## 5 Influence of the Solvent (Porogen)
### Solvent Type
#### 5.4.2 Inorganics  
As stated earlier, various inorganic porogens have been studied such as sodium chloride, silica and sodium hydrogen carbonate but with limited application [22, 24, 46]. However, SCD [25, 26, 27, 46] is a particularly interesting inorganic porogen. SCD serves as an alternate type of porogen as compared with conventional solvent porogens that are often challenging to remove from the polymer matrix, polymers that are sensitive to very small changes in the composition of the porogen mixture, or polymeric materials that are highly compressible. Therefore, SCD offers fine control over average pore-sizes and PSD by altering the density of SCD through pressure-temperature conditions. [PERSON] and coworkers [28, 29, 30] are the pioneers of this methodology, as evidenced by their studies on the effect of SCD; where suspension polymerization is carried out in a pressurized reactor containing liquid carbon dioxide. All the other variables are kept constant and only the pressure is varied. The maximum pressure is restricted to the type of reaction vessel in use. The final pressure is variable because (_i_) the thermodynamics of polymerization (_i.e._, exothermic _vs._ endothermic) affect the pressure conditions, (_ii_) thermal equilibrium may not be have been met at the beginning of the reaction, (_iii_) some degree of polymerization may have occurred and (_iv_) the solutions are not ideal which was confirmed by the variation of change in molar volume (mixing of co-monomers and liquid carbon dioxide) as a function of initial pressure conditions. The porogen was removed through  
Figure 5: Plot of the surface area of macroopores (%) _vs._ PPG-1000 co-porogen content (vol %) in the dry state for poly(DVB) resins. (Reprinted (adapted) with permission from [18]. Copyright 2004 American Chemical Society). Figure 6: Plot of surface area (N\({}_{2}\) BET) _vs._ PPG-1000 co-porogen content (vol %) for dry state poly(DVB) resins showing a maximum \(\sim\)6 vol % PPG-1000. (Reprinted (adapted) with permission from [18]. Copyright 2004 American Chemical Society). depressurization of the reaction mixture because of its unique P-T behaviour. _In situ_ supercritical fluid extraction can be used to remove residual monomers where the relative surface area and micropore structure varies with changes in the final pressure, as shown in Figures 7 and 8 [30]. Figure 7 shows that the total BET and micropore surface area decreases at pressure conditions of 2200-2600 psi and increases between 2600-4000 psi. The same trend is observed in Figure 8 for the percentage of micropore surface area relative to the total surface area. The results indicate that phase separation of SCD from the resin occurs at early stages for conditions between 2200-2600 psi. The opposite case is true between 2600-4000 psi and is supported by the results for conventional solvent porogens, as outlined in Sections 5.1 to 5.3. According to the authors, the solvation strength of carbon dioxide increases due to an increase in pressure \(\sim\)2600-4000 psi, due to changes in its density. Moreover, the overall solvent properties of SCD change with an increase in pressure similar to the change in conventional solvent properties, as described in Section 5.1, and parallels the change in the properties of the resins. The exclusive relationship of an increasing SP of SCD with an increase in pressure is limited in its interpretive value, as indicated by [PERSON] and [PERSON] [47]. The relationship does not account for the changes in surface area with an increase in pressure conditions. The surface area will increase or decrease in one direction with an increase in SP. This raises some questions about the potential effect of solvent polarity of SCD with changes in its properties on the mechanism of pore formation with resins [47].