CRYSTALLIZATION OF HYDROXYAPATITE ON POLYMERS

Four copolymers were prepared through the reaction of N,N'-bis[(diethoxyphosphinyl)methyl]-1,4-benzenediamine with 1,4- or 1,3-benzenediamine and terephthaloyl or isophthaloyl or sebacoyl dichloride at varying molar ratios. A fifth copolymer was prepared by copolymerization of bis(3-aminophenyl)phosphinic acid and 1,3-benzenediamine with isophthaloyl dichloride in dimethylamine. All five copolymers contained phosphinyl groups and upon introduction in calcium phosphate aqueous solutions, supersaturated only with respect to beta-tricalcium phosphate and hydroxyapatite, three of them induced the precipitation of stoichiometric hydroxyapatite, (Ca5(PO4)3OH, HAP). The investigation of the crystal growth process was done at constant supersaturation, which enabled us to measure even very low rates of crystallization. The apparent growth order was found to be 1.25 +/- 0.18, a value found for the crystallization of HAP on various substrates. The surface energy estimate of 92 mJ m-2 from the kinetic data was of the same order of magnitude for the overgrowth of HAP on both inorganic and polymeric substrates. The rates of precipitation showed a linear dependence on the phosphate content of each copolymer, while their structure played also a decisive role. Thus, "open" structured copolymers yielded high rates of precipitation while copolymers in which the phosphorus-containing groups were "buried", not being accessible to the Ca2+ ions in solution, failed to induce hydroxyapatite formation. It is suggested that the phosphorus-containing groups act as the active centers for nucleation of the inorganic phase.