Hydroxyapatite precipitation on a carboxylated vinyl chloride-vinyl acetate copolymer

The kinetics of crystal growth of hydroxyapatite [Ca₅(PO₄)₃OH, HAP], the thermodynamically most stable calcium phosphate phase, on a commercially available carboxylated (1%) vinylchloride (83%) vinyl acetate (13%) copolymer was investigated at relatively low ionic strength, at 37 °C and pH 7.4. Stable supersaturated solutions were seeded with HAP seed crystals as reference and with the polymer in powder form. Well defined induction times preceded the growth of HAP on the polymeric substrate. The induction times measured were inversely proportional to the solution supersaturation. The rates of crystallization both on HAP and on the polymer were measured at conditions of constant supersaturation. HAP growth took place on both substrates and the parabolic dependence of the rates of crystal growth on the relative solution supersaturation suggested a surface diffusion controlled mechanism. The rates of HAP crystal growth on the polymeric substrate were found to depend on the amount of the seed material suggesting selective overgrowth of HAP. A possible explanation for this selectivity is the development of active growth sites through the formation of surface complexes of the calcium ions with the dissociated carboxyl groups.