Strain-crystallizing materials

A strain-crystallizing material like NR shows much better autohesion. It can be processed to a relatively low viscosity for easy wetting on contact, and still exhibit green strength due to strain-induced crystallization. Several other strain-crystallizable elastomers have been synthesized and shown to exhibit autohesion and green strength comparable or superior to that of NR. These include rran.s-polypen-tenamer, fran -butadiene-piperylene elastomers, and uranium-catalyzed polybutadiene. 

Epoxidized natural rubber is still a strain crystallizing mbber and therefore retains the high tensile strength of natural rubber. However, as can be seen from Table 5, in other respects they have very little in common. The epoxidation renders a much higher damping mbber, a much-improved resistance to oil swelling (insofar as a 50 mol % modified natural mbber has similar oil resistance to a 34% nitrile mbber), and much-reduced air permeability. This latest form of modified natural mbber therefore widens the applications base of the natural material and enables it to seek markets hitherto the sole province of some specialty synthetic mbbers. 

The cohesive fracture of conventional, non-strain crystallizing, unfilled elastomers is sensitive to rate and temperature 32.4-1.48-53) exhibiting increased values of 2J with increasing rate and decreasing temperature. The basic viscoelastic nature of the fracture of these materials is evidenced by the fact that it can be described over wide ranges of temperature and rate by time-temperature superposition as described by the WLF Equation... 

Selected classes of asymmetric crystal structures exhibit the property of piezoelectricity. With the application of a mechanical strain, piezoelectric materials develop an electrical potential difference across them conversely, when a potential difference is applied to these materials, a displacement occurs. The efficiency of the conversion between mechanical energy and electrical energy is described by the electromechanical coupling constant, which practically ranges to values as high as 0.7 a value of 1 would imply complete conversion between mechanical and electrical energy. 

Inelastic deformation can occur in crystalline materials by plastic flow . This behavior can lead to large permanent strains, in some cases, at rapid strain rates. In spite of the large strains, the materials retain crystallinity during the deformation process. Surface observations on single crystals often show the presence of lines and steps, such that it appears one portion of the crystal has slipped over another, as shown schematically in Fig. 6.1(a). The slip occurs on specific crystallographic planes in well-defined directions. Clearly, it is important to understand the mechanisms involved in such deformations and identify structural means to control this process. Permanent deformation can also be accomplished by twinning (Fig. 6.1(b)) but the emphasis in this book will be on plastic deformation by glide (slip). 

---