Sulfonated polystyrene-co- divinylbenzene cross-linked cation exchange resin: a potential membrane material in electrochemical applications

Polymeric materials have always attracted high scientific attention due to their outstanding performance and high potential applications in variety fields such as ion exchange membranes, acid catalysis and separation processes [۲]. Cross- linker polymers (CP) are a class of microporous polymers that are prepared via extensive crosslinking of linear or lightly cross-linked precursor polymers. They can be usually obtained by a two- step or one- pot process, starting from several monomers [۱]. The most drastic features of mentioned network polymers are their extremely high surface area and porosity, outstanding chemical stability and their desire rigidity. Polystyrene- divinylbenzene (PS-DVB) based sulfonic resins is an appropriate candidate among so-called polymers especially under the highly oxidizing environment [۶].In particular, several synthetic routs have been explored to improve the electrochemical properties of these rigid polymeric resins [۵,۷]. It has been experimentally supported to shifting the resin morphology from a compact heterogeneous to a highly swollen semi-gelled phase. Thus, it is preferred to incorporate the PS-DVB resins in the presence of a suitable polar organic solvent to swell the polymer framework and to generate the extensive nanometer size porosity with a high specific surface area of hundreds per gram of the polymer [۳,۴]. Furthermore, this facilitate the locally aligned ion-conducting channels or functional groups in the polymer complex framework and affects the electrochemical performance that have been elucidated in terms of ion exchange capacity (IEC), transport number, etc. Herein, sulfonated cation exchange PS-DVB resins have been prepared andsome electrochemical properties especially in terms of sulfur distribution were evaluated. In all sulfonating process,styrene-co- divinylbenzene polymer with a nominal DVB content of ۲% mole, sulfuric acid (۹۵- ۹۸) % and chlorosulfonic acid were purchased from Sigma- Aldrich and used in the experiments. For both agents, Ca. ۲ g of polymer was swelled overnight in the least volume of ۱,۲- dichloroethane (DCE) as swelling solvent of ۲۵ mL. All procedures were performed in a jacketed glass reactor equipped with a back condenser. In case of H۲SO۴, the sulfonating process was conducted in the acid: solvent of ۵:۱ v/v and ۸۰°C for ۸ h and in case of HClO۳, the experiments were implemented in two acid concentrations of ۴۰% and ۶۰% at ۴۲°C and ۶۰°C for ۸ h. In all experiments, the acid added dropwise to the swollen polymer and finally, the reaction mixture was diluted with five portions of acid with a progressively lower concentration. Then, the resins removed via filtration, washing until neutral pH and dried at ۱۱۰°C overnight. For chlorosulfonic acid, an eventual AgNO۳ test was required for chloride ion removal (alkaline hydrolysis). The direct sulfonation process of PS-DVB resins can be conducted with either sulfuric acid or chlorosulfonic acid agents and subsequent alkaline hydrolysis. The sulfonation process of the mentioned resins illustrate not only the sulfonic groups but also sulfone bridges per phenyl rings. Regarding to the approximate formula weight of PS-DVB of C۲H۳-C۶H۵.SO۳, the maximum IEC amount is ۵.۴ mmol. g-۱ corresponding to the full functionalization of all aromatic rings with sulfonic acid. The IEC amounts reveal that SPS-DVB-H۲SO۴ was rather quantitative for this polymer. Nevertheless, sulfonation was not effective in HCLO۳ sample. The difficulty of sulfonation with chlorosulfonic acid is due to the diffusion of sulfonating agent into the onion-liked porous PS-DVB polymer that H۲SO۴ can penetrate feasibly in the shell of the polymer due to its swelling characteristic. Also, as it is evident in Table.۱, some sulfone bridges probably formed and consequently mitigate the effective degree of sulfonation and indeed, the additional sulfone- bridge cross-links appear in the swollen state of ion exchange resins with different sulfonating agents. Also, the comparison of the IEC amount with elemental analysis of C and S can give detailed information of different types of sulfo- groups in the polymer framework containing sulfonic (- SO۲OH), sulfone bridge (-SO۲-) and chlorosulfonic (-SO۲Cl). In the following table, x presents the ratio percentage of C and S in the resin that imply the average number of sulfo- groups per phenyl ring.Furthermore, the elemental data denoted that the formation of more sulfone bridges in HCLO۳ samples that consistence with ۷۸% and ۸۶% of the phenyl rings bearing only one sulfonic group. This despite the fact that in H۲SO۴, ۹۸% of the phenyl rings were functionalized and the amount of sulfo groups in the form of SO۲OH was drastically in comparison with sulfone bridges. The above mentioned results confirm that HCLO۳ is a weaker sulfonating agent for the cross- linked polymer.

کلیدواژه ها:

polystyrene-co- divinylbenzene ، sulfonation ، sulfo-group ، sulfone bridge ،