Effects of component molecular weight on the viscoelastic properties of thermoreversible supramolecular ion gels via hydrogen bonding

We describe a thermoreversible supramolecular ion gel system 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 a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]). Aggregates of intermolecularly hydrogen-bonded PVPh cross-linkers form cross-links, connecting P2VP-PEO-P2VP triblock chains into a transient polymer network through hydrogen bonding with the P2VP endblocks. The viscoelastic properties of the resulting gels are strongly temperature-dependent, as manifested by the 15 orders of magnitude variation in the longest relaxation time (τ ₁) from the gel temperature (T _gel) down to room temperature. Wide temperature- and frequency-independent rubbery plateaus are distinct, indicating the formation of a well-defined network structure. The applicability of time-temperature superposition to this system is striking, suggesting the invariance of the underlying relaxation mechanism. The terminal relaxation dynamics of these hydrogen-bonded supramolecular networks is dramatically retarded upon increasing the P2VP endblock length. In contrast, their viscoelastic properties are relatively insensitive to the PVPh cross-linker length. These results are consistent with the hypothesis that τ ₁ is determined by the average lifetime of a P2VP ↔ PVPh association, which is directly related to the number of hydrogen bonds involved. Besides their potential applications in various electrochemical devices, these materials might also be used in surface coatings, where thermally reversible and sensitive viscoelastic properties are highly advantageous.