Rajeswari Kasi, PhD, University of Connecticut
In recent years, the combination of ionic liquid (ILs) and polymer is promising for the development of novel materials because certain polymers are compatible with ILs. The development of polymer electrolytes with unique properties such as thin film formability, flexibility, and transparency in addition to high ionic conductivity has been an intensive topic of many researchers attempting to realize novel electrochemical device. Polymeric ion-gels consisting of a swollen polymeric network in an IL are among the most conductive solid state electrolytes due to their good mechanical properties and high ionic conductivity which can be used in Li-ion batteries, electrochemical devices, sensors, electromechanical actuators, gas separation membranes, and dye-sensitized solar cells. Most of the studies on ion-gels derived from polymers are based on (1) doping of ILs with polymers, (2) in situ polymerization (or cross-linking) of vinyl monomers in ILs. Among them, the utilization of block copolymers is of great interest since this methodology may have the potential to afford easily processable and mechanically strong ion gels by utilizing self-assembly of ABA triblock copolymer in a B-block compatible IL. It is widely acknowledged that systematic and comprehensive mechanical and electrical property evaluation of ion-gels via self-assembly of block copolymers is desirable. Liquid crystalline block copolymers comprising cholesterol (LC BCPs) are synthesized to harness both the mechanical properties of physically or chemically cross-linked polymer networks as well as the unique features of cholesterol such as chirality, amphiphilicity, and liquid crystallinity. These polymers self-assemble due to supramolecular and van der Waals interactions to form a plethora of interesting and exotic LC structures in the neat- and solution states. We hypothesize that polymers which comprise of LC and brush-like molecules covalently linked through block-type architecture (LC brush BCPs) will self-assemble in ILs form nano- and mesostructures or hierarchical structures at different length scales. This is due to the interplay of microphase separation, LC characteristics, and solvophobic interactions in ionic liquids, and brush-type architecture and will result in very unique morphologies. In addition, using LC BCPs as template for incorporation various nanoparticles (metal, inorganic, etc.), the mechanical properties or/and unique optical, electric properties of resulting ion-gels can be improved. To the best of our knowledge, there has been no research effort directed to the synthesis and investigation of the combined influence of LC cholesterol mesophases and brush-like moieties in block copolymer that self-assemble in ILs, also incorporation of nanoparticles as dual-physical cross-linkers for ion-gels. Thus the current study focuses on designing highly conductive polymeric ion-gels in ILs through the self-assembly of block copolymer which are promising candidates for solid-state electrolyte applications.
Results and discussions
Firstly, we synthesized and characterized a brush diblock copolymer (PMA-g-PEO)-b-PC5MA by reverse addition fragmentation transfer polymerization (RAFT) using AIBN as initiator with dioxane as a solvent. Here, PMA-g-PEO refers to methyacrylates bearing polyethylene glycol and PC5MA refers methacrylates bearing cholesteryl molecule through five methylene units. [BMIM][PF6] known as the good solvent for polyethylene oxide block forms ion-gels by non-covalent intermolecular aggregation of the diblock copolymer. Thereafter we investigated the frequency- and concentration dependence of the linear viscoelastic modulus of the diblock copolymer solutions and gels and effect of copolymer composition) on the gelation characteristics.
Additionally, we are able to convert the chain ends of (PMA-g-PEO)-b-PC5MA to its SH version, which can be used to template gold nanoparticles (AuNPs).
We investigated the initial viscoelastic moduli (G’ and G”) of the ion gels (PMA-g-PEO)-b-PC5MA and (PMA-g-PEO)-b-PC5MA-AuNPs as a function of angle frequency at 25ºC. The presence of Au NPs in this ion-gel will help increasing mechanical properties. In addition, we are investigating the thermal properties of these ion-gels via DSC and TGA method.
Molecular characterization of as-synthesized brush-chol-BCPs polymers Polymer: (PMA-g-PEO)-b-PC5MA-2 Mn ~ 40 kg/mol, PDI~ 1.16, wt % PMA-g-PEO ~ 70, Gel in IL (c ~ 5 wt %).
Summary of results
We describe a new ion-gels prepared via self-assembly of (PMA-g-PEO)-b-PC5MA diblock copolymer in a room temperature ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6]. At 5 wt% concentration of the polymer in IL, formation of strong ion-gel is noted. However, the polymer can also be used to template gold nanoparticles. The presence of Au NPs within the block copolymer increases the mechanical properties of ion-gel. Work in progress includes synthesis of series of polymers with different compositions, molecular weights and AuNP loading and structure-morphology and property characterization. The nanoparticle-ionic liquid gels obtained via gelation of [BMIM][PF6] utilizing a brush copolymer encapsulated nanoparticle is an alternative and attractive way for development of novel solid state electrolytes and offer exciting possibilities of their application in electrochemical devices.