Solvent-Induced Self-Assembly of Mixed Poly(methyl methacrylate)/Polystyrene Brushes on Planar Silica Substrates: Molecular Weight Effect

Abstract

The effect of molecular weight on the solvent-induced self-assembly of mixed poly(methyl methacrylate) (PMMA)/polystyrene (PS) brushes on silicon wafers was studied. For a series of mixed brushes with a fixed PMMA M_n and systematically changed PS M_n, a transition in water advancing contact angle (θ_a) from 74°, the value for a flat PMMA surface, to 91°, the value for a flat PS film, was observed with increasing PS M_n after treatment with CHCl₃. Atomic force microscopy studies showed smooth surfaces for all samples. While no significant changes in surface morphologies were observed after treatment with cyclohexane, a selective solvent for PS, contact angle and XPS studies indicated that the mixed brushes with a PS M_n slightly smaller than that of PMMA underwent self-reorganization, exhibiting a different θ_a. Intriguing surface morphologies composed of relatively ordered nanoscale domains were found from mixed brushes with PS M_n slightly smaller than or similar to that of PMMA after treatment with acetic acid, a selective solvent for PMMA. The nanodomains are speculated to be of a micellar structure, with PS chains forming a core shielded by PMMA chains.