Surface‐modified hydroxyapatite linked by L ‐lactic acid oligomer in the absence of catalyst

A new surface modification method of hydroxyapatite nanoparticles (n‐HA) by surface grafting reaction of L‐lactic acid oligomer with carboxyl terminal (LAc oligomer) in the absence of any catalyst was developed. The LAc oligomer with a certain molecular weight was directly synthesized by condensation of L‐lactic acid. Surface‐modified HA nanoparticles (p‐HA) were attested by Fourier transformation infrared spectroscopy, 31P MAS‐NMR, and thermal gravimetric analysis (TGA). The results showed that LAc oligomer could be grafted onto the n‐HA surface by forming a Ca carboxylate bond. The grafting amount of LAc oligomer was about 13.3 wt %. The p‐HA/PLLA composites showed good mechanical properties and uniform microstructure. The tensile strength and modulus of the p‐HA/PLLA composite containing 15 wt % of p‐HA were 68.7 MPa and 2.1 GPa, respectively, while those of the n‐HA/PLLA composites were 43 MPa and 1.6 GPa, respectively. The p‐HA/PLLA composites had better thermal stability than n‐HA/PLLA composites and neat PLLA had, as determined by isothermal TGA. The hydrolytic degradation behavior of the composites in phosphate buffered saline (PBS, pH 7.4) was investigated. The p‐HA/PLLA composites lost their mechanical properties more slowly than did n‐HA/PLLA composites in PBS because of their reinforced adhesion between the HA filler and PLLA matrix.