Comparative Study on Factors Governing Binding Mechanisms in Polylactic Acid-Hydroxyapatite and...

Binding mechanisms in polylactic acid-hydroxyapatite (PLA-HAp) and polyethylene-hydroxyapatite (PE-HAp) systems are comparatively elucidated on HAp (110) surfaces in unprecedented detail using molecular dynamics simulations conducted with the systematically varying number of monomers (N) between 10 and 400 at 310 K (NVT). Although PE seems to gradually cover the HAp surface more effectively compared to PLA, evident from the corresponding radius of gyration and occupied area values, the interface density and total binding energy in PLA-HAp systems is higher compared to those of PE-HAp systems. It is shown that a linear relationship between the binding energy and the surface area occupied by the monomer exists, consistent with our finding that binding energy converges to a limiting value with respect to monomer size on a constant surface area. The major constituent of the total binding energy is, rather surprisingly, shown to be the energy change in the bulk structure in HAp upon interaction; the next most important contributor is found to be the energy corresponding to surface-polymer interactions. The interplay between mainly these two contributors, acting in different fashions in two systems investigated here, seems to control the total binding energies. Increasing monomer size N initially results in enhanced densification of the interface in the HAp-PLA system up until N ≈ 200 with the positioning of mainly ═O units of PLA onto the HAp surface, consistent with the increasing Ca-O coordination numbers. Further increases in PLA size (N > 200) result in decreasing intensities of the peaks in the concentration profile consistent with the decreasing surface-polymer interaction energies while increased stabilization of the energy of the bulk is pronounced in this region. On the other hand, increasing N leads to a constantly increasing concentration at the interface in PE-HAp systems; -H atoms of the PE chain are positioned closer to the HAp surface than are -C atoms. These changes are coupled with increasing surface-polymer interaction energies in PE-HAp complexes, while slight destabilization in the energy of the bulk is observed for N > 100. A detailed examination of binding mechanisms in these technologically important systems as presented here is essential in material discovery; this valuable information, that will not be available from experiments can be attained through molecular simulations. The current study, to the best of our knowledge, comprises one of the first steps in achieving this goal for PLA/PE-HAp systems.