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

Abstract

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 pul...