Effect of side-chain lengths of imidazolium cation on the properties of carboxyl-functional poly(ionic liquid)-based elastomers

Abstract

The electrostatic interaction between cation/anion pairs of poly(ionic liquids) (PILs) endows great self-healing potential. However, its low bond energy leads to low load rating and weak mechanical strength. Hence, it is a great challenge to balance the mechanical strength and repair performance by designing and regulating the molecular structures of PILs. The configuration of cations and anions in ionic liquid monomers is the decisive factor for the segmental motion of PILs. Thus, we started from the design of monomeric molecules and introduced functional group (-COOH) onto the side chain of the cationic. Herein, a series of carboxyl-functionalized imidazolium ionic liquid monomers (denoted as IL-m) with different side-chain lengths were synthesized via acid–base neutralization and ion exchange reactions. Then, a series of novel and stretchable carboxyl-functional liquid-free PIL-based elastomers based on single component were prepared by photo-initiated polymerization. In this paper, we investigated the relationship between the properties and chemical composition and microstructure of PIL-based elastomers (especially the side chain length) in detail. ATR-FTIR verified the micro-structures and the interactions between the polymer chains, which comfirmed the H-bond and ion–dipole interaction, serving as physical crosslinking points and endowing the elastomers with tunable properties. By optimizing polymerization conditions and side-chain length, tunable ionic conductivity (9.89 × 10^–7 to 2.10 × 10^–5 S cm⁻¹) and self-healing performance (8 h at room temperature or 20 min at 50 °C for PIL-10) were achieved. Increasing the imidazolium side-chain length from 1 to 9, the stretchability of PIL-based elastomers increased from 124.58% to 1336.17%. The PIL-based elastomers also exhibited adjustable glass transition temperatures (-16.50 to 45.72 °C), high transparency (84.8% ~ 92.5%), excellent thermal stability (T_d > 310 °C, up to 348.13 °C), and tunable hydrophilicity or hydrophobicity. Remarkable adhesion strength (500 kPa for PIL-10 on iron) and recyclability were also demonstrated. This strategy will boost the facile fabrication of liquid-free PIL-based elastomers holding great promise in next-generation soft electronics and would provide new opportunities for the development of novel self-healing ionotronics.