# Effect of the Sulfonation on the Swollen State Morphology of Styrene Cross-Linked Polymers
## 4 Conclusions  
The sulfonation of a 2% cross-linked, gel-type polystyrene-divinylbenzene resin with concentrated sulfuric acid, oleum and chlorosulfonic acid produces ion-exchange resins (_Gel_oleum_, _Gel_H\({}_{2}\)SO\({}_{4}\)_, _Gel_HCISO\({}_{3}\)_) with different properties. The conditions employed allow to achieve full sulfonation of the resin and, with oleum even oversulfonation. Hence _Gel_oleum_ has a PEC slightly higher than the ceiling value for a fully mono-sulfonic resin and, of course, higher than the PECs of _Gel_H\({}_{2}\)SO\({}_{4}\)_ and _Gel_HCISO\({}_{3}\)_. In view of its relatively high PEC, _Gel_oleum_ is expected to be the most hydrophilic material hence to have the largest swelling ability in aqueous environment. In spite of this its swollen gel volume, as measured by ISEC, is somewhat lower than the swollen gel volume of _Gel_H\({}_{2}\)SO\({}_{4}\)_. The detailed morphological ISEC analysis also shows that in _Gel_oleum_ ca. 73% of the gel volume is represented by the relatively most dense fraction (0.4 nm\(\cdot\)nm\({}^{-3}\)), while it is only 44% in _Gel_H\({}_{2}\)SO\({}_{4}\)_. This counter-intuitive result can be attributed to the presence of sulfone bridges in _Gel_oleum_. From the mass balance of sulfur, it can be estimated that they connect ca. 30% of the aromatic rings of _Gel_oleum_ while there are none in _Gel_H\({}_{2}\)SO\({}_{4}\)_. This implies that the actual cross-linking degree of _Gel_oleum_ is higher (and hence its swelling ability is lower) than that of _Gel_H\({}_{2}\)SO\({}_{4}\)_. These conclusions are also supported by CP-MAS \({}^{13}\)C-NMR spectroscopy. _Gel_HCISO\({}_{3}\)_ has the lowest swelling ability in aqueous  
Figure 9: Chemical structures of the samples, expected on the basis of comparison between elemental analysis and ion exchange capacities. The aromatic carbon atoms with similar chemical shifts are labeled with the same letters. The R group is either the \(-\)OH moiety (in the case of sulfonic group) or an aromatic ring (in the case of sulfone bridge).  
environment and the highest proportion of sulfone bridges relative to both the number of aromatic rings and the total sulfur amount. However, it also has the lowest PEC (hence hydrophilicity), which is mostly the consequence of the aptitude of HClSO\({}_{3}\) to give a high proportion of sulfone bridges and its inability to give oversulfonation (the contribution of the incomplete hydrolysis of the intermediate \(-\)SO\({}_{2}\)Cl groups to \(-\)SO\({}_{3}\)H is almost negligible). The reduced swelling ability of \(Gel\_HClSO_{3}\) could therefore arise from one or both these causes. Further investigation is needed to find which one plays the most important role.  
The following are available online at [[http://www.mdpi.com/2073-4360/12/3/600/s1](http://www.mdpi.com/2073-4360/12/3/600/s1)]([http://www.mdpi.com/2073-4360/12/3/600/s1](http://www.mdpi.com/2073-4360/12/3/600/s1)).  
Conceptualization, P.C. and M.Z. methodology, P.C., M.Z. and F.R. investigation, C.D.V. and F.R. writing-original draft preparation, P.C. writing-review and editing, C.P., M.Z., F.R. and C.T. supervision, C.P. funding acquisition, P.C. and C.T. All authors have read and agreed to the published version of the manuscript.  
This research was funded by University of Padova, PRAT project CPDA123471.  
We are grateful to [PERSON] for inspiring discussions on the swollen state morphology of ion-exchange materials.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.