Rheology of the lamellar liquid-crystalline phase in polyethoxylated alcohol/water/heptane systems

Resum

Linear viscoelastic tests as well as transient and steady flowexperiments were carried out on lamellar liquid crystalline samples ofpoly(oxyethylene) alcohol/water/heptane systems. The effect ofsurfactant and heptane concentrations on the rheological properties ofthe lamellar mesophase was investigated. The mechanical spectruminside the linear viscoelastic regime shows, in all cases, awell-developed plateau region in the whole frequency range studied.The values of the dynamic functions were higher for intermediatesurfactant or heptane concentrations indicative of a major developmentof the elastic network in the midrange of existence of the lamellarphase. Transient and steady flow experiments point out ashear-induced evolution of the lamellar microstructure. Different powerlaw regions with different values of the flow index were detected inthe viscosity versus shear rate plots. These shear-inducedstructural modifications were confirmed by using polarizingmicroscopy in an optical shearing cell. Structural modificationsappear to be highly influenced by shear rate. In general, applyingrelatively high constant shear rates, the alignment of the bilayersfollowed by the appearance of the «oily streaks» structure wasobserved. Appearance of shear-induced vesicles occurs at highheptane content, as indicates the texture of close-packedmonodisperse spherulites detected by polarizing microscopy.