Chain order in filled SBR elastomers: a proton multiple-quantum NMR study

A series of cross-linked styrene-butadiene rubbers (SBR) filled with different amounts of carbon black and silica are investigated by proton multiple-quantum nuclear magnetic resonance (NMR). The method yields reliable information on residual dipolar couplings and their distribution, which in turn are related to local chain order and the effective cross-link density in these systems. Fundamental differences between the response of a linear precursor, which undergoes reptational motion, and vulcanized SBR are discussed. For the latter, it is found that the average chain order parameter as well as its distribution does not change significantly with the amount and the type of filler. This is in surprising contrast to recent results from Hahn-echo relaxometry applied to the same samples, which indicated a significant filler effect on the cross-link density.