Shear-induced Onion Formation of Triblock Copolymer-embedded Surfactant Lamellar Phase

Abstract

Shear-induced onion phase formation is an unsolved topic in soft matter. It has been widely reported that many different surfactant lamellar systems show the shear-induced onion phase formation, since Roux et al. found it. In order to understand the shear-induced onion phase formation mechanism, Nettesheim et al. traced the formation process by using small angle neutron scattering under shear (RheoSANS) and found an intermediate structure during the structural transition. Kato et al. presented that the coherently buckled lamellae is plausible as such intermediate structure by carefully observing a small angle X-ray scattering profiles under shear (Rheo-SAXS). Although new experimental results have been steadily accumulated, the comprehensive studies on the critical condition and membrane flexibility necessary for the onion phase formation are lacked. Our finding in our previous study is that the development of the oily streak network density, which is a typical structural defect found in the smectic phase, is essential to achieve the shearinduced onion phase formation. As the oily streak network density is increased, the focal conic domains (FCD) with a negative Gaussian curvature are crowded and fulfill the space. FCDs have unfavorable energy contribution originating from the layer compression because of the inhomogeneous layer spacing. Excess increase in the FCD density seems to destabilize the lamellar phase and trigger the realignment of the layer orientation from FCD with the negative Gaussian curvature to new state so that the inhomogeneous layer spacing is eliminated. This could be a driving force for the shear-induced onion formation. The bilayer membrane curvature has to be reorganized from negative to positive for the onion formation (Fig. 1). We have previously studied the shear-induced lamellar/ onion transformation behavior of the amphiphilic triblock copolymer-grafted lamellar phase systems. In such block Shear-induced Onion Formation of Triblock Copolymer-embedded Surfactant Lamellar Phase