Crystallisation of hydroxyapatite in phosphorylated poly(vinyl alcohol) as a synthetic route to tough mechanical hybrid materials

- Hydroxyapatite was crystallised in phosphorylated poly(vinyl alcohol) gels.
- Crystallite size of hydroxyapatite decreased as phosphate density was increased.
- The hybrid specimens prepared in 10% phosphorylated gel was the toughest.
- Phosphate density in organic matrix regulated the mechanical properties of the hybrid.

Partially phosphorylated poly(vinyl alcohol) was prepared by treating poly(vinyl alcohol) with 100% phosphoric acid, and 5, 10 and 20% of the hydroxyl groups were converted to phosphoric acid ester. Addition of Ca2 + to an aqueous solution of phosphorylated poly(vinyl alcohol) gave a transparent gel. Five cycles of alternate soaking of the gel in aqueous CaCl2 and aqueous (NH4)2HPO4 were carried out to crystallise hydroxyapatite (HAP) in the phosphorylated poly(vinyl alcohol) matrix. The X-ray diffraction peaks of HAP formed in 5% phosphorylated PVA were sharp, while those of HAP formed in 20% phosphorylated PVA were broad. The contents of inorganic phase in the hybrid powder were increased from 58 to 76 wt% as the fraction of phosphate groups in the gel was decreased from 20% to 5%. The hybrid powder was first subjected to uniaxial pressing, followed by cold isostatic pressing (CIP) and warm isostatic pressing (WIP) at 120 °C at pressures of 300-980 MPa, to obtain the specimens for three-point bending test. These hybrid specimens showed bending strengths of 15-53 MPa. The hybrid compacts prepared from 10% phosphorylated poly(vinyl alcohol) showed the smallest Young's modulus, the largest displacement at break, and the largest fracture energy, showing that it has the highest toughness among the hybrid materials prepared from poly(vinyl alcohol) with varying degrees of phosphorylation.

Densification of hydroxyapatite crystallised in 10% phosphorylated poly(vinyl alcohol) gave the toughest compact.136