## 4 Discussion  
The use of solvents to modify and control porosity of silica aerogels is well documented in the literature [13; 14; 15]. Solvents impact reaction rates during gel formation, capillary stresses during drying, and, ultimately, the structure of final products. Mixtures of polar and non-polar solvents have been used to modify the gelation process and tailor pore structures of silica xerogels [13]. No similar data exists in the literature for polymer aerogels. In search of a rationale for the data collected in Table 1, we related various solvents properties to the aerogel characteristics (SSA, average pore diameter, pore volume). Among those, vapor pressure of solvent correlates best with SSA of the aerogel; see Figure 5. However, vapor pressure alone does not offer a rationale to explain the observed trends. Figure 4: Pore size distributions of PMHS/DVB polymer aerogels synthesized in cyclohexane (**top left**), tetrahydrofuran (**top right**), acetone (**bottom left**), and n-hexane (**bottom right**). Figure 5: SSA plotted as a function of vapor pressure (at 25 \({}^{\circ}\)C) of solvent for the four solvents used in this study. The straight line is given to guide the eye. We became more successful by considering solubility of the PMHS/DVB polymer system in the respective solvent. Solubility is intertwined with the degree of swelling of a polymer network in a solvent. We will use both concepts--solubility and swelling--to provide a rationale for porosity and surface characteristic of the PMHS/DVB polymer aerogels. Solubility of polymers is best described using [PERSON]'s approach to solubility [16; 17]. The three [PERSON] parameters, \(\delta_{d}\), \(\delta_{p}\) and \(\delta_{h}\), provide a semiquantitative measure of nonpolar (\(\delta_{d}\)), polar (\(\delta_{p}\)), and hydrogen-bond (\(\delta_{h}\)) interactions for a system. They span the three-dimensional Hansen Solubility Parameter (HSP) space, and any molecular compound is represented by a point in HSP space. Mutual solubility is quantified by the distance \(R_{A}\) between two compounds (index 1 and 2) in HSP space: \(R_{A}=\sqrt{4{\left({\delta_{d1}-\delta_{d2}}\right)^{2}}+{\left({\delta_{p1}- \delta_{p2}}\right)^{2}}+{\left({\delta_{h1}-\delta_{h2}}\right)^{2}}}\). Essentially, for a specific solute a \"good\" solvent is found in short distance \(R_{A}\), while a \"bad\" solvent has a large \(R_{A}\). The set of \"good\" solvents for a given solute falls within a sphere, termed Hansen sphere, while solvents considered \"bad\" are located outside that sphere. Note that the radius of Hansen spheres of different solutes can be different. Moreover, since the parameters are related to Gibbs energies, Hansen spheres ultimately depend on temperature and pressure. For solvents used in this study, HSPs are listed in Table 2. Unfortunately, no solubility parameters exist of the PMHS/DVB system. We estimate the parameters from small molecule data (e.g., styrene) and data for polydimethylsiloxane (PDMS). Hansen parameters of styrene are \(\delta_{d}=18.6\), \(\delta_{p}=1.0\), \(\delta_{h}=4.1\)[18]. Multiple parameters have been reported for PDMS, for which solubility in organic solvents depends on the degree of cross-linking (molecular weight) and temperature. For a high-weight (long chain) PDMS polymer, Hansen parameters of \(\delta_{d}=17\), \(\delta_{p}=4\), \(\delta_{h}=4\) have been suggested [19]. Combining data for styrene and PDMS, we find that solubility parameters of the PMHS/DVB system likely fall into the range \(\delta_{d}=17\)-\(18.5\), \(\delta_{p}=1\)-\(5\), \(\delta_{h}\approx 4\), with some uncertainty about the exact values. It turns out that even with this uncertainty, the distinction between THF and cyclohexane as solvents for the PMHS/DVB system on one side, and acetone and n-hexane on the other side, are well explained by their respective distance \(R_{A}\) to PMHS/DVB in HSP space.