Crosslinked poly(ionic liquid)s enabled with dication ionic liquid moieties: adaptable lubricant performance under severe and electric field conditions

Abstract

With the aim of developing more efficient lubricant systems to further reduce energy consumption and provide more satisfactory lubricant performance under severe (high load, broad temperature range) and electric field conditions, herein, three crosslinked polymeric ionic liquids (PILs) {PS(NP_i8-6-NP_i8), PS(NSP₈-6-NSP₈), and PS(NSP₁₂-6-NSP₁₂)} enabled with dication ionic liquid monomers were synthesized successfully via radical copolymerization. Their physical properties, viscosity–temperature behavior, and tribological performance were systematically evaluated. The results demonstrated that all the PILs exhibited high thermal stability (T_5% up to 245.4 °C). And, compared with PS(NP_i8-6-NP_i8) and PS(NSP₈-6-NSP₈), mixing 5 wt% PS(NSP₁₂-6-NSP₁₂) with 500SN could improve the viscosity index (VI) of 500SN base oil to 197 ± 2, representing a 101% improvement. Besides, it could also reduce the wear volume by about 80% under a 200 N load {(2.076 ± 0.049) × 10⁻⁴ mm³ vs. (0.416 ± 0.098) × 10⁻⁴ mm³)}. Moreover, the system maintained effective lubrication under loads greater than 600 N and at temperatures up to 225 °C in short-duration friction tests. The excellent lubricating performance compared to the other prepared PILs demonstrates that the long alkyl chain substitutes of the anion moiety play a key role in enhancing the viscosity index and maintaining oil film continuity under severe conditions. Electric field experiments further confirmed that long-chain structures stabilize interfacial ion distribution via a spatial shielding effect, effectively reducing friction coefficient fluctuations and enhancing electric field adaptability. XPS and ToF-SIMS analyses revealed the formation of a dense, stable composite lubricating film on the steel surface, composed of FeS, SO₄²⁻, and PO₄³⁻ species. This study offers a novel strategy for developing multifunctional lubricant additives with enhanced viscosity, unique lubricity, and adaptive responsiveness, providing efficient lubricant systems for energy saving.