Modification of hydroxyapatite nanosurfaces for enhanced colloidal stability and improved interfacial adhesion in nanocomposites

This work describes a rational approach of hydroxyapatite (HAp) nanosurface modification for graft polymerization of epsilon-caprolactone (CL). The ring-opening polymerization of CL on HAp surfaces was carried out using three types of HAp with different surface hydroxyl functionality: unmodified HAp (surface OH), HAp modified with L-lactic acid (secondary OH), and HAp modified with ethylene glycol (primary OH). The grafting efficiency and the amount of grafted poly(epsilon-caprolactone) (PCL) were dependent on the nature and steric environment of the hydroxyl groups on the HAp surfaces. Transmission electron microscopy measurements and time-dependent phase monitoring indicated that surface-modified HAp could be more uniformly dispersed in methylene chloride than unmodified HAp, and its colloidal stability increased dramatically as the amount of grafted PCL increased. The nanocomposites of PCL and PCL-grafted HAp showed enhanced tensile strength and toughness, compared with that of unmodified HAp and PCL. Increased interfacial interaction parameters (B-sigma y) for the composite of PCL and PCL-grafted HAp strongly supported the enhanced mechanical strength of the nanocomposites. The use of HAp modified with a larger amount of PCL was found to be much more effective in improving mechanical properties of the nanocomposites.