Nanoindentation analysis of αtricalcium phosphate-poly(lactide-co-glycolide) nanocomposite degradation

The internal mechanical property characteristics as functions of position and degradation time of PLGA(50:50)-alpha TCP nanocomposites of varying ceramic-polymer ratios degraded in an aqueous medium have been assessed using depth-sensing nanoindentation. The addition of nanoparticulate alpha TCP increases the elastic modulus of undegraded specimens from 3.72 +/- 0.12 GPa for pure PLGA(50:50) samples to 723 +/- 0.16 GPa recorded for undegraded 40 wt%TCP nanocomposites. Additionally ciTCP incorporation decreases the viscoelastic loss tangent from 0.189 +/- 0.040 measured for pure undegraded PLGA(50:50) to an average of 0.091 +/- 0.006 for undegraded ceramic-polymer composites. No variation in viscosity for the composites with ceramic loading was evidenced. The stiffening effect of alpha TCP addition closely conforms to the lower Hashin-Shtrikman bounds demonstrating that an evenly dispersed nano-filler is the least amenable ceramic configuration to enhance the mechanical properties of PLGA-alpha TCP nanocomposites. The mechanical property evolution for all composite types in an aqueous degradation medium is dominated by material hydration which effects reduced material stiffness and increased specimen viscosity generating a core-periphery mechanical property distribution in terms of elastic modulus and viscoelastic phase angle. The mechanical property core-periphery structure correlates strongly with the core-periphery density structure characterized using X-ray microtomography. Hydrated regions exhibit significant reductions in elastic modulus and viscosity increases which are typical of elastomers. (C) 2014 Elsevier B.V. All rights reserved.