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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.