Structural Properties of Polymer Monoliths

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

The monomers commonly used for the preparation of polymer monoliths are either hydrophobic, for example, styrene/divinylbenzene and alkyl methacrylates, or hydrophilic, for example, acrylamides. The polymerization is usually accomplished by radical chain mechanisms with thermal or photochemical initiation, as detailed in the reviews (Eeltink et al., 2004 Svec, 2004a and b). Internal structures of polymer monoliths are described to be corpuscular rather than spongy this means through-pores were found to be interstices of agglomerated globular skeletons as shown in Fig. 7.1 (Ivanov et al., 2003). Porosity is presumably predetermined by the preparation 

FIGURE 7.1 Scanning electron micrographs of a polystyrene-divinylbenzene monolithic column prepared in a 20-pm fused silica capillary tube (reproduced from the reference, Ivanov et al. (2003), with permission from American Chemical Society). 

It is of much interest to compare polymer monoliths with monolithic silica columns for practical purposes of column selection. Methacrylate-based polymer monoliths have been evaluated extensively in comparison with silica monoliths (Moravcova et al., 2004). The methacrylate-based capillary columns were prepared from butyl methacrylate, ethylene dimethacrylate, in a porogenic mixture of water, 1-propanol, and 1,4-butanediol, and compared with commercial silica particulate and monolithic columns (Chromolith Performance). 

A rather limited range of mesopores in terms of size and volume were observed in the skeletons of polymer monoliths. The porosity of the polymer monolith seems to be lower than that of silica monolith. The total porosity of these monoliths is in the range of 0.61-0.73, whereas interstitial (through-pore) porosity and mesopore porosity are 0.28-0.70 and 0.03-0.24, respectively. In the case of poly(butyl methacrylate-co-ethylene dimethacrylate), the observed porosity is around 0.61-0.71, resulting in permeability 0.15-8.43 x 10 14 m2, whereas the observed porosity of silica monoliths prepared in a capillary is 0.86-0.96 and the permeability is 7-120 x 10 14 m2. Higher permeability will be advantageous for 2D applications, as mentioned later. 

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