Glycerol-induced swollen lamellar phases with siloxane copolymers

Phase behavior is established for a block copolymer polyethyleneoxide–b–dimethylsiloxane–polyethylenoxide (EO)15–(PDMS)15–(EO)15 (IM-22) a in glycerol/water mixed solvent. In water alone, the block copolymer forms biphasic micellar/lamellar (L1/Lα) systems over the range 10–70 wt%, with single Lα-phases between 70–90 wt%. Strong solvent effects on the phase behavior were noted. For example, using a mixed 60:40 vol% glycerol/water solvent, the single Lα-phase region appears at much lower concentrations, only 20 wt% IM-22, as compared to the biphasic L1/Lα system observed in water alone. This interesting observation of Lα-phase swelling on addition of glycerol may be explained by a decrease in attraction between the bilayers, as it is also found that in this mixed glycerol/water solvent there is a close refractive index matching with IM-22. Rheological measurements show the Lα-phases with added glycerol have low shear moduli. The influence of added ionic surfactant sodium dodecylsulfate (SDS) on these swollen IM-22 Lα-phases was studied. Small-angle X-ray scattering (SAXS) indicated the interlamellar distance d remains essentially constant up to 3 mM SDS, and then decreases with increasing SDS content. This weak effect is consistent with the fact that the Lα-phases are most swollen when the mixed solvent contains 60 vol% glycerol. The results suggest that glycerol/water solvent mixtures can be used to tune the refractive index of the background solvent, modifying DLVO-type interactions, and causing significant effects on the phase stability of simple block-copolymer systems.

Birefringence from 20 wt% siloxane copolymer in glycerol/water mixtures as a function of vol% glycerol in the solvent blend.Figure optionsDownload as PowerPoint slide