INSTABILITIES OF THIN POLYMER-FILMS ON LAYERS OF CHEMICALLY IDENTICALGRAFTED MOLECULES

We report autophobic behavior of end-functionalized polystyrene molecu les. By grafting such molecules from a melt onto the surface of a sili con wafer these molecules formed a ''brushlike'' layer which entropica lly repelled the nongrafted but otherwise identical molecules. The bru sh became nonwettable by the same molecules. Thin polystyrene films on top of dense polystyrene brushes were unstable and ruptured as a cons equence of long-range van der Waals interactions which tried to make t he films thinner. The films dewetted the polymer brush by the growth o f cylindrical holes. We investigated this dewetting process in detail as a function of grafting density, which was varied by annealing thin films of end-functionalized molecules for different times. The contact angle formed by the nongrafted molecules on top of the brush, and als o the dewetting velocity, increased significantly with increasing graf ting density. Neutron reflectometry results showed that the brush laye r could be notably penetrated, regardless of the observed dewetting. I mplications of these results to the success of block; copolymers and e nd-functionalized molecules for improved adhesion or for stabilization of thin liquid films are discussed.