| Abstrakt: |
We have examined the thermal stability and compressive strength of a composite material comprised of hydroxyapatite (HAp, Ca10-x(HPO4)x(PO4)6-x(OH)2-x) and the polyphosphazene poly[bis(carboxylatophenoxy)phosphazene]. The HAp is synthesized in the presence of the polyphosphazene utilizing dicalcium phosphate dihydrate (DCPD), CaHPO4·2H2O, and tetracalcium phosphate (TetCP), Ca4(PO4)2O, as the inorganic precursors. Calcium from the inorganic precursors participates in the formation of a polymeric network via ionic cross-linking through the pendent carboxylate groups. The degree of cross-linking of the polyphosphazene and its bonding to the HAp increases the overall thermal stability and changes the mode of failure of the final composite material.The thermal behavior of the polyphosphazene in its protonated, sodium salt, and calcium cross-linked forms was examined utilizing (1) thermogravimetric analysis at temperatures between 50 and 1000 °C, (2) electron impact mass spectrometry up to 550 °C, and (3) isothermal thermolysis in a closed system. The thermal stability of the polyphosphazene was increased by sodium salt formation and was increased further by calcium cross-linking and by bonding to the HAp matrix phase. With heating, the polyphosphazene undergoes both cross-linking, to form a three-dimensional network, and random chain scission of the backbone. The compressive strengths of HAp and the composites constituted of (DCPD+TetCP)-to-polyphosphazene weight ratios of 20-to-1, 10-to-1, and 5-to-1 were examined. The reaction conditions were chosen to obtain a composite material with approximately 65% porosity. An increase in compressive strength, compared to that of HAp, was detected only for the 20-to-1 weight ratio. Further increases in polymer content decreased the compressive strength. In general, as the polymer content of the composite was increased, the composite strength decreased and the strain increased before failure. Thus the mode of failure changed from that of a brittle ceramic to that of a ductile composite. |
