Microscopic order and mechanical response in polymeric liquids, glasses and crystals

During my Ph.D. I have worked on several topics, all connected by the same purpose to understand, from microscopic basis, the relaxation processes in glass-forming liquids. Following interesting and recent works, particular attention has been addressed to the link between fast vibrational dynamics on picosecond time scales and the slow relaxation. The first part of my work has been devoted to the study of the local order and a possible correlation with the fast dynamics. Then I focused on the extended processes involving the whole system such as the spatial-temporal displacements correlations and the elastic modulus. In the last period I have studied and characterized the geometry and the elastic properties of the polymer crystals, glasses and atomic crystals. The study of the glass-forming liquids is approached here from a numerical point of view. By performing Molecular Dynamics (MD) simulations, I was able to study the dynamics on the microscopic level and to collect informations on every observable of interest with a good precision level, while the same process in experiments would require much more effort. To study the phenomenology of the glass-forming liquids, I have chosen as prototype of viscous liquid the simple beads and springs model for polymeric chain liquids. Polymers represent indeed a central class in this kind of research because of their natural disorder: in most cases a polymer liquid, rather than crystallize in a regular lattice, reaches an amorphous glassy state. However for a few purposes, even the atomic and binary mixtures liquids have been studied.