# Methacrylate Polymer Monoliths for Separation Applications
## 3 Applications of Methacrylate Polymer Monoliths in Separation Science
### Challenges for the Development of Methacrylate Monoliths
#### 3.3.3 Physical Architecture Variations  
The effect of monomer and crosslinker concentration on the phase separation and resulting physical architecture, including porosity of the monolith, are well documented [161; 164]. Increased  
Figure 15: **(a)** Separation of peptide mixture on acrylate monolith. Peptides are (1) papain inhibitor; (2) proctolin; (3) Opioid peptide (R-casein fragment 90–95); (4) Ileangiotensin III; (5) angiotensin III; and (6) GGG; **(b)** Separation of amino acid mixture on same monolith. Image adapted with permission from [159]. Copyright (2002), American Chemical Society. temperature generally leads to a reduction in the average pore diameter, due to increased polymer solvation, as well as lower void volumes due to an increased number of nuclei, due in turn to higher polymerization rates at increased temperature. Macropores generally allow for better fluid flow, however they are only a small percentage of the overall porosity. Mesopores contribute to the bulk of the porosity observed and may also form connecting channels between macropores. However, there are conflicting views on whether mesopores actively contribute to a more efficient separation or whether it is surface area and therefore the number of interaction sites, which is the dominant factor. It has been noted that flow near the walls can also play a role in the separation efficiency due to the degree of bonding of the monolith to the capillary walls and the potential for variations in porosity level. Therefore, while a particular morphology may appear to be well oriented and particularly uniform, the true measure of its chromatographic suitability can only be assessed from the efficiency of the resulting separation, in terms of theoretical plate count or the corresponding van Deemter plot. The size of the polymer globules formed can affect the porosity, and therefore the flow characteristics. If this increases dispersion, then peak broadening will occur and resolution will be diminished. It can be stated that while the chemical functionalization dictates the interaction and retention of analytes on the monolith and the elution order, the physical architecture of the monolith is also hugely important as this affects the flow dynamics, dispersion, peak shape and retention. In order to understand whether any stationary phase is suitable for a desired separation application, the flow dynamics and mass transfer in the media should be well understood. The effect of the architecture on any separation is complex and multivariate in nature. Tortuosity, pore size distribution and overall porosity will affect the flow dynamics and mass transport. Tortuosity refers to the path a molecule takes through the porous structure. Smaller molecules tend to be trapped in pores more easily and take a longer time to elute than do larger molecules. The somewhat random polymerization nature of current methods of monolith fabrication means that the architecture and flow through two monoliths, produced using nominally similar conditions, will be different. This will result in differences in the corresponding flow dynamics through these. One of the main differences between polymer monolithic stationary phases is the eddy and random diffusion rates due to the high degree of tortuosity. Considering the van Deemter equation, (Equation (2)), this means that the \"A\" and \"B\" terms are always going to be high which contributes to a higher theoretical plate height. SEM and TEM data, while useful, only show a small part of the overall monolith in most cases and therefore have limited ability to predict the physical structure of the rest of the monolith, and therefore its effect on the separation performed. However, 3D imaging techniques, such as with serial-block face SEM, have demonstrated the potential to capture the monolith physical architecture including the porosity distribution [82; 165].