Ionically Modulated PIMs: Balancing Mobility and Cohesion for Optimal Self-Healing and Stiffness

Polymers of intrinsic microporosity (PIMs) are valued for their large free volume, making them suitable for gas separation membranes. However, their mechanical fragility and physical aging can hinder long-term performance. To improve durability, we added the ionic liquid [BMIM⁺][TFSI⁻] to a sulfonyl-bridged PIM-PA-ionone backbone. Through all-atom molecular dynamics (MD) simulations, we explore how ionic modulation influences porosity, mechanical response, and self-healing.

Our simulations showed a paradox: increasing IL content led to electrostatic-driven densification, confirmed by small-angle X-ray scattering (SAXS) measurements, without changing polymer chain conformation, while enhancing segmental mobility. The ions interpose between polymer chains, disrupting internal cohesion by replacing strong hydrogen bonding with weaker ion-dipole interactions. This loss of structural order compromises stress transmission and reduces the material's stiffness.

During healing, increased molecular motion accelerated structural relaxation and network repair. However, functional recovery was not a simple trend. Healing efficiency, defined by restoring the Young's modulus, was non-monotonic, reaching an optimum at intermediate IL concentration of 0.5 cations per monomer. This reveals a core design principle: optimal resilience requires tuning the ionic content to balance the molecular mobility for structural repair with the cohesive forces essential for mechanical integrity.