Combining Reptation Dynamics and Percolation in Modelling Viscoelastic Response of Collagen Based Nanocomposites

For engineering bone tissues, a suitable scaffold provides initial mechanical stability and supports even cell distribution. Collagen filled with hydroxyapatite nanoparticles exhibits the required balance of physical and biological properties. The mechanical performance of this complex bionanocomposite is commonly measured using meso-scale sized specimens, however, its structure can be controlled on the nano-scale. In this paper, the deformation response of model collagen/hydroxyapatite nanocomposites at small shear deformations was analyzed considering processes occuring at the dimensions of individual collagen molecules. Effect of content of hydroxyapatite (HAP) nanoparticles with specific surface area of 190 m²/g incorporated into collagen I (Co) on the storage moduli (G′) of model nanocomposites was investigated. Unlike for micrometer sized HAP, existing micromechanics models grossly underestimated the experimentally measured G′ in the case of nanocomposites. Substantial retardation of the chain reptation dynamics due to the presence of nanoparticles was proposed to describe the molecular reinforcing mechanism at the nano-scale. The role of the chain-filler interface area for the chain stiffening has been confirmed. An "immobilized chain percolation network" with individual chains obeying Langevin rubber elasticity filled with randomly distributed HAP nanoparticles 10 nm in diameter was demonstrated to provide a reasonable means for analyzing the meso-scale mechanical response of the Co/HAP nanocomposite considering nano-scale structural information.