Dewetting Behavior In PS-b-PMMA Thin Films Induced By Acetone Vapor

Polymer films have many important applications in the fields of science andtechnology. Dewetting is a crucial phenomenon of the polymer films, on the onehand dewetting restrict the applications of the films, on the other hand it can beused to fabricate ordered patterns in thin film. Thus, the research on thedewetting in the polymer film has considerable practical significance. In thispaper, the dewetting behavior in ultrathin films of diblock copolymerpolystyrene-block-poly(methyl methacrylate)(PS-b-PMMA) with differentmolecule weights (coded as SMMA34K and SMMA51K) induced by acetonevapor was studied. Thin films of SMMA34K with different thicknesses andSMMA51K with a thickness of4nm were prepared by spin coating andcharacterized by atomic force microscopy (AFM). The morphology evolutions ofthe films with different thicknesses in acetone vapor were mainly investigatedand the probable mechanisms of the dewetting were discussed. Furthermore, theeffects of residual solvent, acetone volume, and block copolymer composition onthe dewetting behavior of thin films were investigated.A discontinuous film of SMMA34K with holes is obtained by spin coating asolution of0.25mg/mL due to the short PMMA blocks. With the further increaseof the concentration, thin flat films are obtained. When the films are annealed inacetone vapor, the dewetting velocity of the films decrease with the increase ofthe film thickness, and the dewetting mechanisms transform from spinodaldewetting into heterogeneous nucleation and growth. The size of the dropletsfirstly increases and then decreases with the increase of the annealing time. Thedewetting velocity of the film decreases after the residual solvent is removed, butthe transition process of droplets needs lees time. Furthermore, the more theacetone is used in the annealing process, the earlier the dewetting behavior in the films occurs.A flat film continuous of SMMA51K is obtained by spin coating a solutionof0.25mg/mL due to the long PMMA blocks. The dewetting in the4-nm-thickfilm of SMMA51K occurs by the spinodal dewetting mechanism. There are twodewetting processes, a slow dewetting of the polymer layer occurs at thesubstrate’s surface, and a fast one occurs on the top of this layer. The size of thedroplets first increases, then decreases, and finally increases again with theincrease of annealing interval. After the films are annealed for24h, the dropletsizes in the4-nm-thick films of SMMA51K and SMMA34K are very close.