Sum frequency generation (SFG) surface vibrational spectroscopy and atomic force microscopy (AFM) studies of the composition, structure, and mechanical behavior of polymers at interfaces

Sum frequency generation (SFG) surface vibrational spectroscopy, x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) were used to obtain experimental measurements of polymer surface composition, surface structure, and surface mechanical behavior. The experiments described in this dissertation focus on answering three questions: (1) How are polymers configured at interfaces? (2) How do changes in environment (air vs. liquid vs. solid) affect polymer interface properties? (3) How does the mechanical environment (i.e. stretching) affect the interface composition and mechanical behavior of polymers?; It is well established that most polymers assume preferred configurations at interfaces. At the polymer/air interface, the configuration and ordering of short methyl side branches was quantified by analyzing SFG spectra obtained from a series of aspecific poly(ethylene-co-propylene) copolymers ( aEPR) with varying ethylene content. Side branch ordering was also characterized at polymer/liquid interfaces. Immiscible polymer and liquid pairs were shown to form ordered interfaces. In contrast, solvents were shown to disorder polymer/liquid interfaces.; The surface morphologies of polyolefin blends were characterized. In blends of atactic polypropylene (aPP) and aEPR, it was observed that aPP, the polymer with lower surface tension, was enriched at the polymer/air interface. The thickness of the aPP surface enrichment layer was shown to depend on the energy of mixing of the two components, with thicker enrichment layers measured from bulk immiscible blends. Similar results were obtained using isotactic polypropylene (iPP)/aEPR blends. It was found that the surface segregation tendencies of the iPP/aEPR blend could be reversed by exposing the blend to n-hexane vapor, which preferentially solvated aEPR component.; Changes in surface composition, texture, and mechanical behavior of polymers were characterized as a function of tensile elongation. The surfaces of low and high density polyethylene, which have spherulitic microstructures, were found to systematically roughen as the surface microstructure evolved into a fiber morphology. For a phase-separated polystyrene(butadiene)styrene triblock copolymer, at low tensile stress the more flexible component absorbs most of the strain in the bulk. The surface was shown to deform inhomogeneously as the polymer is elongated.; Finally, a method was developed, using AFM, to probe the surface mechanical properties of hydrogel materials, used as contact lenses. The surface mechanical properties of methacrylate hydrogels were shown to be highly dependent on hydration. It was found that the region near the hydrogel/air interface is significantly stiffer than the bulk, indicating the interface region is dehydrated, relative to the level of bulk hydration.