Composites between hydroxyapatite and poly(epsilon-caprolactone) synthesized in open system at room temperature

The hydroxyls present on the surface of hydroxyapatite (HA) granules, annealed at 700°C, 900°C and 1100°C, are able to initiate the polymerization of epsilon-caprolactone (CL), not only at 185°C under vacuum, but also at room temperature in open system. A polymer layer ionically linked to the substrate is formed on HA surface, enhancing the compatibility between the organic phase and the inorganic one in composite biomaterials. We studied the characteristics of the polymer, produced by the reaction carried out at room temperature in open system, as well as the percentages of the poly(epsilon-caprolactone) (PCL) ionically bonded to the HA structure and of the “free” one. Both percentages appear very dependent on the annealing temperature; in particular, HA annealed for 1 hour at 1100°C is the most efficient initiator of the reaction leading to ionically bonded PCL. The percentages of “free” polymer are much higher than at 185°C under vacuum. Its formation is attributed to the role of water in opening the CL rings, and to the presence of carbonate and hydrogen-phosphate ions in the HA annealed at lower temperatures. The presence of water appears to be the limiting factor for the production of PCL not bonded to the HA structure.