Microstructural investigation of hydroxyapatite-polyelectrolyte composites

Hydroxyapatite-poly-L-aspartic acid (HA-PASP) and hydroxyapatite - polyacrylic acid (HA-PAA) composite crystals have been prepared by direct synthesis in aqueous solution. The polyelectrolytes are quantitatively incorporated into the crystals up to about 8 - 9 wt%, as a function of their concentration in solution. The structural and morphological properties of the crystals vary as a function of polyelectrolyte content. TEM images show that the composite crystals display a greater length/width ratio with respect to the control HA crystals. The broadening of the X-ray diffraction reflections on increasing polyelectrolyte content was investigated using three different methods: (i) the Scherrer method, (ii) the Warren - Averbach approach, and (iii) a whole pattern analysis approach. Both polyelectrolytes induce a greater reduction of the mean crystallite size along a direction perpendicular to the c-axis direction, suggesting a preferential interaction of the polymers with the crystal faces parallel to the c-axis. PASP interaction with HA structure provokes a greater increase of strain in comparison to PAA. The data indicate that anionic polyelectrolytes can be usefully applied to modulate the structural and morphological properties of hydroxyapatite crystals.