The scaling behavior of critical adsorption in critical polymer solutions

The critical adsorption ellipsometric measurements of five solutions of polystyrene in cyclohexane for different polystyrene molecular weights collapse to a single universal curve when scaled as a function of n(B) xi/lambda, where n(B) is the polymer solution refractive index, xi = xi(o)N(n)t(-nu) is the correlation length, and lambda is the wavelength of incident light in vacuum. From this universal feature we deduce the value of the polymerization critical exponent n = 0.258 +/- 0.017. We consider both the volume fraction order parameter (phi) and a symmetrized order parameter (psi(s)) together with both the renormalization group (RG) and Monte Carlo (MC) simulation forms for the surface scaling function P-+(x). The symmetrized order parameter gives significantly better agreement with experiment than the volume fraction order parameter. The combination of RG and psi(s) provides better agreement with experiment than does the combination of MC and psi(s). (C) 1996 American institute of Physics.