PhD Thesis Defense | VASILIKI MARIA STAVROPOULOU

Non-equilibrium glass dynamics of amorphous polymers and other materials

Glasses are prototypical out-of-equilibrium systems whose dynamic and thermodynamic properties are still not fully understood. This thesis investigates the mechanisms governing the glass transition and the subsequent structural relaxation toward equilibrium, known as physical aging, in different glass-forming materials, including Van der Waals molecular glasses, polymer glasses, and vitrimeric polymer networks. To this end, advanced calorimetric techniques, mainly Fast Scanning Calorimetry (FSC), were employed to study the kinetics of structural relaxation over broad time and temperature ranges.

The results show that physical aging in deeply quenched molecular glasses cannot be described solely in terms of the α-relaxation, as additional low-energy-barrier mechanisms contribute to the early stages of equilibration. Isoconversional kinetic analysis reveals a progressive increase in the activation energy during aging, supporting a multistep relaxation scenario. Furthermore, complementary dielectric spectroscopy experiments confirm the validity of density scaling under out-of-equilibrium conditions.

In addition, FSC-based protocols were developed to investigate vitrimeric polymer networks, enabling the decoupling of the glass transition from the topology freezing transition and the determination of the activation energies associated with dynamic bond-exchange processes.

Schematic illustration of the (a) enthalpy and (b) specific heat capacity recovery of a glass-forming system during physical aging (Ta: aging temperature, Tf: fictive temperature, Tg: glass transition temperature).