Structural behavior and interactions of dendrimer within lyotropic liquid crystals, monitored by EPR spectroscopy and rheology.

Micro- and macrostructural behaviors of three different lyotropic liquid crystals (LLCs) loaded with a dendrimer, namely second generation poly(propylene imine) (PPI-G2), were studied by means of rheology and electron paramagnetic resonance (EPR). The three mesophases were L(α), Q(224), and H(II) composed of glycerol monooleate (GMO) and water-PPI-G2 solution (and d-α-tocopherol (vitamin E) in the case of H(II)). We characterized the impact of PPI-G2 interactions with the components of the host mesophases on their structural characteristics on different length scales. The incorporation of PPI-G2 within the L(α) and H(II) systems induced the formation of more elastic hexagonal systems with a "solidlike" behavior, while in the Q(224) system a different trend with a "liquidlike" behavior was observed. As a result, the dendrimer induced a remarkable change in both the structural and viscoelastic properties of the systems. Hence, the microenvironment in the interface region within the systems was monitored by computer-aided EPR using 5-doxylstearic acid (5-DSA) as a pH-dependent probe. The microviscosity (τ) and order (S) of systems were found to be sensitive to the PPI-G2 presence: when PPI-G2 concentration increased, τ and S increased in both the L(α) and Q(224) systems. In the H(II) systems two trends were observed, reflecting a decrease in τ and S up to 10 wt % PPI-G2 and subsequently their increase at higher dendrimer concentrations. It was assessed that PPI-G2 interacted strongly with the GMO hydroxyl groups in the L(α) phase, with the water molecules in the Q(224) systems. In the H(II) mesophase strong interactions with both the water and GMO hydroxyl molecules were detected.