Diffusion of Decafluoropentane in Amorphous Glassy Perfluorodioxole Copolymer by Pulse Field Gradient NMR Spectroscopy

Abstract

Pulse field gradient diffusion measurements were made on the decafluoropentane molecule, CF3CHFCHFCF2CF3, in the copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD). The proton spectrum consisted of two overlapping line shapes, and the decay of the echo amplitude with increasing gradient required the use of two apparent diffusion constants. The two apparent diffusion constants differed by 1.5−3 orders of magnitude depending on the length of time over which diffusion occurred. Both apparent diffusion constants were also functions of the diffusion time, indicating the presence of structure in this system that obstructs the translational motion of the penetrant molecules. The slower diffusion process ranged from 10-8 to 10-10 cm2 s-1, while the faster diffusion process ranged from 10-6 to 10-8 cm2 s-1. The product of the slower diffusion constant and the diffusion time was constant within experimental error, which is typical of restricted diffusion. Assuming a s...