## 4 Discussion
Assuming a simple average of PDMS and styrene, we obtain solubility parameters for the PMHS/DVB polymer system of \(\delta_{d}=17.8\), \(\delta_{p}=2.5\), \(\delta_{h}=4.0\). Using these values, \(R_{A}\) to cyclohexane, THF, acetone, and n-hexane, is 5.0, 5.5, 9.6 and 7.5 respectively, and a clear distinction between cyclohexane and THF on one side, and n-hexane and acetone on the other side emerges. Variations of the parameters for PMHS/DVB within the proposed range do not change the grouping. Consequently, we establish that cyclohexane and THF are better solvents for PMHS/DVB than acetone and n-hexane. For the rest of our discussion, we may simply call them \"good\" and \"bad\" solvents, respectively. However, how does solubility of the polymer impact pore formation in the corresponding aerogel? We emphasize here the importance of swelling of a polymer for pore modification of an aerogel. Swelling of a polymer and its solubility in a solvent are intertwined. Indeed, swelling data is used to determine solubility parameters [20; 21]. There is ample evidence in polymer chemistry that the higher the solubility, the higher is the degree of absorption of the solvent into the polymer and, as a consequence, the higher is the swelling of the polymer. This obviously happens for hydrogels (e.g., used in diapers). Linear swelling of a siloxane polymer (e.g., PDMS) can differ by more than a factor of two depending on the solvent [22]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Solvent** & \(\delta_{T}\) & \(\delta_{d}\) & \(\delta_{p}\) & \(\delta_{h}\) \\ \hline cyclohexane & 16.8 & 16.8 & 0.0 & 0.2 \\ tetrahydrofuran & 19.4 & 16.8 & 5.7 & 8.0 \\ acetone & 19.9 & 15.5 & 10.4 & 7.0 \\ n-hexane & 14.9 & 14.9 & 0.0 & 0.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Hansen parameters (units MPa\({}^{0.5}\)) of solvents used in this study [17]. \(\delta_{T}\) is the total solubility parameter, \(\delta_{T}^{2}=\delta_{d}^{2}+\delta_{p}^{2}+\delta_{h}^{2}\). To provide a rationale for pore formation with swelled polymers, we assume that the size of precipitating microgel particles--their \"radius of gyration\"--depends only on the polymer itself and is independent of the solvent used. Hence, the characteristic size of the microgel particles is the same in all our experiments, no matter whether we use acetone or THF as solvent. Since a good solvent causes a high degree of swelling, the volume content of a \"good\" solvent--alternatively, the better swelling agent--within a gel particle is larger than that of a \"bad\" solvent. If the solvent is removed, the microgel particles shrink, and the higher the degree of swelling, the higher the shrinkage of the particles. The concept of microgel particle formation and shrinkage is illustrated in Figure 6. This model agrees with the data of linear shrinkage in Table 1: the higher shrinkage occurs for the \"good\" solvents, THF and cyclohexane, and lower shrinkage is observed for the \"bad\" solvents. Since the wet gel is build up by coalesced particles, the smallest particles, which originated from the better swelling agent, build up the structure with smallest average pore diameter. This conclusion agrees with the observed average pore diameters in Table 1. Since the PMHS/DVB polymer aerogels are essentially build up by similar units in the same way, and the only difference after solvent extraction is the size of the gel particles, a simple scaling yields that structures with larger shrinkage and smaller pore diameter also have the smaller SSA. This consequence should not be confused with trends according to which smaller particles yield higher SSA. SSA is a quantity specific per unit mass, and not per unit volume. According to the data in Table 1, densities of the polymer aerogels in this study differ by up to a factor of two. If indeed we are looking for the surface area per volume of sample, then we find that THF yields the highest surface area per volume of polymer aerogel.