NMR relaxation-time studies of poly(N-vinyl carbazole) and sorbed-O2 effects

Abstract

Proton spin-spin (T2) and spin-lattice (T1) relaxation times have been measured in order to elucidate the molecular dynamics in poly(N-vinylcarbazole) (PVK). Measurements were obtained on samples covering a broad range of molecular weights in air, O2, N2, and in vacuo. Correlation frequencies νc determined from NMR data were used in conjunction with dielectric data to construct transition maps. The α,β relaxations associated with main-chain and segmental motion exhibit temperature-dependent behavior very similar to that observed in polystyrene (PS). The γ relaxation in PVK is highly unusual in that it is dielectrically active and manifested in the T2 data only in the presence of O2. A detailed interpretation of this effect is provided, leading to the conclusion that O2 diffusion is severely restricted at low temperatures and that torsional oscillation of the carbazole group is responsible for the γ relaxation. It is proposed that the presence of O2 leads to relaxation by slow spin diffusion to the paramagnetic site at low temperatures but by direct paramagnetic relaxation by diffusing O2 molecules at high temperatures. This effect is related to the temperature-dependent lifetime of the O2-carbazole complex.

An investigation of 1H and 13C longitudinal relaxation and relaxation in the rotating frame in solid poly(N‐vinylcarbazole)

“…Using air, 1 H longitudinal relaxation was exponential with a T 1 value of 0.80 s. However, on commencing the use of nitrogen gas, the value of T 1 increased rapidly, reaching a limiting value of 53 s after about 6 h exposure, indicating that 1 H longitudinal relaxation in air is controlled almost entirely by interaction with the electronic magnetic moment of absorbed paramagnetic oxygen. This behaviour is similar to that observed previously for 1 H longitudinal relaxation in 1 15. Because of the large magnitude of the electronic magnetic moment, relatively small amounts of oxygen can have a substantial effect on nuclear relaxation.…”

Unexpected proton spin‐lattice relaxation in the solutions of polyolefin and tetrachloroethane

“…So the oxygen effect is the most apparent factor to consider when comparing to the literature. It is well known that paramagnetic oxygen dissolved in solutions causes a major shortening of T 1 relaxation times 14–19. To see the specific oxygen effect on our samples, we degassed the EO‐5 copolymer solution by subjecting it to three freeze–pump–thaw cycles before sealing the NMR tube.…”

“…In the solutions of E/O copolymer and tetrachloroethane without strict degassing treatment, possible contributions other than nuclear dipole–dipole interactions (DD) to the proton relaxation are paramagnetic oxygen effect,14–17 scalar coupling between proton and chlorine nuclei (SC),20 chemical shift anisotropy (CSA), and spin rotation (SR) 22. The total relaxation time T 1 is determined through the following equation: where DD includes contributions from both nuclear and electron spins.…”

Radiation‐induced conductivity in polystyrene as a hopping phenomenon

“…The spectrum of the molecular relaxations in PS a t temperatures lower than the glass transition temperature (T, = 80°C) is now known in detail (see, for example, If one assumes vo = 2.5 [45]). B u t the smallest value of t h e activation energy found for t h e y-relaxation a t t r i b u t e d t o rotatioil of t h e phenyl groups happens t o h e 32 I d (0.33 eY).…”