Spin echo diffusion molecular weights

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 ... 

In many products, the spin-relaxation properties of the components can be different due to molecular sizes, local viscosity and interaction with other molecules. Macromolecules often exhibit rapid FID decay and short T2 relaxation time due to its large molecular weight and reduced rotational dynamics [18]. Mobile water protons, on the other hand, are often found to have long relaxation times due to their small molecular weight and rapid diffusion. As a result, relaxation properties, such as T2, have been used extensively to quantify water/moisture content, fat contents, etc. [20]. For example, oil content in seeds is determined via the spin-echo technique as described according to international standards [64]. 

Because T2 tends to be longer for molecules of lower molecular weight, spin-echo experiments tend to be very sensitive to contamination by light, diffusing impurities often found in polymer preparations. Care must be taken to remove these 32) to avoid inviting misinterpretation of experimental data. 

Ewen B, Maschke U, Richter D, Farago B (1994) Neutron spin echo studies on the segmental diffusion behaviour in the different chain sections of high molecular weight poly(dimetylsiloxane) melt. Acta Polym 45 143-147 Faitelson LA (1995) Some aspects of polymer melts rheology. Mech Compos Mater 31(1) 101—116... 

Though fo is normally determined by mechanical measurements, it is evident from its definition that it is related in a simple manner to translatory diffusion, and in principle it can be determined from self-diffusion measurements in a polymer with sufficiently low molecular weight to avoid entanglements. Some data of this sort have been obtained from diffusion 6f radioactively tagged molecules - and spin-echo nuclear magnetic resonance measurements, as well as from diffusion of deuterated molecules whose concentration can be followed by infrared absorption. They yield the product Pfo. since the frictional resistance for the entire molecule is measured. 

B. D. Boss, E. O. Stejskal, and J. D. Ferry. Self-diffusion in high molecular weight polyisobutylene-benzene mixtures determined by the pulsed-gradient, spin-echo method. J. Phys. Chem., 71 (1967), 1501-1506. 

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