Surface Morphology of Mechanically Heterogeneous Ultrathin Polymer Films

We studied the formation and morphology of mechanically heterogeneous ultrathin film consisting of a rubbery matrix and stiff domains deposited on silicon surface. Polystyrene (PS) was used for the rigid part of the thin polymer coating. A triblock copolymer, poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS), functionalized with 2% maleic anhydride (MA) randomly imbedded into the hydrocarbon chains, served as the rubbery element of the heterogeneous film. The block copolymer/homopolymer (SEBS/PS) combination allowed formation of stable nanocomposite films consisting of elevated hard polymer domains located on the top of an elastomeric layer. All films produced were stable and did not dewet the silicon surface, even though they were heated above the glass transition temperature of PS. This observation implied that SEBS ensured a strong enough interaction between the heterogeneous coatings and the inorganic surface to prevent the dewetting. For the thicker films (18−150 nm) the copolymer possessed the lamellae structure at the PS/SEBS interface, which was formed during spin-coating. Conversely, for the thinnest (9 nm) films the copolymer tended to surround the PS domains with a stabilizing “cage”. The formation of the cage apparently prevented significant enlargement of the film structure during annealing.