Neutron reflection study of a water-soluble biocompatible diblock copolymer adsorbed at the air/water interface: the effects of pH and polymer concentration.

The effect of varying both the solution pH and copolymer concentration on the structure of layers of poly[2-(methacryloyloxy)ethyl phosphorylcholine-block-2-(dimethylamino)ethyl methacrylate] copolymer (denoted as MPC(30)-DMA(60), M(n) = 18,000) adsorbed at the air/water interface is studied using surface tension and specular neutron reflection. The surface structure of the adsorbed diblock copolymer is represented by a dense layer of 10-15 A on the air side, accompanied by a loose layer of 20-30 A extending into the aqueous phase. Although the uniform layer model generally provided a reasonable description of the adsorbed copolymer chains, some deviations were observed. A more detailed analysis showed that the distribution of the copolymer across the interface required a minimum of three layers to take into account the structural inhomogeneities. Refinement of the structural distributions involved the combined fitting of partially deuterated copolymer in null reflecting water and D(2)O and the fully hydrogenated copolymer in D(2)O, leading to a substantial improvement in the reliability of the structural profiles obtained. The data analysis showed an increase in surface excess at higher copolymer concentrations and at more alkaline pH. However, the copolymer layer was fully immersed in water under all conditions studied. Because the surface excess showed a steady increase across the cmc over the high pH range, we speculate that copolymer adsorption above the cmc involves the formation of surface micellar aggregates under these conditions.