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

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 1ρθ) 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 1ρ90) 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 1ρ90) 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 1ρ90) 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 1ρ90 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.

Characterization of annealed isotactic polypropylene in the solid state by 2D time-domain1H NMR

“…These magnetization fractions were attributed to crystalline and amorphous phases. 11,12 In later work, 16,17,19 The minima in T 1 of these fractions were attributed to phase transitions occurring in respective phases of the polymer.…”

“…11 More recently, advanced studies involving numerical solutions of the Bloch equations with the diffusion term and simulations for lamellar structures representing semicrystalline polymers have been reported. 19 In this research, a three-region model (crystalline, intermediate, and amorphous) was deemed consistent with three magnetization fractions in the rotating frame and only one fraction in the laboratory frame. Further analysis of the simulated results 20 revealed, however, that even if only two regions are considered (e.g., crystalline and amorphous regions), the observed magnetization relaxation functions may contain up to four observable fractions.…”

The hydration of paper studied with solid-state magnetisation-exchange 1H NMR spectroscopy

“…In multi-phase systems such as polymer blends, the magnetisation exchange process is particularly sensitive to the amount of interfacial area between two phases (i.e., the surface/volume ratio or the phase morphology; Schmidt-Rohr and Spiess 1994) and the nature of the interface (degree of mixing) between the two phases (Wang 1996). The exchange process is energy-conservative in terms of magnetisation, but there will be some loss of magnetisation to the lattice due to spinlattice relaxation during the experiment (Packer et al 1984;Friebel et al 1997).…”

“…Goldman and Shen (1966) reported a pulse sequence used to select magnetisation on the basis of differences in T 2 . The Goldman-Shen pulse sequence has been successfully applied to select magnetisation of mobile protons in heterogeneous polymeric solids (Packer et al 1984;Kenwright and Packer 1990;VanderHart and McFadden 1996;Friebel et al 1997) but multiple quantum coherences may lead to distortions of spectral line shapes . The dipolar filter pulse sequence (Figure 1b;Schmidt-Rohr and Spiess 1994;Clauss et al 1998) is similar to the Goldman-Shen experiment, but consists of multiple cycles of a similar pulse sequence.…”

“…Selection can be based on differing chemical shifts in the proton spectrum (Caravatti et al 1985), T 1 relaxation times (Packer et al 1984) or rotating frame spin-lattice relaxation times . Goldman and Shen (1966) reported a pulse sequence used to select magnetisation on the basis of differences in T 2 .…”