Watching How Molecules Orient in a Surface Forces Apparatus, Using Confocal Raman Spectroscopy

ORAL

Abstract

Much is known about surface forces, less about where they come from. This laboratory is engaged in direct vibrational spectroscopic measurements of how molecules orient in confined geometries. Regarding force measurements, PDMS (polydimethylsiloxane) was a model system for many years. In this study, we describe direct experiments using a novel version of confocal Raman spectroscopy. This experiment allows direct measurement of how the PDMS molecules orient under confinment as well as under subsequent shear. When the thickness of the fluid film is less than the unperturbed radius of gyration of the polymer, we obtain two novel findings: (a) linewidth analysis of peaks reveals that vibrational relaxation times are perturbed in this confined geometry; (b) orientation of the chain backbone is not everywhere the same within the molecularly-thin film; domains of various orientation are observed instead.

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Authors

  • Shan Jiang

    UIUC, Materials Science and Engineering Department

  • Minsu Kim

    UIUC, Physics Department

  • Sung-Chul Bae

  • Steve Granick

    Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Departments of Physics, Chemistry, and Materials Science and Engineering, University of Illinois at Urbana-Champaign, University of Illinois at Urbana-Champaign, Dept. of Materials Science \& Engineering, of Physics, of Chemistry, and of Chemical \& Biomolecular Engineering, UIUC, Departments of Materials Science and Engineering, of Chemistry, of Physcis, University of Illinois, Department of Materials Science and Engineering, Chemistry, Physics, Chemical Engineering, University of Illinois at Urbana-Champaign, UIUC, Materials Science and Engineering Department, Department of Materials Science \& Engineering, of Chemistry, of Physics, and of Chemical \& Biomolecular Engineering, UIUC, Departments of Chemistry and of Material Science, University of Illinois