Diffusion of decafluoropentane in amorphous glassy perfluorodioxole copolymer by pulse field gradient NMR spectroscopy

Pulse field gradient diffusion measurements were made on the decafluoropent ane molecule, CF3CHFCHFCF2CF3, in the copolymer of tetrafluoroethylene (TFE ) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD). The proton s pectrum consisted of two overlapping line shapes, and the decay of the echo amplitude with increasing gradient required the use of two apparent diffus ion constants. The two apparent diffusion constants differed by 1.5-3 order s of magnitude depending on the length of time over which diffusion occurre d. Both apparent diffusion constants were also functions of the diffusion t ime, indicating the presence of structure in this system that obstructs the translational motion of the penetrant molecules. The slower diffusion proc ess ranged from 10(-8) to 10(-10) cm(2) s(-1), while the faster diffusion p rocess ranged from 10(-6) to 10(-8) cm(2) 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 spherical geome try, the length scale was 0.7 mum. The fast diffusion constant appeared to decrease toward a plateau at large diffusion times, which is typical of tor tuous diffusion in a porous medium. The slower diffusion process was consid ered to involve molecules in low-free-volume regions as this process was as sociated with the broader proton resonance and a longer spin-lattice relaxa tion time. The faster diffusion process was considered to involve penetrant molecules in high-free-volume regions that were interconnected. A narrower resonance with a shorter spin-lattice relaxation time was associated with this process.