A comparison of the volume occupied by macromolecules in the adsorbed state and in bulk solution: Adsorption of narrow molecular weight fractions of poly(vinyl alcohol) at the polystyrene/water interface

Abstract

A commercial sample of poly(vinyl alcohol) ( M w of 45,000) has been fractionated on a preparative scale by gel permeation chromatography. Several fractions were obtained and these were characterized by ultracentrifugation, acetate analysis, and viscometry. The M w of the fractions ranged from 4,000 to 70,000. The acetate content and values for the Huggins/Martin constant, calculated from the viscosity data, showed no variation with molecular weight. The following Mark-Houwink equation was established: [η] 25°C = 2.7 × 10 −4M w 0.71 . By use of the Stockmayer-Fixman equation the polymer-solvent interaction parameter, χ 1, was found to be 0.465. The effective hydrodynamic dimensions and the rms dimensions of the polymer coils in solution were calculated from the ultracentrifugation and viscosity data, respectively, for each fraction. These fractions were used for a study of the dependence of adsorption on molecular weight. The amount adsorbed at the plateau of the isotherm, Λ, and the adsorbed layer thickness, δ, calculated from ultracentrifugation, both increased linearly with M w 1 2 . The volume occupied by each polymer molecule in the adsorbed state was found to be approximately the same as that occupied by the effective hydrodynamic sphere in bulk solution. However, the adsorbed layer thickness was found to be greater than the effective hydrodynamic diameter of the polymer coils in solution. Thus the adsorption was interpreted in terms of a possible deformation of the random coil but with no interpenetration with or compression of adjacent adsorbed molecules.