# Effect of the Sulfonation on the Swollen State Morphology of Styrene Cross-Linked Polymers
## 3 Results and Discussion
### Morphological Characterization in the Swollen State  
The swollen state morphology of cross-linked polymers can be conveniently investigated by means of ISEC [7; 22; 23]. This kind of materials can have specific surface areas and pore volumes much larger in the swollen state than in the dry state. Gel-type resins, for instance, are essentially non-porous and have specific surface areas of a few square-meters per gram only when dry. However, they can swell extensively in proper liquids. The intrusion of the molecules of the swelling agent into the polymer framework separates the polymer chains from one another. The framework expands and the space between the separated chains is filled by the molecules of the swelling agent. This process produces a gel in which the cavities, created by the separation of the chains, have characteristic size comparable to those of micropores and small mesopores. The specific gel volume and surface area can amount to a few cubic centimetres per gram and several hundreds of square meters per gram, respectively. Under these conditions, the interior of the polymer framework can be accessed from the liquid phase in contact with it and exploited for chemical purposes. In this case, the resin morphology is best described with the Ogston model, which depicts the polymer chains as randomly oriented rigid cylindrical rods, with high aspect ratios, and the pores as void spaces among them [24]. In this model the characteristic quantity is the polymer chain concentration, the sum of the lengths of the rods which are contained in a unit volume of gel and which is generally expressed in nm\(\cdot\)nm\({}^{-3}\). Real swollen materials are generally described as a number of discrete gel fractions, each with its volume and characteristic polymer chain concentration. The result of ISEC analysis based on the Ogston model is therefore a volume distribution of different values of polymer chain concentration. As an alternative the cylindrical pore model could also be used, where the pores are depicted as cylinders with a specific pore diameter. This provides a much more direct estimation of the sizes of the cavities in the expanded polymer framework, although it is clear that nothing similar to pores in rigid materials can hardly exist in this case and that the Ogston model is closer to the real situation. The general relationship between the two models is that higher polymer chain concentrations correspond to smaller \"pore diameters\" and vice versa. The ISEC characterization of _Gel_H\({}_{2}SO_{4}\)_, _Gel_oleum_ and _Gel_HCl\({}_{3}\)_ provides important information on the effects of the sulfonation method on the nanometer scale morphology of the swollen gels. They are presented herein using the Ogston model, hence each material is divided in a set of six discrete domains with polymer chain concentrations ranging from 0.1 to 2 nm nm\({}^{-3}\), which approximately correspond to pore diameters from 6 to 1 nm. The results of ISEC characterization are summarized in Table 2 and illustrated in Figure 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Polymer Chain Concentration**} & \multicolumn{2}{c}{**Polymer Chain Concentration (nm nm\({}^{-3}\))**} \\ \cline{2-4}  & _Gel\_H\({}_{2}\)SO\({}_{4}\)_ & _Gel\_oleum_ & _Gel\_HClSO\({}_{3}\)_ \\ \hline
0.1 & 0.44 & 0.10 & 0.27 \\
0.2 & 1.30 & 0.68 & 0.37 \\
0.4 & 1.41 & 2.13 & 1.92 \\
0.8 & 0.00 & 0.00 & 0.00 \\
1.5 & 0.00 & 0.00 & 0.00 \\
2.0 & 0.00 & 0.00 & 0.00 \\ \hline
**Total gel volume (mL g\({}^{-1}\))** & **3.15** & **2.91** & **2.56** \\ \hline \hline \end{tabular}
\end{table} 
Table 2: ISEC characterization of _Gel_H\({}_{2}SO_{4}\)_, _Gel_oleum_, _Gel_HClSO\({}_{3}\)_. Due to the hydrophilic nature of the sulfonic PS-DVB resins, an aqueous solution of sodium sulfate (0.2 M) was used as the mobile phase in the ISEC runs. The presence of the electrolyte is necessary to suppress enthalpic contributions to the partition of the standard solutes between the stationary and mobile phases.