Tensile properties, water absorption and enzymatic degradation studies of polyethylene/starch filled hydroxyapatite blend for orthopaedic applications

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June, 2016.
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Linear low-density polyethylene (LLDPE)/starch blends filled with hydroxyapatite have been synthesized by injection moulding. The aim was to control the rate of biodegradation of LLDPE/starch blends for bone screw fixation using hydroxyapatite (HA). Hydroxyapatite contents were varied from 1.0% to 3.0% in intervals of 0.5% by parts and the blend phases were characterised using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Biodegradation was studied by performing water absorption and enzymatic tests. Water uptakes by the samples were carried out according to ASTM D570 and enzymatic test was carried out on samples in phosphate buffered saline (PBS) containing α-amylase. Tensile properties of the samples before and after enzymatic degradation were determined using Titan and Testometric’s universal testing machine while the surface changes were determined with Meiji Techno optical microscope. Seven different samples were formed for the study, two of the samples; one composed of LLDPE only and the other of 60% LLDPE, 40% starch and 0% hydroxyapatite, were used as controls. The results obtained show that the incorporation of starch granules into the LLDPE reduces the tensile strength but almost doubles the tensile modulus and this was attributed to starch granules expanding the amorphous tie chain of LLDPE. Addition of hydroxyapatite into the blend gave an increase in the tensile strength. The increase in strength with increasing HA content was statistically significant at a p-value of 0.0008 and the improvement slowed the rate at which the blend degraded. Hydroxyapatite is suspected to have affected the intermediate phase of the LLDPE by the hydroxyl group through hydrogen bonding. The water absorption by the blends showed that as hydroxyapatite content increased, the moisture uptake of the blends increased and enzymatic degradation rate increased, giving rise to high percentage loss in tensile strength and modulus. Conversely there was a high gain in percentage elongation. Optical micrographs of the surfaces of the degraded samples showed surface erosion and agglomerates. The samples that showed higher erosion and more agglomerates had the highest water uptake and highest percentage loss in tensile strength and those with less erosion and fewer agglomerates had less water uptake and less percentage loss in tensile strength.
A thesis submitted to the Department of Physics,Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY (Materials Science).