Instabilities of Thin Polymer Films on Layers of Chemically Identical Grafted Molecules

Abstract

We report autophobic behavior of end-functionalized polystyrene molecules. By grafting such molecules from a melt onto the surface of a silicon wafer these molecules formed a “brushlike” layer which entropically repelled the nongrafted but otherwise identical molecules. The brush became nonwettable by the same molecules. Thin polystyrene films on top of dense polystyrene brushes were unstable and ruptured as a consequence of long-range van der Waals interactions which tried to make the films thinner. The films dewetted the polymer brush by the growth of 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 also the dewetting velocity, increased significantly with increasing grafting density. Neutron reflectometry results showed that the brush layer could be notably penetrated, regardless of the observed dewetting. Implications of these results to the success of block copolymers and end-functionalized molecules for improved adhesion or for stabilization of thin liquid films are discussed.