This thesis is inspired by a persistent limitation in the use of composite biomaterials for orthopaedic applications, namely the agglomeration of reinforcing particles in these composites, which has resulted in poor mechanical properties. The use of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and hydroxyapatite (HA) nanoparticles to produce biodegradable nanocomposites is investigated. More specifically, the thesis investigates different methods of composite processing, and interfacial modifying agents and the effect that these have on the nano- and micro- scale structure of composites and their mechanical properties. PHBV and HA were chosen because PHBV is a biodegradable/biocompatible polymer and it has a relatively high stiffness when compared to other biodegradable polymers frequently used in orthopaedic applications. HA is chemically similar to ceramic phase found in bones and hard tissues and the inclusion of HA into biomedical materials has been shown to enhance the rate of osteoconduction. HA/PHBV composites were produced using different dispersing agents including poly(acrylic acid) (PAA), a model dispersing agent, polyethyleneimine (PEI) which allowed for the development of a single solvent system for composite preparation, and heparin (Hep), a macromolecule which is produced in vivo. Additionally, HA/PAA/PHBV composites were prepared from both sonicated and non-sonicated HA/PAA suspensions up to approximately 17% by weight (wt %) of HA content. Attempts to prepare composites with higher HA loadings led to inhomogeneous composite mixtures, which were caused by the dual solvent system used for the composite preparation. The HA/PEI/PHBV and HA/Hep/PHBV composites were produced up to approximately 75 wt % of HA content. It was found that the HA/PEI/PHBV and HA/Hep/PHBV composites could be prepared at higher loadings than HA/PAA/PHBV composites due to the single solvent system used for the preparation of the HA/PEI/PHBV composites and the better dispersion of HA/Hep particles in precursor suspensions. Finally, selected HA/PEI/PHBV composites were further processed using a twin screw extruder. All of the composites were characterised in terms of their dispersion levels as well as their compressive mechanical properties. In addition, HA/PEI/PHBV composite reinforced with 20 wt % of HA content was also tested for its mechanical properties using three different test types; compression, three-point bending, and tensile tests. Finally, the HA/PAA/PHBV, HA/PEI/PHBV, and HA/Hep/PHBV composites were tested their compressive mechanical properties in wet state. It was found that the sonicated HA/PAA suspensions in general had better colloidal stability than non-sonicated ones and that this yielded composites with superior compressive moduli than those prepared from non-sonicated suspensions. In addition, the better dispersion of the particles in the composites prepared from the sonicated suspensions, as confirmed by transmission electron microscopic (TEM) images, led to higher percentage crystallinities when compared to the composites prepared from non-sonicated suspensions. It is likely that the greater number of individual HA particles and smaller HA agglomerates observed in the composites prepared from sonication treatment are acting as nuclei for crystal growth more effectively than large HA agglomerates. The largest modulus and yield strength that could be achieved with this system were approximately 1.45 GPa and 80 MPa, respectively. Composites of HA/PEI/PHBV and HA/Hep/PHBV with approximately 55 wt % of HA content were found to exhibit the largest compressive moduli of approximately 2.5 and 2.8 GPa, respectively. Moreover, the yield strengths for the same materials were found to be approximately 123 and 120 MPa, respectively. This was found to correlate with the better levels of dispersion within the nanocomposites that could be achieved using these stabilisers. The extruded samples were found to have an even greater degree of particle dispersion when compared to the unextruded ones. This improved degree of particle dispersion of the extruded samples resulted in higher moduli in comparison to unextruded samples. The largest compressive modulus and yield strength of the extruded samples were found to be approximately 3.2 GPa and 125 MPa, respectively. The compressive moduli of the composites produced in this thesis are significantly greater than that of cancellous bone (0.4 GPa), but significantly lower than that of cortical bone (12.8–17.7 GPa). However, maximum yield strengths of the HA/PEI/PHBV and HA/Hep/PHBV composites match to cortical bone (120–180 MPa), which is a noteworthy finding in this thesis. The wet mechanical results of all composites as well as pure PHBV polymer showed a reduction in both moduli and yield strengths when compared to dry state. In addition, after 2 weeks in wet state both moduli and yield strengths of the composites and pure polymer converged to approximately the same values. Finally, the HA/PEI/PHBV composite samples tested by tensile testing showed the highest Young’s modulus and those tested by compression testing possessed the lowest Young’s modulus. This resulted from the difference in periods of time for heating exposure and void contents of the tested samples, which were prepared by different methods. However, toughness values obtained from the samples tested using three-point bending and tensile tests, was not significantly different.
| Identifer | oai:union.ndltd.org:ADTP/286136 |
| Creators | Wadcharawadee Noohom |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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