# Methacrylate Polymer Monoliths for Separation Applications
## 2 Methacrylate Polymer Monoliths
### Structural Properties  
There are a number of factors that will impact the resulting physical structure of the polymer, reaction temperature, monomer ratios, cross-linker ratios, time of polymerization, composition of porogenic solvent solution, as well as the presence and nature of functional groups on the surface. The subject of functionalization shall be discussed in greater length in the next section. The preparation method utilized is also an important factor. As noted in Section 2.1, the preparation method can affect the resulting structure, such as the effect of increased polymerization time on reducing surface area and porosity. Monolithic structure is routinely investigated using techniques such as SEM, Brunauer-Emmett-Teller (BET) Assay, to obtain surface area and porosity values. More recently, the use of focused ion beam (FIB) sectioning of carbon monoliths has been reported by [PERSON] _et al._[80]. The 3D physical structures of carbon monoliths were measured using a serial FIB and SEM procedure in order to obtain monolith porosity level, pore volume and pore size. These section images (100 images at 100 nm spacing) were compiled into a 3D rendering of the porous structure. The calculated macroporosity of the monolith agreed well with the measured porosity from Mercury Intrusion Porosimetry (MIP). These 3D reconstruction methods should become very useful in future characterizing and understanding of how monolithic structures are generated and how they affect the flow dynamics and mass transport within the monolith. [PERSON] _et al._ used a similar approach for the characterization of PEGDA monoliths, [78], the data gathered from the sectioning and 3D reconstruction of the monolith was used to extrapolate values for tortuosity, porosity and homogeneity of the monoliths. This data could then examined to identify which factors affected chromatographic performance of the monoliths. Three-dimensional SEM analysis of the PEGDA structure showed short range heterogeneity, which should have given a higher interchannel eddy diffusion, and, therefore, a reduction in separation efficiency [78]. However, in such analysis the trans-channel and trans-column dispersion mechanisms at larger dimensional scales also need to be taken into account. In the case of the PEGDA structure, the high macro-scale dispersion is expected to have resulted in a lower overall interchannel eddy diffusion value. This would results in a lower value for the \"A\" term in the van Deemter equation (Equation (2)), _i.e._, a lower separation efficiency. \[\mathrm{HETP}\,=\,\mathrm{A}\,+\,\mathrm{B/u}\,+\,\mathrm{C}\times\mathrm{u} \tag{2}\]  
Figure 4: Chemical structures of commonly used cross-linking agents for polymerization reactions. Agents are: (**12**) ethylene glycol dimethacrylate; (**13**) 2-methyl-1, 8-octanediol dimethacrylate; (**14**) methylene bis acrylamide; (**15**) tri methylol propane trimethacrylate. where HETP is the Height Equivalent to a Theoretical Plate, A is the Eddy Diffusion Term, B is the Random Diffusion Term and C is the Mobile Phase induced Mass Transfer within the particle, u is linear velocity in ms\({}^{-1}\). Recently, the first use of Scanning Transmission X-ray Microscopy (STXM) to analyze methacrylate monolith structures was reported [81]. From the STXM data, it was also possible to distinguish the Butyl Metheacrylate (BuMA)-EDMA co-polymer from the di-vinyl benzene nanoparticles. This work represents an interesting characterization tool however it remains a specialized technique at this time. Porosity and surface area are conventionally measured via a number of techniques including Brunauer-Emmett-Teller (BET) and MIP measurements [80]. A more recent technique applied for monolithic characterization is serial block face SEM. This has been utilized as a promising analytical tool for the analysis of polymer monolith internal structures [82]. This latter work showed two different macropore types and an increasing porosity moving from the outer rim of the column to the middle.