par Li, Chun 
Président du jury Tlidi, Mustapha
Promoteur Napolitano, Simone;Bi, Kedong
Co-Promoteur Mognetti, Bortolo Matteo
Publication Non publié, 2025-04-16
Président du jury Tlidi, Mustapha
Promoteur Napolitano, Simone
;Bi, KedongCo-Promoteur Mognetti, Bortolo Matteo
Publication Non publié, 2025-04-16
Thèse de doctorat
| Résumé : | This dissertation focuses on elucidating the molecular origin of the slow Arrhenius process (SAP), a previously unrecognized relaxation mechanism recently identified in glassy polymers. To this end, we employed large-scale molecular dynamics simulations and complementary numerical analyses to investigate SAP behavior in model polymer systems under both bulk and confined conditions.Our results reveal that SAP emerges prominently during the desorption of polybutadiene (PB) chains from solid interfaces. The process exhibits an Arrhenius-type temperature dependence over a broad temperature range, with a stable activation energy comparable to that observed in shear relaxation of bulk PB melts. Interestingly, variations in statistical analysis methods and interfacial roughness had minimal effect on the activation energy, indicating that SAP is governed primarily by intramolecular conformational transitions, rather than interfacial characteristics.We further demonstrate a direct correlation between SAP and internal molecular forces. By tuning the dihedral angle barriers within the PB chains, we establish that SAP activation energy, shear viscosity, and glass transition temperature evolve in a coupled manner. These findings confirm that SAP and other macroscopic relaxation phenomena share a common molecular origin rooted in the torsional dynamics of polymer backbones.To validate these insights experimentally, we analyzed dielectric relaxation behavior of various polymer thin films. The dielectric response of SAP in poly(4-bromostyrene) remained invariant across film thicknesses ranging from 10 to 800 nm and at temperatures down to 70 K below the glass transition, reinforcing the simulation-based conclusion that SAP dynamics are largely unaffected by confinement. A single set of thermodynamic parameters was sufficient to describe the process in both the supercooled and glassy states, supporting its enthalpic–entropic origin.Finally, we developed a simplified theoretical model to explain crystallinity gradients in polymers confined in nanopores. Our results challenge the conventional notion of interfacial 'dead layers' and underscore the critical role of polymer adsorption in governing confined crystallization. The model highlights a fundamental interplay between adsorption and ordering processes near interfaces.Altogether, this work offers a comprehensive molecular-level framework for understanding long-time polymer relaxation dynamics and interfacial behavior, with implications for the design of high-performance polymer materials in applications ranging from dielectric devices to nanoconfined systems. |
