Silica-theta

An illustrative example is the work of Clark et al, on the conformation of poly(vinyl pyrrolidone) (PVP) adsorbed on silica 0). These authors determined bound fractions from magnetic resonance experiments. In one instance they added acetone to an aqueous solution of PVP in order to achieve theta conditions for this polymer. They expected to observe an increase in the bound fraction on the basis of solvency effects as predicted by all modern polymer adsorption theory (2-6), but found exactly the opposite effect. Their explanation was plausible, namely that acetone, with ability to adsorb strongly on silica due to its carbonyl group, would be able to partially displace the polymer by competing for the available surface sites. 

The adsorption of block and random copolymers of styrene and methyl methacrylate on to silica from their solutions in carbon tetrachloride/n-heptane, and the resulting dispersion stability, has been investigated. Theta-conditions for the homopolymers and analogous critical non-solvent volume fractions for random copolymers were determined by cloud-point titration. The adsorption of block copolymers varied steadily with the non-solvent content, whilst that of the random copolymers became progressively more dependent on solvent quality only as theta-conditions and phase separation were approached. 

In the absence of polymer the sediment volume of silica depends on the non-solvent fraction of the medium as shown in Figure 6. The sediment volume assessment of steric stabilization behavior of the copolymers is illustrated in Figures 7a to 7c. At low styrene contents, both the random and block copolymers show a steady increase in sediment volume as the non-solvent content is raised up to the phase separation value. With polystyrene and random copolymers of high styrene content, the sediment volume stays largely constant with alteration in the non-solvent fraction until the theta-point is approached and then continues to become larger as the limit of solubility is reached. In Figure 7b only the data points of RC 86 are shown, RC 94 giving almost identical values. 

For sub-micron silica particles with grafted PDMS (up to 22 K), a different result was obtained (Yates and Johnston, 1999). The particles were unstable and flocculated well above the UCSD of the PDMS-C02 binary system. These results may suggest that it is necessary to raise the density to the UCSD for PDMS at infinite molecular weight (theta density). Another possibility is that the parameters used in the theory and simulation are not applicable to PDMS, since the polymer-polymer interactions are far stronger than the polymer-C02 interaction, unlike the case for PFOA. 

Direct experimental data providing the temperature dependence of are not available in the literature. However, as discussed earlier, the dependence of 0 on the quality of the solvent (change in the values of the polymer-solvent interaction parameter) is expected to suggest the trend with temperature also. The experimental determination of

silica particles having polystyrene as the free polymer, indicated [5] that the amount of polymer required to produce phase separation decreased by a factor of three when the theta solvent cyclohexane (x = 0.5) is replaced by the good solvent toluene (x < 0.5). This implies that increased temperatures (reduced values for x) should lead to lower values of the amounts of polymer required for phase separation. It can be safely concluded that the available experimental and theoretical information thus far, exhibits the trend of smaller values of the limiting polymer concentration at higher temperatures. 

At higher surface coverages, the mean-field treatment noted in Section IV indicates that worse than theta conditions are required before an attractive minimum appears in the potential. This phenomena has been observed and termed enhanced steric stabilization by Napper (1983). Recent experiments by Edwards et al. (1984) clearly show that grafting PDMS chains onto silica particles at nl2a 12 — 18 extends the stable range substantially beyond theta conditions. 

Figure 4 X-ray diffraction patterns contrasting various crystallinities of silica (a) radiolarian silica, Porcelanite (opal-CT) and a-Cristobalite (made by heating silica gel at 1,350 °C for 4 h) from Calvert (1983) (b) diatom assemblage from Antarctic plankton tow, deep-sea siliceous ooze (Holocene in age) from beneath the Antarctic Polar Front, and two chert deposits from state of New York. The sharpness of the silica peak(s) between 20° and 26° two theta increases as silica undergoes diagenetic transformation from a fresh-diatom assemblage to buried sediment for...

One system with these desirable features is a suspension of small a 50 nm) silica particles onto which octadecyl chains have been densely grafted (Stober et al. 1968 van Helden et al. 1981 Woutersen and de Kruif 1991). The octadecyl chains have a theta point near room temperature in various solvents, including benzene, dodecane, and hexadecane. 

F was used for the first time as mineralizer in the synthesis of zeolites under neutral or acidic conditions by Flanigen and Patton1941 Subsequently, Guth and Kessler further studied this new synthetic method.[95] Many high- or all-silica zeolites such as ZSM-5, ZSM-23, Theta-1, ferrierite, / , MTN, AST, UTD-1, ITQ-3, ITQ-4, and heteroatom (B, Al, Fe, Ga, and Ti)-substituted high-silica zeolites could be synthesized from an F synthetic system. In addition, most of the open-framework gallophosphates could be hydrothermally synthesized under the influence of F. ... 

The temperature programmed desorption (t.p.d) of n-hexane from the sodium and hydrogen forms of ZSM-5. ZSM-11 and THETA-1 have been studied. The t.p.d profiles have been analysed by a newly developed method. From these analyses peak temperatures, peak widths, maximum rates of desorption and activation energies of desorption as a function of coverage have been obtained. The saturation capacities of these high silica zeolites for n-hexane have also been determined. The effect of change of cation on all of these quantities is demonstrated. 

Figure 9.5 Example of typical powder x-ray diffractogram measured for an ordered mesoporous silica. The diffraction pattern shows reflections at low 2 theta angles indexed to the cubic laid space group (3D cubic MCM-48 phase). The diffractogram is recorded at low angles with a conventional laboratory instrument.

Figure 5.19. The dependence of the amount adsorbed on the relative molecular mass for polystyrene adsorbing onto silica from theta solvents (o, cyclohexane and A, decalin) and a good solvent (+ and X, carbon tetrachloride). Adapted from Fleer et...

Figure 19-11. Reduced viscosities as a function of the reduced shear stress of colloidal silica suspensions (diameter of 100 nm) in the presence of addedpolymer (polystyrene). The solvent used is decalin which is a near theta solvent for polystyrene. The size ratio of the polymer radius of gyration to the colloid radius (Rg/R) is 0.02S. The colloid volume fraction ((f>) is kept fixed at 0.4. In the absence of added polymer (Cp/c = 0), the particles behave as hard spheres and as more polymer is added to the system, the particles begin to feel an attraction. The colloid-polymer suspensions at (p of 0.4 shear thin between a zero rate viscosity of r o and a high shear rate plateau viscosity r]x,. The shear thinning behavior (in the absence and presence of polymer) is well captured by equation (19-10) with n = 1.4. Note rjo, rjao and cTc are functions of volume fraction and strengths of attraction but weakly dependent on range of attraction (Shah, 2003c Rueb, 1997).

Various synthesis methods of mesoporous carbon based on different mesoporous silicate or aluminosilicate templates have been developed [5,7]. The first report on the synthesis of OMC used mesoporous silica MCM-48 with the bicontinuous cubic Ia3d symmetry as the template the as-prepared OMC was denoted as CMK-1. Thereafter, various OMCs with different pore topologies have been actively investigated. For these OMCs, the imiform mesopores are interconnected, resulting in the appearance of distinct X-ray diffraction lines below 2 theta of 5. Meanwhile, they have a large siu ace area and high pore volume. The other structural parameters such as pore diameters, particle morphologies and sizes, and microstructures of carbon frameworks could be tuned by... 

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