Light-sensitive lamellar phases

Light sensitive lamellar (L-alpha) phases have been generated in glycerol/water mixtures from blends of an inert ABA tri-block co-polymer (dimethylsiloxane-polyethylenoxide, (EO)(15)-(PDMS)(15)-(EO)(15)), and a photodestructible anionic surfactant C(6)PAS (sodium 4-hexylphenylazosulfonate). These L-alpha systems have been formulated in a 6/4 glycerol/water (v/v) mixed solvent. Rheology, small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS) and dynamic light scattering (DLS) have been used to characterize the change in phase behavior and structure after the incorporated C(6)PAS is selectively degraded by UV light incident on the L-alpha phases. The ABA co-polymer alone forms weakly structured lamellar mesophases, which are stabilized by thermal fluctuations and characterized by low shear moduli (L-alpha phase-A). Addition of CO6PS (10 mM) introduces charge stabilization, generating more ordered and stiffer L-alpha systems (L-alpha phase-B). After UV irradiation (Hg lamp) of the L-alpha-B phases, and subsequent degradation of the ionic C(6)PAS, the results of rheological and scattering studies are consistent with an irreversible change back to L-alpha phase-A type systems. These formulations display a novel transition between electrostatically- and thermally-stabilized lamellae, which may be controlled by incident UV light. (C) 2008 Elsevier Inc. All rights reserved.