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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 (theta(a)) from 74 degrees, the value for a flat PMMA surface, to 91 degrees, the value for a flat PS film, was observed with increasing PS M(n) after treatment with CHCl(3). 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 theta(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.
Zhao et al. (Tue,) studied this question.