Polyethylene proton relaxation

Proton NMR relaxation parameters have also been determined for polyethylene ( ) and polyethylene oxide (39) in the melting region. The apparent contradiction between the proton spin-lattice relaxation parameter for a high molecular weight linear polyethylene sample at its melting point, with the relaxation measurements, has previously been pointed out. (17) This discrepancy is still maintained with the more detailed results reported here for both types of polyethylene. For the proton relaxation a small, but distinct, discontinuity is reported at the melting teirperature. (38)... 

On the other hand, Connor and Hartland ( ) have reported results of a proton NMR study for a series of polyethylene oxide samples by rotating frame proton relaxation time T p meastirements. Ti was also determined. For their lowest molecular weight sam-... 

Proton relaxation under multiple pulse conditions has also been used to characterise phase composition in, for example, PET [95,96], and polyethylene [120]. The technique is particularly useful in the case of PET because the phases present generally do not show large differences in the decay times of their FID components, so FID analysis would be particularly problematic. Although the problem of assessing the extent to which spin diffusion is suppressed also applies to relaxation under multiple pulse conditions, the available experimental evidence suggests that they may be more effective in practice than the corresponding off-resonance spin-locking experiment [96]. This is almost certainly due to practical considerations rather than theoretical ones. 

Koch H, Bachus R, Kimmich R. Molecular fluctuations in polyethylene melts. Dependence of the longitudinal and transverse proton relaxation on the chain length. Polymer 1980 21 1009. 

Voigt G, Kimmich R. Chain fluctuations in the amorphous regions of polyethylene as indicated in proton relaxation spectroscopy. Polymer 1980 21 1001. 

The transition of polyethylene from the orthorhombic to the hexagonal phase was observed at very high pressures (3000-5000 bar) by proton NMR and nuclear magnetic relaxation time measurements the phase diagram derived from this spectroscopy is in substantial agreement with DSC data. According to these results (see Fig. 1), the hexagonal phase can be observed above the triple point at 490 K and 3000 bar [13,14]. 

Information on the local structure of polymers is not limited to proton-proton spin diffusion experiments. Spin diffusion among the carbons can also be used [97], However, because of the low gyromagnetic ratio and natural abundance of the carbon-13 nuclei, it is less efficient than proton-proton spin diffusion. At natural abundance, the rate of spin diffusion is usually too low to compete with the rate of spin-lattice relaxation in most polymers. However, an interesting exception is that of semi-crystalline polymers such as linear polyethylene and cellulose which have very long longitudinal relaxation times and for which natural-abundance spin... 

D.L. VanderHart, A.N. Garroway, C NMR rotating frame relaxation in a solid with strongly coupled protons polyethylene,. Chem. Phys. 71 (1979) 2773-2787. 

At temperatures above their Tg, the resonance spectrum of noncrystalline polybutadiene (PB) (Fig. 8a) is clearly different from that of the semicrystalline polyethylene (Fig. 8c). Amorphous PB exhibits a narrow Lorentzian line shape with a width of 0.2 G. In contrast, the PE spectrum comprises two components, ie, narrow and broad line shapes. When the spectra of semicrystalline polymers are recorded in the glassy state (Figs. 8b and 8d), only abroad component is observed. This indicates that the line shape corresponds to molecular mobility and the line width reflects a correlation (or relaxation) time. Therefore, the broad and narrow components of semicrystalbne PE (Fig. 8c) are related to protons of methylene groups in rigid and mobile (amorphous) environments, respectively. On the basis of this, it was proposed that the degree of crystallinity could be determined by resolving the area of the broad component (rigid phase) from the spectrum. 

Decomposition of proton NMR spectra of linear polyethylene fractions reveals an almost quantitative agreement with the analyses of the Raman internal modes [227]. Both chemical shifts and relaxation times, obtained by solid-state NMR,... 

Brereton MG, Ward IM, Boden N, Wright P. Nature of the proton NMR transverse relaxation function of polyethylene melts. 1. Monodispersed polyethylenes. Macromolecules 1991 24 2068. 

Figure 16 shows typical proton spin-lattice relaxation dispersion data for polyethylene melts as an illustration of the three-component behavior of polymer melts. For comparison with model theories the chain-mode regime represented by component B is suited best and will be discussed in detail. It will be shown that the NMR relaxometry frequency window of typically 10 Hz< V <10 Hz (for proton resonance) almost exclusively probes the influence of chain modes represented by component B (compare Fig. 5). That is, the correlation function experimentally relevant for spin-lattice relaxation dispersion may be identified with component B according to...

Permanent or thermoreversible cross-links mediate the opposite effect on chain dynamics compared with dilution by a solvent instead of releasing topological constraints by dilution, additional hindrances to chain modes are established by the network. With respect to NMR measurands relatively large cross-link densities are needed to affect chain modes visible in the experimental time/frequency window, as demonstrated with proton spin-lattice relaxation dispersion of polyethylene cross-Unked by 10-Mrad irradiation with electron beams [123] and with styrene-butadiene rubbers [29]. However, there is a very strong effect on the dipolar correlation effect which probes much slower motions and can therefore be used favorably for the determination of the cross-Hnk density [29, 176, 177]. 

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