Calcium phosphate mineralization beneath monolayers of poly(n-butylacrylate)-block-poly(acrylic acid) block copolymers

Amphiphilic poly(acrylic acid)-block-poly(n-butylacrylate) block copolymer films at the air-water interface have been mineralized with calcium phosphate. The polymers form stable monolayers at the free surface. Their stability is virtually independent of ion strength or pH changes in the subphase. The outcome of calcium phosphate crystallization depends on the calcium and phosphate concentrations, the stirring rate of the subphase, and the subphase pH. At low calcium and phosphate concentrations (2 mM), uniform polymer-calcium phosphate hybrid films form. Higher calcium and phosphate concentrations yield less ordered films, which often contain large blocks of material beneath the polymer monolayer. Occasionally, also filaments similar to samples described by Peytcheva et al. (Colloid Polym. Sci., 2002, 280, 218) are observed. Films mineralized at pH values below ca. 6 contain particles that are a few nanometers apart and the resulting films retain some flexibility. Films grown above pH 6 have a higher degree of mineralization. They are stiff and tend to break upon mechanical stress. Overall, the paper illustrates that low calcium phosphate supersaturation in the subphase and a well-defined (but not crystalline) interface are crucial for controlling calcium phosphate mineralization. As a result, the current study could serve as a model for biological mineralization which is more closely related to Nature than films made from e. g. detergents or low molecular mass compounds.