Photoinitiated synthesis of mixed polymer brushes of polystyrene and poly(methyl methacrylate)

We describe the synthesis of mixed, or binary, polymer brushes containing polystyrene (PS) and poly(methyl methacrylate) (PMMA). These films were synthesized by a grafting-from (GF) strategy from gold substrates that were coated with a bis-thiol free radical photoinitiator, similar to 2,2'-azobis-(isobutyronitrile) (AIBN). The PS layer was 70 nm thick for all substrates, and the PMMA was varied up to 700 nm. The number-average molecular weight (M.) of the ungrafted polymers was determined by gel permeation chromatography (GPC) and multiangle light scattering (MALS); the M-n for bulk PS and PMMA was similar to57 000 and similar to515 000 Da, respectively. The air/liquid interface was probed by static water contact angles (theta) and X-ray photoelectron spectroscopy (XPS). We found that both theta and the oxygen concentration varied depending on whether the substrate was immersed in a nonselective solvent (tetrahydrofuran) or selective solvent for PS (cyclohexane) or PMMA (isobutanol). The morphology was monitored by atomic force microscopy (AFM), and we observed a dimple-type structure in cyclohexane and isobutanol, with less nanophase separation after immersion in tetrahydrofuran. Reflection-absorption infrared spectroscopy (RAIRS) was used to quantify the % PMMA in the binary brushes, which ranged from 0 to 90%, and was confirmed by XPS. These substrates exhibited reversible switching, and both lateral and layered nanophase separation were observed. The elasticity of a mixed brush was probed by AFM and was found to lie between that of neat PS and PMMA.