(14ax) Polymerized Ionic Liquids As a Platform for Novel Functional Materials

Research Interests: block copolymers, soft condensed matter, chemistry, physics, carbohydrate-based functional materials

Teaching Interests: chemical engineering thermodynamics, analysis of transport phenomena, separation processes, electrochemical systems, polymer chemistry, polymer physics

Polymerized Ionic Liquids (i.e., PILs) are an emerging class of materials with ionic liquid moieties covalently attached to a polymer backbone. As such, they synergistically combine the processability of polymers with the versatile chemical functionality of ionic liquids. PILs can exploit a vast array of synthetically accessible polymer chemistries and architectures, with functionalities ranging from ionic conductivity, and magnetism. In this work, we seek a fundamental understanding of structure-property relationships in this exciting class of frontier materials.

Carbon-linked imidazolium moieties represent a promising chemistry for proton conducting PILs due to the amphoteric nature of the nitrogen atoms located in the aromatic ring. Herein, the effect of lamellar domain spacing on ionic conductivity is investigated for a model system of hydrated diblock copolymer based on a protic PIL. We present a strategy that allows for the synthesis of a well-defined series of narrowly dispersed poly(styrene-block-ionic liquid) (i.e., PS-b-PIL) with constant volume fraction of ionic liquid moieties (f_IL = 0.39) and with two types of mobile charge carriers: trifluoroacetate anions and protons. These materials self-assemble into ordered lamellar morphologies with variable domain spacing (ca. 20-70 nm) as demonstrated by Small-Angle X-Ray Scattering. PS-b-PIL membranes exhibit ionic conductivities above 10^-4S/cm at room temperature, which are independent of domain spacing consistent with their nearly identical water content. The conductivity scaling relationship demonstrated in this paper suggest that a mechanically robust membrane can be designed without compromising its ability to transport ions. In addition, PIL-based membranes exhibit low water uptake (λ = 10) in comparison with many proton-conducting systems reported elsewhere. The low water content of the materials described herein makes them promising candidate for electrochemical devices operating in aqueous electrolytes at low current densities where moderate ion conduction and low product crossover are required.

 Ionic liquids based on transition metals are suitable for magnetic PILs due to the paramagnetic nature of the atoms with partially filled d-orbitals. We developed a paramagnetic and biocompatible PIL based on poly(ethylene glycol) (i.e., PEG) and the ionic liquid anion tetrachloroferrate(III) (i.e., FeCl₄^-). We illustrate a molecular design strategy based on a combination of epoxide ring opening anionic polymerization and thiol-ene click chemistry that allows for exquisite control over the molecular structure of the polymer and guarantees scalability appropriate for extensive physical characterization. PIL containing FeCl₄^- is paramagnetic with a magnetic susceptibility at 30 ^oC of 30x10^-6 emu/g.Oe, as characteristic of materials based on anions containing transition metals with partially filled d-orbitals. Additionally, the PIL significantly modifies the magnetic environment experienced by the ¹H of water, as revealed by a decrease in the spin-lattice relaxation time (i.e., T₁) of ¹H determined from Nuclear Magnetic Resonance (i.e., NMR) from 8 to 0.3 seconds. The structure-property relationships demonstrated in this work makes PILs based on PEG and FeCl₄^- suitable as a T₁-weighted MRI contrast agent. The magnetic and biocompatible nature of PILs is promising for development of novel therapeutic agents (i.e., drugs, proteins, antibodies) based on polymer conjugates that exhibit controlled residence time within the human body; and magnetic functionality for MRI imaging, and facilitated targeting.

Elucidating design rules for novel functional materials using a modular synthetic platform needs to be extended beyond a PIL platform given the grand challenges faced nowadays by society. We will present research strategies to engineer better medicines, provide access to clean water, and manage the nitrogen cycle.