Multi-length scale evaluation of the temperature-tunable mechanical properties of a lyotropic mesophase

The thermoreversible mechanical properties of a non-ionic polymer, lipid-based complex fluid were investigated by oscillatory and steady shear rheology, and analyzed in the context of mesophase architecture and polymer-water interactions. The sol phase (G″>G′) is observed at lower temperatures (5-17 °C) than the gel phase (G′>G″; 20-50 °C) and is driven by the temperature-induced changes in water solubility of polyethylene glycol (PEG). In the sol state, water solvated, extended PEG chains cannot interact within the confines of a two-dimensional hexagonally ordered array of prolate micelles that possess sufficient lattice dimensions to accommodate the polymer chains. With increasing temperature, the less water-soluble PEG adopts a compacted conformation that shields the polymer from bulk water. The densely coiled PEG chains localized within the interstitial water layers of a multi-lamellar structure promote chain entanglement and formation of a hydrogen-bonded network with the surrounding water layer, resulting in a soft gel. The strength of the hydrogen bonds within the network and therefore gel strength can be adjusted over an order of magnitude by temperature modulation between 20 and 50 °C. These studies identify principles useful in the preparation of stimuli-responsive mechanical materials.