Kaori Nakaya

Kinetic pathway of lamellar → gyroid transition: Pretransition and transient states

The kinetic pathway of a lamellar → gyroid transition has been investigated by means of a small angle x-ray scattering (SAXS) and a new computer simulation scheme developed by Saeki. Approaching temperature to a lamellar → gyroid transition temperature (TLG) from the lamellar phase, SAXS profiles reveal that modulation fluctuations with local hexagonal arrangement develop in the lamellar state. The modulation fluctuation layer (MFL) structure is an equilibrium structure and corresponds to the most unstable fluctuation modes of lamellar structure. The MFL transforms to a unique three dimensional network lattice having R3c symmetry, which agrees with the prediction of the computer simulation. This rhombohedral structure is a transient structure and transforms to the gyroid structure with elapse of time.

Static and dynamic structures of spherical nonionic surfactant micelles during the disorder-order transition

We have investigated the static and dynamic structures of nonionic surfactant micelles, a C(12)E(8)/water binary system, during the disorder-order transition using small angle x-ray scattering, static light scattering, and dynamic light scattering techniques. In the disordered phase, the micelles have spherical shape and intermicellar interactions are governed by the hard core and weak long ranged attractive potentials. With increase of the micellar concentration, the disordered micelles transform to the three characteristic ordered micellar phases, a hexagonally close packed lattice, a body centered cubic lattice, and an A15 lattice having area-minimizing structure. The stability of these phases is well explained by balance of a close packing rule and a minimal-area rule proposed by Ziherl and Kamien [Phys. Rev. Lett. 85, 3528 (2000)]. The role of hydrodynamic interactions in surfactant micellar solutions was compared with that in hard sphere colloidal particle suspensions.

Effects of grafted polymer chains on lamellar membranes

We have investigated the effects of grafted polymer chains [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)] on the bending modulus and the intermembrane interactions of lamellar membranes (C(12)E(5) water) by means of a neutron spin-echo and a small-angle x-ray scattering technique. In this study the hydrophilic chain takes the mushroom configuration on the membrane. The bending modulus of the polymer-grafted membranes increases in proportion to the square of the end to end distance of the polymer chain, which agrees well with the theoretical prediction of Hiergeist and Lipowsky [J. Phys. II 6, 1465 (1996)]. From the interlamellar interaction point of view, the mushroom layer is renormalized to the membrane thickness, which enhances the repulsive Helfrich interaction. When the size of the decorated polymer chain increases to the interlamellar distance, however, the mushroom is squeezed so as to optimize the interlamellar potential. Further increase of the grafted polymer size brings a lamellar-lamellar phase separation, where the grafted polymer chains are localized in the dilute lamellar phase and the concentrated lamellar phase forms the onionlike texture.

Kinetic pathway to double-gyroid structure

We have investigated the structural development during order-order transitions to the double-gyroid (DG) phase of nonionic surfactant/water systems based on two-dimensional small-angle x-ray scattering patterns from highly oriented ordered mesophases. The lamellar (L) to DG transition proceeds through two intermediate structures, a fluctuating perforated layer structure having ABAB stacking and a hexagonal perforated lamellar structure with ABCABC stacking (HPLABC). For a hexagonally packed cylinder (H) to DG transition, we also observed the HPLABC structure as the intermediate phase, thus the HPLABC is an entrance structure for the DG phase. The hexagonal perforated lamellar (HPL) structure consists of hexagonally packed holes surrounded by the planar tripods, and the transition from HPL structure to the DG phase proceeds by rotation of the dihedral angle of connected tripods. A geometrical consideration shows that large deformations of HPL planes are necessary to form the DG structure from the HPLABC structure, whereas the transition from a HPL structure with ABAB stacking (HPLAB) to the DG structure is straightforward. In spite of the topological constraints, the HPLABC structure is observed in the kinetic pathway to the DG structure.

