Rubber stretched/relaxed

Figve 7.14 The FT Raman measurements on crosslinked natural rubber stretched and relaxed and recorded in two ways as illustrated. Laser polarised vertically. 

When the stretched rubber band is relaxed, the signs of the thermodynamic state functions change ... 

Observe the properties of a stretched and a relaxed rubber band. 

For example, the volume change of an acrylonitrile-butadiene rubber (NBR)40 sample at X = 2 relative to the volume of its undeformed state was about 5 x 10 4, and the values for the other vulcanizates were less than this. We therefore assumed that the use of Eqs. (34) and (35) is warranted for the computation of dW/dlt for our rubber samples, except at very small deformations for which// < 3.02. In most cases, stress relaxation was allowed to proceed at given stretch ratios and 1- and 10-min isochronal stress values were taken for the calculations. 

Under the electron microscope the myosin heads can sometimes be seen to be attached to the nearby thin actin filaments as crossbridges. When skeletal muscle is relaxed (not activated by a nerve impulse), the crossbridges are not attached, and the muscle can be stretched readily. The thin filaments are free to move past the thick filaments, and the muscle has some of the properties of a weak rubber band. However, when the muscle is activated and under tension, the crossbridges form more frequently. When ATP is exhausted (e.g., after death) muscle enters the state of rigor in which the crossbridges can be seen by electron... 

A good elastomer should not undergo plastic flow in either the stretched or relaxed state, and when stretched should have a memory of its relaxed state. These conditions are best achieved with natural rubber (ds-poIy-2-methyl-1,3-butadiene, ds-polyisoprene Section 13-4) by curing (vulcanizing) with sulfur. Natural rubber is tacky and undergoes plastic flow rather readily, but when it is heated with 1-8% by weight of elemental sulfur in the presence of an accelerator, sulfur cross-links are introduced between the chains. These cross-links reduce plastic flow and provide a reference framework for the stretched polymer to return to when it is allowed to relax. Too much sulfur completely destroys the elastic properties and produces hard rubber of the kind used in cases for storage batteries. 

This model is based on classic rubber theory, suggesting that elastin is made up of a network of random chains that are kinetically free and exist in a high entropic state. Stretching orders the chains and limits their conformational freedom, thus decreasing the overall entropy of the system (Hoeve and Flory, 1974). This provides the restoring force to the relaxed state. 

One of the prominent features of polymeric liquids is the property to recover partially the pre-deformation state. Such behaviour is analogous to a rubber band snapping back when released after stretching. This is a consequence of the relaxation of macromolecular coils in the system every deformed macro-molecular coil tends to recover its pre-deformed equilibrium form. In the considered theory, the form and dimensions of the deformed macromolecular coil are connected with the internal variables which have to be considered when the tensor of recoverable strain is to be calculated. Further on, we shall consider the simplest case, when the form and dimensions of macromolecular coils are determined by the only internal tensor. In this case, the behaviour of the polymer liquid is considered to describe by one of the constitutive equations (9.48)-(9.49) or (9.58). 

Katz demonstrates by X-ray diffraction that natural rubber is amorphous in the relaxed state and crystalline upon stretching Meyer and Mark show that the crystallographic and the chemical evidence for the chain concept are in agreement... 

Stress relaxation occurs when a molecular chain carrying a load breaks. This occurs, e.g., during the oxidation of rubbers. When a stretched chain segment breaks, it returns to a relaxed state. Only stretched chains carry the load, and a load on a broken chain in a network cannot be shifted to other chains. It may be assumed that the rate at which stretched network chains are broken is proportional to the total number of chains (n) carrying the load ... 

I 14. Stretch a rubber band while holding it gently to your lips. Then slowly let it relax while still in contact with your lips. 

Fig. 51. Change of modulus (0 and draw ratio (X) for natural rubber The Vidicon pictures show the onset of crystallization during stretching (X,), the maximum of the crystallization (X ) and the melting of the last crystallites upon relaxation (X.,)...

Host outstanding properties of these products were high resilience and good resistance to stress decay. Resilience Is Illustrated In Figure 1 which shows the stress-strain relationship of a poly[(plvalolactone-b-lsoprene-b-plvalolactone)-g-plvalo-lactone] fiber as It was stretched 300% and then allowed to relax. The shaded area Is the work lost as the fiber was loaded and then unloaded. This area amounts to 13% of the total, which shows that work recovered was 87%. Such high resilience compares very favorably with that of chemically-cured natural rubber. 

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