Studies of Rubberlike Polymers by Nuclear Magnetism

The actual width of the proton resonance line in uncured natural rubber has been determined at room temperature to be 0.06 gauss. Curing of polymers increases the line width at a given temperature. The small increase in natural rubber is possibly compatible with a physical bonding rather than the usually assumed cross-linking. For a butadiene-styrene copolymer, the increase of line width due to cure is somewhat larger. Carbon-black loading increases the line width to a lesser degree than does cure. This is compatible with the concept of physical bonding between the blacks and the polymer chain molecules. Variations of line width caused by changes in chemical composition and copolymerization were also investigated. Polypropylene and polypropylene oxide of roughly the same average molecular weight are compared. The polypropylene oxide exhibits a greater degree of “rotation” about its C—O bonds than polypropylene does about its C—C bonds. Two butadiene-acrylonitrile copolymers also have been studied as a function of temperature. Finally, spin-lattice relaxation time vs. temperature studies are reported for a butadiene-acrylonitrile copolymer and for raw Butyl, over the temperature range from −50° to 70° C. Estimates of the magnitude of the barriers hindering “rotation” are made.