Repulsive interlamellar interaction induced by addition of colloidal particles

The effects of colloidal particles confined between lamellar membrane slits on interlamellar interactions have been investigated by small-angle neutron scattering. On addition of colloidal particles to a lamellar phase composed of a non-ionic surfactant, the first lamellar peak becomes sharper and higher-order peaks appear. Thus the colloidal particles suppress undulation fluctuations of lamellar membranes by their steric hindrance, which results in a repulsive interlamellar interaction. As the interlamellar distance decreases, the position of the Bragg peak shifts towards higher q [where q is the magnitude of scattering vector, given by q = (4\pi/\lambda)\sin\theta, where 2\theta is the scattering angle and λ is the wavelength] and the peak intensity weakens. This tendency is completely opposite to the behavior of non-ionic surfactant lamellar phases, where the interlamellar interaction is governed by the Helfrich interaction. A phenomenological free-energy model is proposed based on the restriction of membrane fluctuations by colloidal particles. This model describes the experimental results well.

Surfactant mesophases mediated by colloidal particles

We have investigated effects of guest colloidal particles on the mesoscopic structures of a surfactant/water system by means of small angle neutron and x-ray scattering techniques. The addition of a small amount of colloidal particles between lamellar slits reduces the undulational fluctuations of lamellae and brings about a lamellar to micelle transition. On the other hand, on addition of colloidal particles into a disordered prolate micelle phase, the micelles deform to a spherical shape. We will discuss these structure changes in terms of the depletion effects due to the presence of colloidal particles.

Dynamical nature of least stable fluctuation modes of lamellar structure observed in a nonionic surfactant/water system

The nature of fluctuation modes of lamellar structure in a nonionic surfactant/water system has been investigated using a small angle x-ray scattering and a neutron spin echo technique. Approaching temperature from lamellar phase to double-gyroid phase, a diffuse scattering peak appears in the small angle scattering profile prior to the transition. This diffuse scattering is originated from the least stable fluctuation modes of lamellar structure predicted by Qi and Wang [Macromolecules 30, 4491 (1997)]. The intermediate scattering profiles of the lamellar phase just before the transition can be described by a undulation fluctuation mode and a least stable fluctuation mode of the lamellar structure. The relaxation rate of the least stable fluctuation mode decreases with approaching temperature to the double-gyroid phase. However, in the vicinity of the lamellar to double-gyroid transition temperature, the decrease of relaxation rate is suppressed and the lamellar structure transforms to the intermediate r...

Inter‐Lamellar Interaction Mediated by Amphiphilic Triblock Copolymer

We incorporated amphiphilic triblock copolymer into a lyotropic lamellar system and studied the effects of the hydrophilic chain length on the elastic nature of the lamellar membranes. The triblock copolymer consists of a hydrophobic chain (PPO) bounded by two identical hydrophilic chains (PEO). The hydrophobic chain anchors in the lamellar membranes and the hydrophilic chain decorates the membrane. We estimated the layer compression modulus by combining a small angle x‐ray scattering (SAXS) and a neutron spin echo (NSE) technique. The increase of the hydrophilic chain length changes the bending rigidity of membrane and enhances the repulsive forces between membranes. We discuss the results in terms of the entropic undulation interaction of lamellar membranes and comformational entropy of the hydrophilic chains.

Fluctuation Modes Prior to Lamellar‐Double Gyroid Transition of a Nonionic Surfactant/Water System

The nature of fluctuation modes of lamellar structure in a nonionic surfactant/water system has been investigated using a small angle x‐ray scattering and a neutron spin echo technique. Approaching temperature from lamellar phase to double‐gyroid phase, a diffuse scattering peak appears in the small angle scattering profile prior to the transition. This diffuse scattering is originated from the least stable fluctuation modes of lamellar structure predicted by Qi and Wang. The intermediate scattering profiles of the lamellar phase can be described by an undulation fluctuation mode and a least stable fluctuation mode of the lamellar structure.

Morphology Transition from Sphere to Rod by Confining the Polymer Chains in a Dilute Microemulsion System

In this study we investigated the morphology transition of microemulsion droplet induced by polymer confinement using a small angle neutron scattering (SANS) technique. By confining the polymer chain strongly the scattering profiles showed the following changes; 1) a characteristic scattering peak corresponding to the size of droplet shifts to the higher q side, and 2) the scattering intensity in the low q region increases considerably. These changes of the scattering profiles can be described by a rod (or sphero‐cylinder) model. Thus, the strong confinement of polymer chains in droplets induces the morphological transition from spherical to rod‐like droplet.