Link Density, Impurities, Fillers

Andrews and Reed [31] observed an increase in radical intensity with increasing cross-link density of sulphur cured natural rubber (Fig. 7.24) using the preorientation-technique described above. This observation is well in keeping with the fact that 

silane treated 15 /xm glass bead filled polyisoprene, 

A slightly different two-phase system with strong bonds at phase boundaries is that obtained from triblock copolymers such as styrene-butadiene. As discussed in Chapter 2 such a copolymer molecule is composed of rigid (styrene) end blocks joined by elastomeric (butadiene) center blocks. The styrene blocks aggregate to form small domains which act as cross-links leading to a rubberlike elasticity of the 

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From a fit of Equation (10) to spatially resolved relaxation curves, images of the parameters A, B, T2, q M2 have been obtained [3- - 32]. Here A/(A + B) can be interpreted as the concentration of cross-links and B/(A + B) as the concentration of dangling chains. In addition to A/(A + B) also q M2 is related to the cross-link density in this model. In practice also T2 has been found to depend on cross-link density and subsequently strain, an effect which has been exploited in calibration of the image in Figure 7.6. Interestingly, carbon-black as an active filler has little effect on the relaxation times, but silicate filler has. Consequently the chemical cross-link density of carbon-black filled elastomers can be determined by NMR. The apparent insensitivity of NMR to the interaction of the network chains with carbon black filler particles is explained with paramagnetic impurities of carbon black, which lead to rapid relaxation of the NMR signal in the vicinity of the filler particles. 

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