Supramolecular polymeric networks via hydrogen bonding in ionic liquids

Abstract

Swelling a cross-linked polymeric network with an ionic liquid to produce an ion gel is a facile and promising strategy to supply ionic liquids with mechanical integrity and persistent structure without sacrificing their fascinating properties. This thesis examines a special system from supramacromolecular assembly via hydrogen bonding, consisting of a poly(2-vinylpyridine-b-ethylene oxide-b-2-vinylpyridine) (P2VP–PEO–P2VP) triblock copolymer, a poly(4-vinylphenol) (PVPh) linear homopolymer, and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIM][TFSA]), where hydrogen bonding between the P2VP blocks and PVPh cross-linkers generates a transient polymer network. This thesis aims to systematically study the structure and dynamics of this versatile hydrogen-bonded supramolecular ion gel system. The substantially wide liquidus temperature range of the ionic liquid affords access to interesting and unprecedented rheological response of the resulting gels. For example, the terminal relaxation time varies by 15 decades from the gel temperature down to room temperature; extremely wide temperature- and frequency-independent rubbery plateaus are pronounced, indicating the formation of a well-defined polymer network structure; the applicability of time–temperature superposition is striking, suggesting the invariance of the underlying relaxation mechanism. In order to elucidate the underlying molecular mechanisms that control the structure and dynamics, viscoelastic properties and morphologies were investigated over wide composition and temperature ranges. In terms of the relaxation mechanism, the macroscopic rheology was qualitatively correlated with the average lifetime of a P2VP↔PVPh association, which is determined by the number of hydrogen bonds involved. This establishes the plausibility of the interpretation that stress relaxation occurs through simultaneous breaking of all hydrogen bonds involved. In terms of mechanical properties, it was demonstrated that the extent of cross-linking, and therefore ultimate mechanical strength, can be modulated through variation in the concentration of the cross-linker added. In terms of morphology, a long-range ordered hexagonal morphology was observed after annealing at elevated temperatures for a sufficient period of time. Overall, this thesis highlights that the utilization of hydrogen bonding to achieve ion gels is an advantageous approach to impart tunability in terms of viscoelastic and mechanical properties. Our strategy should be potentially useful in designing tailor-made smart materials.