THE BIMODAL ENERGY LANDSCAPE WHEN POLYMERS ADSORB

We describe the histogram of chain conformations that results when fle xible polymers adsorb more quickly than the surface structure equilibr ates. By infrared analysis of conformational substates in a model syst em (poly(methyl methacrylate) adsorbed primarily by hydrogen bonding o nto oxidized silicon), this histogram was measured and found to be bim odal. One large population of chains spread into flattened conformatio ns. A second large population of chains, finding less free surface ava ilable at the time of adsorption, became attached more tenuously. The history dependence of adsorption could be understood as the irreversib le adsorption of objects with flexible shape. This interpretation diff ers decisively from the traditional considerations of minimizing the f ree energy.