The role of a single end group in poly(ethylene oxide) adsorption on colloidal and film polystyrene: complimentary sedimentation and total internal reflectance fluorescence studies

Abstract

This work examines “apparent equilibrium” and kinetic features of adsorbing homopolymers with a modification at one end of each chain to enhance chain-end adsorption. The end groups combined fluorescent and hydrophobic units so that polymers could be easily assayed and label-induced artifacts avoided. A series of end-modified poly(ethylene oxide)s (PEOs) of constant backbone length (molecular weight 100 000) but with varied end-group hydrophobicity were synthesized and their adsorption on polystyrene (PS) latex and films examined. Isotherms generated by the centrifugation of PS latex revealed that hydrophobically modified PEO adsorbed up to five times more than unmodified PEO. Total internal reflectance fluorescence (TIRF) spectroscopy revealed faster accumulation of hydrophobe-containing PEO compared with less hydrophobic samples. All layers, however, appeared equally stable to washing in pure solvent. Hydrophobe-containing PEOs could incorporate into or displace chains from preadsorbed layers of less hydrophobic PEO, but a preadsorbed layer of end-hydrophobic PEO excluded chains from a less hydrophobic sample. The impact of the small hydrophobic end groups on surface coverages and kinetic features was greater than expected for brush-forming polymers and PEO-surfactant systems. The enhanced end-group effects were attributed to the adsorption of the main backbone, a feature lacking in many previous polymer brush or surfactant studies. A comparison of surface coverages measured by TIRF spectroscopy and centrifugation methods revealed inaccuracies in the internal TIRF calibrations. The centrifugation method was thought to give a more accurate determination of the surface excess, while TIRF spectroscopy is better suited to kinetic and dynamic studies.