Molecular mobility of nitroxide spin probes in glassy polymers. Quasi-libration model

ESR spectra of three spin probes with different molecular volumes: 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl, di-p-anisylnitroxide, and nitroxide derivative of fullerene in glassy polystyrene, polyvinyl trimethylsilane, and Teflon AF-2400 were calculated numerically within the model of quasi-libration motions. Temperature ranges, where the model is capable to reproduce spectra within experimental errors, were defined. It was found that simulation of X-band ESR spectra allows to determine quasi-libration amplitudes around molecular axes X and Y with accuracy ~ 3° and around Z axis with accuracy ~ 15-20°. A shape of distribution of quasi-libration amplitudes was also determined qualitatively by ESR spectra simulations. It was established that the average amplitude of quasi-libration motion depends on the free volume of each polymer and geometrical molecular volume of a spin probe. Quasi-libration amplitudes increase as the temperature increases, and reach the value of 40 degrees. We found that upon further temperature increase, quasi-libration model becomes inapplicable for quantitative numerical spectra simulation.