Chitosan-hydroxyapatite nanocomposites: Effect of interfacial layer on mechanical and dielectric properties

The aim of this work is to investigate properties of interfacial layer in chitosan-hydroxyapatite nanoparticles (CS-nHAp) films. Simultaneous mechanical and dielectric percolation phenomena in films are observed for 30-40% of nHAp loadings. Structure, electrical and mechanical properties of composites and their dependencies on the hydroxyapatite concentrations are investigated by SEM, thermogravimetry, FTIR spectroscopy, nanoindentation and dielectric spectroscopy measurements. Hardness, reduced elastic modulus, conductivity and dielectric constant exhibit a strong dependence on nanoparticle concentration such that maximum values of referred properties are obtained at 30-40 wt % of nHAp. A three-phase model is used to include chitosan matrix, nanoparticles of hydroxyapatite and interfacial layer with dielectric constant higher than that of chitosan and hydroxyapatite. This layer between nanoparticles and matrix is due to strong interactions between chitosan's side groups with PO43- of hydroxyapatite. By performing simulations of spectroscopy measurements in the Hz-GHz frequency range we show that a maximum in all properties is traceable to the existence of overlapping and decreasing of thickness of interfacial layer. Our study proposes a systematic approach to design smart materials for bone tissue engineering.