Defects in technical elastomers

Elastomers are cross-linked macromolecules above the glass-transition temperature. They are entropy elastic and free of viscous flow. For most applications, the rubber is blended with filler material such as silicates and carbon black before vulcanization. Carbon black is an active filler which introduces physical cross-links of macromolecular chains in addition to the chemical cross-links formed during the vulcanization process. The chemical cross-link density is temperature independent, while the strength of the physical cross-links varies with temperature. 

The chemical cross-link density is a central quantity of interest in the elastomer industry, which, in a car tyre for instance, may vary in a well-defined fashion across the tyre tread. Both types of cross-links together determine the moduli of shear and elasticity, G and E, respectively. At small elongations A = L/Lq the tensile stress th is given by [Elil] 

The same expression is obtained for the shear modulus G at small deformations under application of a shear stress oji. Conventional measurements of the cross-link density are, therefore, performed by defined sample deformation, for instance, in a rheometer during the vulcanization process of a test sample. The maximum rheometer moment for a given deformation amplitude is a direct measure of the cross-link density. Another, but invasive method for measuring cross-link densities in unfilled elastomer samples is by swelling in chloroform or toluene. In practice, spatial variations in cross-link density 

Sample heterogeneities can arise in different stages of elastomer production and product use. Even in a perfectly prepared homogeneous elastomer product, unavoidable ageing processes induce space-dependent defects. Examples for sources of defects are  

At high cross-link densities the residual dipolar interaction of the protons on the network chains increases, and the linewidth broadens beyond 3 kHz. Then line-narrowing techniques from solid-state NMR, such as MAS and multi-piil.se excitation must be applied to reduce the ani.sotropy of the dipole-dipole interaction during the space encoding steps of the imaging sequence. In this way, signal-to-noise ratio as well as spatial resolution can be improved jCorll. 

---