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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