Sensitivity of high-field electron paramagnetic resonance to the reorientation of molecular guests in glassy polymers

The reorientation of one paramagnetic molecule in glassy polystyrene is studied by high-field electron paramagnetic resonance spectroscopy at four different Larmor frequencies between 9.5 and 285 GHz. On increasing the Larmor frequency, the lineshape exhibits larger sensitivity to the rotational motion of the radical. Unfreezing the rotational dynamics by increasing the reorientation rate leads to considerable line shifts, whereas broadening effects are not dominant. The finding is evidence that the paramagnetic Molecule undergoes small-angle reorientation in the glassy matrix.