INDEX titania

Fig. 5.34 Refractive index variation of 450°C annealed nanotubular titania film, and for comparison a glass (Corning 2947) substrate, in the range 380 to lOSOnm. The Ti02 film has an average refractive index in the visible range of 1.66.

The oxide titania, Ti02, because of its high refractive index, is used in high-grade white pigments of great opacity and covering power. The... 

Unreacted ester groups are then burned away during the calcining. Owing to its simplicity this method permits commercial silicas to be treated with any amount of titania up to 5-6% Ti, at which point saturation is usually reached.6 The method increases melt index and tends to broaden the molecular weight distribution, which can be useful in some applications such as blow molding. 

Fig. 14. The melt index of the polymer, which reflects the catalyst termination rate, is also promoted by titania, at least at the lower activation temperatures.

This explains the melt index behavior of coprecipitated silica-titania catalysts which is shown in Fig. 16. With each catalyst, the MI rises with increasing calcining temperature until sintering begins, then it drops. The... 

Fig. 16. The relative melt index potential (RMIP) of a series of cogelled Cr/silica titania catalysts rises and then falls with calcining temperature, indicating first dehydroxylation then sintering. However, the more titania in the catalyst, the more easily it sinters and therefore the lower the temperature at which peak RMIP develops.

Fig. 19. The termination rate, plotted here as relative melt index potential (RMIP), reflects the extent of surface dehydroxylation in two series of Cr/silica-titania catalysts, calcined in (Y) air or ( ) CO and then air to reoxidize the chromium, both at the temperatures shown. The third series ( ) shows the additional benefit of low-temperature attachment. It was calcined in CO at the temperatures shown, then air at a lower temperature (760°C).

Many methods have also been demonstrated to process materials other than polystyrene and silica into spherical colloids as monodisperse samples. For example, a new method was recently reported that allowed for the generation of titania spherical colloids with uniform diameters controllable in the range of 200 to 500 nm.66 Titania is a semiconductor and has a much higher refractive index (2.6 for anatase and 2.9 for rutile) when... 

UV light induced refractive index change has been observed in organic polysilane (PS)-silica and PS-titania hybrid thin films prepared by the sol-gel method.135 The magnitude of the refractive index change was found to be 0.16 for 50 wt % PS-silica and 0.18 for 50 wt % PS-titania hybrid thin films. It was demonstrated that the refractive index change of the hybrid thin films is due to the photodecomposition of PS. These findings indicate potential usefulness of the hybrid thin films as optical devices with refractive index modulated structures. 

Fig. 8a-c. Electron microscopy images of hollow titania spheres prepared by calcination of PS spheres coated with seven layer pairs of TALH and PDADMAC at a,b 450°C c 950°C. a and b are hollow anatase (refractive index, n 2.5) titania spheres and c hollow rutile (n 2.6-2.9) macroporous titania spheres. (Adapted from [65], by permission of Wiley-VCH)... 

Titania most often appears in the form of one of two predominant crystallographic polymorphs, rutile and anatase. At least seven other distinct phases have been characterized by x-ray diffraction [3], but none are sufficiently commonplace to play a role in pigmentation using titanias. Rutile possesses a higher refractive index than anatase. It is also inherently considerably less photoactive than anatase. For these reasons, rutile has become the predominant polymorph of titania for pigmentary uses in polymers. 

Similarly, CdS NCs capped with mercaptopro-pyltrimethoxysilane (MPTMS) were added to alcohohc solutions of TEOS or TMOS and subsequently incorporated into sihca matrices. Pb and Hg-based NCs have also been synthesized with MPTMS as the capping agent, and the role of the pendant siloxy groups is again to anchor the NCs to the sihca, silica-titania, or zirconia networks. Zirconia films doped with PbS NCs were prepared with refractive indices up to 1.53 and a third-order susceptibility value of 1.3 X 10 esu. Higher refractive index sihca-titania sol gel films doped with PbS (n = 1.63) were also fabricated. ... 

The photochemical properties of titania surfaces are of interest for several reasons. They determine the stability of pigmented paint systems [76], the rate at which pollutants can be degraded in systems designed to purify air and water [77], and are the root cause of poorly understood phenomena such as water photolysis [78] and super hydrophilicity [79]. Using thin rutile epilayers with five low index orientations, it has been shown that the relative rates of photochemical reactions catalyzed by titania depend on the surface orientation [80]. In this chapter, experiments used to map the complete orientation dependence of the relative photochemical reactivity of TiO are described [81-83]. In this case, the relevant reactions are carried out at room temperature and this gives us the opportunity to fix both the surface morphological structure and stoichiometry. For the reactions described here, all of the surfaces were... 

FIGURE 40 Melt index of polymers made at 105-109 °C with Cr/silica-titania catalyst (870 °C) in the presence of various amounts of butene isomers. 

FIGURE 49 The influence of the mesoporosity of the catalyst on its activity and on the polymer melt index and MW. Cr/silica-titania hydrogel (450 m2g ) was dried by first replacing the pore water with a solvent of varying surface tension to produce catalysts having a variety of pore volumes that were then activated at 800 °C and tested at 105 °C. 

FIGURE 50 Pore volume distribution of Cr/silica-titania catalysts dried by various methods and then activated at 800 °C. The mesoporosity of the catalyst influences its activity and the polymer melt index (tested at 105 °C with 1.5 mol C2H4 L ). 

FIGURE 77 Impact resistance of films as a function of the polymer melt index. The addition of poisons to the reactor affects LCB levels in the polymer. In contrast to 02, CO diminishes elasticity, which in turn results in less orientation in the blown film, and therefore improved impact and tear resistance. (Cr/silica-titania catalyst, activated at 650 °C, polymer density of 0.938 g mL, film thickness 25 pm). 

FIGURE 82 Response to shear stress, shown here as the polymer HLMI/MI ratio, as a function of catalyst activation temperature for polymers made in the slurry process with Cr/silica-titania catalyst. Reaction temperature was varied (102-110 °C) to produce three series of polymers of constant melt index. (Compare with Figure 83.)... 

FIGURE 107 Fluidity (inverse of melt viscosity) of polymers made with cogelled Cr/silica-titania catalysts of varying titania contents. In this plot, the fluidity (similar to melt index) is increased (lower MW) by titania, but titania also promotes sintering at high temperatures. (Ti02 listed in mol%.)... 

FIGURE 123 Melt index potentials of Cr/silica-titania catalysts that were activated in various gases by the R R process. Catalysts were reduced 3 h in the gas shown at the temperature shown, then reoxidized 2 h in air at the same temperature. Reduction in CS2 produced polymers of the highest melt index. 

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