SHEAR MELTING AND ORIENTATION OF A LYOTROPIC CUBIC PHASE

A bicontinuous lyotropic cubic phase, composed of the nonionic surfact ant pentaethylene glycol dodecyl ether (C(12)E(5)), heavy water (D2O) and tetradecane (C14H30), was investigated by small angle neutron scat tering (SANS). The cubic phase is stable only in a narrow temperature and composition range. Upon increasing the oil content the phase melts to an L(3) phase and at higher temperature it is in equilibrium with a lamellar (L(alpha)) phase. The scattering function of the powder cub ic sample is dominated by a strong correlation peak at q = 0.057 Angst rom(-1) in addition to a small hump at q approximate to 0.11 Angstrom( -1). From the known area per C(12)E(5) molecule at the polar/apolar in terface the lattice parameter is calculated for different minimal surf ace model structures. The position of the dominating reflection from t he cubic phase is found to be consistent with the spacing between the 211 planes of the Gyroid minimal surface structure. The effect of shea r on the scattering pattern was investigated using a Couette shear cel l. Under shear a weakly anisotropic scattering pattern was obtained. F rom a separate rheology experiments it was concluded that shearing the sample has the effect of melting the long range cubic order, presumab ly into a fluid disordered L(3) phase. When turning off the shear, the cubic phase recrystallises with a preferred orientation The diffracti on pattern is consistent with the diffraction from 211 planes with one major and one minor population in the orientation of the <111> axis. A major population having the <111> axis parallel to the shear velocit y (i.e. the tangent direction), and a minor component having the <111> axis parallel to the velocity gradient.