Increasing 3D Supramolecular Order by Decreasing Molecular Order. A Comparative Study of Helical Assemblies of Dendronized Nonchlorinated and Tetrachlorinated Perylene Bisimides

A nonplanar, twisted, and flexible tetrachlorinated perylene bisimide (Cl4PBI) was functionalized with two AB3 minidendrons containing hydrogenated or semifluorinated dodecyl groups. The hydrogenated dendron was attached to the imide groups of Cl4PBI via m = 0, 1, and 2 methylenic units, whereas the dendron containing semifluorinated groups was attached via m = 3 or a di(ethylene oxide) linker (m = 2EO). The supramolecular structures of these compounds, determined by a combination of differential scanning calorimetry, X-ray diffraction, and solid-state NMR, were compared with those of nonchlorinated planar and rigid PBI reported previously, which demonstrated the thermodynamically controlled formation of 2D periodic arrays at high temperatures and 3D arrays at low temperatures. The molecularly less ordered Cl4PBI containing hydrogenated dendrons self-organize into exclusively 3D crystalline periodic arrays under thermodynamic control for m = 0 and 2, while the more highly molecularly ordered PBI produced less stable and ordered 3D crystals and also 2D assemblies. This induction of a higher degree of 3D order in supramolecular assemblies of the less well-ordered molecular building blocks was unanticipated. The semifluorinated dendronized Cl4PBI with m = 3 formed a 2D columnar hexagonal array under kinetic control, whereas the compound with m = 2EO formed an unusual 2D honeycomb-like hexagonal phase under thermodynamic control. These Cl4PBI compounds provide a new route to stable crystalline assemblies via thermodynamic control at lower temperatures than previously obtained with PBI, thus generating 3D order in an accessible range of temperature of interest for structural analysis and for technological applications

Growing Correlation Length on Cooling Below the Onset of Caging in a Simulated Glass-Forming Liquid

We present a calculation of a fourth-order, time-dependent density correlation function that measures higher-order spatiotemporall correlations of the density of a liquid. From molecular dynamics simulations of a glass-forming Lennard-Jones liquid, we find that the characteristic length scale of this function has a maximum as a function of time which increases steadily beyond the characteristic length of the static pair correlation function $g(r)$ in the temperature range approaching the mode coupling temperature from above

Site-selective growth of colloidal crystals with photonic properties on chemically patterned surfaces

Site-selective growth of colloidal crystals with photonic properties on chemically patterned surfaces was investigated. Substrates with a silane layer pattern of hydrophobic and hydrophilic regions were used as crystallization template. Close-packed structures with excellent optical properties that vary only slightly with the feature dimensions were obtained

Resolution enhancement in MRI of laser polarized 3He by control of diffusion

Diffusion of atoms or molecules in presence of magnetic field gradients not only attenuates the NMR signal but also leads to distortions close to restricting boundaries. This phenomenon is most evident in imaging with laser polarized (LP) noble gases. Diffusion of gases can be manipulated, however, by admixing inert gases of different molecular weight. In this work we analyze the effect of mixing LP-(3)He with SF(6) on the image quality of a phantom consisting of an arrangement of capillaries with different diameters. Admixing buffer gases of higher molecular weight changes the contrast and offers a means to record images with high spatial and time resolution. Additionally we demonstrate how distortions due to edge enhancement can be reduced even for long timed MRI-sequences

Le roman noir (littérature "contre", contre-littérature)

