The reorientation of guest paramagnetic molecules free radicals spin probes in the host polymers and glass formers is studied by High-Field High-Frequency Paramagnetic Resonance ((HFEPR)-E-2) spectroscopy at three different Larmour frequencies, 95, 190 and 285 GHz. The structural change of the host that occurs close to glass transition temperature changes the reorientation regime of the molecular spin probe. For the slow tumbling regime of the rotational motion, the line shape exhibits larger sensitivity of the free radicals spin probe to both the static and the dynamics of the environment. Change of the reorientation rate lead to considerable line shifts, whereas broadening effects are not dominant. Discussion is focused on the suitable choice of the spin probe used in the (HFEPR)-E-2 spectroscopy in the study of the polymeric and glass formers systems.
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