Marek W. Urban

When ionic liquid (IL) monomers are chemically bonded into a polymer, either as pendant groups or within the main chains, the resulting poly(ionic liquids) (PILs) retain the key properties of ILs while offering the mechanical integrity and processability of polymers. Their dynamics and rheological behaviors often differ from neutral polymers due to factors such as ionic interactions, charge density, and morphology. Here, a new class of PILs composed of imidazole-bis(trifluoromethyl-sulfonyl)imide cation–anion pairs with especially bulky side chains containing aromatic, sulfonamide, and aliphatic building blocks were synthesized to investigate the effects of bulky side groups on viscoelastic and glass-forming behaviors. Being unentangled with a degree of polymerization in the 40–50 range and low dispersity, they exhibit unusually broad transitions from segmental to terminal relaxation, spanning 7 decades of frequency, behaving as viscoelastic fluids. Structural variations, including methyl, phenyl, and tolyl side-chain endcaps, exhibited a tunable array of glass transition, fragility, and relaxation time. Extended backbone conformation stabilized by the ionic nature of PILs, combined with dipolar/quadrupole inter- and intrachain interactions, and steric effects of bulky side groups dominate the viscoelastic spectra, offering an opportunity to create distinctive viscoelastic properties. As intermediate glass formers, their low fragility relative to most linear polymers can be traced to low B-fitting parameter values in the Vogel–Fulcher–Tammann relation.

This Letter outlines the synthesis of imidazolium (Im⁺)-(bis((trifluoromethyl) sulfonyl)amide) (TFSI^–)-based poly(ionic liquid)-based covalently adaptable networks (PIL-CAN) containing variable length aliphatic spacers terminated with acetoacetate (AcAc) and cross-linked by tris(2-aminoethyl) amine (TREN). These PIL-CANs exhibit the storage modulus in the order of ∼2.5–3.0 GPa and can undergo multiple reprocessing by mechanical damage and compression molding at 120 °C. The recovery of physical and chemical properties involves exchange reactions between imine and enamine, and keto and enol tautomers. Upon reprocessing, storage moduli, junction densities, and entropic energy are preserved. These thermally stable CAN-PILs exhibit storage modulus values comparable to those of high-performance phenolic or epoxy networks but significantly lower glass transition temperatures (T_g). While storage moduli and junction densities in the glassy region are functions of molecular weight and aliphatic spacer length, the entropy remains unchanged due to polar and dipolar contributions. If backbone and side chains are “locked” in place, no structural changes will occur to enable configurational changes, and the thermal motion and/or the free volume will dominate entropy rather than the backbone structure.

In these studies, we developed a new generation of polymeric materials capable of electrical energy storage and forming higher‐than‐binary logic circuits. Under non‐equilibrium conditions, polar–dipolar interactions facilitate t ion‐lock energy storage by emplacement of cation–anion pairs along the aliphatic side groups. Maintaining their locations for extended times facilitates energy storage. These properties were obtained by polymerizing ionic liquid monomers composed of covalently attached dipolar aliphatic variable‐length spacers (S) and tails (T) on both sides of a cation‐ anion pairs, enabling cation–anion polarization and precise fixation. When an electric field (EF) is applied, polarization of ion pairs occurs, and the ratio of aliphatic S‐to‐T controls ion locking . The degree of polarization governs the energy storage beyond classical approximations, whereas ion–dipole, dipole‐induced dipole, and dipole–dipole coupling facilitate multi‐logic circuitry. Integrating these materials into multiple series, in parallel, or a combination thereof provides the opportunity for development of higher‐than‐binary logic systems with enhancedr information density and multi‐valued logic gates. Among others, these materials may find numerous applications in quantum computing, fuzzy logic, or modeling uncertainty, just to name a few.

In these studies, we developed a new generation of polymeric materials capable of electrical energy storage and forming higher‐than‐binary logic circuits. Under non‐equilibrium conditions, polar–dipolar interactions facilitate t ion‐lock energy storage by emplacement of cation–anion pairs along the aliphatic side groups. Maintaining their locations for extended times facilitates energy storage. These properties were obtained by polymerizing ionic liquid monomers composed of covalently attached dipolar aliphatic variable‐length spacers (S) and tails (T) on both sides of a cation‐ anion pairs, enabling cation–anion polarization and precise fixation. When an electric field (EF) is applied, polarization of ion pairs occurs, and the ratio of aliphatic S‐to‐T controls ion locking . The degree of polarization governs the energy storage beyond classical approximations, whereas ion–dipole, dipole‐induced dipole, and dipole–dipole coupling facilitate multi‐logic circuitry. Integrating these materials into multiple series, in parallel, or a combination thereof provides the opportunity for development of higher‐than‐binary logic systems with enhancedr information density and multi‐valued logic gates. Among others, these materials may find numerous applications in quantum computing, fuzzy logic, or modeling uncertainty, just to name a few.

The ability to modulate the ionic conductivity of poly(ionic liquids) (PILs) through the molecular design of copolymer architectures or topologies provides an opportunity to advance the limited understanding of polar–dipolar interactions and their role. In these studies, using the reversible addition–fragmentation chain transfer (RAFT) method, we copolymerized a series of imidazolium-based ionic monomers to form random or block copolymers with star and linear topologies and variable aliphatic side-chain lengths. Using an array of spectroscopic, thermal, and conductivity measurements, these studies show that block star PILs exhibit 2–3 orders of magnitude higher ionic conductivity compared to their random star counterparts. Given similar cation–anion content, the conductivity enhancement is attributed to multiple inter- and intrachain dipolar interactions, which facilitate greater mobility in the proximity of ionic domains. Variable-length alkyl side groups also empower the tunability of the frequency-dependent ionic conductivities in block and random or star and linear PILs. Tweaking ionic conductivity through dipolar moieties of copolymer topologies may enable numerous applications where soft synthetic and biologically conductive interfaces are advantageous.