Le roman noir contemporain a toujours été envisagé par la critique comme un avatar du roman policier. Or, si les histoires de détective dont il est en partie constitué peuvent justifier un tel rapprochement, il met pourtant en œuvre un univers fictionnel radicalement différent, et une manière d'opposition qui fait d'abord du roman noir une " littérature contre ". Mais l'étude d'une quinzaine de ses productions majeures, américaines et françaises, montre bien qu'il déborde largement le cadre trop étroit que fixent les contours d'une catégorie générique. Un registre thématique de la trivialité qui relève du grotesque littéraire, une écriture paradoxale du banal et de l'excès qui donne à voir la dualité essentielle de l'individu et la duplicité du réel, une stratégie textuelle du brouillage des sens et du sens, qui saisit le lecteur et l'amène à douter de ses propres repères, autant d'éléments caractéristiques qui trouvent des résonances dans d'autres domaines du littéraire et étendent le caractère subversif du roman noir de sa thématique à son esthétique. Le roman noir renverrait ainsi à une conception de l'humanité qui conditionne la manière de penser le littéraire. C'est alors comme " contre-littérature " qu'il convient de l'appréhender, parce qu'il révèle le caractère arbitraire des frontières dressées par l'institution littéraire et remet en cause la conception dominante de la littérature.The contemporary "Roman Noir" has always been presented by the critic as an avatar of the detective stories. But if detective stories constitute a large part of the genre, justifying in a way a such approach, it uses in fact a very different fictional universe based on the opposition, which makes the Roman Noir the expression of an against literature. The research done on the main fiveteen American and French productions shows that it widely overflows the limits fixed by the too narrow frame of a generic category. A trivial thematic register inherited from the grotesque literary, a paradoxical description of banality and excess, which shows the essential duality between the human being and the duplicity of the reality, a writing strategy, which scrambles feelings and meanings to seize the reader and makes him/her doubt of his/her own references, are many characteristic elements, which find echoes in other literature fields and increase the subversive character of the Roman Noir from its thematic to its esthetic. The Roman Noir would be a vision of the humanity, which influences our conception of literature. So it is most as a counter-literature that we have to approach it, because it reveals the arbitrary character of the literary institution drawn up frontiers and amends the prevailing conception of the literature.NANTERRE-BU PARIS10 (920502102) / SudocSudocFranceF

^2H-NMR detection of the pressure effect on packing modulations in a columnar liquid crystal

The effect of high pressure on the columnar density modulation of the hexaoctylether of rufigallol, resulting from the incommensurability of the aromatic and aliphatic packing, is studied by 1D and 2D 2H-NMR spectroscopy. The mobility of the side chains can be used to probe the local packing density along the columns. We show that the numbers of molecules in regions of high and low packing density are strongly dependent both on temperature and on pressure. 2D-exchange NMR spectra show that the rate of exchange between these regions decreases considerably at increasing pressure, which is attributed to an increase of the length scale of the pinched regions. Our findings are consistent with the model proposed by de Gennes to resolve the incommensurability problem. A large decrease in the rate of molecular reorientation about the column axis is observed at the II-III phase transition.

Hierarchical self-assembly in diblock copolypeptides of poly(γ-benzyl-l-glutamate) with poly poly(l-leucine) and poly(O-benzyl-l-tyrosine)

Block copolypeptides with their inherent nanometer length scale of phase separation, provide means of manipulating the type (α-helices, β-strands) and persistence of peptide secondary structures. Two such examples are employed based on the α-helical poly(γ-benzyl-l- glutamate) (PBLG) polypeptide as one block and poly(l-leucine) (α-helical) or poly(O-benzyl-l-tyrosine) (POBT) (β-strands) as the second block. Although both secondary structures are present in the copolypeptides the effect of nano-scale confinement is to induce folding in the POBT β-sheets and to maintain the defected α-helices of PBLG and PLEU with a limited lateral coherence. © 2010 Elsevier Ltd. All rights reserved

Heterogeneity in polymer melts from melting of polymer crystals

Semi-crystalline polymers containing amorphous and crystalline regions usually have intimately mixed chains. The resulting topological constraints (entanglements) in the amorphous regions limit the drawability in the solid state. By controlled synthesis the number of entanglements can be reduced. Ultimately, crystals composed of single chains are feasible, where the chains are fully separated from each other. If such separation can be maintained in the melt a new melt state can be formed. Here we show that through slow and carefully controlled melting such polymer crystals form a heterogeneous melt with more entangled regions, where the chains are mixed, and less entangled ones, composed of individually separated chains. Chain reptation, required for the homogenization of the entanglement distribution, is found to be considerably hindered. The long-lived heterogeneous melt shows decreased melt viscosity and provides enhanced drawability on crystallization. This novel route to create heterogeneous melt should be applicable to polymers in general