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This book presents cutting-edge experimental and computational results and provides comprehensive coverage on the impact of non-equilibrium structure and dynamics on the properties of soft matter confined to the nanoscale. The book is organized into three main sections: · Equilibration and physical aging: by treating non-equilibrium phenomena with the formal methodology of statistical physics in bulk, the analysis of the kinetics of equilibration sheds new light on the physical origin of the non-equilibrium character of thin polymer films. Both the impact of sample preparation and that of interfacial interactions are analyzed using a large set of experiments. A historical overview of the investigation of the non-equilibrium character of thin polymer films is also presented. Furthermore, the discussion focuses on how interfaces and geometrical confinement perturb the pathways and kinetics of equilibrations of soft glasses (a process of tremendous technological interest). · Irreversible adsorption: the formation of stable adsorbed layers occurs at timescales much larger than the time necessary to equilibrate soft matter in bulk. The question is posed as to whether this process could be considered as the driving force of equilibration. In this section, the investigation of the physics of irreversible chain adsorption is accompanied by a detailed analysis of the molecular dynamics, structure, morphology, and crystallization of adsorbed layers. · Glass transition and material properties: the discussion covers a broad range of non-equilibrium phenomena affecting different families of soft materials – polymers, low molecular weight glass formers, and liquid crystals. In these systems, geometrical confinement induces the formation of non-equilibrium phases, otherwise not achievable via processing of bulk samples. The examples illustrated in this section show how non-equilibrium phenomena can be exploited as innovative processing parameters to fabricate novel nanomaterials with improved performance. Finally, the differences between experiments performed under equilibrium conditions and temperature scans from equilibrium to non-equilibrium states at the nanoscale are discussed.

Discrete mathematics --- Mathematical statistics --- Mathematics --- Classical mechanics. Field theory --- Fluid mechanics --- Statistical physics --- Matter physics --- Physicochemistry --- Macromolecules --- Chemical structure --- Chemistry of complexes --- Electrical engineering --- complexen (chemie) --- grafentheorie --- nanotechniek --- statistiek --- materialen (technologie) --- fysica --- polymeren --- fysicochemie --- dynamica --- vloeistoffen

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The goal of the experimental investigations in the past two years was to understand the role of interfacial interactions on the structural relaxation process of polymers. To achieve our objectives, we measured and analyzed by means of broadband relaxation spectroscopy the dynamics of bulk and ultrathin films (thickness < 200 nm) of several amorphous and semicrystalline polymers [poly(vinyl acetate), PVAc; atactic polypropylene, a-PP; poly(ethylene terephthalate), PET; poly(3-hydroxybutytate), PHB; polystyrene, a-PS, atactic poly (2-vinyl pyridine), a-P2VP]. Owing to the wide frequency range of the principal technique (1 mHz - 10 MHz) the complex relaxation scenario above and below the glass transition temperature in those systems could be investigated in great detail. In this dissertation, we started with an introduction to the phenomenology of the structural relaxation process (alpha-relaxation) via the concepts of cooperative motion and dynamical heterogeneity (CHAPTER 1). The discussion was exemplified by the analysis of the relaxation behavior of a polar model polymer, PVAc. In CHAPTER 2 we described a recently introduced methodology for the investigation of non polar systems by means of dielectric probes. The experimental data reported filled the gap in the characterization of a-PP, being considered a model system for the study of the dynamics in amorphous polymers in the liquid state. The relatively low value of the intrinsic dipole moment of this polymer does not permit an easy analysis of its dielectric relaxation. Notwithstanding this, it was possible to characterize the glass transition dynamics of a-PP by broadband dielectric spectroscopy (DS) using small amounts of DBANS, an organic molecule with a high dipole moment. After an introduction to the phenomenology of deviations from bulk behavior in ultrathin polymers films (via six different key themes in CHAPTER 3), we reviewed the contribution of dielectric spectroscopy to the debate on confinement effects in the last ten years. In this experimental work, the dynamics of ultrathin polymer films was explored at different time and length scales. We extended the conventional dynamic range of dielectric relaxation spectroscopy by introducing real-time crystallization experiments in a confined geometry (motions of the whole polymer chain up to 100000 s). To understand the impact of interfacial interaction on the structural relaxation we characterized the changes in the alpha-process of PET down to 13 nm (CHAPTER 4). Ultrathin films were capped between aluminum electrodes, often discussed as the geometry of model nanocomposites, and the formation of stable chemical bonds between the polymer and the metal ensured the presence of a layer with slower dynamics at the interface. Deviations from bulk behavior, appearing as an increase in the relaxation time at a fixed temperature, a simultaneous reduction of the dielectric strength and a broadening of the structural peak, are observed for films of a thickness below 35 nm. The slowing-down acts as a shift factor slightly decreasing with the temperature, and does not affect the fragility. We introduced the concept of reduced mobility layer, RML, and showed how the dielectric response of ultrathin films is altered by the presence of layers with a different mobility. A simple explanation for the dual character (static – dynamical) of the confinement effects was proposed. As regarding crystallization phenomena, we proved the feasibility of DS as a tool to monitor the crystallization kinetics also in ultrathin polymer films (CHAPTER 5). We combined the real time monitoring of the conversion of the amorphous phase into crystalline structures with an analysis of the thickness