Midblock-sulfonated triblock ionomers derived from a long-chain poly[styrene-b-butadiene-b-styrene] triblock copolymer

Abstract

Selective sulfonation of a poly[styrene-b-butadiene-b-styrene] (SBS) triblock copolymer has been performed with a sulfur trioxide-dioxane (SO 3 -dioxane) complex to generate midblock-sulfonated triblock ionomers possessing varying degrees of sulfonation (DOS). Products of the sulfonation reaction have been characterized by proton nuclear magnetic resonance and Fourier-transform infrared spectroscopies to ensure sulfonation only involves the B midblocks. Due to this targeted sulfonation, the lower glass transition temperature (T g ) disappears completely in all the ionomers examined, indicating that the formation of ionic aggregates restricts the mobility of the B midblocks. Such aggregates hinder microphase separation and promote a diffuse interface, as established by progressive broadening of the upper T g with increasing DOS. Additional evidence of diffuse interfaces and matrix densification is provided by small-angle X-ray scattering, which reveals a concurrent size reduction in microdomain spacing with increasing DOS. A sulfonation-induced order-order morphological transition from cylinders to lamellae is likewise observed. Due to the retention of microphase-separated glassy microdomains that serve as physical crosslinks, these triblock ionomers are capable of absorbing remarkably high solvent levels and forming highly swollen gel networks in polar media, thereby making them suitable for use in hygiene and healthcare applications, as well as in devices requiring ion transport.

Molecular and morphological characterization of midblock‐sulfonated styrenic triblock copolymers

“…A solution to this challenge is sulfonation of the copolymer midblock, rather than the endblocks. Seminal studies of midblock‐sulfonated copolymers intended for use as surfactant molecules and physical hydrogels pave the way for the present study, wherein we extend our previous investigation regarding midblock sulfonation of a long‐chain poly( p ‐ tert ‐butylstyrene‐ b ‐styrene‐ b ‐ p ‐ tert ‐butylstyrene) (TST) triblock copolymer. To ensure selective sulfonation of the S midblock and avoid undesirable reaction of the T endblocks, we utilize the acetyl sulfate route depicted in Figure instead of the triethylphosphate:SO 3 complex introduced for short TS and TST chains.…”

“…Images were collected on a JEOL 2000FX microscope operated at 200 kV. Positron annihilation lifetime spectroscopy (PALS) was conducted on select specimens with ortho ‐positronium ions generated from a Na source at ambient temperature. Spectra were collected for a minimum of 3.5 million annihilation events.…”

“…Such physical, rather than chemical, crosslinking constitutes the underlying design principle of ABA thermoplastic elastomers, which possess a soft (rubbery) B midblock and hard (glassy or semicrystalline) endblocks. Functionalization of conventional thermoplastic elastomers has extended this paradigm initially developed for nonpolar species by incorporating a charged midblock into a multiblock copolymer.…”

Solvent‐Templated Block Ionomers for Base‐ and Acid‐Gas Separations: Effect of Humidity on Ammonia and Carbon Dioxide Permeation

“…Technological interest in such sulfonated block ionomers (SBIs) ranges from fuel cells and proton‐exchange membranes to water‐desalination membranes, electroactive media, and organic photovoltaics, as well as molecular transport . While morphological development in nonionic block copolymers is well understood and largely controllable, a longstanding challenge of charged block copolymers in general has been kinetic entrapment due to the presence of ionic clusters that serve as additional physical crosslinks and thwart morphological equilibration . Mineart et al have established that bulk SBI morphologies can be templated by judicious choice of casting solvent and brought to equilibrium through the use of solvent‐vapor annealing …”

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

“…However, the amount of water swelling, which increases with sulfonation level, could hinder some of the mechanical properties of ionomers. One approach for influencing this effect involves the selective sulfonation of the mid‐blocks, rather than the end‐blocks, to maintain the mechanical stability of the polymers . Another approach includes the use of organic or inorganic additives to crosslink the micro domains .…”