Midblock sulfonation of a model long-chain poly(p-tert-butylstyrene-b-styrene-b-p-tert-butylstyrene) triblock copolymer

Abstract

Homopolymers containing ionic moieties such as sulfonic acid groups are suitable as fuel-cell, waterpurification and ion-exchange membranes or, alternatively, as electroactive or otherwise stimuliresponsive soft materials. Block ionomers constitute a class of ionomers in which (i) long sequences of repeat units are sufficiently incompatible to microphase-separate and (ii) at least one sequence possesses ionic groups along its backbone. In this study, the midblock of a model poly(p-tert-butylstyrene-bstyrene-b-p-tert-butylstyrene) (TST) triblock copolymer has been selectively sulfonated to different degrees by reaction with acetyl sulfate. Self-assembly of the unaffected endblocks prevents the resultant block ionomers from dissolving in water and imparts water-swollen ionomer films with sufficient mechanical integrity to remain intact. The degree of sulfonation is controlled by varying the concentration of acetyl sulfate, as confirmed by spectroscopic analyses. An increase in ionic content promotes a reduction in thermal stability due to sulfonic acid cleaving, but a significant increase in water sorption that increases with increasing temperature. Although the parent TST copolymer is disordered according to small-angle X-ray scattering, moderate levels of midblock sulfonation induce microphase ordering and simultaneously increase the intermicrodomain distance due to swelling considerations.

Morphological Investigation of Midblock‐Sulfonated Block Ionomers Prepared from Solvents Differing in Polarity

“…[ 33 ] An alternative route by which to produce network-forming amphiphilic block copolymers is through the introduction of a charged middle block. [ 34,35 ] The presence of ionic groups (e.g., sulfonates) in well-defi ned, connected microdomains provides a practical route to multifunctional polymeric materials with high ionic conductivity for use in fuel cells and other ion-exchange applications. [ 36 ] While the technological advantages of ion-containing block copolymers, or block ionomers, are apparent, understanding the phase behavior of this class of materials presents new challenges due to the existence of highly repulsive forces between ionic and nonionic groups in adjacent blocks (and a correspondingly high interblock incompatibility).…”

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.…”

“…The purified products were lyophilized to remove moisture. For consistency with our previous study, the sulfonated TST copolymers are designated as TsST x , where x denotes the degree of midblock sulfonation (DOS, in mol %). Films for analysis were prepared by first dissolving each ionomer at a concentration of 2 wt % in THF.…”

“…Details regarding the midblock‐selective sulfonation of the TST copolymer have been previously reported and are only briefly reproduced here for that reason. The parent (unsulfonated) TST copolymer was dissolved at 5 wt % in an 18:1 v:v mixture of DCE and AA.…”

“…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. The sulfonated TST copolymers generated here are analyzed for comparison with our earlier report and subjected to solvent‐vapor annealing (SVA) to permit morphological refinement …”