Angle resolved X-ray photoelectron spectroscopic (XPS) studies of a series of perfluoropolyethers of general formula F[CF(CF3)CF2O] n CF2CF3, where n = 27, 65, and >70, show that there is preferential ordering of-CF3 groups at the liquid surfaces. The C 1s intensity corresponding to the -CF3 groups increases with decrease in the electron take-off angle. No measurable change in the oxygen or fluorine intensity is observed upon varying the electron take-off angle. However, experiments show that no adsorption site exists at the surfaces, indicating that the ether oxygens are not available for any of the surface processes. Relative enhancement of the C 1s intensity of the -CF2/CF3 group with decreasing take-off angle is different for different liquids, with smaller chain length liquids showing a more rapid change. This is explained as due to an increase in helicity and tilt of molecular chains from the surface normal. Computational studies have been performed at the semiempirical level to understand the mo...

http://www.dstuns.iitm.ac.in/listpdf/57.pdf

Tilted orientation of molecular chains at the surface of a series of perfluoropolyether liquids: an X-ray photoelectron spectroscopic investigation

Perfluoropolyether (PFPE) polymers potentially improve the thermodynamic stability, compatibility, and ionic conductivity of sulfide-based solid-state electrolyte (SSE)/Li-metal interfaces. However, ion-transport kinetics of PFPE polymers remain inadequately understood. Using molecular dynamics, we characterized the coordination structures and diffusion in PFPEs terminated with hydroxyl or carboxyl moieties and poly(ethylene oxide) (PEO) embedded with lithium and bis(trifluoromethylsulfonyl)imide ions. The mechanism underlying Li+ diffusion in the PFPE polymers was found to differ from that in PEO, and their respective ion–polymer contact strengths and durations were analyzed. Furthermore, the terminal structure of PFPE polymers was shown to significantly alter the cation–polymer and anion–polymer coordination structures, which in turn accounts for the different extents of the Li+ coordination strength in PFPE-diol and PFPE-dimethylcarbonate (DMC). Finally, PFPE-DMC favors longer durations of contact with cations and shorter ones with anions, whereas this relationship is reversed in PFPE-diol. Thus, we provide a novel way to explain experimentally the observed differences in their ionic conductivities, and we illustrate a general principle for increasing Li+ diffusivity in PFPE polymers, providing an avenue for understanding ionic migration in PFPE–SSE composite electrolytes. 

Computational Investigation of Lithium-Ion Transport Mechanisms in Perfluoropolyether Polymers

Angle resolved x-ray photoelectron spectroscopic studies of a series of liquid mixtures of a perfluoropolyether (Krytox 16350) and a polyphenylether (Santovac-5) show that the former spreads on the latter and the film thickness could even be smaller than the photoelectron mean free path. The molecular ordering in krytox remain as that of the free liquid which is manifested in the enhancement of the relative intensity of specific features in the photoelectron spectrum as the electron take-off angle is decreased. The preferential ordering is limited to the very top and an upper limit of this thickness is estimated to be about 8 A. This observation is in accordance with molecular dynamics simulations of long chain hydrocarbon liquids.

http://www.dstuns.iitm.ac.in/listpdf/53.pdf

V. Bindu

Papers

Investigation of the depth of preferential surface ordering in liquids: A photoelectron spectroscopic investigation of liquid mixtures