Stability and collapse of amphiphilic copolymer aggregates in contact with hydrophilic mica surfaces

Amphiphilic copolymers with a main chain containing polyethylene glycol (PEG) and side chains of different molecular weight and hydrophobicity form spherical aggregates (micelles) in aqueous solution. The suitability of these for incorporation and delivery of small hydrophobic molecules depends strongly on their stability in solution and on their structural changes in contact with surfaces. Atomic force microscopy (AFM) in tapping mode in liquid was used to characterize adsorbed aggregates of a series of copolymers on a model hydrophilic surface (mica). Adsorbed aggregates did not retain the core-shell character they had in solution, but opened up completely to form a layer only 0.5-1 nm thick which corresponds to one or two cross-sectional diameters of individual PEG chains. A good correlation was found between the number of polymer chains in individual adsorbed aggregates as determined with AFM and that known from light scattering studies on the corresponding structures in solution, indicating that each aggregate on the surface originated from one aggregate in solution. Adsorption could be reduced with a more stable hydrophobic core and some copolymers did not adsorb on mica within the time studied.