HIGH-RESOLUTION SOLID-STATE NMR INVESTIGATION OF THE FILLER-RUBBER INTERACTION .1. HIGH-SPEED H-1 MAGIC-ANGLE-SPINNING NMR-SPECTROSCOPY IN CARBON-BLACK FILLED STYRENE-BUTADIENE RUBBER

This present work, dealing with filler/rubber interactions, provides a new approach for investigating the behaviour of elastomeric chains (s tyrene-butadiene rubber, SBR) in the close vicinity of carbon black su rfaces. Pulsed nuclear magnetic resonance measurements have been carri ed out on pure SBR, and on carbon gels obtained by solvent extraction of the carbon black filled elastomers. The initial concentration of ca rbon black in the filler/rubber mixtures was varied between 20 and 100 parts per hundred parts by weight (phr). The high resolution proton s pin-spin relaxation time, T-2, has been measured for each of the indiv idual resonance species belonging to the SBR chain. High resolution wa s achieved by rapidly spinning the sample at the magic angle. It has b een observed that a very high magic-angle spinning (MAS) rate (> 18 kH z) is necessary to achieve a fully resolved spectrum of SBR. However, at a spinning rate of similar to 15 kHz it is possible to avoid all of the spinning side bands and to deconvolute, unambiguously, the differ ent resonance peaks that are present in the spectrum. Tn the unfilled elastomer, at temperatures much higher than the T-g, the chain segment al motions are anisotropic and deviate from true liquid-like behaviour . The adsorbed rubber chains are found to consist of loosely and tight ly immobilized segments. The relative immobilization of the different protons has evidenced the methine H-1 to be much more immobilized than the aromatic or methylene species. Therefore, the olefinic part of th e butadiene segment of the elastomeric chain appears to be the most af fected by the carbon black surface. Moreover, T-2 is found to be indep endent of the filler concentration in the 30-80 phr range, and the rel ative concentration of the tightly bound rubber in the composite shows a maximum at a filler content of similar to 50 phr, where maximum rei nforcement is normally observed. As far as the molecular dynamics is c oncerned, highly filled systems (> 80 phr carbon black) behave differe ntly from low and medium filled systems (< 80 phr).