Morphological and structural investigation of octacalcium phosphate hydrolysis in the presence of polyacrylic acids: Effect of relative molecular weights

The results of this paper indicate that poly(acrylic acid)s of different molecular weights (PAA2, M(w) = 2000; PAA250, M(w) = 250 000) inhibit the hydrolysis of octacalcium phosphate (OCP) into hydroxyapatite (HA) to an extent that depends on their concentration in solution, as well as on their molecular weights. This inhibition, which takes place through poly(acrylic acid) adsorption on OCP crystals, is more effective for PAA2 than for PAA250. The significant increase of the roughness of the OCP crystal surface after adsorption of PAA250, put into evidence by AFM images, and the decrease of the coherence lengths of the perfect crystalline domains along the c-axis direction of OCP crystals after adsorption of PAA2 suggest that the shorter length of PAA2 molecules allows a greater and more ordered interaction with the hydrated layers of the (100) face of OCP. The results of TEM investigation suggest that OCP hydrolysis occurs according to a layer-to-layer mechanism that implies splitting into smaller crystals and accounts for the platelet-like morphology of the final apatitic crystals.