Sulfonation-Induced Cross-Linking and Nanostructural Evolution of a Thermoplastic Elastomer for Ordered Mesoporous Carbon Synthesis: A Mechanistic Study

Abstract

Direct pyrolysis of self-assembled block copolymers (BCPs) is a resource-efficient method for synthesizing ordered mesoporous carbons (OMCs), through which the resulting pore textures and properties are often collectively determined by the precursor identity and processing pathways. Previous works in this area heavily rely on the use of polyacrylonitrile-based BCP systems, which employ a high-temperature cross-linking reaction that can impact the degree of ordering in their nanostructures. Recently, thermoplastic elastomers have been employed as an emerging OMC precursor, demonstrated by commodity grade polystyrene-block-poly(ethylene-ranbutylene)-block-polystyrene (SEBS). This method requires solid-state, sulfonation-induced cross-linking, involving simultaneous reactions with both the majority poly(ethylene-ran-butylene) phase and the polystyrene segments. This work elucidates the fundamentals of how the reaction mechanism and condition govern SEBS nanostructure development, which deconvolutes distinct contributions from sulfonation and cross-linking. Specifically, small-angle X-ray scattering results, in conjunction with chemical evolution investigations, indicate that polystyrene sulfonation is primarily responsible for increased domain spacing that is mediated through competition between thermodynamically driven nanostructure rearrangement and kinetic trapping from cross-linking. The conversion of cross-linked SEBS, obtained from varying reaction conditions, to OMCs is also studied for establishing critical process−structure relationship. These fundamental understandings provide key insights about rational system design of SEBSderived OMCs, prepared through two steps of sulfonation-induced cross-linking and direct pyrolysis.

Harnessing the power of thermoplastic elastomer-derived ordered mesoporous carbons through functionalization

“…The sulfonation of PS at early stages of the reaction, followed by the sulfonation of the olefinic majority phase, result in a significantly swelled nanostructure. As crosslinking in the majority phase becomes more significant, the matrix begins to densify, and the polymer domains decrease slightly in size, as described in Scheme S1 † 38 . We note that upon crosslinking, the degree of ordering of SEBS may be reduced, while the domain spacing expands from 21.0 to 40.2 nm.…”

“…Recent works have demonstrated a cost-effective strategy for fabricating OMCs using styrenic thermoplastic elastomers (TPE) as precursors, in conjunction with sulfonation-induced crosslinking chemistry. 36–38 Specifically, the sulfonation-induced crosslinking reaction selectively crosslinks the polyolefin majority phase of triblock copolymer TPEs, enabling them to become efficient carbon precursors. Upon pyrolysis, the minority PS phase that does not undergo crosslinking is thermally decomposed while the crosslinked olefinic majority phase is converted to carbon, resulting in the formation of OMC.…”

A general strategy to prepare macro-/mesoporous materials from thermoplastic elastomer blends

“…This change is due to both the swelling of polystyrene and polyolen domains upon reaction. 40 With 1 wt% SIS loading, the domain spacing increases from 31.1 nm to 40.3 nm within the rst hour of sulfonation-based crosslinking. Following 2 h of reaction time, the domain spacing further increases to 42.2 nm; this indicates the majority of the structural rearrangement occurs within the rst hour, which is faster compared to the bulk SEBS.…”