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 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 µm. 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.

Distribution and Effect of Water Content on Molecular Mobility in Poly(vinylpyrrolidone) Glasses: A Molecular Dynamics Simulation

“…10%) of the simulated PVP due to the faster cooling rate (0.1 K/ps); and (2) the short length of time (0.1 2s) during which the diffusion coefficients were calculated. The latter argument is supported by the fact that solute diffusion in amorphous polymers and other similar systems (e.g., lipid membranes) may exhibit two or more diffusion coefficients differing by one to three orders of magnitude depending on the time scales of measurement methods such as NMR, fluorescence and neutron scattering (71,72). The short simulation length also caused anomalous nonEinsteinian diffusion behavior as demonstrated by curvature in plots of log r 2 vs. log t with slopes <1 (Fig.…”

A Lattice Model for the Simulation of Diffusion in Heterogeneous Polymer Systems. Simulation of Apparent Diffusion Constants as Determined by Pulse-Field-Gradient Nuclear Magnetic Resonance

“…This is a very permeable polymer at temperatures well below the glass transition. [14][15][16][17][18][19][20] The fractional free volume in this high-perme- ability polymer is ∼0.3, whereas the fractional free volume [17][18][19][20] associated with the larger free-volume elements is considered to be ∼0.03. Thus, 10% of the free volume is associated with the large-free-volume elements.…”

Effects of Conditioning, Aging, and Copolymer Content on the Diffusion of Propane and Pentane in Amorphous Glassy Perfluorodioxole by Pulse Field Gradient NMR

“…Another polymer with nearly as large free volume elements and about as high a fractional free volume is the random copolymer of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (PDD). − However, this polymer is less permeable and ages less rapidly. Diffusion of small molecules in this polymer has been characterized using pulse field gradient (PFG) NMR − which determines the self-diffusion constant. In the TFE/PDD copolymer which is 35% TFE, the apparent diffusion constant in the PFG NMR experiment depends on the time over which diffusion is allowed to occur. − This behavior is indicative of an impediment to the random walk a penetrant would undergo in a homogeneous system.…”