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Nanostructured hydroxyapatite (HA) functionalized with poly(ethylene glycol) (HA-g-PEG) of different molar mass was used as a thermal stabilizer to prepare polyoxymethylene (POM) composites by a melt processing method. The chemical and crystalline structure of (HA-g-PEG) and POM/HA-g-PEG composites was investigated by means of FTIR and XRD. The thermal properties, degree of crystallinity, and melting behaviour of POM-based composites were analysed with TG, DSC, and TOPEM DSC methods. The tensile strength, Young’s modulus, toughness, and wettability of POM were investigated as well. A preliminary assessment of bioactivity, in vitro chemical stability, and formaldehyde release from POM/HA-g-PEG composites by Schiff’s test was also performed. An SEM/EDX method was used to observe the morphology of POM/HA-g-PEG composites. The results indicate that the addition of 1% HA-g-PEG slightly increases the melting temperature and degree of crystallinity. In small amounts, HA-g-PEG particles probably act as nucleating agents for the POM crystallization process. Incorporation of 5% HA-g-PEG to POM caused a decrease in the crystallinity of the polymer matrix, as a result, some mechanical properties of POM/HA-g-PEG composites also decreased. The thermal stability of POM/HA-g-PEG composites improved significantly from 309°C for unmodified POM to 342°C for POM/10.0% HA-g-PEG 600. The most effective thermal stabilizer was synthesized with the lowest mass-average molar mass PEG. The in vitro bioactivity test confirmed that, as the average molar mass of PEG in HA-g-PEG hybrids increased, POM-based composites indicated higher bioactivity. The in vitro chemical stability analysis results showed that both the POM matrix and the HA-g-PEG additive remain stable during the whole incubation time. Importantly, after seven days of dynamic incubation, no formaldehyde was detected in all filtrates, which is crucial in biomedical applications, among others.

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In this work, the effect of functionalized hydroxyapatite (HAp-g-PEG) on the thermal degradation behaviour and flammability of polyoxymethylene copolymer (POM_C) was investigated. Organic–inorganic hybrid additive, obtained by grafting of poly(ethylene glycol) on HAp surface using 1,6-hexamethylene diisocyanate as a linker, has been incorporated into POM matrix through an extrusion process. TG analyses show that the incorporation of HAp-g-PEG into POM_C leads to the significant improvement in the thermal stability of the polymer matrix. This is an important finding as POM, being an engineering polymer, suffers from a relatively low thermal stability that limits processing options to develop advanced composite materials. Based on TG results kinetic parameters of the thermal degradation process using Friedman isoconversional method and nonlinear regression method have been calculated; an increase in activation energy of POM degradation process after incorporation of HAp-g-PEG has been found. Thermoanalytical analyses (TG-FTIR/MS) show that incorporation of HAp-g-PEG has no significant effect on the composition of the thermal degradation products. Flammability investigations showed small effect of HAp-g-PEG 600 on the burning time and rate, however, there was a significant reduction in the amount of dripping during the material’s burning process.

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