Silica with polymers

Figure 18. Method of coating ultratransparent light guide fibers of silica with polymer films for strength and stability.

The data in Figure 185 allow a comparison of the MW distributions of polymers made from the 250 and 400 °C silicas, with polymer produced by chromium oxide on silica, as shown in Figure 19 or 24, for example. They are quite similar. This comparison supports the argument that both types of catalyst contain the same, or at least similar, active species. In both cases, this may be interpreted as the di-attached species shown in Scheme 38. 

YMC-Pack CM Silica with polymer shell 6,5 Carboxymethyl 2.6 mg (BSA)... 

Fig. 3-1. Separation of racemic 3,5-dinitrobenzamido leucine Al.A -diallylamide on silica and polymer-based chiral stationary phases. Conditions column size 150 x 4.6 mm i.d. mobile phase 20 % hexane in dichloromethane flowrate 1 mL min injection 7 pg. Peaks shown are l,3,5-tri-rert.-butylbenzene (1), R-enantiomer (2) 5-enantiomer (2 ). (Reprinted with permission from ref. [8]. Copyright 1997 American Chemical Society.)...

The drawback of the described adsorbents is the leakage of the bonded phase that may occur after the change of eluent or temperature of operation when the equilibrium of the polymer adsorption is disturbed. In order to prepare a more stable support Dulout et al. [31] introduced the treatment of porous silica with PEO, poly-lV-vinylpyrrolidone or polyvinylalcohol solution followed by a second treatment with an aqueous solution of a protein whose molecular weight was lower than that of the proteins to be separated. Possibly, displacement of the weakly adsorbed coils by the stronger interacting proteins produce an additional shrouding of the polymer-coated supports. After the weakly adsorbed portion was replaced, the stability of the mixed adsorption layer was higher. 

Large yields of polymer seem to be obtained only when polymerization proceeds on the outer catalyst surface, because the transport of high molecular polyethylene from catalyst pores is impossible (112). The working part of the specific surface of the catalyst can be expected to increase with diminishing strength of links between catalyst particles (112). Therefore, to obtain a highly active catalyst a support with large pore volume should be used (e.g. silica with pore volume >1.5 cm8/g). 

Perhaps the most important waterside problems relate to the likelihood of boiler surface deposits and their control. High concentrations of caustic or salines only occur if porous deposits are present. It is much better to remove the cause of deposition problems than to try to manage their effects, and modem iron and silica transport polymers, together with improved cleaning protocols, have done much to limit deposition in large boilers. 

The obtained gels were purified by Soxhlet extraction with chloroform to remove the unreacted polyoxazoline. Table 6 summarizes the results of the preparation of polymer hybrids together with their water adsorptions. In comparison with the silica gel without polyoxazoline segments, the modified silica with 50% polyoxazoline was found to show higher water adsorption. 

The Wittig reaction has been carried out with polymer-supported ylids (see p. 509). It has also been done on silica gel. " ... 

Apart from styrene oligomers [514], it appears that OPLC analysis of polymer additives has not been reported. However, the technique has been used for analysis of food antioxidants (BHA, BHT, NDGA and propyl, octyl, and dodecyl gallate) on silica with five different solvent mixtures and densitometric detection [479],... 

Well-dispersed silica and polymer/Co(salen) segments at a molecular level were obtained. Nondestructive immobilization of Co(salen) complexes within silica aero- and xerogels was also achieved with the sol-gel method using silylether-appended salen.1227... 

If the hydrolyses in organosilicate-polymer systems are carried out with increased amounts of the silicate, bicontinuous phases can be obtained (with the silica and polymer phases interpenetrating one another) [213]. At still-higher concentrations of the silicate, the silica generated becomes the continuous phase, with the polymer dispersed in it. The result is a polymer-modified ceramic, variously called an "ORMOCER" [214,215], "CERAMER" [216,217], or "POLY-CERAM" [218,219]. It is obviously of considerable importance to determine how the elastomeric phase modifies the ceramic in which it is dispersed. 

It should be pointed out that the addition of substances, which could improve the biocompatibility of sol-gel processing and the functional characteristics of the silica matrix, is practiced rather widely. Polyethylene glycol) is one of such additives [110— 113]. Enzyme stabilization was favored by formation of polyelectrolyte complexes with polymers. For example, an increase in the lactate oxidase and glycolate oxidase activity and lifetime took place when they were combined with poly(N-vinylimida-zole) and poly(ethyleneimine), respectively, prior to their immobilization [87,114]. To improve the functional efficiency of entrapped horseradish peroxidase, a graft copolymer of polyvinylimidazole and polyvinylpyridine was added [115,116]. As shown in Refs. [117,118], the denaturation of calcium-binding proteins, cod III parvalbumin and oncomodulin, in the course of sol-gel processing could be decreased by complexation with calcium cations. 

In common with some other authors (18-20), Napper removed excess stabilizer from the dispersion medium so as to give the dispersed particles full surface coverage, leaving negligible amounts of free polymer in solution. As the solvency was worsened, no more polymer could be adsorbed, so that critical flocculation conditions do not necessarily correspond to surface saturation. In the present work, which may refer more closely with some practical applications, the stabilizer is kept at the plateau adsorption level but at the expense of complicating the system by the presence of free polymer. Clarke and Vincent (21) have reported on the effect of free polystyrene on the stability of silica with terminally-attached sytrene chains, but the very considerable differences to our studies make an assessment of the possible role played by unadsorbed polymer unproductive. 

The pH value and the electrolyte concentration show decisive influences on the flocculation and the sedimentation behaviour of precipitated silica. Precipitated silica could not be flocculated at high pH values without the addition of an electrolyte. At pH> 3.5 no change in the state of the suspension was observed, e.g. the sedimentogramms of the suspensions with polymer addition up to saturation (0 1) have the same shapes as that of the sedi-mentogram of the polymer free suspension. At pH 2.3 and even at a very low polymer concentration (c(40,000/PEG) > 3 10-5 mg/ml,... 

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