Adsorption of Hydrophilic-Hydrophobic Block Copolymers on Silica from Aqueous Solutions

Adsorption of a water-soluble diblock copolymer, poly(tert-butylstyrene)-sodium poly-(styrenesulfonate) (PtBS-NaPSS), on silica surfaces in aqueous solutions was studied using ellipsometry and atomic-force microscopy (AFM). The block copolymers used weee compositionally asymmetric, with large, hydrophilic, PSS blocks and small, hydrophobic, PtBS blocks. Materials with molecular weights of 87 000 and 160 000 weee used. Adsorption could not be observed in pure water without added salt (NaCl). When the NaCl concentration was increased to 1 M, adsorption could be readily observed. The measured adsorbed amount at long times was significantly larger for the 87 000 diblock compared with that for a polyelectrolyte homopolymer of comparable molecular size, demonstrating the role played by the uncharged block in anchoring the diblock at the solid surface. The kinetics of adsorption showed a two-stage process: an initial diffusion-limited stage, followed by a slower buildup of surface coverage in a brush-limited stage. The number density of molecules at the surface was smaller for the higher molecular weight species, in agreement with simple scaling arguments.