END-ATTACHING COPOLYMER ADSORPTION - KINETICS AND EFFECTS OF CHAIN ARCHITECTURE

In this study we report on the adsorption behavior of end-attaching triblock and diblock copolymers. The triblock copolymers consist of relatively short PEO end blocks and a much longer PS middle block or, in one case, of zwitterionic end groups with a PS middle block. The diblock materials consist of a PEO block of fixed length and varying polystyrene lengths. A silicon wafer with an oxide layer is used as the surface, and adsorption takes place from toluene; only the end groups adsorb, leaving the polystyrene block dangling in solution. We find that diblock copolymers which range from symmetric to moderately symmetric obey the scaling law for the surface coverage which was proposed in the theory of Marques and Joanny (i.e., sigma is-proportional-to 1/N(A), where N(A) is the number of segments in the adsorbing block). However, we find that for triblock copolymers of comparable asymmetry the surface coverage scales according to the relationship sigma is-proportional-to 1/beta2, where beta is the ratio of the size of the nonadsorbing block to that of the adsorbing block. This implies that moderately symmetric triblock copolymers behave like highly asymmetric diblock copolymers; the adsorption behavior is dominated by the nonadsorbing block size. Our experiments demonstrate the observation of extremely long-lived nonequilibrium states. On the basis of these observations we propose a simplified version of a recent theory of adsorption which is capable of qualitatively describing much of the experimentally observed phenomena.