Structural correlation study of pulsed plasma-polymerized fluorocarbon solids by two-dimensional wide-line separation NMR spectroscopy

Two-dimensional wide-line separation (WISE) NMR spectroscopy has been applied to obtain structural correlation in fluorocarbon films deposited via a pulsed plasma enhanced chemical vapor deposition. Films from two pulsed deposition conditions, 10/20 and 10/400 ms on/off times, were analyzed in detail. One-dimensional F-19 and C-13 NMR results agreed well with prior studies of these films using X-ray photoelectron spectroscopy, which showed greater CF2 fractions with decreasing pulsed deposition duty cycle. C-13 CPMAS studies, in addition, distinguished between unsaturated and saturated fluorocarbon species in these films. However, F-19-C-13 WISE allowed further differentiation of fluorocarbon moieties by the difference in nearest-neighbor connectivity. The 10/20 film showed a prevalence of at least two inequivalent CF3 species. A weak presence of unsaturated =CF- species was also detected. No specific CF2 moiety was resolved, which was attributed to significant chemical shift dispersion and heterogeneity in CF2 bonding environment. In contrast, the 10/400 film displayed an increased proportion of CF2 species, predominantly as -CF2CF2- and CF3CF2- groups, and less CF3 termination, indicative of a structure approaching that of poly(tetrafluoroethylene). These observations support the hypothesis of a less aggressive deposition environment, inducing less cross-links and unsaturation and promoting linear chain propagation, with a longer pulse off-time during deposition. Spinning sideband patterns in the F-19 dimension of NMR spectra did not severely hinder spectral interpretation but instead offered a means of revealing motional differences in the fluorocarbon moieties.