Rotational mobility of guest molecules studied by method of oriented spin probe

Abstract

Oriented spin probe method was used for study of rotational mobility of di-p-anisylnitroxide in low-temperature ethanol, toluene and isopropylbenzene glasses as well as in glassy polystyrene, poly-(N-vinylpyrrolidone) and polyvinyl butyral matrixes. Alignment of probe molecules in initially isotropic samples was induced by polarized light irradiation (photoselection phenomenon). The temperature dependencies of relaxation of alignment were measured. The rotational relaxation of light-induced alignment in low-temperature glasses was found to complete near the glass transition temperature. A step-type relaxation of alignment on a time scale of minutes has been found in the temperature range 150–300 K for polystyrene, poly-(N-vinylpyrrolidone) and polyvinyl butyral. The results are in dramatic contradiction with the commonly known rotational correlation times of probe molecules in glassy polymers obtained by the traditional spin probe method. The nonlinear least-squares fitting of experimental EPR spectra has been used for treatment of this contradiction. Quantitative agreement of simulated and experimental EPR spectra in the temperature range 77–300 K has been achieved simply by using the model of quasi-libration. By this term we mean that the probe molecules are in stochastic fast angular motion in the anisotropic cage of the matrix. The average angular displacement at ∽300 K has been estimated as 5–15°.