Polyoxymethylene-copolymer based composites with PEG-grafted hydroxyapatite with improved thermal stability

In this work hydroxyapatite (HAp) was chemically modified by poly(ethylene glycol) (PEG) in a grafting reaction and then incorporated into polyoxymethylene (POM) matrix using melt processing methods. The obtained POM/HAp-g-PEG composites for biomedical applications were investigated using FTIR, DSC, TOPEM DSC, WAXD and TG. Mechanical properties and formaldehyde release during incubation were studied, too. It was found that introduction of HAp-g-PEG to POM matrix has a little influence on the melting temperature as well as on degree of crystallinity. The highest degree of crystallinity and the lowest supercooling were found for POM/0.5% HAp-g-PEG system which suggest that HAp-g-PEG can act as a nucleating agent for POM crystallization process. Importantly, in the contrast to unmodified HAp, HAp grafted with PEG significantly increases the POM thermal stability-from 296 °C for POM to 326 °C for POM modified with 10% of HAp-g-PEG. Much smaller formaldehyde release has been recorded for composites with high HAp-g-PEG in comparison to POM containing unmodified HAp, lower even that for pristine POM, which is of crucial importance for biomedical applications. HAp-g-PEG hinders POM thermal decomposition during processing and enables to introduce high amounts of HAp without losing composite properties.