Kinetics of Polymer Adsorption on Colloidal Surfaces

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KINETICS OF POLYMER ADSORPTION ON COLLOIDAL SURFACES Z. GAO AND H. D. OU-YANG Physics Department and Center for Polymer Science and Engineering Lehigh University, Bethlehem, PA 18015

INTRODUCTION behavior has been one of the current focuses of polymer Interfacial 2 research. ' The practical importance of the problem is directly related to coating, colloidal stability, etc. Several of the important issues on the kinetics of the polymers at the interface such as reversibility, equilibrium and polymer rearrangement at the surface have drawn particular interests recently.3. For example, there is a long standing speculation that due to the very slow kinetics in the processes of polymer adsorption and desorption that in many cases polymers at interfaces will not reach equilibrium. If this were indeed true one would like to understand in more details as to how and what are the kinetic processes which prevent the equilibrium. Furthermore, it is found experimentally that most adsorbed polymer layers are non-washable by physically-adsorbed possible to exchange pure solvents, while it is still 56 It is important to polymers with other similar polymers in solution. ' understand why invading polymers facilitate the desorption of chains already on the surface. Frantz and Granick have recently reported a carefully tailored experimental study on the exchanging kinetics of homopolymers at interfaces which addressed issues on homopolymer adsorptions.' Using Fourier-Transform Infrared Spectroscopy (FTIS) from attenuated total reflection they studied adsorption and desorption of homopolymers in theta solvent. They found that The desorption is initially the kinetics are controlled by desorption. The decay following a single exponential decay e4"' with rffgoes like ea". time increases rapidly with the culture period of the pre-adsorbed layer due The measured relaxation to annealing process of chains near the surface. times are on the order of hours, orders of magnitude slower than the times In order for the whole required for chains to diffuse through the layer. chain, with a large number of surface bounded segments, to leave the interface, the segments have to desorb in small numbers at a time. In this study we investigate the kinetics of polymer exchange at the One would expect that the problem for surface for end-adsorbing chains. end-adsorbing polymers is different from that of homopolymers because layer In contrast to the flat structures are very different for the two cases. configuration at interfaces formed by homopolymers, end-adsorbing polymers form a brush-like configuration with stickers at interfaces and backbones 6 Based on the theory by Ligoure et al. the stretched into the solution. kinetics for end-adsorbing polymers with surface repelling backbones are characterized by the penetration of chains through the progressively In their calculation the surface coverage increasing polymer layer. increases logarithmically with time. The characteristic time, however, goes exponentially with the chain deformation energy. We used D