Stretching rubbery materials

At room temperature, PE is a semi-crystalline plastomer (a plastic which on stretching shows elongation like an elastomer), but on heating crystallites melt and the polymer passes through an elastomeric phase. Similarly, by hindering the crystallisation of PE (that is, by incorporating new chain elements), amorphous curable rubbery materials like ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), ethylene-vinyl acetate copolymer (EVA), chlorinated polyethylene (CM), and chlorosulphonated polyethylene (CSM) can be prepared. 

This experiment provides fundamental information on the stiffness or tensile strength of a material. (Tensile is defined as the capability of being stretched or extended.) Consider by contrast the bottom curve in Figure 7-3, which describes the behavior of a rubbery material. Minimal stress results in substantial elongation, signifying a low tensile modulus (linear portion of curve). Elastomers commonly can be stretched several hundred percent before they break. In the middle is the plot of a tough plastic, a material that combines properties of both of the other two classes. Tough plastics often have a modu-... 

Elastomers Rubbery materials. The restoring force comes from uncoiling or unkinking of coiled or kinked molecular chains. They can be highly stretched. 

Figure 58 shows the isochronal VL function behavior for the polycarbonate as a function of the stretch X. Clearly, the behavior is very different for the material below the glass transition, that is the solid-like or glassy polycarbonate, than it is for the rubbery material in Figure 32. 

In general, the transverse strain of semi-crystalline polymers (perpendicular to the stretching direction under uniaxial elongation) decreases with deformation. For a uniform deformation, the thickness of the sample decreases. For rubbery materials, at low deformations, the thickness t decreases as f=fo where Iq and... 

In general, the transverse strain (perpendicular to the stretching direction under uniaxial elongation), decreases at a constant rate with deformation. The Poisson s ratio is defined as the ratio of the transverse strain to the applied straiit. The Poisson s ratio of rubbery materials is in the range of 0.4 to 0.5 (compared to 0.3 for hard materials such as metal or glass). As described above, most semicrystalline polymeric materials, however, show inhomogeneous deformation accompanied with neck formation. The evaluation of the... 

Two other important classes of polymer are thermosets and elastomers. Thermosets harden when the temperature is increased due to crosslinking and cannot be processed by melting. Thermosets are often formed by resins such as epoxy adhesive which, when mixed with hardener, becomes solid ( sets ) very rapidly if heated. Elastomers are rubbery materials that can be stretched to many times their original dimension and that recover... 

Natural rubber (NR) is a well studied elastomer. Of particular interest is the ability of NR to crystallize, specifically the strain-induced crystallization that takes place whilst the material is stretched. Moreover, in many elastomer applications, network chain dynamics under external stress/strain are critical for determining ultimate performance. Thus, a study on how the strain-induced crystallization affects the dynamics of a rubbery material is of outmost importance. Lee et al [1] reported their initial findings on the role of uniaxial extension on the relaxation behavior of cross-linked polyisoprene by means of dielectric spectroscopy. Nonetheless, to our best knowledge no in-depth study of the effects of strain induced crystallization on the molecular dynamics of NR has been undertaken, analyzing the relaxation spectra and correlating the molecular motion of chains with its structure. Broadband dielectric spectroscopy (BDS) has been chosen in order to study the dynamic features of segmental dynamics, because it is a comparatively simple technique for the analysis of the relaxation behaviour over a suitable frequency interval. This study is important from a basic and practical point of view, since an elongated crosslinked polymer at equilibrium may be considered as a new anisotropic material whose distribution of relaxation times could be affected by the orientation of the chains. 

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