Probing Porous Polymer Resins by High-Field Electron Spin Resonance Spectroscopy

Dynamical Line-Shifts in High-Field Electron Spin Resonance: Applications to Polymer Physics

High-field high-frequency Electron Paramagnetic Resonance spectroscopy (HF 2 -EPR) is a powerful tool to investigate, with ultra-high angular resolution, the rotational dynamics of complex systems like polymers, viscous fluids and glasses. Usually, information is drawn by detailed numerical analysis of the overall lineshape. Here, we present a simplified analytical model of the line shifts due to the rotational dynamics of the paramagnetic centre. The model captures the basic features of the reorientation process (time scale and size of the angular jump). It is compared with experimental results concerning the reorientation of a paramagnetic guest molecule dissolved in polystyrene. We find that, if the rotational model to describe the reorientation of the radical is consistent, the best-fit parameters yield equally acceptable best-fits of the overall spectrum by numerical simulations and dynamical line shifts by independent analytic expressions.

Signatures of the fast dynamics in glassy polystyrene: First evidence by high-field Electron Paramagnetic Resonance of molecular guests

The reorientation of one small paramagnetic molecule (spin probe) in glassy polystyrene (PS) is studied by high-field electron paramagnetic resonance spectroscopy at two different Larmor frequencies (190 and 285 GHz). Two different regimes separated by a crossover region are evidenced. Below 180 K the rotational times are nearly temperature independent with no apparent distribution. In the temperature range of 180-220 K a large increase of the rotational mobility is observed with the widening of the distribution of correlation times which exhibits two components: (i) a deltalike, temperature-independent component representing the fraction of spin probes w which persist in the low-temperature dynamics; (ii) a strongly temperature-dependent component, to be described by a power distribution, representing the fraction of spin probes 1-w undergoing activated motion over an exponential distribution of barrier heights g(E). Above 180 K a steep decrease of w is evidenced. The shape and the width of g(E) do not differ from the reported ones for PS within the errors. For the first time the large increase of the rotational mobility of the spin probe at 180 K is ascribed to the onset of the fast dynamics detected by neutron scattering at T(f)=175+/-25 K.