Contribution of the methine proton to the transverse 1H NMR relaxation in vulcanized natural rubber

C. A. STEREN; G. A., MONTI; A. J. MARZOCCA; S. CERVENY

MACROMOLECULES

Año: 2004 vol. 37 p. 5624 - 5629

0024-9297

Transverse 1H NMR relaxation experiments in vulcanized Natural Rubber (NR) samples above the glass transition temperature (Tg) are reported on. Natural Rubber samples with the same  initial chemical composition but different times of cure were studied. The distinct feature of this study as compared to previous works, is that the contribution to the total transverse 1H magnetization of the methine group of the monomeric unit and, the contribution of the methyl and methylene functional groups are analyzed independently. The results of these experiments show that for short delay times the decay of the 1H transverse magnetization of the methine groups is slower than that of the methyl/methylene groups. Consequently, the methine 1H contribution to the total transverse magnetization can be distinguished from the contribution of the other functional groups. A slow relaxing contribution to the total transverse 1H magnetization in NR samples, similar to the one reported on in this work, has been described in the literature.1   However, the contribution was attributed to 1H in dangling chain ends experiencing fast anisotropic motions. Here, we show that, on the contrary, the contribution is indeed produced by the methine groups in cross linked chains. Values for the residual dipolar second moment, fraction of dangling chain ends and spin-spin relaxation constant time T2 were obtained from the fit of the transverse 1H  relaxation curves of the methine and methyl/methylene groups independently. The values obtained for these parameters suggest that the protons in the polymer chains sense the same microscopic properties of the material independently of the functional group where they are. The slower decay of the methine magnetization is caused in part by a weaker dipolar interaction for the methine proton with its nearest 1H neighbors as compared to the intragroup dipolar couplings in methyl or methylene groups.