The effects of morphology on 1H NMR spectra and relaxation in semicrystalline polyolefins

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Abstract
A brief résumé is given of the role of structural heterogeneity, magnetic dipolar couplings, molecular structure, and molecular motion in determining the 1H NMR spectra and relaxation properties of heterogeneous solids such as semicrystalline polymers. Measurements of 1H spin‐lattice relaxation in laboratory (T1) and rotating frames (T) are reported for a number of solid polyolefin samples. These include solution‐crystallized and melt‐crystallized polyethylene, annealed and quenched isotactic polypropene, and isotactic polybut‐1‐ene. In addition, broad‐line 1H spectra, both normal and partially (T) relaxed, are reported for these materials as well as a number of pulsed NMR experiments having the philosophy of the so‐called Goldman–Shen experiment. Spin‐lattice relaxation (T1) for all samples is a single exponential process, whereas rotating‐frame relaxation comprises three exponential processes both on‐resonance (θ = 90°) and off‐resonance at the magic angle (θ = 54.7°), with the latter generally being much slower. The spectra show clearly the existence of components having differing degrees of mobility and, with the exception of the solution‐crystallized polyethylene, the partially (T) relaxed spectra indicate a correlation between breadth of resonance line and magnitude of T1ρ. The Goldman–Shen‐type experiments indicate a spin‐diffusional transport of magnetization between the different spectral and (T) components. A computer program has been used to simulate the NMR behavior of a three‐region system comprising repeating units of infinite lamellae of different widths, each region having different intrinsic relaxation times and spin diffusion coefficients. The results demonstrate that the observed 1H NMR behavior of these samples can be interpreted in terms of this model and that, inter alia, the long‐time T behavior reflects, qualitatively, the time taken for magnetization to diffuse a distance of the order of the dimensions of the region to which it corresponds.