Polyethylene spin diffusion

Fig. 5.16. Imaging polymer morphology by spin-diffusion contrast on a sample of electrically aged polyethylene, (a) Sample for electrical aging in needle-plate geometry and region cut out for spin diffusion imaging with one-dimensional spatial resolution, (b) Spatially resolved distribution of the domain sizes derived from fitting theoretical diffusion curves of a sandwich layer model to the experimental data. Pronounced changes in the thickness for crystalline, interfacial and amorphous layers are obtained [58].

In some cases it is not possible to fit the data to a simple model in which there are only two phases, crystalline and non-crystalline it is necessary to assume the presence of an intermediate interfacial phase. For instance, in a particular set of spin-diffusion experiments on polyethylene the data could be fitted to a three-phase model in which the thickness of the interfacial region was found to be 2.2 0.5 nm both for low- and for high-density polymers, compared with thicknesses of about 9 and 40 nm, respectively, for the crystallites. 

In a spin-diffusion experiment on a sample of polyethylene the magnetisation of the crystalline phase was found to have risen to 0.57 of its maximum value 120 ms after the start of the FID. Assuming that the magnetisation of this phase is essentially zero at the start of the FID and that the appropriate diffusion coefficient Z) = 8.3 x 10 m s , calculate the mean thickness of the non-crystalline layers, assuming that the pol5mier consists of lamellar stacks of crystalline and non-crystalline material. 

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... 

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. 

In the field of solid polymers, reviews by Schaefer and Stejskal and Lyerla of recently developed techniques for obtaining high-resolution spectra of solids are particularly welcome. High-resolution techniques are included in a wide-ranging article that also covers the use of more traditional broadline n.m.r. in studying molecular motion, spin-diffusion, self-diffusion, and phase structure in both synthetic and biological polymers. Useful specialized reviews have been published on phase structure in polyethylene, elastomers, interfacial effects, and radiation effects. ... 

A one-dimensional diffusion model was used for n-paraffins and has also been applied to chain folded lamellae in polyethylene [50]. If Tisd is the contribution to the relaxation time due to spin diffusion for the first case it can be shown that ... 

The first direct spin-echo diffusion measurement in a polymer melt was reported by McCall, Douglass, and Anderson34) in low-molecular weight polyethylenes. Because of the limitations of the SGSE method the authors found themselves restricted (D > 1(T7 cm2/sec) to molecular weights M < 10 and to temperatures T above 130 °C. They observed an Arrhenius behavior ... 

Variable-temperature NMR experiments provide information about the dynamic processes that occur in solids. It is well known that, for a semicrystalline polymer such as polyethylene (PE), a higher rate of cooling from the melt leads to lower crystallinity C CP/MAS NMR establishes that the cooling rate also affects the structure and motion of the amorphous domain [217]. Many of the dynamic properties of PE, such as jump rates and activation energy, can be explained in terms of chain diffusion between the phases, as detected by 2-D exchange C NMR [218]. Very slow motions in the crystalline a-form of deuterated poly(vinylidene fluoride) have also been detected by exchange NMR [219]. Double-quantum transitions can occur in materials in which spin pairs are only a few bonds apart this... 

Experimentally, we do not have any data on the diffusion of trapped chains in a network. What is available is the diffusion coefficient of a labeled chain in polyethylene melts, measured by two techniques 1) from nuclear spin resonance data, and 2) using deuterated chains as labels, the local concentrations of deuterated/protonated species being probed by infrared measurements. ... 

Two-dimensional NMR experiments can also be used in the solid state to study the structure and dynamics of polymers, and 2D solid-state NOESY has been used to provide a molecular-level assignment for the polyethylene a transition observed by dielectric and dynamic mechanical spectroscopy [32]. One proposal is that this transition can be assigned to chain diffusion between the crystalline and amorphous regions [33], Two peaks are observed in the C-CPMAS spectra of polyethylene that can be assigned to chains in crystalline and amorphous environments. Figure 3.27 shows the 2D spin exchange... 

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