Origins of Unusual Phase Behavior in Polymer/Ionic Liquid Solutions

Influence of ionic liquids on the chain dynamics and enthalpy relaxation of poly(methyl methacrylate)

Imidazolium ionic liquids (ILs) with various alkyl chain lengths on the cations ([C_nmim]⁺, n = 2, 4 and 8) and different combined anions ([TFSI]^- and [PF₆]^-) were blended with poly(methyl methacrylate) (PMMA), and the effects of the IL structure on the chain dynamics of PMMA were experimentally investigated by rheology and DSC measurements combined with a simulation method. The results indicate that the interaction between PMMA and ILs becomes stronger as the alkyl chain length on the imidazolium ring increases or the anion changes from [PF₆]^- to [TFSI]^-. As a result, a higher critical entanglement concentration and a larger entanglement molecular weight of PMMA were found in [C₈mim][TFSI] due to the stiffer conformation. Molecular dynamics (MD) simulations further demonstrated stronger interactions between PMMA and ILs with longer cationic alkyl chain lengths or [TFSI]^- anions, which showed smaller Flory-Huggins interaction parameters and larger radii of gyration, R_g. However, the larger size of alkyl chains or [TFSI]^- anions produced a larger free volume in the system as evidenced by positron annihilation lifetime spectroscopy (PALS), which competed with the molecular interaction and dominated the segmental motion. Therefore, a lower T_g and accelerated segmental relaxation were observed. Compared to alkyl chain length, the effect of anions on the interactions between ILs and PMMA is more prominent.

Unusual Lower Critical Solution Temperature Phase Behavior of Poly(benzyl methacrylate) in a Pyrrolidinium-Based Ionic Liquid

Polymer/ionic liquid systems are being increasingly explored, yet those exhibiting lower critical solution temperature (LCST) phase behavior remain poorly understood. Poly(benzyl methacrylate) in certain ionic liquids constitute unusual LCST systems, in that the second virial coefficient (A₂) in dilute solutions has recently been shown to be positive, indicative of good solvent behavior, even above phase separation temperatures, where A₂ < 0 is expected. In this work, we describe the LCST phase behavior of poly(benzyl methacrylate) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide for three different molecular weights (32, 63, and 76 kg/mol) in concentrated solutions (5–40% by weight). Turbidimetry measurements reveal a strong concentration dependence to the phase boundaries, yet the molecular weight is shown to have no influence. The critical compositions of these systems are not accessed, and must therefore lie above 40 wt% polymer, far from the values (ca. 10%) anticipated by Flory-Huggins theory. The proximity of the experimental cloud point to the coexistence curve (binodal) and the thermo-reversibility of the phase transitions, are also confirmed at various heating and cooling rates.

Phase Behavior of Poly(ethylene oxide) in Room Temperature Ionic Liquids: A Molecular Simulation and Deep Neural Network Study

The phase behavior of polymers in room temperature ionic liquids is a topic of considerable interest. In this work we study the phase diagram of poly(ethylene oxide) in four imidazolium ionic liquids (ILs) using molecular simulation. We develop united atom models for 1-butyl-2,3-dimethylimidazolium ([BMMIM]), 1-ethyl-2,3-dimethylimidazolium ([EMMIM]), and 1-ethyl-3-methylimidazolium ([EMIM]) in an analogous fashion to previously developed models for 1-butyl-3-methylimidazolium ([BMIM]) and tetrafluoroborate ([BF₄]) using symmetry-adapted perturbation theory. At high temperatures we obtain the coexistence concentrations using an interface method where the polymer and IL are simulated in a large elongated box, and an interface between coexisting phases is formed. At lower temperatures we use a deep neural network (DNN) method. The input descriptors for the DNN are the cohesive energy of mixing, the volume change of mixing, and the coordination numbers between cation and polymer, all of which are obtained from simulations of mixed systems at a series of temperatures. The DNN is trained by using the phase-separated systems at high temperatures and a mixed phase at low temperatures. The method predicts a lower critical solution temperature which decreases as the alkyl chain length on the cation is decreased, consistent with experiment. The simulations show that methylation of the cation has little effect on the phase diagram. This is in contrast to what is seen in experiments but could be because the polymer chains in the simulations are too short. At low temperatures the chains display two conformational motifs, namely a crown ether conformation and a ring conformation, each of which can wrap the chain around a single cation. This provides the entropic penalty for mixing and a reason for demixing as the temperature is raised. Such conformations might not be possible for longer chains. The combination of data-driven techniques and molecular simulation shows promise in the study of the phase behavior and physical properties of complex fluids.

