Gelation Mechanism of Thermoreversible Supramacromolecular Ion Gels via Hydrogen Bonding

A series of supramacromolecular ion gels were prepared from blends of a poly(2-vinylpyridine)-b-poly(ethyl acrylate)-b-poly(2-vinylpyridine) (VEAV) triblock copolymer and a poly(4-hydroxystyrene) (H) homopolymer in all ionic liquid. The VEAV concentration was held at 10 wt %, and the H concentration was varied from 0 to 8 wt %. The solvent was the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide (EMITFSI). Above 160 degrees C all samples formed homogeneous solutions, but upon cooling gels were formed due to increased hydrogen bonds between the V blocks and H cross-linkers. The gelation mechanism was investigated in detail by rheology and small-angle X-ray scattering. The gel point (gel-liquid transition temperature) increased with the concentration of the cross-linkers. The dynamic moduli, G' and G '', showed excellent time-temperature superposition, and gave master curves extending over 15 orders of magnitude in reduced frequency. The shift factors a(T) showed the same strong temperature dependence for all the samples, which is attributed to the increasing number of hydrogen bonds between a given V block and H cross-linker upon cooling below the gel point. The gel quality was assessed in terms of the modulus value, G(x), the sharpness of the change in dynamic moduli with temperature near the gel point, and the minimum value of tan delta in the gel regime. By all three measures the best gel was formed at the mole ratio closest to 1:1 of pyridine:phenol units, i.e., stoichiometric balance. Furthermore, the maximum, value of G(x), corresponded to 50% of the EA blocks being elastically effective, consistent with expectation given the tendency for midblocks to loop back into the same micelle. A gel-like response was also obtained upon mixing 10% of a poly(2-vinylpyridine)-b-poly(ethyl acrylate) (VEA) diblock copolymer and H homopolymers in EMITFSI. However, in this case the rheological properties resulted from congestion of micelles rather than bridging chains. Small angle X-ray scattering revealed the presence of a nanophase-separated state, but without any significant change oil traversing the gel point, or between diblock and triblock samples.