Fluctuations of lamellar structure prior to a lamellar -> gyroid transition in a nonionic surfactant system

Fluctuations of lamellar structure prior to a lamellar-->gyroid transition in a nonionic surfactant-water system has been investigated by means of sma ll-angle x-ray scattering (SAXS) and differential scanning calorimeter meas urements. For large DeltaT (DeltaT=T-T-LG, where T is the temperature and T -LG the lamellar-->gyroid transition temperature) in the lamellar phase, th e SAXS profiles can be described by a Caille correlation function for undul ating lamellar structure. Approaching the temperature to the T-LG, an exces s diffuse scattering grows at the lower Q (Q is the magnitude of scattering vector) side of the first lamellar peak. Highly oriented lamellar samples revealed that the excess diffuse scattering arises from in-plane density fl uctuations. We attribute this diffuse scattering to the perforation fluctua tion layer (PFL) structure and we show that the PFL is an equilibrium struc ture. At T-LG, the PFL transformed to the gyroid phase through a transient ordered structure having a rhombohedral symmetry.