Synthesis and Characterization of Poly(ether ether ketone) Derivatives Obtained by Carbonyl Reduction

Hydroxylated poly(ether ether ketone) (PEEK-OH) samples, with different degrees of functionalization, have been synthesized by selective carbonyl reduction. The ATR-FTIR and NMR spectra corroborated the reduction success, showing a decrease in the intensity of the peaks related to the ketone group and simultaneously the appearance of new signals associated with the hydroxyl group. TGA curves indicated a reduction in thermal stability with increasing hydroxylation degree (HD), attributed to changes in the spatial organization of the molecules and a less intense resonance effect. DSC experiments showed a progressive diminution in the crystallization and melting temperatures as the number of hydroxyl groups increased due to the restrictions on polymer chain diffusion imposed by the strong hydrogen bonds. Furthermore, the level of crystallinity of the derivatives drastically decreased upon increasing HD, as evidenced from the X-ray diffraction spectra. The random distribution of this functional group inhibits the molecular packing, leading to the formation of smaller and less perfect crystals. DMA studies revealed a decrease in both storage and loss moduli as the extent of modification increased. However, the presence of the hydroxyl group caused a significant improvement in the glass transition temperatures of the polymers, particularly at higher degree of functionalization, where the increment was close to 40 °C. Such enhancement is attributed to a decrease in mobility and flexibility induced by the strong intermolecular interactions between the chains.