Mechanical and chemical properties of poly(styrene‐isobutylene‐styrene) block copolymers: Effect of sulfonation and counter ion substitution

Abstract

ABSTRACTIn this study, the mechanical and chemical properties of a series of sulfonated poly(styrene‐isobutylene‐styrene) (SIBS) block copolymers were evaluated using a combination of nanoindentation, dynamic mechanical analysis (DMA), elemental analysis (EA), Fourier transform infrared spectroscopy (FTIR), water absorption, and small angle X‐ray scattering studies (SAXS). The materials properties were characterized as a function of the sulfonation percent in the block copolymers, as well as a result of the counter‐ion substitution with Mg2+, Ca2+, and Ba2+. Nanoindentation studies revealed that the elastic modulus (E) and hardness (H) increase with sulfonation up to a certain level, at which point, the effect of water content further hinders any mechanical reinforcement. The incorporation of counter‐ions increases E and H, but the results are dependent upon the size of the counter‐ion. DMA results showed that the polymer maintained the glass transition temperature (Tg) of the polyisobutylene (PIB) segment (−60°C) regardless of the sulfonation level or counter‐ion substituted. However, both the shoulder of the PIB Tg (−30°C), which was probably caused by a Rouse‐type motion, as well as the Tg of polystyrene (105°C) disappeared upon sulfonation. Counter‐ion substitution increased the storage modulus of the rubbery plateau, which is indicative of a stronger and more thermally stable crosslinked complex formation. Additional unique relaxations were observed with the counter‐ions, and could be attributed to the stretching/rotation of the SO bond and the interaction of the cations with the oxygen in the sulfonic group. FTIR results also revealed a unique shifting of the asymmetric SO band when counter‐ions were added. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40344.