NMR Studies of the Thermal Degradation of a Perfluoropolyether on the Surfaces of γ-Alumina and Kaolinite

Abstract

Solid-state nuclear magnetic resonance (NMR) methods are used to follow the thermal degradation of Krytox 1506, a common perfluoropolyether, following adsorption onto the surfaces of gamma-Al2O3 and a model clay (kaolinite). The alumina studies are complemented with thermogravimetric analysis (TGA) to follow the degradation process macroscopically. Molecular-level details are revealed through 19F magic-angle spinning (MAS), 27Al MAS, and 19F --> 27Al cross-polarization MAS (CPMAS) NMR. The CPMAS results show the time-dependent formation of probable VIAl(O6 - nFn) (n = 1, 2, 3) species in which the fluorine atoms are selectively associated with octahedrally coordinated aluminum atoms. For the alumina system, the changes in peak shapes of the CP spectra over time suggest the early formation of catalytically active degradation products, which in turn lead to the formation of additional perfluoropolyether degradation products. Similar to the alumina system, the kaolinite system also displays new resonances in both the 27Al MAS and 19F --> 27Al CPMAS spectra after thermal treatment at 300 degrees C for up to 20 h but reveals a more distinct species at -15.5 ppm that forms at the expense of an initial species (3 ppm), which is in greater abundance at shorter heating times.

Lithium Insertion Mechanism in Iron Fluoride Nanoparticles Prepared by Catalytic Decomposition of Fluoropolymer

“…However, disappearance of any intense NMR signal from the precursor mixture as soon as 300 °C was reached without any dwelling, indicates PFPE decomposition followed by random orientation of fluorine nuclei on iron in FeF 3 and FeF 2 that possessed strong magnetic field inhomogeneities. The resonances of 19 F NMR at −80.9, −82.9, −88.6, −89.8, −130.9, and −145.3 ppm can be assigned to the −OCF­(CF 3 ) 2 , CF 3 CF 2 CF 2 O–, −OCF 2 CF 3 , −OCF 2 CF 3 , CF 3 CF 2 CF 2 O–, and −OCF­(CF 3 )­CF 3 , respectively, and the others are spinning side bands …”

2D‐NMR studies of a model for Krytox® fluoropolymers

“…[23] This is due to a large 13 C isotope shift of the 19 F resonance for the former correlation. In the 1 J CF HSQC, the cross-peaks come from 19 F directly bound to 13 C. By comparison, in the 2 J CF HSQC experiment, cross-peaks are from 19 F directly bound to 12 C and are two bonds away from 13 C. This isotope shift is identical for the resonances of all the structure units in this compound and is easily taken into account when interpreting the NMR data for this class of fluoropolymers. Two-bond 19 F-12 C/ 13 C isotope shifts for similar structures are reported to be 0.010-0.015 ppm and are below the error of the chemical shift measurements in these 2D experiments.…”

“…[8] Relatively simple NMR methods have been used to detect and quantify proton-containing fragments in PFPEs, [9 -11] but the structural identities of these end-groups have not been determined. Recently, [12] solid-state NMR has been used to study Krytox  -PFPE decomposition products captured on the surface of a solid support.…”

“…[8] Relatively simple NMR methods have been used to detect and quantify proton-containing fragments in PFPEs, [9 -11] but the structural identities of these end-groups have not been determined. Recently, [12] solid-state NMR has been used to study Krytox  -PFPE decomposition products captured on the surface of a solid support. While many additives have been used to modulate and enhance the performance of PFPEs, the structural characteristics and nature of the Krytox  fluoropolymer repeat unit is the most critical factor in affecting polymer morphology and physical properties of these materials.…”

Fluoride ions as structural probe-ions in 19F MAS NMR studies of cement materials and thermally activated SCMs

“…This experiment is very useful in studies of the local environments for fluoride guest ions in the calcium-silicate-hydrate (C-S-H) phases of hydrated Portland cement. 19 F MAS NMR has earlier been employed in several studies of clay minerals including 2:1 layer silicates such as montmorillonite and hectorite (Huve et al, 1992;Reinholdt et al, 2001Reinholdt et al, , 2005, kaolinites (Cochiara and Phillips, 2008;Denkenberger et al, 2007;Labouriau et al, 1995;Sanders et al, 2010) and mica minerals (Cattaneo et al, 2011;Fechtelkord and Langner, 2013). These investigations have characterised the generally low fluorine content in natural mineral samples, the role of fluorine as a mineraliser in clay syntheses performed in a fluoride medium, and the reactive and accessible surface sites for the attachment of fluoride ions.…”