We have used a combination of surface plasmon resonance (SPR) and the quartz crystal microbalance (QCM) to monitor in situ the solution-phase adsorption of the perfluoropolyether lubricant Fomblin ZDOL onto a silver surface. This dual-probe technique was then extended in a novel way by the addition of electrochemical control and used to monitor the electrochemically induced solution-phase desorption of octadecanethiol (C18S) from a gold surface as well as the electrochemical polymerization of polypyrrole (PPY) on a gold surface. The experimental results obtained by the joint technique compare favorably with those obtained using SPR and QCM independently. The combination allows us to measure simultaneously the optical and acoustic properties of these materials as they interact with the metallic surface. While SPR and QCM have similar resolution and are both able to follow deposition in real time, there are distinct advantages to the simultaneous measurement. These advantages allow one to (1) test the validity of the governing equations often used to analyze data collected using the two techniques, bringing to light weaknesses in the assumptions inherent in these equations, (2) calculate interfacial density and refractive index values in a system where the bulk values are known and the physical state of the adsorbed material is similar to that of the bulk, (3) show that the viscoelastic properties of an adsorbed material change significantly as the material desorbs from an interface, and (4) observe the evolution in the electronic and chemical properties of a conducting polymer film as it is being deposited while precisely monitoring the mass of the deposited film.
The in situ study of phase transitions in polymers by real-time atomic force microscopy (AFM) has received much attention recently. In this paper we report on the accuracy of surface temperatures measured during variable-temperature AFM experiments. In AFM studies on organic and polymeric samples at elevated temperatures, the presence of an unheated AFM cantilever and tip close to the sample surface can result in a significant depression of the surface temperature. This effect was estimated by measuring the temperature depression quantitatively for a series of n-alkanoic acids in different gases, i.e., argon, air, and helium. We developed an analytical expression by modeling the observed surface temperatures and their distance dependence using heat transfer theory, which allows us to predict the temperature effects in different experiments. For poly(ethylene oxide) (PEO) we predict that no temperature correction is necessary for films thinner than 500 nm. To test the temperature calibration, we have acquired quantitative data on the crystallization of individual PEO lamellae in thin films on silicon surfaces. The lamellar growth rates, lamellar thicknesses, and melting ranges obey the typical dependencies on crystallization temperature and supercooling known from bulk polymer crystallization. The values for equilibrium melting temperatures, as well as surface free energies of the fold surfaces, determined by Hoffman−Weeks extrapolation, the Gibbs−Thompson equation, and the Hoffman−Lauritzen theory, compare favorably with values published in the literature and hence validate our model calculations.
We have used a quartz crystal microbalance (QCM) to study the solution-phase adsorption of the perfluoropolyether lubricants Fomblin ZDOL and Demnum SA from perfluorohexane and perfluorobutylmethyl ether onto model carbon-coated hard-disk surfaces. The validity of the QCM results and the applicability of the well-known Sauerbrey equation have been verified through combined QCM and surface plasmon resonance experiments. The adsorption isotherms are relatively simple and exhibit a Langmuirlike behavior. However, the plateau region is sometimes ill-defined, and the adsorption process is strongly dependent on the history of the system. The adsorbed mass at a particular concentration shows a pronounced dependence on the equilibration time between successive concentration changes, even though the adsorption kinetics appears to have equilibrated. In the first few seconds, the adsorption process is diffusion limited, with the adsorbed mass quickly increasing to a maximum before decaying to the equilibrium value. To interpret these results, we present an adsorption model involving a site-specific adsorption process, fast initial adsorption, and surface reorganization that is able to qualitatively reproduce the observed kinetic and equilibrium behavior.
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