• Pressure
• Johari Goldstein β-relaxation
• Glass transition
• Dispersion
• Dielectric probe
• Apolar polymer
• Structural α-relaxation

Binary mixtures of rigid polar molecules dissolved in apolar solvents are excellent model systems to deepen the understanding of the relaxation dynamics in glass forming systems. In the first part of this thesis, we investigate the molecular dynamics of polar rigid molecules (heterocyclic compounds: tert-butylpyridine, Quinaldine) in apolar solvent (tristyrene). Our result show that the excess wing (characteristic of the polar rigid molecules) is nothing but a Johari Goldstein secondary relaxation so close in the time scale to the structural relaxation that its low frequency side is hidden beneath the structural peak. Analyzing the temperature and pressure behaviour of the two processes, a clear correlation has been found between the Johari- Goldstein relaxation time, the structural relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). These results support the idea that the Johari-Goldstein relaxation acts as a precursor of the structural relaxation and therefore of the glass transition phenomenon.
In the second part of this thesis, we use large rigid polar molecules (4,4-(N,N-dibutylamino)-(E)-nitrostilbene (DBANS)) as a dielectric probe to investigate the molecular dynamics of apolar polymer (poly(styrene) and poly(propylene)). The results show that, the probe molecule motion is coupled to the cooperative α- molecular dynamics of the polymer matrix, while it is not affected by additional processes, like intramolecular defect motions (due to helices in PS) or β-relaxation (for PP). The variations of the glass transition and of the steepness index with molecular weight revealed by the dielectric probe match exactly with what reported in literature for the undoped polymeric matrix.