NMR relaxation-time studies of poly(<i>N</i>-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.