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 (styrene-butadiene rubber, SBR) in the close vicinity of carbon black surfaces. Pulsed nuclear magnetic resonance measurements have been carried out on pure SBR, and on carbon gels obtained by solvent extraction of the carbon black filled elastomers. The initial concentration of carbon black in the filler/rubber mixtures was varied between 20 and 100 parts per hundred parts by weight (phr). The high resolution proton spin-spin relaxation time, T-2, has been measured for each of the individual resonance species belonging to the SBR chain. High resolution was achieved by rapidly spinning the sample at the magic angle. It has been observed that a very high magic-angle spinning (MAS) rate (> 18 kHz) 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 different resonance peaks that are present in the spectrum. Tn the unfilled elastomer, at temperatures much higher than the T-g, the chain segmental motions are anisotropic and deviate from true liquid-like behaviour. The adsorbed rubber chains are found to consist of loosely and tightly 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 the butadiene segment of the elastomeric chain appears to be the most affected by the carbon black surface. Moreover, T-2 is found to be independent of the filler concentration in the 30-80 phr range, and the relative concentration of the tightly bound rubber in the composite shows a maximum at a filler content of similar to 50 phr, where maximum reinforcement is normally observed. As far as the molecular dynamics is concerned, highly filled systems (> 80 phr carbon black) behave differently from low and medium filled systems (< 80 phr).