Effect of temperature and pressure on secondary relaxation in glass former system

Glass former materials are characterized by a complex relaxation pattern, which evolves over several time decades. Dielectric spectroscopy has proven particularly useful for studying such scenario as it is able to monitor the dielectric dynamics of a system over a range up to 16 time decades. It has turned out that in such a broad dynamic range several molecular processes take place, and usually most of them are characterized by nonexponential relaxation functions. In polymeric materials the slowest of these processes is called normal mode: simplifying, if we consider a vector connecting the two ends of a polymeric chain, the normal mode reflects the motion of such vector. In non polymeric materials the slowest process is usually called main, structural or - relaxation. It reflects the cooperative motion of the molecules and its characteristic time can be related to the overall viscosity of the material. The origin of the structural a-relaxation is ascribed to cooperative motions that involve an increasing number of molecules and slow down dramatically when the glass transition is approached, for example either by decreasing temperature T or increasing pressure P (i.e., density) [1,2]. Even in case of materials composed by rigid molecules, on shorter timescales than the structural and normal mode processes, additional processes, called secondary processes, appear in dielectric measurements in the frequency interval that opens up in between the main relaxation and the vibrational dynamics. In the case such processes depend on the local motion of whole molecule (intermolecular process), they are usually called Johari-Goldstein, JG, (secondary) relaxation, [3,4], otherwise they are called secondary or non-JG relaxation or intramolecular secondary relaxation. Until now there is no general consensus about the identification of the microscopic origin of secondary relaxation. It has been suggested that the connection or the similarity of dynamic properties (dependence of relaxation time on temperature, pressure, thermodynamic history of glass formation) of the secondary relaxation with those of the structural one can be used as criterium to distinguish JG and non-JG relaxations [5]. However, the existence of such connection is still questioned and the debate about the validity of these criterium is debated. This thesis concerns about the characterization of secondary processes in the deep glassy state and near the glass transition and about the investigation of the connection of relaxation time of secondary and structural processes. The experimental results of this thesis will be reported in chapter 3 and 4. In chapter 3, we will discuss the effect on the secondary relaxations of several glass formers of the pressure, temperature, and the thermodynamic history. We considered both glass formers having secondary relaxation of the JG type and glass formers with intramolecular secondary relaxation. Regarding the isothermal pressure dependence of secondary processes we tried to related the observed behavior with isothermal compressibility of the material. Regarding, the effect of thermodynamic history on the dynamic properties (relaxation time at fixed value of pressure and temperature, activation volume determined in fixed isothermal condition) most of the work on this topic until now considered thermodynamic history differing only for the cooling rate applied to the sample. Instead, in this thesis we used thermodynamic histories differing for the sequence of temperature and pressure variations applied to the sample. Our investigation tried to improve the knowledge of this phenomenology, and to interpret it in terms of complexity of the secondary process and expansivity of the materials. In chapter 4, we contribute to the discussion about the relation between secondary and structural processes. A great advantage in such a study is provided by the possibility of studying the variation of relaxation dynamics with temperature and pressure. By controlling both those thermodynamic parameters it is possible to study the relaxation dynamics of the same system at different densities and temperatures but the same value of structural relaxation time. In such conditions we evidenced a clear relation between the dynamics properties of the structural and secondary relaxation processes. Instead, regarding the dielectric strength of the two processes no relation was found. The experimental finding of a dynamic relation between the two processes extends in the high-pressure region previous results obtained at ambient pressure only and agrees with those of similar works performed in the last years mainly by the group of Prof. Paluch in Katowice. We propose that the experimental procedure herein applied can be used to distinguish secondary processes of intramolecular and intermolecular origin.