Langmuir 1994,10, 241-245
241
Kinetics, Isotherms, and Competition in Polymer Adsorption Using the Quartz Crystal Microbalance Hong Xu and Joseph B. Schlenoff Department of Chemistry and Center for Materials Science and Technology (MARTECH), The Florida State University, Tallahassee, Florida 32306 Received May 11,1993. I n Final Form: August 3 , 1 9 9 9 The kinetics and isotherms of adsorption of polystyrene and poly(ethy1eneoxide) at the solution/metal interface have been measured in situ using the quartz crystal microbalance. For polystyrene, cyclohexane at the 8 temperature was used as the solvent and gold as the surface. Isotherms for polystyrene over the molecular weight range 10.'to 106 were confiied to be of the high-affinity type. The effect of solution viscosity on frequency measurementswas considered in detail and shown to be negligible over a range of conditions. The surface displacement of lower molecular weight polymer by higher molecular weight polymer was shown to be rapid. Using polymers having a wide molecular weight distribution, it was demonstratedthat a limiting, stable value for surface coverage is attained more slowly,suggestingcontinuous competition for sites among molecules of different sizes. Poly(ethy1ene oxide) adsorbed onto gold from a good solvent, whereas polystyrene did not.
Introduction Polymer adsorption processes are central to phenomena such as colloid stability,' adhesion,2 lubrication? and corrosion protection. Many techniques are available for probing the adsorption of polymers to the solid/liquid interface. Classical methods include spectroscopic difference measurements on the supernate following the introduction of an adsorber. These techniques rely on the substrate having a high specific surface area, and materiale such as carbon black,' metal powder? and glass powdefl have been employed. Planar surfaces, though of low specific area, are, in many respects, better characterized. Polymer/surface interactions are better defined: for example, it is possible to produce clean metal surfaces under vacuum. These substrates can offer the possibility of self-assembly using specific molecule/surface interactions.'-"J The choice of techniques for studying adsorption at planar solid/solution interfaces is limited. Radiolabeling offers the required sensitivity,l@-l2 as does reflectance FTIR.lS One of the most popular approaches is ellipsometry,1C18in which one can measure in situ adsorption of polymer layers a few nanometers thick. If some assumptions concerning the density profile are made, the
* To whom correspondence should be addressed.
AbstradpubliahedinAduance ACSAbstracts, October 15,1993.
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apparent thickness can be translated into a surface coverage. Ellipsometry is particularly useful when determining the effect of various experimental parameters, such as solvent strength, temperature, molecular weight, and charge density (for polyelectrolytes), on the surface conformation. In fact, much of the theoretical work on polymer adsorption specifically addresses the thickness of the adsorbed 1ayer,1s-21and ellipsometry has thus been used to test theory. In our work we are more concerned with adsorption/ desorption phase transitions and competition for surface sites between polymers having different molecularweights. We describe the use of the quartz crystal microbalance (QCM) for direct in situ measurements of the amount of adsorbed polymer a t metal/solution interfaces. In our experiments we use a well-studied system-polystyrene in cyclohexane at 35 OC-which is known to be a 8 condition.22 In verifying that the QCM can be used to study polymer adsorption, we illustrate the utility of the technique in determining kinetics and isotherms. We also perform displacement studies.
Experimental Section Oscillators were prepared by evaporating 150-nm gold over 5-nm chrome contact pads onto 5-MHz AT cut quartz, supplied "overtone" polished (Valpey Fisher), using an Edwards 306A evaporator operating at
