Modulation of proton NMR free induction decay by spin diffusion

The effects of spin diffusion on the free induction decay (FID) of protons associated with the noncrystalline domains of two polymers, polyethylene and blended Nylon 66 (ZYTEL‐408), have been examined using Goldman–Shen NMR pulse sequence ${\text{[(1/2)πx−t}}_0\text{−(1/2)πx̄−τ−(1/2)πx−t].}$ At $\text{τ≲100\hspace{0.17em}μ}\mathrm{s},$ the FID is drastically distorted from the exponential form while at longer $\text{τ(τ≳500\hspace{0.17em}μ}\mathrm{s})$ an exponential FID is observed; but its transverse spin relaxation rate is smaller than that of the FID obtained after a single $\text{(1/2)πx}$ pulse and approaches this value at the limit of large τ. These findings are interpreted in terms of the theory of rapid spin diffusion. The spatial variation in the spin relaxation rate within the noncrystalline domain is inferred.