Functional stability of a ferromagnetic polycrystalline Ni2MnGa high temperature shape memory alloy

Electrocaloric Ni 2 MnGa is of interest for solid state refrigeration applications, as well as a high temperature thermal shape memory alloy. Here, polycrystalline Ni 54 Mn 25 Ga 21 is examined using in situ synchrotron X-ray di raction. The initial martensite ( M f ) and austenite ( A f ) finish temperatures were found to be 232 C and 298 C respectively. M f was observed to decline by 8 C / cycle and A f increased by 1 C / cycle. Both below and surprisingly, above the Curie temperature, the application of an e.m.f. was found to a ect the lattice parameters measured. A change in the thermal expansion of the two phases was found around the Curie temperature

Interaction of Aspirin (Acetylsalicylic Acid) with Lipid Membranes

We studied the interaction of Aspirin (acetylsalicylic acid) with lipid membranes using x-ray diffraction for bilayers containing up to 50 mol% of aspirin. From 2D x-ray intensity maps that cover large areas of reciprocal space we determined the position of the ASA molecules in the phospholipid bilayers and the molecular arrangement of the molecules in the plane of the membranes. We present direct experimental evidence that ASA molecules participate in saturated lipid bilayers of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) and preferably reside in the head group region of the membrane. Up to 50 mol% ASA molecules can be dissolved in this type of bilayer before the lateral membrane organization is disturbed and the membranes are found to form an ordered, 2D crystal-like structure. Furthermore, ASA and cholesterol were found to co-exist in saturated lipid bilayers, with the ASA molecules residing in the head group region and the cholesterol molecules participating in the hydrophobic membrane core

Synthesis and properties of main chain discotic liquid crystalline polyethers based on rufigallol

Two series of thermotropic main chain discotic liquid crystalline polyethers, PR4m-n, based on rufigallol were prepared starting from the symmetric tetraethers of rufigallol, R4m; m and n represent the number of carbon atoms in the side chain and spacer segment, respectively. The symmetric tetraethers were in turn readily prepared by selective alkylation of rufigallol under controlled phase-transfer conditions. GPC analysis of the polymers suggested that they were all of moderate molecular weights, with M-n varying between 5400 and 17 000. The length of the spacer segment n in these polyethers was systematically varied, and its effect on the phase transition temperatures and the mesophase structure was examined using DSC, polarized light microscopy, and X-ray diffraction. It is noticed that when the spacer lengths are relatively long(n greater than or equal to 2m), the isotropization temperature (TD-i) decreases as the spacer length n increases, an observation that is in accordance with those previously made. However, when the spacer lengths are relatively small (n < 2m), the dependence of TD-i is quite the opposite; TD-i actually increases with an increase in spacer length. Furthermore, X-ray diffraction studies indicate that, in the discotic columnar mesophases that are formed, the columns pack in a hexagonal manner when n greater than or equal to 2m, while they do so in a rectangular lattice when n < 2m, leading to the formation of Dh and Dr mesophases, respectively. Finally, comparison of the discotic polyethers with their low molar mass analogues confirms the role of polymerization in stabilizing the mesophase; while all the polymers exhibit columnar mesophases, some of their low molar mass analogues are not liquid crystalline

Structures of some surfactant-polyelectrolyte complexes

Structures of complexes formed in aqueous solutions by some anionic polyelectrolytes (double and single stranded (ds and ss) DNA, poly(vinyl sulfonate) (PVS), and poly(styrene sulfonate) (PSS)) with a cationic surfactant system consisting of cetyltrimethylammonium bromide (CTAB) and sodium 3-hydroxy-2-naphthoate (SHN) have been determined using small angle Xray diffraction. All complexes are found to have a two-dimensional (2-D) hexagonal structure at low SHN concentrations. Analysis of the diffraction data shows that the ds DNA-CTAB complex has an intercalated structure, with each DNA strand surrounded by three cylindrical micelles. On increasing SHN concentration, DNA-CTAB-SHN complexes exhibit a hexagonal -to- lamellar transition, whereas PVS complexes show a hexagonal --> centered rectangular --> lamellar transition. PSS complexes show yet another sequence of structures. These results indicate the significant influence of the chemical nature of the polyelectrolyte on the structure of the complexes

Asymmetric discotic liquid crystals based on rufigallol

Abstract: Two series of asymmetric hexaethers (asymmetric by virtue of having alkyl chains of unequal lengths) of rufigallol, viz., R442n and R482n (1,5-dialkoxy-2,3,6,7-tetrabutoxy-9,10-anthraquinone and 1,5-dialkoxy-2,3,6,7-tetraoctyloxy-9,10-anthraquinone, respectively) were synthesized. These hexaethers exhibit a typical hexagonal columnar mesophase (D-h) as evidenced from X-ray diffraction and polarized light microscopic studies. Differential scanning calorimetric (DSC) studies suggest that the temperatures of isotropization (TD-i) and the molar isotropization entropy (Delta S-i) fall with increase in asymmetry. The fall in Delta S-i with increase in asymmetry is significantly larger than what may be expected merely from increase in the relative alkyl chain content, suggesting that asymmetry causes a significant decrease in the extent of ordering in the columnar mesophase. However, X-ray diffraction studies suggests that the intercolumnar spacing (D) is not affected by the presence of asymmetry but depends only on the total number of alkyl chain carbon atoms (C-n) present in a molecule. Furthermore, from the linear variation of the disk area, pi D-2/4, versus C-n (which was shown to be system independent), it was possible to calculate the volume occupied by a single alkyl chain methylene (CH2) unit in the columnar mesophase. This value is significantly smaller than that estimated in liquid n-alkanes, suggesting that the alkyl chains are not truely liquidlike in such discotic coulmnar mesophases. A further interesting observation in these systems is that when the alkyl chains are very long, an additional ordering of the side chains suggestive of side-chain crystallization is observed

