Adsorption behavior of polymers with ionic end groups : mono- and difunctional polystyrenes in toluene

The adsorption behavior of mono- and difunctional telechelic polymers, omega- and alpha,omega-sulfonato polystyrenes, from dilute toluene solution on the surface of a silicon wafer is investigated by ellipsometry. Polystyrene without ionic end groups does not adsorb, and the attachment of molecules to the silicon dioxide surface proceeds via the functional ionic ends. Already a single ionic group promotes adsorption, and adsorption kinetics at small times is found to be diffusion limited. The approach of an equilibrium adsorbed amount at later times is controlled by the hindered diffusion of chains through the already existing layer. With monofunctional chains a thin stable monolayer at the surface is formed. With difunctional chains there is strong indication for coupling of ionic ends and the formation of additional layers. It is caused by the interaction of dangling ends, when only one end of the chains is adsorbed at the surface. The molecular weight dependence of the grafting density is consistent with scaling predictions, and differences in the adsorbed amount of telechelics with different counterions are explained on the basis of different valencies.