Laser Light Scattering Spectroscopy Combined With Brewster Angle Microscopy: nCB and Polymer Monolayers

Abstract

Brewster Angle Microscopy (BAM) is a powerful tool to image the structure of monomolecular films1,2. Laser Light Scattering Spectroscopy (LLSS) is used to measure the surface visco-elastic coefficients Ke=G¯+K¯ and KV=η¯+ζ¯ as a function of surface density. In addition, since the surface pressure vs. surface density is known, the isotherm compressibility vs. surface density can be estimated. These numbers will be compared for the liquid crystal, nCB systems that collapse to multilayers reversibly and for certain polymer systems to be used as alignment layers in liquid crystal video display applications. The footprints of the incident beams for BAM and LLSS are superposed so the Ke, Kv coefficients can be associated with the morphology displayed by the BAM image. The BAM images show that convective transport is usually present unless the monolayer is very rigid. In many cases the monolayer in the footprint of the laser beams is not either homogeneous or isotropic. Indeed, the molecular director may be aligned in domains and there can be a distribution of defects all observable by BAM. This fact must be taken into account when interpreting LLSS autocorrelation functions. Since the grating technique is used with normal incidence, it is easy to make measurements with LLSS as a function of direction on the surface by simply rotating the grating. Experimental techniques and results will be discussed. We conclude that LLSS data is incomplete for monomolecular films unless BAM images of the footprint are also recorded. Two literature references to BAM images of nCB from our group follow.