Influence of substrate hydrophobicity on the adsorption of an amphiphilic diblock copolymer

Abstract

The adsorption of poly( tert-butylmethacrylate)-block-poly(2-(dimethylamino-ethyl) methacrylate) (PtBUMA- b-PDMAEMA) was studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis performed on dried samples. The copolymer was dissolved in toluene at concentrations below (0.01 wt%) and above (0.05 and 1 wt%) the CMC; silicon (SiOH) and CH 3-grafted silicon (SiCH 3) were used as substrates. Whatever the concentration and the substrate, a layer of individual copolymer molecules, 1.5–3 nm thick, formed rapidly. The adsorbed amount was slightly higher and the resistance to AFM tip scraping was stronger on SiOH than on SiCH 3. This is attributed to hydrogen bonding between the PDMAEMA block and the OH groups of the silicon surface, leading to polarization of the adsorbed layer. Above the CMC, on SiOH, randomly scattered dot-like features (about 5 nm high) observed by AFM were attributed to individual micelles, which were not displaced by drying. On SiCH 3, the particles found on the top of the adsorbed layer were micelle aggregates, about 50 nm thick, the lateral size of which was strongly influenced by the rate of drying. This further difference between SiCH 3 and SiOH is tentatively attributed to the exposure of PDMAEMA by the adsorbed layer formed on SiCH 3, while only PtBUMA would be exposed by the layer adsorbed on SiOH. The red blood cell shape and the size of the micelles observed in single layers indicate that the PtBUMA corona was not made compact as a result of drying.