Evidence for a t−a(a∼1/2) time dependence of chain fluctuations as observed by NMR on cis-1,4-polybutadiene

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Abstract
From a detailed analysis of spin -echo envelopes, the transverse magnetization of protons bound to cis 1–4 polybutadiene chains is found to vary according to M(t)=M0exp(−ξ tα) (5/4≲ α ≲ 3/2). It is shown that this function may be related to a tα−2 time dependence of chain fluctuations. The effects of the partial chain ordering on the fluctuations of the remaining disordered chains, permit the ordering process to be followed by NMR. The ordering rates (k1/2≳ 0.5 min−1) are found to be much higher than those which have been measured on natural rubber. In the presence of good solvents such as CFCl3, C7D8, and CS2 the time constant of the transverse magnetization exhibits an exponential concentration dependence that we have related to the increase of microscopic jump rates, We, which govern chain fluctuations; We∼1012 sec−1.