Morphologies in Acetone-Annealed Polystyrene-Poly(methyl methacrylate) Diblock Copolymer Thin Films

The time development of morphologies and structures of symmetric and asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer thin films after annealing in acetone vapor was investigated by tapping mode atomic force microscopy (AFM). Film thickness played an important role in the morphology evolution of symmetric PS-b-PMMA (f(PS)approximate to f(PMMA), f(PS) is the volume fraction of the PS in PS-b-PMMA, f(PMMA) is the volume fraction of the PMMA) thin film. When film thickness was controlled according to their equilibrium bulk lamellar period, all the investigated thin films of PS-b-PMMA with different molecular weights can change from disordered pattern to nanoscale depression and at last to striped structures. In the case of asymmetric PS-b-PMMA systems, when the content of PS segment was low (25%, 36%), film morphology changed from disordered state to regularly packed spheres and directly to stripes, respectively; when the content of PS segment was high (70%), film morphology changed from disordered pattern to relatively flat state. The formation mechanisms of different film morphologies have been discussed in detail.