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Digital archive of theses discussed at the University of Pisa


Thesis etd-05102007-155111

Thesis type
Tesi di dottorato di ricerca
Bercu, Ionut Vasile
Thesis title
High field EPR spectroscopy studies in polymers and small molecule glass-formers
Academic discipline
Course of study
Relatore Martinelli, Massimo
Relatore Leporini, Dino
  • characteristics temperatures
  • Electron paramagnetic resonance and relaxation
  • glass transition
  • polymers and molecular glass formers
Graduation session start date
In recent years, an increased interest in the Electron Paramagnetic Resonance (EPR) spectroscopy technique has been observed. As in Nuclear Magnetic Resonance (NMR), there is a tendency in EPR to go to higher microwave frequencies (95 GHz and higher) to obtain enhanced spectral resolution and sensitivity. In this thesis, the potentialities of high field high frequency EPR (HF2 EPR) are exploited in the study of the dynamics of a paramagnetic probe guest molecules (spin probe) in disordered matrices such as polymers and molecular glass formers. The high magnetic fields involved, offer a unique angular sensitivity to the reorientation motion, while by increasing the microwave frequency the dynamics appear more and more in the slow motion regime. The information from the slow motion HF2 EPR spectra analysis, are obtained from the line shifts rather than the lines widths. Moreover a multi-frequency approach is adopted (95 GHz, 190 GHz and 285 GHz) which offers great advantages in the dynamics studies. In fact the motional model chosen in order to describe the reorientation of the spin probe molecules must satisfactorily fit the three sets of spectra.
The focus of this work was to investigate the characteristics temperatures of the polystyrene (PS) well below the glass transition temperature Tg down to cryogenic temperatures as well as the characteristics temperatures in the molecular liquid ortho-terphenyl (OTP), and in the polymer polybutadiene (PB) both above Tg.
The slow motion spectra of a small, stiff, spherical spin probe in glassy polystyrene (PS) were obtained. A fully analytical and numerical simulation analysis was carried out. Two different regimes separated by a crossover region were evidenced. Below 180 K the spin probe is trapped, the rotational times are nearly temperature independent with no apparent distribution. In the temperature range, 180-220 K a large increase of the rotational mobility is observed with a widening of the distribution of correlation times which exhibits two components: i) a delta-like temperature-independent component representing the fraction of spin probe w still trapped; ii) a strongly temperature-dependent component representing the fraction of un-trapped spin probe 1-w undergoing activated motion over an exponential distribution of barriers heights. Above 180 K a steep decrease of w is evidenced. The de-trapping of spin probe and the onset of its large increase of the rotational mobility at 180 K are interpreted as signature of the onset of the fast motion detected by neutron scattering in PS at 175 ± 25 K. By the analytical evaluation of the frequency shift an alternative approach to characterize the spin probe dynamics is found that confirms the results from full numerical simulation.
In the temperature range T>Tg, an optimal choice of the spin probes allowed the investigation of the molecular glass former ortho-therphenyl (OTP) and polymer polybutadiene (PB) by studying the slow motion regime of HF2 EPR spectra. As a function of temperature, the frequency shift of the HF2 EPR spectra exhibit several well distinct regimes with a characteristic cusp-like behaviour. The cusp is found to be very close to the so-called critical temperature Tc which is predicted by the mode coupling theory developed by Götze and co-workers.