THE DETERMINATION OF SEGMENT DENSITY PROFILES OF POLYETHYLENE OXIDE LAYERS ADSORBED AT THE AIR-WATER-INTERFACE

The adsorption of polyethylene oxide of molecular weight 17 800 and 87 000 at the air/water interface has been studied by neutron reflection and surface tension measurements. Over the concentration range 10(-4) -10(-1) wt% of polymer the surface excesses determined by neutron refl ection were consistent with the results derived from the surface tensi on using the Gibbs equation. Reflectivity measurements were made on th ree samples of different isotopic composition at a fixed concentration of 0.1 wt%. The adsorbed polymer layers were found to be essentially completely immersed in the water, contrary to earlier suggestions that a significant fraction of the polymer protrudes into the vapour phase . The distribution profiles of the polymers with respect to the surfac e normal direction are composed of two major blocks, one constituting the layer at the top surface and the other forming a diffuse region ex tending into the bulk solution. Over 90% of the homopolymer is in the first region with a thickness of 18+/-2 Angstrom, and the remaining di ffuse region has a thickness of 35+/-5 Angstrom. Alternatively, the vo lume fraction profile of the polymer can be described as a 1:1 sum of two half Gaussians of widths 10 and 35 Angstrom. Change of molecular w eight and addition of salt do not change either the surface coverage o r the segment density profiles significantly.