The formation kinetics and self-assembly of multilamellar tubules of the diacetylenic phospholipid 1,2-bis(tricosa-10,12-diynoyl)-sn-glycerol-3-phosphocholine formed under controlled cooling rates were studied by x-ray diffraction and optical, atomic force, and scanning electron microscopy. Tubule formation was driven by a reversible first-order phase transition from an intralamellar, chain-melted L(alpha) phase to a chain-frozen L(beta), phase. These observations are the basis of a highly efficient method of tubule production in which tubule lengths can be controlled, between 1 and 100 micrometers, by varying the cooling rate. These tubules can be made in suspensions with 10 percent lipid by mass, far exceeding the lipid solubility limit.

We have performed x-ray scattering studies of freely suspended, thick (∼2μ), oriented dimyristoyl–phosphatidyl–choline (DMPC)–water multilamellar films with the emphasis on the ordered phases. These stable films were drawn in situ on a triple axis x-ray spectrometer at a controlled, continuously variable temperature (T) and relative humidity (RH). The structure was determined at several temperatures and humidities and a T–RH phase diagram was constructed. We have found that the previously labeled ordered Lβ′ phase is in fact three distinct two-dimensional phases differentiated by the direction of chain tilt with respect to the in-plane lattice. The line shapes of the scattering data indicate that a lower limit of ∼200 Å may be placed on the in-plane chain–chain correlation length, while the chain positions are uncorrelated across the water layers.

We report on x-ray scattering studies of the nematic to smectic-^4 transition in 4-cyano-4'-octylbiphenyl under nonequilibrium shear-flow conditions. As the transition is approached, the interplay between the viscous frictional and the flow-induced-fluctuation forces on the nematic director leads to a series of regimes whose occurrence results from the divergence in one of the viscosities due to the critical slowing down of the smectic-,4 order-parameter fluctuations. The experiments demonstrate that, under flow, synchrotron x-ray-diffraction techniques provide a powerful structural probe of steady-state dynamical behavior.PACS numbers: 61.30. Eb, 64.70.Md, 82.70.Kj Maxwell and Reynolds realized that the microscopic structure of a fluid should be distorted when yx becomes comparable to 1. ] Here, y is the shear rate and r is a relevant relaxation time; in a simple fluid it would correspond to exceedingly fast diffusion times, of order 10~1 2 -10~1 4 s. The required shear rate for observable effects is thus extremely difficult to achieve. In elegant experiments, using large particles to slow down the interparticle diffusion times to about 10 ~2 s, large distortions have been observed at moderate shear rates, in the structure of a fluid of colloidal suspensions. 2 Aside from simple fluid systems where single-particle diffusion sets the time scale, de Gennes and Onuki and Kawasaki were among the early workers to point to the effects of shear flow on pretransitional fluctuations associated with continuous transitions. 3 For these systems, the relevant time scale is the relaxation of a collective fluctuation domain which is readily varied by temperature or pressure, and diverges at the transition.In this paper, we report on a synchrotron x-ray scattering study of the nematic (TV) to smectic-,4 (Sm-A) phase transition in 4-cyano-4'-octylbiphenyl (8CB), under shear flow conditions. The TV-Sm-^4 transition corresponds to the onset of a one-dimensional mass density wave along the nematic director n, a unit vector which specifies the average orientation of the molecules. This transition has been studied extensively at equilibrium primarily because it is a simple example of an almost second-order freezing transition. 4 As the transition is approached in the TV phase, the pretransitional Sm-A fluctuation domains, which are directly probed by x rays, grow to length scales of order microns before the Sm-A phase sets in. According to dynamical scaling, 5 the fluctuation relaxation time T~~ £ 3/2 , where £ is an effective correlation length for critical fluctuations; thus, we chose to study this system because the effective control parameter yx which drives the system away from equilibrium should increase substantially, near the TV-Sm-^4 transition. We emphasize that our experiments are among the few microscopic studies, probing length scales < 100 A and as large as ~-jum, of an ordering transition under nonequilibrium steady-state conditions. Such studies have important bearings on our understanding of phase transitions away...

Synchrotron x-ray scattering studies were performed to probe the nonequilibrium structures of two layered systems at high shear rates: the smectic-A phase of the thermotropic liquid crystal 4-cyano-4'-octylbiphenyl (8CB) and the lamellar L(alpha) phases of surfactant membranes composed of sodium dodecyl sulfate and pentanol. Whereas the lamellar surfactant phases oriented primarily with their layers parallel to the shearing plates, as expected intuitively, in the corresponding high shear regime, the smectic-A liquid crystalline material oriented with the layers perpendicular to the shearing plates. A careful numerical study revealed that this surprising layer orientation results from nonlinear dynamics of the liquid crystal director and is caused by the flow distortion of thermal fluctuations.

Polyethylene propylene films of various molecular weights, which would normally wet native-oxidecovered Si surfaces, were observed to dewet the surface when the film thickness became less than the polymer radius of gyration. These films could be made to wet either by increasing the annealing temperature or by chemically modifying the surface. The results are shown to be consistent with an expression for the spreading parameter that incorporates a stretching free energy term for the polymer chains. Measurements of the diffusion constant of the polymer on the silica surface indicated that an activation energy was required to desorb the polymer segments from the surface before diffusion could occur.