Surface Light Scattering and its Application to Polymer Monolayers.
Laser heterodyne spectroscopy of light scattered from capillary waves on the air-water interface is used to study the surface properties of monolayers and thin films Optical imaging of a diffraction grating is employed to provide a stable local oscillator field and to select a precise capillary wave number k. The power spectrum in frequency domain is well approximated by a Lorentzian profile for the range of capillary wave number 260 (LESSTHEQ) k (LESSTHEQ) 390 cm('-1). The measured spectrum at each k is corrected for the finite aperture size and the finite incident angle. The surface tensions and the kinematic viscosities of water, anisole, and ethanol are deduced from the dispersion relation and found to be within 5% of the literature values, confirming the approximations made for the spectral analysis. The light scattering spectroscopy together with the Wilhelmy plate method have been applied to poly(vinyl acetate) (PVAc) and poly(ethylene oxide) (PEO) monolayers spread at the air-water interface. The surface pressure isotherms obtained from the Wilhelmy method as a function of surface concentration exhibit five distinct power law regions for PVCc and three regions for PEO. Besides the dilute, the concentrated, and the collapsing regions, the scaling exponent of the two dimensional semi-dilute, good solvent condition is observed for both polymers, but PVAc shows an additional power law region identical to the three dimensional semi-dilute, good solvent condition. The wave frequency is found to follow roughly the surface tension, and the damping coefficient shows two maxima for PVAc and a single maximum for PEO. The analyses indicate that the dynamic elasticity contributes most to the visoelastic portion of the wave frequency, and the first damping maxima of PVAc and PEO are caused by the mode coupling of longitudinal and capillary waves. The electric field induced surface wave apparatus utilizes the generation of waves by an electric field and the detection by a reflection of a laser beam. This has just been implemented to be used for the study of surfaces of polymer solutions. Its instrumentation and the preliminary results are discussed.
- Pub Date:
- September 1987
- Physics: General