The Influence of Surface Hydrophobicity on Polyacrylamide Adsorption

The adsorption of a modified polyacrylamide on gold surfaces coated with varying proportions of -CH3- and -OH-terminated alkanethiols (producing substrates of varying hydrophobicity: theta(c) = 75, 98, and 119 degrees), was investigated using quartz crystal microbalance with dissipation (QCM-D), tapping-mode atomic force microscopy (TM-AFM), and captive bubble contact angle measurements. The QCM-D data for the polymer adsorbing on the different substrates indicates that the polymer adsorbs faster and to a greater extent on surfaces with higher hydrophobicity. Dissipation data from the QCM-D suggests that the adsorbed polymer undergoes a conformational change when adsorbing onto the substrates of higher hydrophobicity, forming a less rigid extended layer as the adsorption progresses toward the maximum adsorbed amount. AFM imaging of the adsorbed layer illustrates that the polymer layer is incomplete on all three substrates, and that the underlying substrate hydrophobicity has a role in determining the morphology (distribution, coverage, and thickness) of the adsorbed layer. Contact angle measurements of the polymer-coated substrates show variation in the ability of the polymer to reduce the hydrophobicity of the substrates. The role of coverage and distribution of adsorbed polymer on the surface hydrophobicity reduction is discussed.