| This dissertation focuses on ultra-thin liquid films, less than a few nanometers thick, of one polymer, polydimethylsiloxane (PDMS). PDMS, a model flexible linear polymer, also plays a very important role in many applications. There have been many speculations about the molecular conformation in these films; the very flexibility of the chains gives many possibilities. This dissertation will present experimental studies exploring both the ordering of the polymer by the substrate, and one of the consequences of such ordering, on water and solid interfaces.; Brewster angle microscopy, surface pressure and surface potential measurements were used to test the reasons for the stability of two-dimensional foams in Langmuir monolayers, which are confined at the air/water interface. Such foams are almost universally observed at gas/liquid coexistence. Our advantage in exploring the mechanism for foam stability was the only known exception: PDMS on a surfactant substrate, in contrast with a pure water substrate. The only difference was the average dipole moment density contrast between liquid and gaseous domains, almost absent in the surfactant case. Such contrast leads to long-range repulsion across the foam. This contrast is universal in gas/liquid coexistence. The difference in dipole moment density contrast for the polymer on the two substrates may result from polymer chain conformation, but no direct technique to verify this on liquid surfaces is available.; Deuterium nuclear magnetic resonance was applied for the first time to such ultra-thin polymer films on solid substrates, to probe for order in such films. Using an Anopore membrane provided 2 m2 surface area for PDMS film deposition, enough to obtain a DNMR signal from a PDMS monolayer. Polymer side group ordering, induced by the surface, was demonstrated directly, DNMR shows different macromolecular-level dynamics depending on the polymer amount, DNMR data gives insight into the molecular picture of the polymer chain conformations and dynamics. The most probable chain conformation for one monolayer is a flattened, caterpillar-like structure. For thicker films, a flat conformation is also most likely. These conclusions appear to hold on both the native hydrophilic substrate and the hydrophobic substrate where molecular interactions are different. |
