Ionic cluster driven polymer network formation: Macroscopic evidence of network reversibility by X-ray microtomography

Abstract

Significant progress has been made in the field of dynamic reversible networks in polymers, especially in unfilled systems. However, achieving similar reversibility in highly filled and mechanically robust elastomers with restricted chain mobility remains a formidable challenge. Furthermore, characterization techniques for assessing network reversibility are not significantly advanced beyond the traditional evaluation of mechanical properties before and after healing. To this end, a dynamic interaction was studied for bromobutyl rubber (BIIR) mixed with alkyl imidazole, which facilitates the formation of reversible ionic clusters within the elastomer matrix. Moreover, a special type of carbon black with graphitic microstructure was used as a filler not only to reinforce the elastomer, but also to form reversible bonds with the alkyl imidazolium ions by cation-π and π-π interactions. Through the synergistic effect of these interactions, the 1-butylimidazole-treated BIIR achieved excellent mechanical properties, including a tensile strength of 14 MPa and an elongation at break exceeding 1000 % with a self-healing efficiency of 71 %. X-ray microtomography studies provided compelling evidence of network reversibility in ionically treated BIIR. 3D reconstructions revealed that approximately 72 % of microcavities disappeared after healing, indicating enhanced material durability. The macro-scale durability was further assessed through tear-fatigue analysis, revealing significantly enhanced resistance to crack growth of the 1-butylimidazole-treated BIIR compared to its sulfur-cured counterpart. The formation of dual dynamic interaction in combination with advanced characterization techniques provides a comprehensive approach for the development of reversible materials and access to their reversibility for real-world scenarios.