ABSTRACT:Proton spin-spin relaxation time (T2) was measured by the pulsed NMR technique for bound rubbers in composites prepared from natural rubber and carbon black at different acid group concentrations. The bound rubber consisted of loosely and tightly bound rubbers. By incorporation of the oxidized carbon black with a high concentration of acid groups into natural rubber, the content of the total bound rubber in the composite increased, and the segmental mobility of the loosely and tightly bound rubber constituting the bound rubber was enhanced. The increase in the segmental mobility of the bound rubber and its content resulted from the formation of the bound rubber with fairly long Ioops·on the oxidized carbon black. The change in reactivity of the carbon black brought about nitric acid treatment is also discussed in connection with the results mentioned above.

SynopsisProton spin-spin relaxation time has been measured by the pulsed NMR technique for the bound rubbers extracted from both silica-filled polyisoprene and polybutadiene composites.Two relaxation times T,, (short) and T,, (long) are observed for all samples. They are ascribed, respectively, to the relaxation of the tightly and loosely bound rubber components. When the silica filled polybutadiene composite is heat treated at 120"C, loosely bound rubber is preferentially formed, which leads to the increase in the total bound rubber fraction in the composite. During the heat treatment of silica-filled polyisoprene composite, a part of the loosely bound rubber phase is transformed into tightly one, and simultaneously the chain mobilities of both phases become more constrained state. These changes are accompanied by the degradation of polyisoprene molecules probably due to the strong chemical interaction of silanol group and rubber molecules. At a prolonged heat treatment, the fraction of total bound rubber in the composite decreases as a result of the degradation of the loosely bound rubber molecules.

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