Poly(ethylene oxide) (PEO) Chains Are Not "Hydrophilic" When They Exist As Polymer Brush Chains

A model poly(ethylene oxide) (PEO) brush system was prepared by spreading a poly(ethylene oxide)-poly(n-butyl acrylate) (PEO-PnBA) amphiphilic diblock copolymer on to an air-water interface. The polymer segment density profiles of the PEO brush in the direction normal to the air-water interface under various grafting density conditions were determined by using the neutron reflectivity (NR) measurement technique. In order to achieve a theoretically sound analysis of the reflectivity data, we developed a new data analysis method that uses the self-consistent field (SCF) theoretical modeling as a tool for predicting expected reflectivity results for comparison with the experimental data. Using this new data analysis technique, we discovered, for the first time, that contrary to what is known for PEO in normal situations, the PEO brush chains are not “hydrophilic” (i.e., χ_PEO/water > 0.5) because of the many body interactions that are forced to be effective in the brush situation, in particular, in the region close to the grafting surface. We suspect that this effect is key to the mechanism by which a PEO brush system, a coating structure commonly used in biomedical materials for protein resistance and biocompatibilization, is able to resist, so effectively, the adsorption of proteins in physiological conditions.