Shear-induced onion formation of polymer-grafted lamellar phase

Abstract

We study the shear-induced lamellar/onion transformation of the polymer-grafted lamellar phase composed of nonionic surfactant and amphiphilic triblock copolymers. Increase in the mole fraction of polymer X_P and the degrees of polymerization of hydrophilic chain N_EO remarkably affects the shear-induced onion phase formation behavior. From the viewpoint of the defect-mediated rheology, the shear modulus G′ is attributed to the oily streak network density. Linear increase in G′ with preshear rate prior to the onion formation indicates that the increase in the oily streak network density is essential as a pretransition. The oily streak is composed of the focal conic domains (FCDs), which locally have inhomogeneous layer spacing. The critical shear stress σ_c is well scaled by the effective increment of the bending modulus theoretically predicted for the polymer-grafted membranes, Δκ ∼ X_PN^1.27_EO. Experimental results suggest that the shear-induced onion phase is achieved by reorganizing the layer orientation of FCDs with the negative Gaussian curvature in order to eliminate the inhomogeneous layer spacing.