Mechanical Properties of Poly(styrene-isobutylene-styrene) Membranes As a Function of Sulfonation Level and Counter-Ion Substitution

Mechanical Properties of Poly(styrene-isobutylene-styrene) Membranes as a Function of Sulfonation Level and Counter-Ion Substitution

Agnes M. Padovani^a,b, David Suleiman^b, and Arnaldo Negrón^b

Department of Engineering Science and Materials^b, University of Puerto Rico

Mayagüez, Puerto Rico, 00681-9000

Department of Chemical Engineering^b, University of Puerto Rico

Mayagüez, Puerto Rico, 00681-9000

In this study, mechanical properties of sulfonated poly(styrene-isobutylene-styrene) (SIBS) were measured using nanoindentation and dynamic mechanical analysis (DMA) as function of sulfonation level (0-97%) and counter-ion substitution (Mg⁺², Ca⁺², Ba⁺²). Nanoindentation results showed that increasing the amount of sulfonic groups did not have a significant effect on the elastic modulus and hardness for the unsubstituted membranes.  However, the elastic modulus showed an optimum level with sulfonation around 64% sulfonation.  The incorporation of counter-ions improved their elastic modulus and hardness, with ion size playing a significant role on their improvement (Ba⁺² > Ca⁺² > Mg⁺²). DMA results showed that sulfonation of the base polymer maintained the glass transition temperature (Tg) of the polyisobutylene (PIB) segment of the membrane at -60°C regardless of sulfonation level.  However, the shoulder of the PIB Tg (-30°C), that was perhaps caused by a Rouse type motion, as well as the Tg of PS (105°C) disappeared upon sulfonation. The substitution of inorganic counter-ions maintained the glass transition temperature of the PIB at -60°C regardless of counter-ion exchanged and increased the modulus of the rubbery plateau indicative of a stronger and more thermally stable cross-linked complex formation. Additional unique relaxations were observed for the sulfonated and counter-ion substituted samples, attributed to the stretching/rotation of the S-O bond and the interaction of the cations with the oxygen of the sulfonate group.