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In this work, the synthesis and characterization of PHEMA-TiO2 nanocomposites starting from three different TiO2 precursors is presented. A simple mixing of the TiO2 particles into the monomers and polymerization of the mixture was not successful in terms of homogeneous distribution of the nanoparticles. The agglomeration of nanoparticles is the major problem to get titanium dioxide into the polymer matrix. Therefore, an in situ synthesis of titanium dioxide in the polymer is considered as an elegant and necessary approach. Embedded titanium complex was transformed into titanium dioxide in boiling water.
All PHEMA-TiO2 nanocomposites are characterized by a refractive index between
1.482 - 1.489 (DSR-λ) and a high transparency in the visual wavelength range, independently of the TiO2 precursor. The lightfastness of TiO2 nanocomposites, without any UV absorbers added, are characterized at blue wool scale 5 which is defined to be good. It shows a slight deterioration to PHEMA but the nanocomposites fulfill the demands for IOL materials. Adding an organic UV absorber leads to an interaction with the complexes which results in a worsened hydrolysis. The reason is that the UV absorber and the titanium central atom are coordinative bonded, which was observed for the 2-hydroxybenzophenone structure. This coordination binding was to be weakened by an extension of the conjugated π-system of the UV absorber structure. It results a UV absorber with an integrated blue filter, but this did not show the desired effect. Furthermore, it was observed that an octahedral coordination of the TiO2 precursor is necessary, because the titanium complex with ethanol ligands led to side reactions at polymerization. Long-term experiments observed the release of TiO2-nanoparticles under physiological conditions. The transition to a metal coordination sphere with a polymerizable ligand improved the TiO2-nanoparticles fixation in the polymer network. After hydrolysis of Ti(acac)2(OiPr)2 the chelating agent acetylacetone is not chemically bound to the polymer backbone and can be extracted with water. The chelating agent 2-(methacryloyloxy)-ethyl acetoacetate in Ti(aaema)2(OiPr)2 is covalently bonded to the polymer backbone and cannot be extracted which results in a better fixation of TiO2-particles. A possible explanation for the particle leakage is the "hopping diffusion". The nanoparticles are enclosed in a cage of polymer chains, whereby the polymer chains are still agile and can slide over the particles. That’s the reason why the particles can move into the next polymer cage. The release of the particles was determined by refractive index measurement of the nanocomposites. It was found that the refractive index of the composites were significantly reduced after 6 months. This is due to the fact that the particles release the polymer under physiological conditions. A release of particles in the water were observed via transmission electron microscopy too and was to be expected after the results of the refractometric measurements.