spontaneous return to the maximal level of entropy [8]. 

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