Mechanical and Chemical Properties of Poly(styrene-isobutylene- styrene) Block Copolymers: Effect of Sulfonation and Counter Ion Substitution David Suleiman, 1 Agnes M. Padovani, 1,2 Arnaldo A. Negr  on, 1 James M. Sloan, 3 Eugene Napadensky, 3 Dawn M. Crawford 3 1 Chemical Engineering Department, University of Puerto Rico, Mayag€ uez, Puerto Rico 00681–9000 2 Engineering Science and Materials Department, University of Puerto Rico, Mayag€ uez, Puerto Rico 00681–9000 3 U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, Maryland 21005-5069 Correspondence to: D. Suleiman (E - mail: David.Suleiman@upr.edu) ABSTRACT: In 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 materi- als 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 Mg 21 , Ca 21 , and Ba 21 . 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 reinforce- ment. 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 (T g ) of the polyisobutylene (PIB) segment (260  C) regardless of the sulfonation level or counter-ion substituted. However, both the shoulder of the PIB T g (230  C), which was probably caused by a Rouse-type motion, as well as the T g 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 SAO bond and the interaction of the cations with the oxygen in the sulfonic group. FTIR results also revealed a unique shifting of the asymmetric SAO band when counter-ions were added. V C 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40344. KEYWORDS: elastomers; mechanical properties; nanostructured polymers; properties and characterization; structure-property relations Received 26 August 2013; accepted 23 December 2013 DOI: 10.1002/app.40344 INTRODUCTION Proton exchange membranes (PEMs), such as the perfluoro- sulfonate, NafionV R , have unique chemical functional groups and free-volume that allows for the selective transport of some substances, while blocking (or partially blocking) others. PEMs are commonly used in fuel cell applications, such as direct methanol fuel cells (DMFC). 1–3 Some of the major chal- lenges for the development of DMFC are to overcome the methanol cross-over, which limits cell efficiency and lifetime, and also to maintain or improve the mechanical properties, which can be subject to aging or degradation. 1–4 Numerous studies have focused on the development of alternative or nanocomposite PEMs that can perhaps allow for significantly different transport mechanism’s for protons and methanol, while still maintaining and/or enhancing the mechanical prop- erties of the material. 2,3,5–7 Alternative PEM’s are the focus of on-going efforts by this group, one of which involves the chemical modification and structural characterization of an ionomeric polymer, the tri- block copolymer poly(styrene-isobutylene-styrene) (SIBS) formed via molecular self-assembly. 8–12 These studies emphasize the dependence of phase structure and transport properties on intermediate range molecular interactions. Self-assembled mor- phologies occur in block copolymers that are composed of ther- modynamically immiscible constituent blocks. Ordered microstructures that include spheres, cylinders, or lamellae can be observed depending on the chemical composition of the blocks, the sample preparation, interactions between the blocks, V C 2014 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2014, DOI: 10.1002/APP.40344 40344 (1 of 8)