Adsorption of Amphiphilic Diblock Copolymer Micelles at the Mica/Solution Interface

The adsorption of the amphiphilic diblock copolymer poly(2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate) (DMA−MMA) at the mica/solution interface has been studied, over a range of solution pH values, using in situ atomic force microscopy. In contrast to the adsorption of the homopolymer poly(2-(dimethylamino)ethyl methacrylate (DMA), which exhibited a featureless adsorbed layer, adsorbed layers of the DMA−MMA copolymer exhibited lateral structure at all pH values studied. At low solution pH, images of the adsorbed layer reveal discrete domelike structures, separated by regions with no lateral features. Force−distance data recorded between these domes indicate adsorption in the form of positively charged material. It is proposed that the adsorbed layer forms via the adsorption of DMA−MMA micelles directly to the mica surface, followed by relaxation of the DMA chains to the surface. This relaxation is driven by electrostatics and results in an adsorbed layer that is essentially a layer of DMA chains with the MMA cores protruding as domes at the sites of the original micelle adsorption. The domes are evident over the entire surface, although arranged in a disordered manner. The distance between the domes is seen to decrease as the pH of the adsorbing copolymer solution is increased from 4 to 7, due to the increased number of negatively charged adsorption sites on the mica surface. Adsorption of charged latex particles shows that objects without molecular freedom adsorb in a close-packed manner. Close packing of adsorbed units is also observed when the DMA−MMA micelles are adsorbed from a solution at natural pH (pH 8.3), where the relaxation of the coronal DMA chains is electrostatically hindered.