Models of slow rotational mobility of paramagnetic probes in glassy media

Abstract

Experimental ESR spectra of the 2,2,6.6-tetramethyl-4-oxopiperidinoxyl (TEMPON) radical probe in the glycerol, polystyrene, and polyvinylbutyral matrices measured in the temperature range 77--373 K were quantitatively compared with the ESR spectra calculated using the known theoretical models of rotational mobility. It was shown that simulation of ESR spectra by the nonlinear least-squares method is an efficient procedure for discriminating ' between theoretical models. The temperature ranges were determined in which it is possible to achieve quantitative agreement between experimental and theoretical spectra as well as the ranges in which theoretical models are insufficient to quantitatively describe the experimental results. It was established that the widely used model of Brownian diffusion in isotropic medium is inadequate to describe the ESR spectra in the case of slow motions of small probe molecules. It was found that specific interactions (formation of weak complexes) between the probe molecules and the molecules of the medium results in strongly anisotropic molecular rotational motions.

Temperature dependence of ESR spectra of nitroxide spin probe in glassy polymers

“…However, a more detailed analysis reveals some characteristic properties of temperature dependencies of ESR spectra in polymer glasses in comparison with ones in low‐molecular glasses. For comparison, the previously observed20 temperature dependence of ESR spectra for TEMPON in glycerol is given in Figure 5. We define three ranges of qualitatively different changes in the shape of ESR spectrum.…”

“…Data obtained by echo‐induced ESR,15, 43–45 electron double resonance,46–49 and also saturation recovery measurements50 indicate that nitroxides undergo libration motions (fast reorientation motions near equilibrium) in glassy materials. The results of numerical CW‐ESR spectra simulations show that the amplitudes of “librations” in low‐molecular glasses are smaller than ones in polymers 18, 20, 51…”

“…To explain observed trends and resolve contradictions, numerical simulations of ESR spectra have been used. At this time, it has been established that the models of Brownian isotropic rotational diffusion, jump diffusion, and free diffusion are unable either to reproduce quantitatively experimental spectra of nitroxide radicals in glassy polymers18, 20 or to get the satisfactory description of temperature dependencies of ESR spectra in two ranges of ESR frequency 19. The use of correlation time distribution in numerical simulation led to a significant improvement in agreement between theoretically calculated and experimental ESR spectra for nitroxides in some polymers 21, 22.…”

“…In spite of the fact that the use of either correlation time distribution or complicated models contributes to significant improvement of agreement between theoretical and experimental ESR spectra, the quantitative agreement is not necessarily obtained. At this moment there is limited amount of data on the application of these approaches for describing experimental ESR spectra of nitroxides in glassy polymers 18, 20–22. The problem of describing the temperature dependence of ESR spectra in the full range of temperatures below T g still has not been resolved.…”

Simulation of Rigid-Limit and Slow-Motion EPR Spectra for Extraction of Quantitative Dynamic and Orientational Information

“…Some authors use mean-squared sine of the displacement angle averaged over all the paramagnetic particles, <sin 2 x>, <sin 2 y>, and <sin 2 z>, for characterization of quasilibration motion [43,44,[48][49][50]. The relations between these characteristics and the quasilibration amplitudes mentioned earlier are given by…”