Hybrid Systems through Composites

Examples of known phosphazene polymer blends are those in which phosphazenes with methylamino, trifluoroethoxy, phenoxy, or oligo-ethyleneoxy side groups form blends with poly(vinyl chloride), polystyrene, poly(methyl methacrylate), or polyethylene oxide).97 100 IPNs have been produced from [NP(OCH2CH2OCH2CH2OCH3)2] (MEEP) and poly(methyl methacrylate).101-103 In addition, a special type of IPN has been reported in which a water-soluble polyphosphazene such as MEEP forms an IPN with a silicate or titanate network generated by hydrolysis of tetraethoxysilane or tetraalkoxytitanane.104 These materials are polyphosphazene/ceramic composites, which have been described as suitable materials for the preparation of antistatic layers in the manufacture of photographic film. 

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Membranes can also be used to purify a mixmre and attain composition beyond the azeotropic composition. The pervaporation process features a liquid feed, a liquid retentate, and a vapor permeate. While gas-phase membrane processes are essentially isothermal, the phase change in the pervaporation process produces a temperature decrease as the retentate flows through the unit. Since flux rates decrease with decreasing temperature, the conventional pervaporation unit consists of several membrane modules in series with interstage heating. The vapor permeate must be condensed for recovery and recycle, and refrigeration is usually required. Hybrid systems of distillation columns and pervaporation units are frequently used in situations where distillation alone is impossible or very expensive. An important application is the removal of water from the ethanol-water azeotrope. Chapter 14 will discuss the details of design and control of such processes. 

Fischer-Tropsch synthesis catalysts when applied in the hydrogenation of C02 perform rather poorly, yielding only small amounts of C2+ hydrocarbons. The development of new catalysts, therefore, is necessary for the production of higher hydrocarbons through the direct hydrogenation of C02. Effective hybrid or composite catalyst systems may be used for this purpose by combining C02 hydrogenation catalysts with an acidic component. Two basic combinations have been studied. 

Usually the silica/polymer composites are prepared with styrene, MMA, BA, or their copolymers. However, few reports cover experiments with less commonly used polymers such as poly(styrene sulfonic acid) (PSSA), poly(hydroxyethylmethacry-late) (PHFMA), poly(aminoethylmethacrylate) PAEMA [133], polyethylene (PE) [134], or polyamides [135]. Using a miniemulsion of nickel-based catalysts for the polymerization of ethylene, which is dispersed in toluene in the presence of hy-drophobically modified silica particles, PEysilica hybrids could be prepared [134]. The ethylene is introduced into the system by bubbling through the miniemulsion. The hydrophobic moiety of the silica particles interacts with the growing polymer and leads to lentil-shaped or isotropic hybrids. Lentil-shaped particles are composed of semicrystalline PE, whereas the isotropic hybrids are composed of amorphous polymer. The crystallinity of the polymer is determined by the choice of polymerization catalyst. Silica/polyamide hybrid nanoparticles were prepared with 3-aminopropyltriethoxysilane (APS)-modified silica particles [135]. These particles were dispersed in sebacoylchloride and the solution miniemulsifled in an aqueous... 

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