Monolithic materials

There is a general consensus that power densities of at least 1 kW/kg for heating or 0.5 kW/kg for cooling are achievable using composite or monolithic materials. 

The role of the matrix is to protect the filler from corrosive action of the enviroment and to ensure interactions between the fibers by mechanical, physical and chemical effects. The mechanical properties of fiber composites are dependent on the mutual position of the fibers in the monolithic materials. 

In another facility, bench tops in the main room were covered with epoxy-treated monolithic material to resist the hard wear anticipated. The side room, where instrument work was to be carried out, was equipped with tops of plastic laminate. This resulted in substantial savings. Somehow it was difficult to convince the engineering firm in charge of building the laboratory that monolithic material, which they had recommended throughout, was not necessary everywhere. 

In a case where a fiune hood was to be placed on a portion of counter faced with plastic laminate, the planner found an economical solution. Since the plastic laminate was not suitable for use with a fume hood, he ordered a sheet of monolithic material Va inch thick and had it cut to the exact dimensions of the hood. The cost was just a fraction of that of a full thickness top. The sheet was put in place and the edges were treated with a silicone compound. This treatment stood up against highly corrosive materials as weU as heat for many years. 

In the second area, improvements to the thermal and mechanical stability of nanoporous materials from ordered block copolymers should be targeted. To expand the application base for these materials, high temperature stability is a key requirement. For example, in templating applications that require elevated processing temperatures in either thin films or monolithic materials... 

Horvath et al. sintered the contents of a capillary column packed with 6 pm oc-tadecylsilica by heating to 360 °C in the presence of a sodium bicarbonate solution [101]. These conditions also strip the alkyl ligands from the silica support, thus significantly deteriorating the chromatographic properties. However, the performance was partly recovered after resilanization of the monolithic material with dimethyloctadecylchlorosilane allowing the separation of aromatic hydrocarbons and protected aminoacids with an efficiency of up to 160,000 plates/m. 

In contrast to the above technologies that involve packing beads, the most appealing aspect of the monolithic materials discussed in this section is their ease of preparation in a single step from low molecular weight compounds. In situ created monoliths can be prepared from both silica and organic polymers. 

Since a comprehensive description of all monolithic materials would exceed the scope of this chapter and a number of other monolithic materials are also described elsewhere in this volume, this contribution will be restricted mainly to monoliths for chromatographic purposes and prepared by polymerization of monomer mixtures in non-aqueous solvents. Monolithic capillary columns for CEC are treated in another chapter and will not be presented in detail here. 

Obviously, the monolithic material may serve its purpose only if provided with a suitable surface chemistry, which depends on the desired application. For example, hydrophobic moieties are required for reversed phase chromatography, ionizable groups must be present for separation in the ion-exchange mode, and chiral functionalities are the prerequisite for enantioselective separations. Several methods can be used to prepare monolithic columns with a wide variety of surface chemistries. 

Because the monoliths allow total convection of the mobile phase through their pores, the overall mass transfer is dramatically accelerated compared to conventional porous structures. Based on the morphology and porous properties of the molded monoliths, which allow fast flow of substrate solutions, it can be safely anticipated that they would also provide outstanding supports for immobilization of biocatalysts, thus extending the original concept of monolithic materials to the area of catalysis. 

In contrast, monolithic materials are easily amenable to any format. This has been demonstrated by using short monolithic rods prepared by copolymerization of divinylbenzene and 2-hydroxyethyl methacrylate in the presence of specifically selected porogens [93]. Table 2 compares recoveries of substituted phenols from both the copolymer and poly(divinylbenzene) cartridges and clearly confirms the positive effect of the polar comonomer. 

F. Svec, T. B. Tennikova, Z. Deyl, Eds., Monolithic Materials Preparation, Properties, and Applications, Elsevier, Amsterdam 2003. 

Monolithic Materials Preparation, Properties and Applications, eds. F. Svec, Z. 

Space limitations unfortunately preclude a full discussion of two more recent approaches to permanently immobilized Ru catalysts, see (a) Monolithic Materials New High-Performance Supports for Permanently Immobilized Metathesis Catalysts, M. Mayr, B. Mayr, M.R. Buchmeiser, Angew. Chem. 2001, 113, 3957-3960 Angew. Chem. Int. Ed. 2001,... 

Fig. 6.12 Scanning electron microscopy images of a sol-gel derived column material (A) a rigid rod and (B) a magnification of the bimodal pore structure in the resulting monolithic material [28]. Adapted with permission from the American Chemical Society.

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

Monolithic materials exhibit rednced flow resistance, which results in high permeability and conseqnently high speed of separation. 

The polymerization mixture for the preparation of rigid, macroporous monolithic materials in an unstirred mold generally contains a monovinyl compound (monomer), a divinyl compound (crosslinker), an inert diluent (porogen), as well as an initiator. The mechanism of pore formation of such a mixture has been postulated by Seidl et al. [101], Guyot and Bartholin [102], and Kun and Kunin [103] and can be summarized as in the following text. 

Increasing the amount of cross-linking agent (divinyl compound) at expense of monomer causes a decrease in pore size, which is accompanied by a distinct increase in surface area [101-104]. Even if this has been observed for macroporous beads prepared by suspension polymerization, the results can directly be transferred to the fabrication of rigid monolithic materials in an unstirred mold by thermally [105,106] as well as photochemically [107] initiated free radical copolymerization. 

---