Light-sensitive lamellar phases

Light sensitive lamellar (Lα) 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 C6PAS (sodium 4-hexylphenylazosulfonate). These Lα 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 C6PAS is selectively degraded by UV light incident on the Lα phases. The ABA co-polymer alone forms weakly structured lamellar mesophases, which are stabilized by thermal fluctuations and characterized by low shear moduli (Lα phase-A). Addition of C6PAS (10 mM) introduces charge stabilization, generating more ordered and stiffer Lα systems (Lα phase-B). After UV irradiation (Hg lamp) of the Lα-B phases, and subsequent degradation of the ionic C6PAS, the results of rheological and scattering studies are consistent with an irreversible change back to Lα phase-A type systems. These formulations display a novel transition between electrostatically- and thermally-stabilized lamellae, which may be controlled by incident UV light.

SANS measurements of photosurfactant/block co-polymer lamellar phases in a mixed glycerol/water solvent. (a) Lα-A phase without photosurfactant C6PAS; (b) Lα-B phase with 10 mM C6PAS before UV irradiation; (c) the Lα-B′ system after 60 min UV irradiation to destroy the photosurfactant.Figure optionsDownload as PowerPoint slide