par Wang, Kai 
Président du jury Tlidi, Mustapha
Promoteur Napolitano, Simone
Co-Promoteur Losada Perez, Patricia
Publication Non publié, 2025-07-10
Président du jury Tlidi, Mustapha
Promoteur Napolitano, Simone
Co-Promoteur Losada Perez, Patricia
Publication Non publié, 2025-07-10
Thèse de doctorat
| Résumé : | A nonequilibrium state refers to a condition in which the system has not yet reached thermodynamic equilibrium, often characterized by the slow evolution of molecular conformations, internal stress, or density distribution. Due to the inherent structural complexity and broad spectrum of relaxation times in polymers, nonequilibrium behavior is especially pronounced. Typical nonequilibrium states include physical aging caused by rapid cooling, chain orientation and residual stress induced during processing methods such as spin-coating or stretching, interfacial heterogeneity arising from adsorption or confinement effects in thin films, and frozen conformations or poorly entangled networks formed during fast solvent evaporation. These nonequilibrium features significantly influence the mechanical, thermal, and interfacial properties of polymeric materials. Therefore, understanding the evolution of nonequilibrium states is a fundamental step toward tailoring polymer performance and enabling functional material design.The equilibration from a nonequilibrium state to an equilibrium state is essentially a progressive evolution from a high free energy state towards a lower one. This evolution is mediated by relaxation processes spontaneously occurring at the molecular level. These microscopic relaxation processes serve both as a bridge and a regulator, determining the rate, pathway, and efficiency by which the system returns to equilibrium. The (α-)relaxation is the primary molecular process in polymeric materials and is associated with the cooperative motion of several segments. This microscopic mechanism dominates the viscoelastic behavior near the glass transition temperature, T_g, and represents the kinetic origin of vitrification. Guided by the α-process, as the temperature increases, polymer segments gain sufficient thermal energy to undergo large-scale rearrangements, leading to observable macroscopic effects such as stress relaxation and viscosity reduction. This process exhibits a strong temperature dependence, typically following a super-Arrhenius trend that is often described by the empirical Vogel-Fulcher-Tammann (VFT) equation. The timescale of the α-relaxation typically hits 100 s around T_g and quickly gets to geological values when the temperature drops by few K. As a consequence, in the glassy state, the α-relaxation becomes too slow and large-scale rearrangements become prohibitive.However, a growing body of experimental evidence points to the existence of an alternative equilibration pathway—one that remains active even below T_g, with a temperature-invariant activation barrier. Our group has experimentally verified the universal presence of this distinct pathway, referred to as the slow Arrhenius process (SAP), in 2022. In contrast to the α-process, the SAP involves localized, small rearrangements rather than large-scale cooperative motions. This enables polymers to relax and approach equilibrium on experimentally accessible timescales, even in regimes where α-relaxation becomes effectively frozen. Moreover, the comparison of the activation energies observed in processes such as physical aging, dewetting, re-entanglement, and adsorption kinetics suggests that SAP serves as the underlying molecular mechanism driving these slow equilibration phenomena. Despite considerable efforts in the last 3 years, we are still far from fully understanding the molecular mechanism by which SAP drives these equilibration processes.In this work, we combine theoretical analysis and experimental approaches to investigate the role of the slow Arrhenius process in various equilibration processes. Chapter 1 introduces the discovery and key properties of the SAP, while Chapter 2 outlines the materials and methods used in this study. In Chapter 3, we develop a model for irreversible polymer adsorption based on SAP dynamics, successfully predicting the adsorption rates in different nonequilibrium states. Chapter 4 applies the SAP to analyze stress relaxation in spin-coated polymer films prepared under various nonequilibrium conditions, revealing that the SAP can function as a molecular-level probe of processing-induced stress. Chapter 5 extends the investigation to other soft matter systems, by examining molecular transfer kinetics between lipid bilayer organized assemblies. Despite the structural differences between lipids and polymers, the observed SAP behavior suggests a common underlying relaxation mechanism. Finally, Chapter 6 synthesizes the major findings of this research and proposes specific directions for future investigations aiming at further unraveling the molecular mechanism underlying the SAP. |
