END-ADSORBED TRIBLOCK COPOLYMER CHAINS AT THE LIQUID SOLID INTERFACE - BRIDGING EFFECTS IN A GOOD SOLVENT

We report direct force measurements on the interactions of end-adsorbed diblock and triblock copolymer layers against a bare mica surface in toluene. Only monotonically repulsive forces are observed when a bare mica sheet interacts with a single layer of polystyrene (PS)-poly(ethylene oxide) (PEO) diblock copolymer, end-adsorbed at a toluene/mica interface. In contrast, clearly detectable attractive forces are found for PEO-PS-PEO triblock copolymers under similar conditions. The attraction is attributed to polymer bridges in which the two PEO end blocks of a single triblock chain simultaneously adsorb onto opposing mica surfaces. The experimentally detected evolution of the force profiles from attractive to repulsive with successive compression-decompression cycles reflects the dynamics of conformational rearrangements and the subsequent migration of chains between the surfaces. The results further indicate that the relative populations of chains adsorbed in a loop or tail conformation depend on the adsorption energy of the anchoring blocks, as does the degree of stretching of the bridging chains before rupture upon separation of the mica surfaces.