| Hydroxyapatite filled nanocomposites are wildly used in the field of biomedical engineering. They are of great significance either for theoretical research or the guidance of practical application to explore the mechanism between organic phase and the hydroxyapatite interface. Thus, it is necessary to study the interaction mechanism between the interface of deficient hydroxyapatite which is closer to the fact and the organic phase, for that the deficient hydroxyapatite has a higher surface activity. In this paper, the methods (GGA-PBE) of density functional theory (DFT) are employed to construct the model of deficient hydroxyapatite (D-HA) and investigate the interactions between the surfaces of polylactic acid (PLA) and it. We are looking forward to a more in-depth understanding to the nature of interaction between organic matrix nanocomposites.To make the surface of D-HA model closer to the fact, a more realistic charge compensation mechanism and a lower surface energy of configuration on the D-HA surface are essential. In view of this, in Chapter III of this article, we firstly abstracted a one-dimensional model of hydroxyapatite based on the experimental fact, which contained the major impurity particles. Starting from this model, we built a rule to describe the mapping from the characteristics of macro parameter to the corresponding number of micro-particles. Further, according to this model as well as the special values of the parameters which are easy to measure and in accordance with the fact, we obtained the two-dimensional deficient hydroxyapatite model, with the molecular formula of Ca8[(HPO4)3(PO4)2(CO3)](OH)2. And then, based on this molecular formula, we gradually searched for the final stable structure of deficient hydroxyapatite by the energy calculation and the structure optimization employing the DFT methods in CASTEP package. The characteristics of the structure and the location of CO32-, etc. are basically the same as which reported in the literature which reported a similar structure. At the same time, we also investigated the geometric characteristics of (001) surface as well as the causes of it.In Chapter IV of this article, we further studied the adsorption behavior of the PLA model on the D-HA (001) surface. 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 unit surface, but only one could 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 were two kinds of hbs interactions: one type was the medium stronger hbs between the C=O and the hydrogen in HPO42- with the bond length of 1.69 A and bond energy of about 48 kJ·mol-1; the other was 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.
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