Molecular insights on the boundary behavior of liquids.

摘要

This thesis concerns the molecular understanding of the solid-liquid interface; one of the central themes in the study of soft materials. While there is general consensus that the liquid in proximity to a solid behaves differently from this same liquid in the bulk, the origin of such differences are poorly understood. In this thesis, work has been carried out to investigate the following three areas: (1) translational diffusion of polymer chain adsorbed on a solid surface, at the dilute surface coverage limit; (2) polymer melt translational diffusion, when the polymer melt is molecularly-thin; (3) solvation forces of simple small-molecule fluids in molecularly-thin films, and how they depend on equilibration.;A large part of my dissertation is related to polymer interfacial dynamics. As one of the center topics of polymer physics, many models have been generated to understand the dynamics of polymer in bulk solutions and in bulk melt. However, to study the dynamics of polymer at the interfaces is more complicated and such understanding is crucial to the comprehension of fracture properties of polymer composites, polymer surface glass transition, biocompatible coatings, friction and lubrication, and adhesion, among others. With Fluorescence Correlation Spectroscopy, the molecular weight dependence of the surface diffusion of polystyrene on various substrates with dilute surface coverage was investigated. The exact form of dependence is highly sensitive to the adsorbing surface used; suggesting that minute details on the surface can be significant to the interfacial dynamics of surface adsorbed polymer.;The second part of the thesis looked at the layering structure of a liquid at a solid boundary. Force distance profiles of octamethylcyclotetrasiloxane (OMCTS) were obtained at various approaching rates and the result shows that the number of layers observed in the confined OMCTS is highly dependent on the approaching rate used during experiments. With decreasing rate, number of layers observed reduces hinting that the outer layers are liquid like, and can relax if given enough time. This is confirmed by the removal of the 6th and the 5th layers of the confined OMCTS when the approaching rates were reduced to 1 nm/s and 0.78 nm/s respectively. Different relaxation time for different layers highlights the heterogeneity of in the structure of a confined liquid.;The last part of my thesis concerns the translational diffusion of molecularly thin polymer melt. While simulations have been performed, to my knowledge no direct measurements exist yet of polymer melt diffusion in molecularly-thin films. This is due to the lack of proper experimental tools and the large number of variables involved. In this work, a new experimental platform was developed by combining the surface forces apparatus with a fluorescent technique called the fluorescence recovery after photobleaching to shed light onto the molecular origin of the unique dynamics of a confined polymer melt. The effects of two very fundamental factors, namely the thickness dependence and the pressure dependence on the centre-of-mass diffusion of the confined polymer melt have been investigated. A strong pressure dependence within a highly confined melt suggests that the dynamics within the molecular thin melt are highly heterogeneous. On the other hand, a weak thickness dependence and also viscosity dependence suggest that the dynamics of the confined polymer melt is governed by the polymer-surface interaction, with the degree of confinement and the viscosity of the solvent playing less important roles.