Schematic diagram of self-healable hydrogen fuel delivery.

This perspective review delineates recent advances in poly(ionic liquids) (PILs) in the context of molecular events and nature‐inspired materials. PIL's structural uniqueness and versatility provide the opportunity to utilize a bottom‐up approach, which, when combined with top‐down nano‐ and micro‐manufacturing, may lead to numerous diversified applications. PILs’ features mark a turning point where biology and materials intersect, which will lead to new materials involving bio‐inspired self‐assembling building blocks, facilitating the platform for top‐down fabrication approaches. To achieve this, it is essential to disentangle the peculiar effects of chain topology from other factors, including but not limited to ion concentration, segmental dynamics, or side‐chain mobility, ultimately leading to a comprehensive model of the topology‐conductivity relationships. The ultimate goal is to relate the molecular events involved in polar and dipolar interactions to PILs' functional integrity, thereby enabling intended performance over time. The aim of this review is not to summarize recent advances in chemical and physical properties, but to identify how molecular events can influence the topology, morphology, and micro/nano‐manufacturing. Furthermore, the aim is to identify guiding principles and a framework for designing PILs by analyzing their structural features and the role of molecular events in various applications.

These studies elucidate the origin of molecular processes involved in the self‐healing of imidazolium‐based poly(ionic liquid) copolymers (PILCs) subjected to dynamic surface oscillating forces (SOF). In contrast to their counterpart homopolymers, which are either brittle or exhibit irreversible responses, PILCs composed of short (‐CH ₃ ; Me) and long (–CH) ₂ ) ₃ CH ₃ ; Bu) aliphatic side chains attached to the cation‐anion pair composed of a 50/50 monomer molar ratio (poly(Me/Bu 50/50)) exhibit remarkable recovery. These dynamic responses arise from the competing polar and dipolar forces attributed to a balanced coexistence of ordered and disordered states involving the reversible rearrangements of H‐bonding, ionic interactions, and London dispersion forces. These copolymer composition‐driven processes exhibit dynamic recovery due to significant entropy increases, causing energy dissipation and reversible segmental rearrangements to achieve energetically favorable states. Spectroscopic FT‐IR measurements combined with 2D correlation spectroscopic (2D‐COS) analysis supported by molecular dynamics (MD) simulations reveal the significance of short‐ and long‐forces involved in polar‐dipolar interactions that enable dynamic recovery. The combination of directional and non‐directional entropy‐driven interchanges appears promising for identifying PILC architectures capable of mechanical adaptability, dynamic self‐healing, and ionic conductivity.

These studies elucidate the origin of molecular processes involved in the self‐healing of imidazolium‐based poly(ionic liquid) copolymers (PILCs) subjected to dynamic surface oscillating forces (SOF). In contrast to their counterpart homopolymers, which are either brittle or exhibit irreversible responses, PILCs composed of short (‐CH ₃ ; Me) and long (–CH) ₂ ) ₃ CH ₃ ; Bu) aliphatic side chains attached to the cation‐anion pair composed of a 50/50 monomer molar ratio (poly(Me/Bu 50/50)) exhibit remarkable recovery. These dynamic responses arise from the competing polar and dipolar forces attributed to a balanced coexistence of ordered and disordered states involving the reversible rearrangements of H‐bonding, ionic interactions, and London dispersion forces. These copolymer composition‐driven processes exhibit dynamic recovery due to significant entropy increases, causing energy dissipation and reversible segmental rearrangements to achieve energetically favorable states. Spectroscopic FT‐IR measurements combined with 2D correlation spectroscopic (2D‐COS) analysis supported by molecular dynamics (MD) simulations reveal the significance of short‐ and long‐forces involved in polar‐dipolar interactions that enable dynamic recovery. The combination of directional and non‐directional entropy‐driven interchanges appears promising for identifying PILC architectures capable of mechanical adaptability, dynamic self‐healing, and ionic conductivity.

Commodity aliphatic and aromatic acrylic‐based copolymers self‐heal due to ubiquitous key‐and‐lock , ring‐and‐lock , and fluorophilic‐σ‐lock van der Waals (vdW) interactions. However, the role of these interactions in the presence of covalently copolymerized ionic liquid (IL) is not known. This study is driven by the hypothesis that covalently incorporated cation–anion pairs to form poly(ionic liquid) copolymers (PILCs) can perturb inter‐ or intra‐chain vdW interactions reflected in mechanical and electrical responses. To test this hypothesis, we synthesized a series of PILCs comprising of pentafluorostyrene (PFS) and imidazolium‐based IL monomers with variable‐length aliphatic tails (methyl and butyl). Using a combination of 2D ¹ H‐ ¹ H and ¹⁹ F ‐ ¹⁹ F NOESY NMR and FTIR measurements supplemented by molecular dynamic (MD) simulations, these studies demonstrate that preferentially alternating/random PILCs topologies facilitate self‐healing. The introduction of cation–anion moieties modifies the fluorophilic‐σ‐lock interactions and, along with longer aliphatic tails ─(CH ₂ ) ₃ CH ₃ covalently attached to the imidazolium cation, enhances cation‐anion mobility, thus faster recovery from mechanical damage occurs. These findings underline how precise control over dipolar and ionic interactions through copolymer composition enables self‐healing in PILCs. These insights may open pathways for designing sustainable, mechanically resilient materials for applications in energy storage and energy harvesting.