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Pulse Field Gradient NMR Study of Diffusion of Pentane in Amorphous Glassy Perfluorodioxole

2001; American Chemical Society; Volume: 34; Issue: 14 Linguagem: Inglês

10.1021/ma010094c

1520-5835

Ghirmai Meresi, Yingzi Wang, Job D. Cardoza, Wen, Alan A. Jones, Jaimie Gosselin, David G. Azar, Paul T. Inglefield,

Advanced Neuroimaging Techniques and Applications

Pulse field gradient diffusion measurements of pentane in a random copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (PDD) were made as a function of the time allowed for diffusion to occur. The initial change in echo amplitude at low values of q = γδg was used to determine the apparent diffusion constant. The apparent diffusion constant determined in this way decreases to a constant value as time increases. At a time of 12.4 ms the apparent diffusion constant was 2.2 × 10-7 cm2 s-1, and it decreases to 5.87 × 10-8 cm2 s-1 at 1 s. This change in the apparent diffusion constant was interpreted in terms of morphological structure on the micron length scale in this completely amorphous glassy polymer. This polymer has been considered to have high free volume regions which lead to the observed rapid permeation of gases and vapors. These regions are interspersed in lower free volume regions, and the low free volume regions constitute the majority component. In the pulse field gradient NMR experiment, regions allowing for rapid diffusion interspersed with regions allowing only slow diffusion can lead to changes in the apparent diffusion constant as a function of diffusion time. The data observed in this system were compared with two simple models for the topology of the regions allowing for rapid diffusion: restricted diffusion and tortuous diffusion. The observed behavior is qualitatively similar to tortuous diffusion and can be fit with an equation describing this type of diffusion.