A simulation method for the phase diagram of complex fluid mixtures

The phase behavior of complex fluid mixtures is of continuing interest, but obtaining the phase diagram from computer simulations can be challenging. In the Gibbs ensemble method, for example, each of the coexisting phases is simulated in a different cell, and ensuring the equality of chemical potentials of all components requires the transfer of molecules from one cell to the other. For complex fluids such as polymers, successful insertions are rare. An alternative method is to simulate both coexisting phases in a single simulation cell, with an interface between them. The challenge here is that the interface position moves during the simulation, making it difficult to determine the concentration profile and coexisting concentrations. In this work, we propose a new method for single cell simulations that uses a spatial concentration autocorrelation function to (spatially) align instantaneous concentration profiles from different snapshots. This allows one to obtain average concentration profiles and hence the coexisting concentrations. We test the method by calculating the phase diagrams of two systems: the Widom-Rowlinson model and the symmetric blends of freely jointed polymer molecules for which phase diagrams from conventional methods are available. Excellent agreement is found, except in the neighborhood of the critical point where the interface is broad and finite size effects are important. The method is easy to implement and readily applied to any mixture of complex fluids.

Unusual phase separation and rheological behavior of poly(ethylene oxide)/ionic liquid mixtures with specific interactions

The phase separation behavior of poly(ethylene oxide) (PEO) in ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) was investigated by rheological, optical microscopy, FT-IR and DSC measurements. It is demonstrated that specific interactions, particularly the hydrogen bonding between PEO and the ionic liquids as evidenced by FT-IR, in which a subtle but apparent absorption peak shift near the phase transition appears, account for the unusual low critical solution temperature (LCST) phase separation. Unlike the typical trend in which the storage modulus G' simply increases with temperature near the phase boundary for polymer blends without specific interaction, in our study, a novel "V-shaped" rheological response is observed, namely a dip in G' followed by an upturn, especially at low PEO concentration (<50 wt%). The magnitude of the "V" dip has heating rate and frequency dependences, while Tr (the phase transition temperature) is almost unchanged with heating rate and frequency. Upon increasing the alkyl chain length on the imidazolium ring from an ethyl to a butyl, the "V-shape" becomes more prominent and shifts to higher temperature, which is consistent with the results of FT-IR and DSC, evidently due to the stronger hydrogen bonding interaction between PEO and [BMIM][BF4] than [EMIM][BF4]. This unusual "V" dip might be tentatively ascribed to the coupling effects of the breaking of the "hydrogen bonding cage" formed between PEO chains and IL molecules and dissolution of the heterogeneous clusters as verified by FT-IR and TEM, respectively, and the following upturn is dominated by the interface formation upon phase separation.

SANS study on the solvated structure and molecular interactions of a thermo-responsive polymer in a room temperature ionic liquid

We have utilized SANS to quantitatively characterize the LCST-type phase behavior of PPhEtMA in d₈-[C₂mIm⁺][TFSA⁻].

Entropic Mechanism for the Lower Critical Solution Temperature of Poly(ethylene oxide) in a Room Temperature Ionic Liquid

Polymers exhibit interesting phase behavior in room temperature ionic liquids. For example poly(ethylene oxide) (PEO) displays a lower critical solution temperature (LCST) in [BMIM][BF₄] with a critical temperature and concentration that are only weakly dependent on molecular weight, contrary to the behavior of polymers in other solvents. To shed light on the mechanism of the LCST, we study the phase behavior of PEO in [BMIM][BF₄] using molecular dynamics (MD) simulations. The simulations show the signature of a phase transition as the temperature is increased. At low temperatures, interactions similar to a hydrogen bond are found between the imidazolium hydrogen and the PEO oxygen (HI-O H-bond) and the imidazolium hydrogen and the anion fluorines (HI-F H-bond). These interactions stabilize the mixed phase. A potential of mean force (PMF) analysis shows an entropic cost associated with the HI-O H-bond, which makes the bond formation unfavorable at higher temperatures, while the HI-F H-bond does not show a significant temperature dependence: This suggests that LCST phase separation is driven by the entropic penalty of the polymer for a PEO-cation hydrogen bond. We test the effect of scaling the charges on the [BMIM][BF₄]. Interestingly, the scaled charge force-field does not predict a phase separation at any temperature, thus, emphasizing the pitfalls of charge scaling for mixtures.

Thermoreversible partitioning of poly(ethylene oxide)s between water and a hydrophobic ionic liquid

We describe a poly(ethylene oxide) (PEO) homopolymer "shuttle" between water and a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). PEO homopolymers with varying molecular weight transferred reversibly and quantitatively between water at room temperature and [EMIM][TFSI] at an elevated temperature. The temperature of the transfer from water to [EMIM][TFSI] shows a linear dependence on PEO molecular weight and a dependence on polymer concentration consistent with expectation based on Flory-Huggins theory. These results are also consistent with the previously observed lower critical solution temperature (LCST) behavior of PEO in water. Dynamic light scattering study of the concentration and temperature dependence of the swelling degree of PEO corona of polybutadiene (PB)-PEO block copolymer micelles indicates that the solvent quality of [EMIM][TFSI] for PEO remains essentially the same as a good solvent over the temperature range of the PEO shuttle. Fundamental understanding of the PEO shuttle is of significance in development of systems for phase transfer of reagents and reaction products between ionic liquids and water.