Solution properties of polymers in ionic liquids

Abstract

Ionic liquids (ILs) are emerging as promising solvents for polymers. They exhibit excellent chemical and thermal stability, favorable ionic conductivity, and negligible vapor pressure, making them attractive alternatives to traditional solvents. Moreover, adding polymers to ILs provides mechanical integrity to these solvents while retaining their transport properties, enabling their use in various applications such as polymer electrolytes, gas separation, and energy storage. However, the use of these materials would benefit from a thorough understanding of the polymer behavior in this relatively new class of solvent. This work focuses on developing a fundamental understanding on solvation behavior of two model polymers in ILs by characterizing their structural, dynamic, and thermodynamic properties. In the past years, various simulation approaches have reported conflicting conformational characteristics of poly(ethylene oxide) (PEO) in an IL. As much as simulation studies are on-going, experimental work to guide simulation models are lacking. This work focused on assessing the excluded volume exponent, v, for PEO in an IL by measuring the dependence of PEO coil size on polymer molecular weight. Since this system is not viable in light scattering, small-angle neutron scattering (SANS) was utilized instead. Therefore, a wide range of accessible molecular weights (10 to 250 kg/mol) of perdeuterated polymers (d-PEO) were synthesized by anionic polymerization, for enhanced contrast in SANS. The dependence of coil size on polymer molecular weight was assessed to yield v ≈ 0.55 at 80 °C, which indicates that PEO adopts a slightly swollen, flexible coil conformation in the IL. The results clarified the uncertainty surrounding PEO coil dimensions in IL, and also successfully guided the development of new simulation models. This work was further extended to examine the tunability of coil dimensions by varying the ionic liquid components. Altering both the cation and anion of the IL changed the solvent quality, but the anion seemed to exert a more pronounced effect. The temperature dependence of coil dimensions was moderate, however, the coil size varied insignificantly in ternary mixtures of PEO, IL, and a binary salt. The static and dynamic properties of a range of molecular weights (20 to 160 kg/mol) of poly(benzyl methacrylate) (PBzMA) were assessed in four different ionic liquids over a wide temperature range (27 °C ‒ 155 °C), primarily using light scattering. The relevant structural, dynamic, and thermodynamic parameters were examined as a function of concentration, temperature, and molecular weight. Some interesting observations were revealed. The phase behavior study showed a potential shift of the critical composition to polymer-rich region, as opposed to lower concentrations of polymer commonly observed in polymer solutions. The second-virial coefficient (A2) remains surprisingly positive, even at temperatures close to the phase separation, where A2 < 0 is anticipated. Furthermore, A2 shows a stronger dependence on molecular weight than commonly observed for polymers in good solvents. On the dynamic side, the diffusion virial coefficients remained mostly positive, further corroborating the good solvent behavior of A2. The excluded volume exponents (v≈ 0.52 ‒ 0.55) obtained from the dependence of hydrodynamic radii on molecular weight also revealed good solvent characteristics of these ILs.