dependence of the structural relaxation of PHB. We concluded that for this polymer the tremendous slowing down of the crystallization kinetics (the mean crystallization time increased by more than one order of magnitude) is not due to a change in the chain mobility on the time and lengthscale of the dynamic glass transition. The increase of the crystallization time was in fact accompanied by a constant value of the glass transition temperature. Arguing with similar examples from the recent literature we concluded that in ultrathin polymer films the slow-down of crystallization kinetics is not simply induced by a size-dependent increase in the glass transition temperature but by a reduction of the chain mobility at the very interface with an attractive substrate. Moreover, we considered the reduction of active nuclei density as a possible element contributing to the increase of the crystallization time in ultrathin films by comparing the effects of temperature and thickness on the diffusion limited crystallization kinetics (CHAPTER 6). We proved that in the regime of cold crystallization, the dynamics of thinner films corresponds to the one in bulk but at lower temperatures. To rationalize these results, we investigated the interplay between crystallization phenomena and structural relaxation arriving to a correlation between the crystallization time and the structural relaxation time (CHAPTER 7). We proposed a model to characterize the deviation from bulk behavior in ultrathin polymer films. As a main result the changes in the crystallization (structural relaxation) time depends on the temperature of the experiments and vanish at sufficiently high temperatures. We arrived to the same conclusions studying the relaxation behavior of extremely thin films (thickness < 6 nm) of a-PS (CHAPTER 8). By characterizing the response of these samples after different thermal histories we showed the possibility to obtain films with different dynamics by simple changing the annealing temperature. We observed an anomalous increase with the temperature of the electric capacitance and attributed it to the defreezing of the dead layer, i.e. the polymer chains absorbed by the substrate. Such defreezing was found to act as a precursor for the dynamic glass transition of the reduced mobility layer, and provides the direct link between interfacial interactions and molecular relaxations. In terms of a simple scaling expression we found an alternative path to show that the deviations from bulk behavior depend on the time scale of the experiment and thus on the temperature. Finally, we experimentally verified in ultrathin films of a-P2VP that the deviations from bulk behavior are temperature dependent (CHAPTER 9). We treated the influence of an absorbing substrate as a small perturbation in the activation energy of the a-modes related to the dynamic glass transition. We proposed that the deviations from bulk behavior derive from the balance of such a perturbation and the energy of the thermal bath. In conclusions we proposed a correlation between the structural time tau_a and the crystallization time, tau_cry, valid for bulk systems, at temperatures just above Tg. In ultrathin polymer films, the coupling between segmental mobility and crystallization rate breaks down because of interfacial interactions. We modeled the properties of the layers at the very interface with an attractive substrate in terms of a reduction of molecular mobility on the time and the length scale of the dynamic glass transition. Further we experimentally verified that the structural relaxation time, tau_a, increases in proximity of the walls and that the relative variations of tau_a vanish at sufficiently high temperatures. We thus support the idea that deviations from bulk behavior originate from changes of chains’ conformation at the interfaces, considering the latter as a particular case of surface ordering effects. We thus think to have (partially) replied to the question “What are the effects of interfacial interactions on the structural relaxation dynamics of ultrathin polymer films?”. Still, we do not know how to reply to questions like: Is the dynamics of polymer chains at a free surface faster than in bulk? If so, why? Can free surfaces be treated as liquid like layers even below bulk Tg? Is the size of cooperative motion tremendously affected by the presence of an interface? What is the penetration depth of this perturbation? What is the profile of the gradients of molecular mobility? Could those be described just in terms of conformational changes and distance from the wall? Are there different profiles of molecular mobility, or the deviations in the different correlated quantities (structural time, crystallization time, physical aging, diffusion ...) are only due to averaging issues? Are the changes in the cold crystallization kinetics and the diffusion dynamics completely governed by the a-modes, or should other molecular processes acting at intermediate length scales be taken into account? Why does an attractive substrate induce changes propagating up to 10 times the radius of gyration? We hope it will be possible to give an answer to those points in the near future. This dissertation aims to understand how and why interfaces may influence the structural relaxation of polymers. To study the interfacial properties of these materials we characterized the dielectric response of ultrathin polymer films (thickness < 200 nm) and monitored the changes in the material performance upon thickness reduction down to 4 nm. Considering the tremendous fundamental and technological relevance of polymer/substrate systems leading to an increase of the glass transition temperature, Tg, we concentrated our attention on the investigation of those interactions causing a partial or total immobilization of the polymer chains. We proposed a correlation between the structural relaxation (segmental) time tau_a and the crystallization time, tau_cry, valid in bulk systems, at temperature just above Tg. We found that in ultrathin polymer films such a coupling breaks down because of interfacial interactions. We thus modeled the properties of the layers at the very interface with an attractive substrate in terms of reduction of molecular mobility on the time and the length scale of dynamic glass transition. We experimentally verified that tau_a increases in proximity of the walls and that the relative variations of tau_a vanish at sufficiently high temperature.