Structure of poly([gamma]-benzyl-L-glutamate) monolayers at the gas[endash]water interface: A Brewster angle microscopy and x-ray scattering study
- Department of Physics and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (United States)
- Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003 (United States)
This paper reports Brewster angle microscopy (BAM), x-ray specular reflectivity (XR), grazing incidence diffraction (GID) and off-specular diffuse scattering (XOSDS) measurements of Langmuir monolayers formed on water by both mono- and polydisperse samples of [alpha]-helical poly([gamma]-benzyl L-glutamate) (PBLG) as a function of area/monomer [ital A]. The microscopic behavior does not exhibit any discernible effects due to differing dispersity. At low surface densities (A[gt][approximately]21 hthinsp;[Angstrom][sup 2]/monomer, surface pressure [Pi]=0), BAM images reveal partial surface coverage by solidlike monolayer islands. GID measurements show an interhelix peak corresponding to a local parallel alignment of rodlike PBLG molecules, indicating their tendency to aggregate laterally without external pressure. Compression to A[lt]21 hthinsp;[Angstrom][sup 2]/monomer first leads to full and uniform surface coverage by the monolayer, followed by a steep rise in [Pi] that is accompanied by a decrease in the interhelix distance. Further compression results in a plateau of constant [Pi] in the [Pi]-A isotherm ([approximately]11.5[lt]A[lt][approximately]18.5 hthinsp;[Angstrom][sup 2]/monomer, [Pi][approximately]9 hthinsp;dyn/cm), which has previously been attributed to a first-order monolayer[endash]bilayer transition. The interfacial electron density profiles determined by the XR measurements on both sides of the coexistence plateau provide direct evidence for this transition. On the basis of x-ray scattering results, the film on the high-density side of the plateau is shown to consist of a newly formed incomplete and incommensurate second layer that sits on top of and has lower average density than a homogeneous first layer. GID measurements indicate that the second layer can be characterized by larger interhelix [ital d]-spacing than the first layer, while XOSDS measurements on the bilayer suggest that the second layer is microscopically inhomogeneous. For both mono- and bilayers, the analysis of observed GID peak widths indicates that the extent of lateral positional correlations between parallel PBLG rods ranges from a few to no more than [approximately]15 interhelix distances, implying short-range order. [copyright] [ital 1999 American Institute of Physics.]
- OSTI ID:
- 6060281
- Journal Information:
- Journal of Chemical Physics, Vol. 111:21; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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