Swelling of Lalpha-phases by matching the refractive index of the water-glycerol mixed solvent and that of the bilayers in the block copolymer system of (EO)15-(PDMS)15-(EO)15.

The swelling of Lalpha-phases from the block copolymer polyethylenoxide-b-polydimethylsiloxane-polyethylenoxide (EO)15-(PDMS)15-(EO)15 in water/glycerol mixtures is reported. At low and medium polymer concentrations (<60%), the block copolymer forms a turbid vesicular dispersion in water. With time, the small unilamellar vesicles (SUV) and the large multilamellar vesicles (MLV) separate into a two phase L1/Lalpha-system. The turbid dispersions of the Lalpha-phase below 60% of the compound become more and more transparent with increasing glycerol and at 60% of glycerol become completely clear. Replacement of water by the solvent glycerol thus lowers the turbidity of the dispersion and swells the interlamellar distance between the bilayers. A 20% aqueous L1/Lalpha-dispersion can thus be transformed into a single birefringent transparent Lalpha-phase. The swelling of the Lalpha-phase in water and the decrease of the turbidity of the dispersion by the addition of glycerol is explained by the matching of the refractive index of the solvent to the refractive index of the bilayers of the block copolymer. The matching of a refractive index lowers the Hamaker constant in the DLVO theory between the bilayers and therefore decreases the attraction between the bilayers what allows them to swell to a larger separation. The microstructures in the phases were determined by cryo- and FFR-TEM. The interlamellar distance between the bilayers was determined by SAXS measurements. The viscous properties of the Lalpha-phases were determined by oscillatory rheological measurements. In comparison to other Lalpha-phases from normal surfactants, the Lalpha-phases from the block copolymer (EO)15-(PDMS)15-(EO)15 have low shear moduli. This is probably due to the high flexibility of the poly dimethylsiloxane block in the bilayers what can be recognized on the non-spherical shapes of the SUV's.