Polymeric monolithic material

M.R. Buchmeiser, Polymeric monolithic materials Syntheses, properties, functionalization and applications, Polymer, 48(8) 2187-2198, April 2007. 

Figure 13.24 Separation of racemic DNZ-leucine on polymeric monolithic material. Conditions polymerization mixture, chiral monomer 76 8wt%, 2-hydroxyethyl methacrylate 24wt% ethylene dimethacrylate 8wt%, 1-dodecanol 45 wt%, and cyclohexanol 15wt% UV-initiated polymerization for 16 h at room temperature ...

The first example in the Hterature of continuous-flow biocatalysis using SILP catalyst with a SCCO2 stream was published by Lozano et al. [60]. They grafted the ILs over a polymeric monolithic material and then immobiUzed the enzyme Candida antarctica lipase B (CALB) by simple adsorption of an aqueous solution of the enzyme. With this system, they performed the transesterification reaction of vinyl propionate and citroneUol (Figure 18.13). 

Finally, dye-doped microparticles of irregular form can be produced by grinding or ball milling of polymeric monoliths provided that the material is brittle enough to... 

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. 

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... 

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. 

The bulk polymeric format, characterised by highly cross-linked monolithic materials, is still widely used for the preparation of enzyme mimic despite some of its evident drawbacks. This polymerisation method is well known and described in detail in the literature and has often be considered the first choice when developing molecular imprinted catalysts for new reactions. The bulk polymer section is presented in three subsections related to the main topics covered hydrolytic reactions, carbon-carbon bond forming reactions and functional groups interconversion. 

When smaller porous particles are used, the result is a highly efficient packed column with smaller interspatial voids however, this leads to lower permeability of the column, and high backpressures are often generated. To overcome these problems, polymeric monoliths have been developed these are a continuous phase of porous material that can be used without generating the high backpressures observed with fine particles. These systems, popularized by Frechet and Svec, have found application both in separation devices [110] and, more recently, as flow reactors [111-113],... 

Molecularly imprinted polymers with a variety of shapes have also been prepared by polymerizing monoliths in molds. This in situ preparation of MIPs was utilized for filling of capillaries [20], columns [21], and membranes [22, 23]. Each specific particle geometry however needs optimization of the respective polymerization conditions while maintaining the correct conditions for successful imprinting. It would be advantageous to separate these two processes, e.g., to prepare a molecularly imprinted material in one step, which then can be processed in a mold process in a separate step to result the desired shape. 

Kunz and Kirschning developed a chemically functionalized monolithic material which is based on a glass/polymer composite [28,29] (refer to Sect. 3.1). This material is available in different shapes including rods, disks, and Raschig rings. The polymeric phase of this composite was chemically functionalized (e.g., substitution of the benzylic chlorine by trimethylamine or sulfonation). Rod-shaped objects were first embedded in a solvent-resistant and shrinkable PTFE tube. This was followed by encapsulation with a pressure-resistant fiber-reinforced epoxy resin housing with two standard HPLC fittings, which created... 

An alternative option to packed channels is the use of monolithic materials, which may have many of the same benefits as packed beds, including high surface area and easily controlled surface chemistry. However, a distinct advantage of monoliths is the ability to prepare them easily and rapidly via polymerization of liquid precursors within the channels of the microdevice without the need for any retaining structures. Despite the popularity of monolithic capillary columns for separations of a variety of low and high molecular weight compounds in HPLC mode, > their first application in microfluidic chips dates back only to 2005. 

The feasibility of synthesizing monolithic material in PDMS, glass, and polymeric substrates has been shown. However, PDMS is not so suited for the analysis of nonpolar analytes. The analytes are absorbed onto and into the PDMS, which results in poor recovery and low enrichment. Without surface modification, PDMS is only suited for polar samples such as genomic DNA. In Figure 50.27, a SEM image of monolithic material inside a Zeonor polymer microchannel is shown... 

The extraction efficiency for nucleic acids using monolithic silica material in microfluidic devices is not nearly as good as what can be achieved using silica beads. However, with monolithic material, better repeatabilities are obtained. For these reasons, Karwa et al. and Wolfe et al. combined the two materials. Both groups used a sol-gel to immobihze silica beads in a channel. This approach has already proven its usefulness in CEC. The channel is packed by a two-step process, where the particles are first introduced into the channel and then held in place with a polymeric phase. This packing technique provided excellent effieieney, repeatability, and stabihty. In fact, Wolfe et al. found that the extraction efficiency for a A.DNA sample improved to a value of about 70%, compared to 57 % for silica beads alone, and 20-30% for the sol-gel matrices tested. Due to the presence of the particles in the column, matrix cracking caused by internal pressure differences within the pores of the sol-gel matrix is reduced. 

As glass and quartz exhibit the same surface property as fused-silica capillary, the monolithic materials could be conveniently prepared in a glass- or quartz-based microfluidic device via the same way of monoliths in the capillary. However, glass/quartz devices are rather expensive, and the need for specialized facilities for their fabrication with conventional photolithography technology hinders any rapid modification of the chip architecture. An attractive alternative is using a variety of polymeric materials, such as poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), polycarbonate (PC), and cyclic olefin copolymer (COC), to fabricate microchips for their mechanical and chemical properties, low cost, ease of fabrication, and high flexibility. 

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