Understanding the roles of nanoparticle dispersion and polymer crystallinity in controlling the mechanical properties of HA/PHBV nanocomposites.

Nano-sized hydroxyapatite (HA) particles stabilized using poly(acrylic acid) (PAA) as a dispersing agent, and sonic energy to further increase dispersion, were blended with poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) using a precipitation/gelation method to produce HA/PHBV nanocomposites with up to 16% by weight of HA content. The level of HA nanoparticle dispersion was monitored in the precursor dispersions prior to composite production and in the nanocomposites by a range of techniques including visual observation, turbidity measurements and electron microscopy, and the roles of the dispersing agent and the sonic energy in controlling the dispersion of HA particles in both the precursor dispersions and the final composites as well as their effects on the compressive strength and Young's modulus were investigated. It was found that HA suspensions treated with both PAA and sonic energy possessed significantly better colloidal stability compared to untreated suspensions or suspensions treated with either PAA or sonic energy. This, in turn, resulted in better dispersion of HA nanoparticles in the composites and higher compressive moduli as a function of the particle loading. This enhancement in stiffness of the composites was attributed primarily to the increased surface area of the HA filler in the more highly dispersed samples, but also to an observed increase in the crystalline content achievable after annealing of the samples. It is proposed that this increase in crystallinity is due to the more highly dispersed particles acting as nucleation sites for the crystallization of the PHBV at the particle interface, which, in turn, leads to enhancement of the bonding between the matrix and filler.