Polyacrylamide adsorption from aqueous solutions on gold and silver surfaces monitored by the quartz crystal microbalance

Viscoelastic properties of ultrathin films are important in such applications as polymer-supported lipid bilayers, and the quartz crystal microbalance (QCM) offers the ability to measure these properties in situ. In this study, polymer films are created by adsorbing polyacrylamide to gold and silver surfaces from aqueous solutions. The molecular weight of the polymers ranges from 10 000 to 1000 000 g/mol, and concentrations range from dilute to the overlap concentration. Using a continuum mechanics viscoelastic film model, we extract film property parameters from frequency and dissipation change measurements for multiple harmonics. The precision of the QCM and the modeling technique are limited, however, leading to large property value ranges for any given film. The adsorbed films range from 10 to 150 Angstrom thick, with moduli ranging from 15 to 228 kPa and viscosities ranging from 1.21 x 10(-3) to 2.85 x 10(-3) Pa s. Meanwhile, the bulk solution properties of the different molecular weight materials lead to drastically different adsorption profiles. While the viscosity differences between water and 10 000 M-w polymer solutions lead to large frequency and dissipation responses, the large relaxation times of 1000 000 M-w polymers make their bulk solution viscosity differences from water largely undetectable by the QCM. In addition, the viscosity of the 10 000 M-w solutions may be frequency dependent over the range of operating frequencies, contrary to the standard QCM assumption of frequency independence.