Preparation and characterization of polyacrylic acid-hydroxyapatite nanocomposite by microwave-assisted synthesis method.

Polyacrylic acid, polyalkenoic acids in general, form the liquid ionomer phase of glass ionomer cements, which are frequently used in root restorations in dentistry. It is possible to obtain these ionomers with a fast, energy-efficient, high reaction efficiency and a clean method with microwave irradiation method. In this study, polyacrylic acid and its composite with nano HAp have been synthesized by microwave (MW) irradiation. The two-process parameters that were tested are MW intensity and reaction time. The polymerization was carried out at 110 °C up to 40 min and yield over 92% was produced in 30 min. The average molar mass of PAA was found as 11960 Da using a high-resolution mass spectrometer (TOF-MS). On the other hand, hydroxyapatite (HAp) nanoparticles have been prepared via the sol-gel procedure using potassium dihydrogen phosphate and calcium nitrate tetrahydrate as the precursors for phosphorus and calcium, respectively. XRD, EDS analysis revealed that the particles contain calcium deficient hydroxyapatite (Ca(HPO)(PO)(OH) (HAp) crystals with beta-TCP phase. Morphological observation by SEM measurement proved that the crystal particles of the HAp are very regular and granular, and their size is 25-45 nm in the longitudinal section. These particles were used in composite preparation with PAA. The yield of the composite obtained by heating at 500 W, 30 min was found to be 90%. From the FTIR and H-NMR results, it was observed that there was not only a physical but also an electrostatic interaction between HAp and PAA. The thermal behavior of PAA and its composite with nano HAp were determined by the thermogravimetric analyzer (TGA). The results showed that anhydride formation or decarboxylation occurred at a lower temperature, confirming the interaction between PAA and HAp. The polymerization rate is much faster with microwave heating than conventional heating. Microwave irradiation enables rapid energy transfer and high-energy efficiency, hence, a faster reaction rate.