Role of Cross‐Link Density in Structural and Functional Characteristics of the <scp>PAA‐co‐PAAm</scp> Ionogels for Acoustic Sensing Applications

ABSTRACT This study investigates the effect of cross‐linker content in tailoring the structure–property correlation of a robust polymeric ionogel for acoustic sensor application. Ionogels are synthesized by incorporating 1‐ethyl‐3‐methyl imidazolium ethyl sulfate [EMIM][EtSO 4 ] inside an acrylic acid‐co‐acrylamide copolymer (PAA‐co‐PAAm). Further, the influence of cross‐linker N, N′ ‐methylene bisacrylamide (MBAA) on various key properties required for ionogel to act as an acoustic sensor is studied by varying the concentration from 0.1 to 2 mol% relative to the monomer concentration. The formation of stable polymer‐ionic liquid networks is confirmed using FTIR, DSC, and TGA. Higher cross‐link density enhances thermal stability, environmental stability, and hardness, while decreasing the water absorption. The systematic constriction of the polymeric network, achieved by increasing MBAA, is quantified by determining cross‐link density and microstructural parameters via rheological analysis. Electrochemical impedance spectroscopy (EIS) studies suggest that increased cross‐link density elevated the bulk resistance (78–182 Ω), lowered the electrical double layer (EDL) capacitance (0.096–0.045 μF/cm 2 at 1 kHz), and ionic conductivity (1.0 × 10 −3 to 4.4 × 10 −4 S/cm). Frequency‐dependent capacitance profiles show stronger drop‐downs at lower frequencies for loose polymer networks formed at low cross‐link density, revealing their enhanced ionic mobility and interfacial charging. Further, in this study, an ionogel acoustic sensor is fabricated, which detected acoustic signals in all the applied frequencies (1–10 kHz). Thus, this work is a novel attempt that highlights the solemn role of cross‐link density in governing the electrochemical responsiveness of an ionogel and presents the potential of ionogel‐based highly sensitive, futuristic, flexible conformal acoustic sensors.