School of Engineering and Materials Science
Research Student Awards
PhD Thesis: Dynamic mechanical characterisation of hydroxyapatite reinforced biomedical composites
Author: NAZHAT, Showan
Supervisor(s): Ray Smith
Dynamic mechanical analysis was used to study the viscoelastic behaviour of two hydroxyapatite-polymer composites developed for bone replacement and augmentation. Hydroxyapatite (HA) reinforced high density polyethylene (HDPE) composites (HA/HDPE) and HA reinforced biodegradable copolymer poly(L, DL lactic acid) (PLDLA), which was also reinforced with a semi-crystalline form of poly(lactic acid) (PLLA) composites (HA/PLLA/PLDLA), constituted the two systems.
For HA/HDPE composites, the effect of HA volume fraction, HA particle size and chemical modification with the use of a silane coupling agent on HA as well as the acrylic acid grafting of the HDPE matrix were investigated. Increasing HA volume fraction increased the storage modulus (EI) and tended to decrease tan . EI was found to be linearly related to Young's modulus values obtained by quasi-static tensile tests for the same composites. Relative modulus and damping studies showed that the properties of unfilled HDPE were different to the filled matrix due to thermally induced stresses near the filler-matrix interface region. Density and inorganic content measurements showed that the HA content in the composites was consistently lower than predicted volume fractions and that smaller particles were more easily incorporated into the matrix. The molecular weight of HDPE was lowered by chemical modification, which altered the damping behaviour of the modified composites. A significantly lower HA volume fraction than predicted accompanied by an inhomogeneous dispersion in these composites led to a lowering of EI.
For HA/PLLA/PLDLA composites, the addition of HA, PLLA fibres and a combination of both reinforcing agents and processing conditions were investigated. The matrix glass transition temperature (Tg) was increased by reinforcement. Both HA and PLLA increased EI and a combination of both fillers proved to be most effective. As the moulding temperature increased, the fibre and matrix partially blended leading to a broadening in Tg.