Segment density profiles of polyelectrolyte brushes determined by Fourier transform ellipsometry

We describe a method for the explicit determination of the segment density profile phi(z) of surface-attached polymer brushes with multiple angle of i ncidence null-ellipsometry. Because the refractive index contrast between t he brush layer and the solvent is weak, multiple reflections are of minor i nfluence and the ellipsometric spectrum is closely related to the Fourier t ransform of the refractive index profile, thereby allowing for explicit inv ersion of the ellipsometric data. We chose surface-attached monolayers of p olymethacrylic acid (PMAA), a weak polyelectrolyte, as a model system and d etermined the segment density profile of this system as a function of the p H value of the surrounding medium by the Fourier method. Complementary to t he Fourier analysis, fits with error functions are given as well. The brush es were prepared on the bases of high refractive index prisms with the "gra fting-from" technique. In water, the brushes swell by more than a factor of 30. The swelling increases with increasing pH because of a growing fractio n of dissociated acidic groups leading to a larger electrostatic repulsion. (C) 1999 American Institute of Physics. [S0021-9606(99)70339-9].