BLOCK COPOLYMER ADSORPTION STUDIED BY DYNAMIC SCANNING ANGLE REFLECTOMETRY

We employ a novel scanning angle reflectometer to study both the kinetics of block copolymer adsorption and the structure of the adsorbed layer. The apparatus measures the intensity of parallel polarized light reflected from a glass-solution interface at and around the Brewster angle. We study the deposition of highly asymmetric styrene/ethylene oxide (S/EO) block copolymers from cyclopentane. When the surface is challenged with a concentrated or moderately dilute polymer solution, a very fast initial chain deposition is observed. This layer is always far from equilibrium, relaxing slowly over periods of days. Stable layers are prepared by using the concentration step procedure: the initial coverage at extremely low polymer solution concentration allows for individual chain relaxations; subsequent increases in polymer solution concentration result in stable, thicker, polymer layers within 1 h. Such layers are in semiquantitative agreement with those calculated from the self-consistent-field theory due to Scheutjens and Fleer. The computations suggest that at very low concentrations both types of segments can absorb, while at higher concentrations the EO units displace the PS segments and the characteristic profile of the highly extended "grafted" PS tails is present. Analysis of the adsorption kinetics shows transport of chains through the brush layer to be relatively fast while chain rearrangements in the adsorbed layer are prohibitively slow. At a coverage on the order of 10 mg/m2, a phase transition in the polymer layer is observed. This new layer has a more compact, less extended structure.