Molecular visualization of individual molecules during flow .

摘要

Wetting and flow properties of molecularly thin polymer films are at the heart of many practical applications such as coatings, composites, microfluidics, and lubrication In some applications, these properties ensure even surface coverage; while in others, molecular flows act as new tools for surface patterning and performing biochemical assays on microchips. Further advances in these fields depend on our understanding (i) the mechanisms controlling the kinetics of flow and (ii) molecular organization of thin polymer films. In thin films polymers can adopt a wide range of conformations, topologies, and packing arrangements. Most of the structures correspond to metastable states that, however, are practically relevant due to extremely slow equilibration of the spatially constrained systems. Equilibration rates depend on the film formation process, which involves spreading of long and flexible macromolecules on a substrate. As such, the goal of this paper is (i) to examine molecular organization of thin polymer films and (ii) to study molecular mechanism of spreading of thin films on a solid substrate.;Traditional techniques lack the resolution to completely map out the conformation of each and every individual molecule in these films, thus our knowledge of the relative shape of neighboring molecules is incomplete. Because of this we turn to polymer bottle brush molecules, with the ability to be visualized using atomic force microscopy, to model linear polymer systems in thin films. We have shown that polymer films are largely composed of two distinct conformations (coiled chain and folded chain) whose relative contribution varies depending on film preparation and spreading coordinate. Although it is known that polymer molecules typically undergo a plug flow, we have discovered and modeled an unusual fractionation when we mix chemically similar linear polymer with brush polymer. In situ studies were also conducted and for the first time we were able to directly observe molecular slip, dissociation and scission during the spreading process. Finally we introduce preliminary studies concerning the control of the molecular properties in these films. In doing so we explore the role of confinement on these polymer films as well as the effects electric fields have when manipulating these flows.