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

Abstract

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 oC) than the gel phase (G′>G″; 20–50 oC) 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 oC. These studies identify principles useful in the preparation of stimuli-responsive mechanical materials. The mechanical characteristics of a non-ionic polymer, lipid-based complex fluid that exhibits a thermoinverted phase transition (sol phase at lower temperature than the gel state) were examined by combined rheology, small-angle X-ray, and neutron diffraction studies and Raman spectroscopy. The temperature-driven water solubility and thus conformational changes of amphiphile-tethered polyethylene oxide (PEO) regulates the formation and strength of the hydrogen-bonded network. These studies identify principles useful in the preparation of stimuli-responsive mechanical materials.