ADSORPTION-KINETICS OF BLOCK COPOLYMERS FROM A GOOD SOLVENT - A 2-STAGE PROCESS

The adsorption kinetics and adsorption isotherm of a polystyrene (PS)-poly(ethylene oxide) (PEO) diblock copolymer onto a silicon wafer from toluene solution was measured in situ by ellipsometry. Both blocks are in good solvent conditions but the small PEO block is adsorbed while the much larger PS block dangles in solution. Thus, PS-PEO behaves like an end-adsorbed chain. The adsorption kinetics shows two processes on a clearly separated time scale. In the beginning, the time behavior of the adsorbed amount can be described as a diffusion-controlled process leading to a surface coverage with small interaction between molecules. A still denser surface coverage is achieved by the penetration of chains through the existing monolayer combined with the conformational rearrangement to a more brushy conformation. Experiments with different molecular weights indicate that the repulsion between the nonadsorbing PS blocks determines the maximal adsorbed amount. The adsorbed monolayer can be rapidly and completely displaced by PEO oligomer with a length comparable to the PEO block in the block copolymer. Models of the adsorption process are discussed in detail.