Chemisorption of thiolated poly(ethylene oxide) to gold: surface chain densities measured by ellipsometry and neutron reflectometry
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Physical property studies of surfaces formed by chemisorption of polyethylene oxide (PEO) onto gold are reported. Such surfaces have potential as model materials for elucidation of the mechanism of resistance to protein adsorption by PEO surfaces. Thiolated monomethoxy poly(ethylene oxide) (PEO) was chemisorbed onto gold-coated silicon wafers under various conditions such that different surface chain densities were achieved. Chain density was varied by controlling PEO solubility (proximity to cloud-point conditions) as well as chemisorption time. Films prepared with PEO of molecular weight 750, 2000, and 5000 g/mol were studied. Chain densities determined in the dry state by ellipsometry were found to be in the range of 0.4-0.7, 0.33-0.58, and 0.12-0.30 chains/nm(2) for MW 750, 2000, and 5000 PEO, respectively. Chain density was found to decrease with increasing molecular weight and to increase as cloud-point conditions were approached. PEO-layer mass densities and chain densities were determined independently by neutron reflectometry. Under low-solubility conditions and for a 4-h chemisorption time, film mass and chain density values of 1.0 +/-0.3 g cm(-3) and 1.8 +/- 0.9 chains/nm(2) were found for MW 750 PEO, and 0.82 +/- 0.02 g cm(-3) and 0.23 +/- 0.07 chains/nm(2) for MW 5000 PEO. Ellipsometry data for these systems yielded graft densities of 0.63 +/- 0.13 and 0.30 +/- 0.02 chains/nm(2), respectively. Using the mass densities obtained from the neutron data in the ellipsometry calculations, chain densities of 0.6 +/- 0.3 and 0.25 +/- 0.02 chains/nm(2), respectively, were obtained for the MW 750 and 5000 films. The ellipsometry and neutron data for the MW 5000 system are thus in agreement within experimental error. In general, the chain-density values are much higher than those corresponding to layers of unperturbed random coil PEO ("mushrooms"), suggesting that the PEO layers are in the brush regime with the chains in an extended conformation.