J. Chem. Phys. 111, 9761 (1999)

Structure of poly(g-benzyl-L-glutamate) monolayers at the gas/water interface: A Brewster angle microscopy and x-ray scattering study

Masafumi Fukuto, Ralf K. Heilmann, and Peter S. Pershan
Department of Physics and Division of Engineering and Applied Sciences,
Harvard University, Cambridge, Massachusetts 02138

Seungju M. Yu,(a) Jennifer A. Griffiths,(b) and David A. Tirrell(c)
Polymer Science and Engineering Department,
University of Massachusetts, Amherst, Massachusetts 01003

(a) Present address: Department of Chemisty, University of Wisconsin, Madison, Wisconsin 53706
(b) Present address: Kimberly-Clark Corporation, Roswell, Georgia 30076
(c) Present address: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadina, California 91125


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 a-helical poly(g-benzyl-L-glutamate) (PBLG) as a function of area/monomer A. The microscopic behavior does not exhibit any discernible effects due to differing dispersity. At low surface densities (A > 21 Å2/monomer, surface pressure P = 0), BAM images reveal partial surface coverage by solid-like monolayer islands. GID measurements show an inter-helix peak corresponding to a local parallel alignment of rod-like PBLG molecules, indicating their tendency to aggregate laterally without external pressure. Compression to A < 21 Å2/monomer first leads to full and uniform surface coverage by the monolayer, followed by a steep rise in P that is accompanied by a decrease in the inter-helix distance. Further compression results in a plateau of constant P in the P-A isotherm (~ 11.5 < A < ~ 18.5 Å2/monomer, P ~ 9 dyn/cm), which has previously been attributed to a first-order monolayer-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 inter-helix 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 ~15 inter-helix distances, implying short-range order.

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