## 3 Results  
After aging in the digestion vessel the samples appear white. The appearance does not change after supercritical drying (Figure 1). Extracting the solvent in the supercritical dryer causes significant shrinkage of samples. To account for the shrinkage, we measured height and diameter of the (almost) cylindrical samples before and after supercritical drying using a caliper. Bulk densities of samples were determined using measured volume and mass. As a caveat, these measurements carry a substantial error margin. Nevertheless, relative shrinkage and density data aligns with other quantitative data shown in Table 1 and displays two groups of polymer aerogels: samples synthesized in cyclohexane and THF exhibit linear shrinkage of over 30%, while samples synthesized in acetone and n-hexane contract only about 25%. Linear shrinkage correlates with bulk density of the aerogel. Densities of the first group (0.61-0.73 g/cm \({}^{3}\)) are about twice as high than those of the second (0.27-0.38 g/cm\({}^{3}\)). The shrinkage data agrees with previous syntheses carried out using acetone and cyclohexane as solvents [7]. Necessary for the later discussion is the observation that in our experiments the shrinkage occurs inside the COreactor already during the first washing with liquid CO\({}_{2}\). Although we cannot measure the linear dimension of the sample inside the reactor, we get the visual impression that supercritical extraction of CO\({}_{2}\) in the drying process does not cause further significant shrinkage, but that all changes in size happened before. Differences in microstructure of the polymer aerogels are discernable in FE-SEM images of the samples shown in Figure 2. The porous microstructure of polymer aerogels is typically explained by aggregation of small particles [10; 11]. Characteristic particle diameters and pore sizes of the samples appear quite different. In particular, the sample synthesized in THF displays a fine microstructure in comparison to the coarser microstructure of the sample synthesized in n-hexane. Nitrogen isotherms of PMHS/DVB polymer aerogels synthesized in various solvents are shown in Figure 3, and pore size distributions are shown in Figure 4. Results of the porosimetry and surface characterization for the different samples are given in Table 1. We find the highest SSA of 392 m\({}^{2}\)/g for the aerogel synthesized in acetone, and the lowest SSA (120 m\({}^{2}\)/g) for using cyclohexane as solvent. The smallest average pore diameter in a sample is 4.2 nm (THF), and the largest 26.5 nm (cyclohexane). The highest pore volume of an aerogel is 1.99 cm\({}^{3}\)/g using n-hexane as solvent, and the lowest occurs for using THF. Qualitatively, the values and trends are consistent with apparent microstructures displayed in FE-SEM pictures (Figure 2). Samples obtained using cyclohexane or THF as solvent exhibit a denser microstructure and smaller pore sizes, consistent with the higher density. This is complemented by small pore volume, small average pore diameter, and small specific surface area. On the other hand, samples synthesized in acetone and n-hexane display a more open porous microstructure, consistent with a lower measured density. Porosimetry of these samples yield larger pore volume, larger average pore diameter, and high specific surface area. The observed porosities are typical for silica-based aerogels [12]. Pore and surface characteristics fall into the range of data \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Sample**} & **Relative Linear** & **Density** & **SSA** & **Pore Volume** & **Average Pore** \\  & **Shrinkage** & **(g/cm\({}^{3}\))** & **(m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** & **Diameter (nm)** \\ \hline cyclohexane & 38\% & 0.61 & 120 & 0.59 & 14.7 \\ tetrahydrofuran & 31\% & 0.71 & 231 & 0.43 & 6.4 \\ acetone & 25\% & 0.31 & 392 & 1.82 & 18.8 \\ n-hexane & 24\% & 0.27 & 303 & 1.99 & 25.7 \\ \hline \end{tabular}
\end{table}