A COMPARISON OF POLYSTYRENE-POLY(ETHYLENE OXIDE) DIBLOCK COPOLYMER AND POLY(ETHYLENE OXIDE) HOMOPOLYMER ADSORPTION FROM AQUEOUS-SOLUTIONS

Although amphiphilic polystyrene-poly(ethylene oxide) diblock copolyme rs (PS-PEO) adsorb from aqueous solutions to hydrophobic surfaces, the y are unable to form an end-anchored brush. Four independent technique s-scanning angle reflectometry, hydrodynamic layer thickness measureme nts, streaming potential measurements, and total internal reflection m icroscopy-indicate that PS-PEO adsorption is indistinguishable from PE O homopolymer adsorption. The surface concentrations attained by three PS-PEO diblocks are independent of molecular weight (between 67 000 a nd 479 000) and are indistinguishable from PEO (420 000 molecular weig ht) surface concentrations. Hydrodynamic layer thicknesses and electro kinetic layer thicknesses are on the order of only a few nanometers fo r both PEO and PS-PEO diblocks, more characteristic of a homopolymer l ayer than an extended brush. Furthermore, adsorbed PS-PEO imparts no d etectable steric repulsion to the energy of interaction between a Brow nian particle and a wall. Apparently, brush formation is prevented bec ause the surface affinity of the large water-soluble PEO block present s a large kinetic barrier to its being completely displaced from the s urface by the insoluble PS block.