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Diffusion of decafluoropentane in amorphous glassy perfluorodioxole copolymer by pulse field gradient NMR spectroscopy
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) cm(2) s(-1), while the faster diffusion process 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 geometry, 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 tortuous diffusion in a porous medium. The slower diffusion process was considered to involve molecules in low-free-volume regions as this process was associated with the broader proton resonance and a longer spin-lattice relaxation 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.
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Reference:
1,3-Benzodioxole – Wikipedia,
,Dioxole | C3H4O2 – PubChem