Ulf Olsson

Time-resolved small-angle neutron scattering (TR-SANS) was employed to observe temperature-induced phase transitions from the sponge (L-3 to the lamellar ( L (alpha) phase, and vice versa, in the water-oil (n-decane)-non-ionic surfactant (C12E5 system using both bulk and film contrast. Samples of different bilayer volume fractions phi and solvent viscosities eta were investigated applying various amplitudes of temperature jump Delta T . The findings of a previous H-2-NMR study could be confirmed, where the lamellar phase formation was determined to occur through a nucleation and growth process, while it was concluded that the L-3-phase develops in a mechanistically different and more rapid manner involving uncorrelated passage formation. Likewise, the kinetic trends of the nucleation and growth transition (decreased transition time with increase of phi and Delta T were witnessed once again. Additionally, NMR and SANS data that demonstrate a strong dependency of that process on solvent viscosity eta are presented. Contrariwise, it is made evident via both SANS and NMR results that the L-alpha-to-L-3 transition time is independent (within experimental sensitivity) of the varied parameters (phi, Delta T, eta) . Unusual scattering evolution in one experiment, originating from a highly ordered lamellar phase, intriguingly hints that a major rate determining factor is the disruption of long-range order. Furthermore, the bulk contrast investigations give insight into structure peak shifts/development during the transitions, while the film contrast experiments prove the bilayer thickness to be constant throughout the phase transitions and show that there is no evidence for a change in the short-range order of the bilayer structure. The latter was considered possible, due to the different topology of the L-3 and L-alpha phases. Lastly, an unexpected yet consistent appearance of anisotropic scattering is detected in the L-3-to-L-alpha transitions.