THERMAL-STABILITY AND COMPRESSIVE STRENGTH OF CALCIUM-DEFICIENT HYDROXYAPATITE POLY[BIS(CARBOXYLATOPHENOXY)PHOSPHAZENE] COMPOSITES

We have examined the thermal stability and compressive strength of a c omposite material comprised of hydroxyapatite (HAp, Ca-10-x(HPO4)(x)(P O4)(6-x)(OH)(2-x)) and the polyphosphazene poly[bis(carboxylatophenoxy )phosphazene]. The HAp is synthesized in the presence of the polyphosp hazene utilizing dicalcium phosphate dihydrate (DCPD), CaHPO4 . 2H(2)O , and tetracalcium phosphate (TetCP), Ca-4(PO4)(2)O, as the inorganic precursors. Calcium from the inorganic precursors participates in the formation of a polymeric network via ionic cross-linking through the p endent carboxylate groups. The degree of cross-linking of the polyphos phazene and its bonding to the HAp increases the overall thermal stabi lity 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) thermo gravimetric analysis at temperatures between 50 and 1000 degrees C, (2 ) electron impact mass spectrometry up to 550 degrees C, and (3) isoth ermal thermolysis in a closed system. The thermal stability of the pol yphosphazene was increased by sodium salt formation and was increased further by calcium cross-linking and by bonding to the HAp matrix phas e. With heating, the polyphosphazene undergoes both cross-linking, to form a three-dimensional network, and random chain scission of the bac kbone. 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 obta in a composite material with approximately 65% porosity. An increase i n compressive strength, compared to that of HAp, was detected only for the 20-to-1 weight ratio. Further increases in polymer content decrea sed the compressive strength. In general, as the polymer content of th e composite was increased, the composite strength decreased and the st rain increased before failure. Thus the mode of failure changed from t hat of a brittle ceramic to that of a ductile composite.