59th Annual Report on Research 2014

Introduction and general goals

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 (LCBBCs) 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 LCBBCs 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 LCBBCs (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 polymethacrylate bearing polyethylene glycol of 2000 Da and PC5MA refers to polymethacrylate bearing cholesteryl unit through 5 methylene spacers. [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.

We explored the initial viscoelastic moduli (G' and G") of the ion gels LCBBC and LCBBC/AuNPs as a function of angle frequency at 25oC. The presence of AuNPs in this ion-gel will help increasing mechanical properties. In addition, we investigated the mesostructure analysis of the ion-gel. The network structure of the ion gel was formed due to the non-covalent interactions and the numerous separated AuNPs were also observed within the nanocomposite ion gel matrix.

The ionic conductivity and electrochemical stability of ion gels were examined to demonstrate the potential of these materials for use as solid polymer electrolytes and electrochemical materials. These results suggest that the LCBBC/AuNPs ion gel could be a promising candidate for applications in various electrochemical devices.

Summary of results

We describe a new ion-gels prepared via self-assembly of LCBBC (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. In addition, the polymer can also be used to template gold nanoparticles. The presence of AuNPs within the block copolymer increases the mechanical properties of ion-gel. 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.

Publications

1. C. T. Nguyen, T. H. Tran, X. L. Lu and R. M. Kasi, Polymer Chemistry, 2014, 5, 2774-2783.

2. C. T. Nguyen, Y. Zhu, X. Chen, G. A. Sotzing, S. G.-Focil, and R. M. Kasi, J. Mater. Chem. C, 2014, DOI: 10.1039/c4tc01702a