Microphase Separation of Ionic Liquid-Containing Diblock Copolymers

The phase separation of salt-doped polymer membranes has continued to draw considerable attention in a broad spectrum of research fields related to electrochemical devices, primarily because of their potential to simultaneously tune the high ionic conductivity and the mechanical robustness. For instance, room-temperature ionic liquids (ILs) are now considered prospective salts for substantially increasing the range of mechanical integrity and ionic conductivity when mixed with block copolymers because a nearly endless number of IL mixtures can potentially be made. Nonetheless, the major driving force for the microphase separation of block copolymers and ILs remains largely unclear. As a result, numerous investigations have been undertaken but to our dismay, no noteworthy result has been presented. Case in point, a comparison between the Flory−Huggins type of mean-field theory and an experiment for a mixture of PEO and EMIM suggested that the strong ion−ion correlation was suppressed in mixtures of an uncharged homopolymer and the IL. This suppression may be caused by the molecular features of ILs, yet the true mechanism and significance of the fluctuation suppression remain unknown.

At present, the Landau theory of Leibler is well-known to qualitatively account for the disorder−order and order−order phase transitions of salt-free, uncharged di-block copolymer melts. This model predicts decreases in the incompatibility between two blocks upon the addition of small amounts of ILs, but experimental observations for a mixture of a di-block copolymer and an IL suggest that the two blocks become significantly immiscible. Consequently, clarification of the driving force for the phase separation of IL-containing block copolymers is greatly needed and is likely to bridge the gap between theory and experiment. On this account, Michigan Technological University physicist: Professor Issei Nakamura developed a new model for microphase separation of IL-containing di-block copolymer melts by considering the phase behaviors observed in experiments, in a bid to address the aforementioned shortfall. His work is currently published in the research journal, Macromolecules 2020.

The main goal of the research was to further develop the mean-field treatment for IL-containing polymers so as to bridge the gap between theory and experiment. To realize this, the author studied the phase behavior of a block copolymer and ionic liquid mixture using the Landau theory of Leibler and field-theoretic techniques in polymer physics. Specifically, he considered the lyotropic phase behavior of a mixture of poly(styrene-block-2-vinylpyridine) (S2VP) copolymers with imidazolium bis(trifluoromethane)sulfonimide ([Im][TFSI]).

The author reported that his weak-segregation theory of microphase separation was able to account for electrostatic screening, the excluded volume effect, and the dielectric contrast between the species. Most important, the results of the phase diagrams for ionic liquid-containing S2VP and poly (ethylene oxide-block-styrene) qualitatively agreed with the observed transitions between the ordered microstructures corresponding to lamellae, hexagonally packed cylinders, and body-centered cubic lattices.

In summary, Professor Nakamura developed a weak-segregation theory for IL-containing di-block copolymers using the Landau theory of Leibler and the Born solvation energy of the ions accounting for the excluded volumes of the species and electrostatic screening between the ions. The presented calculations showed that the effect of electrostatic screening on the microphase separation may not be as significant as deduced from the traditional polymer theory. In a statement to Advances in Engineering, Professor Nakamura pointed out that despite its simplicity, the Born solvation energy of the ions can be used to consider the dissolution of an ionic liquid in block copolymers when the dielectric contrast is large.