Hydroxyapatite polyacrylic acid nanocrystals

Hydroxyapatite nanocrystals have been synthesized in the presence of polyac rylic acid in solution. The method leads to apatite crystals with a polyele ctrolyte relative content which increases as a function of its concentratio n in solution and reaches a limit value of about 15% wt. Polyacrylic acid a ffects the degree of crystallinity, the coherent length of the perfect crys talline domains, as well as the dimensions and the morphology of the crysta ls. TEM images indicate that on increasing polyetectrolyte content, the dim ensions of the crystals are reduced, while their morphology changes from pl ate-like to acicular, and, at high polyacrylic acid content, the crystals t end to aggregate in clusters. In agreement, the broadening of the X-ray dif fraction reflections indicate a reduction of the coherent length along the long dimension (002) and the cross section (310) of the apatite crystals. T he reduction is greater along the direction orthogonal to the c-axis, which can be rationalized by a preferential interaction of the carboxylate group s of the polyelectrolyte with the phosphate sites on the crystalline faces parallel to the c-axis.