Modeling and investigation of interfacial interaction between PLA and one type of deficient hydroxyapatite.

A model of one type of deficient hydroxyapatite (D-HA) was constructed, and the interaction mechanism between polylactide acid (PLA) and the (001) surface of D-HA was also investigated for the first time employing density functional theory (DFT) in the Perdew, Burke, and Ernzerhof (PBE) generalized gradient approximation (GGA). First, a mathematical model of D-HA was abstracted from experimental facts, through which a group of special values and the nonstoichiometric formula, Ca(8)[(HPO(4))(3)(PO(4))(2)(CO(3))](OH)(2), were obtained. Next, a stable configuration of D-HA was achieved with the method of searching stable structure gradually. After the most stable configuration was identified, our attention mainly turns to the results concerning the (001) surface of D-HA. Methyl lactate was employed to act as PLA monomer estimating the interaction behavior between PLA and D-HA. Simultaneously, to achieve an accurate description of hydrogen bonds (hbs), a plane wave energy cutoff of 700 eV was used. Significantly, we observed that there were two P-OHs on the surface, but only one can form a hydrogen bond with PLA; also, besides the interaction between carbonyl oxygen (C=O) in the PLA and calcium ions in the D-HA, there are two kinds of hbs interactions: one type is the medium stronger hbs between the C=O and the hydrogen in HPO(4)(2-) with the bond length of 1.69 A and bond energy of about 48 kJ mol(-1); the other is the weak hbs between the oxygen in phosphate and the hydrogen in methyl/methylene with the average bond length of about 2.48 A and bond energy of about 9 kJ mol(-1). PDOS was also employed to characterize the existence of hbs. Our results may have potential promotion to investigate the properties of polymeric nanocomposites.