Reentrant phase transitions of DNA-surfactant complexes

Complexes of double-stranded DNA with the cationic surfactant cetyltrimethylammonium bromide have been studied using small angle x-ray diffraction at varying concentrations of DNA and the cosurfactant hexanol. At low DNA concentrations, an intercalated hexagonal (HIc)-lamellar (L ac )-inverted hexagonal (HIIc ) transformation is found on increasing hexanol content. The HII c structure is converted into L a c on adding more DNA. Further increase in hexanol content leads to a phase separation in the surfactant solution, and a reentrant L a c-HII c -L a c transition is observed as DNA concentration is increased. Such structural transformations of DNA-surfactant complexes, driven by DNA concentration, have not been reported until now

Tuning the structure of surfactant complexes with DNA and other polyelectrolytes

We have carried out small-angle X-ray diffraction studies on complexes formed by the anionic polyelectrolytes, namely, sodium salts of double and single stranded (ds and ss) DNA, poly( glutamic acid) ( PGA), poly( acrylic acid) (PAA), and poly( styrene sulfonate) (PSS) with a cationic surfactant system consisting of cetyltrimethylammonium bromide ( CTAB) and sodium 3-hydroxy-2-naphthoate (SHN). All complexes have a two-dimensional (2D) hexagonal structure at low SHN concentrations. DNA-CTAB-SHN complexes exhibit a hexagonal to lamellar transition near the SHN concentration at which CTAB-SHN micelles show a cylinder to bilayer transformation. On the other hand, PGA and PAA complexes form a 2D centered rectangular phase at higher SHN concentrations, and PSS complexes show a primitive rectangular structure. These results provide a striking example of polyion specificity in polyelectrolytesurfactant interactions

Phase behavior of concentrated aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium hydroxy naphthoate (SHN)

We have characterized the phase behavior of mixtures of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN) over a wide range of surfactant concentrations using polarizing optical microscopy and X-ray diffraction. A variety of liquid crystalline phases, such as hexagonal, lamellar with and without curvature defects, and nematic, are observed in these mixtures. At high temperatures the curvature defects in the lamellar phase are annealed gradually on decreasing the water content. However, at lower temperatures these two lamellar structures are separated by an intermediate phase, where the bilayer defects appear to order into a lattice. The ternary phase diagram shows a high degree of symmetry about the line corresponding to equimolar CTAB/SHN composition, as in the case of mixtures of cationic and anionic surfactants

Power-law forces between particles in a nematic

Macroscopic particles of size a immersed in liquid crystalline solvents (e.g. polystyrene spheres in a lyotropic nematic) will exert long-range forces on each other by distorting the order-parameter field around them. As a first step towards a full theory of this novel class of suspensions, we show how to calculate the energy of interaction as a function of the separation r for r much greater than a between particles of various shapes and orientations in a nematic liquid crystal. Our approach can be understood (i) as a continuum description of the concentration field of the particles, with symmetry-allowed couplings between concentration gradients and director fields or (ii) in terms of an electrostatic analogy due to Brochard and de Gennes, in which the far-field transverse components of the director distortion are treated as a two-component Coulomb field of which particles of various shapes and orientations are elementary multipolar sources. The results with the greatest potential relevance to the experiments of Raghunathan et al. are for spheres: these act like quadrupoles, and the interaction is proportional to Ka(6)/r(5), and attractive for r oriented in a range around 50 degrees to the nematic axis (n) over cap, although it is repulsive for r normal or parallel to (n) over cap. The error in a recent calculation in the literature, claiming an always repulsive, r(-3) interaction, is pointed out

Re-entrant Phase Behavior of a Concentrated Anionic Surfactant System with Strongly Binding Counterions

The phase behavior of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the strongly binding counterion p-toluidine hydrochloride (PTHC) has been examined using small-angle X-ray diffraction and polarizing microscopy. A hexagonal-to-lamellar transition on varying the PTHC to SDS molar ratio (alpha) occurs through a nematic phase of rodlike micelles (N-C) -> isotropic (I) -> nematic of disklike micelles (N-D) at a fixed surfactant concentration (phi). The lamellar phase is found to coexist with an isotropic phase (l') over a large region of the phase diagram. Deuterium nuclear magnetic resonance investigations of the phase behavior at phi = 0.4 confirm the transition from N-C to N-D on varying alpha. The viscoelastic and flow behaviors of the different phases were examined. A decrease in the steady shear viscosity across the different phases with increasing alpha suggests a decrease in the aspect ratio of the micellar aggregates. From the transient shear stress response of the N-C and N-D nematic phases in step shear experiments, they were characterized to be tumbling and now aligning, respectively. Our studies reveal that by tuning the morphology of the surfactant micelles strongly binding counterions modify the phase behavior and rheological properties of concentrated surfactant solutions