High Refractive Index Hybrid Polymer Composites

Transparent hybrid polymers created through thiol-ene click chemistry were obtained. Methacrylate and acrylate capped zirconium oxo-clusters (ZOCs) were incorporated into two thiol-ene polymer networks. The vinyl capping groups potentially enable the ZOCs to become an integral part of the thiol-ene network. The ZOCs are reasonably soluble in the thiol and ene monomers yielding a loading of 1 wt. -%. The resulting polymer composites were highly transparent in the visible region and exhibited significant enhancement in the refractive indices with measured values. Moreover, dynamic mechanical analysis (DMA) revealed that all the composites exhibited good mechanical strength and were robust enough to undergo machining and/or polishing without fracture. In a subsequent study, a series of hard transparent thermoset polymers samples made of tetravinylsilane (TVS) and 1.3-benzenedithiol (BDT) with varying loadings of a zirconium oxo-cluster Zr6(OH)4O4(OMc)12 (ZOC) was synthesized. The resulting polymers exhibited an increase in the refractive index (n) when compared to the parent polymer made from TVS and BDT, which refractive index is n = 1.699. The increase in refractive index reached a maximum value of 1.711 at a loading of 3 wt.% and then began to decrease as the concentration of ZOC increases in the polymer matrix. The refractive index of ZOC itself was determined to be between 1.539 and 1.542 when measured by the Becke line test. Notably these values are lower than the TVS-BDT polymer matrix into which it is incorporated. As such, the observation that the addition of small amounts of ZOC can increase the refractive index when incorporated into the TVS-BDT polymer is unexpected. It was found that this behavior was the result of changes in the density of the resultant polymer. Changes in glass transition temperature (Tg) and refractive index of the composites were shown to correlate with the trend in the degree of cross-linking, as monitored by Raman spectroscopy, which occurred to be increased at low loadings then began to decrease as the ZOC concentration got larger. The origin of the observed trend in cross-linking is not fully apparent, however solid state 29Si NMR show evidence for specific interactions between the silicon in the polymer matrix and the ZOC. It is suggested that these interactions happen through coordination of thioether and thiol groups to empty d orbitals in the zirconium which reduces its ability to cross link. Motivated by the discoveries on the aforementioned work, two series of poly (methyl methacrylate) containing increasing loadings of 1,3,5-trithiane-based additives were produced. The refractive index of samples containing the additives were higher than the parent polymer, and the respective densities tended to be slightly lower if, again, compared to the parent polymer. The results seem to indicate that the tested additives contribute to an increase in refractive index of the polymers but its presence in the material compromises its molecular packing, that generates less dense composites.