Molecular dynamics simulation of hydroxyapatite–polyacrylic acid interfaces

Polymer–hydroxyapatite (HAP) composites are widely studied as potential bone replacement materials. The HAP–polymer interfacial molecular interactions have significant role on the mechanical response of composite systems. We have used molecular dynamics (MD) simulations to evaluate the nature of these interfaces in polyacrylic acid–hydroxyapatite composites. We have obtained the parameters for monoclinic hydroxyapatite in CVFF (consistent valence force field) from the known potential energy function of apatites. Our simulations indicate that potential sites for chelation and hydrogen bond formation between HAP and polyacrylic acid (PAAc) exist. Earlier, we have synthesized in situ HAP–polymer composites wherein intimate interaction between HAP and polymer is enabled through participation of polymer during HAP mineralization. Our simulations indicate that for in situ HAP, the most favorable orientation of PAAc for attachment with HAP is along the c-axis of HAP aligned parallel to polymer chains. Also, binding energy for ex situ HAP composites is found to be lower as compared to that of in situ HAP.