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

Abstract

Abstract A brief résumé is given of the role of structural heterogeneity, magnetic dipolar couplings, molecular structure, and molecular motion in determining the 1 H NMR spectra and relaxation properties of heterogeneous solids such as semicrystalline polymers. Measurements of 1 H spin‐lattice relaxation in laboratory ( T 1 ) 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 1 H 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 ( T 1 ) 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 T 1ρ . 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 1 H 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.