Solid-State Dynamics in the closo-Carboranes:A (11)B MAS NMR and Molecular Dynamics Study

This work explores the dynamic behaviour of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B0 = 9.4 T, the 11B MAS linewidths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (tauc) calculated by molecular dynamics are on the picosecond timescale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the 11B spin-lattice relaxation times (T1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the 11B MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter PQeff derived from “dynamic shift” measurements using 11B MQMAS NMR spectroscopy. The behaviour of PQeff as a function of temperature for para-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures and this was supported by the molecular dynamics simulations

Chiral molecular imprinting in liquid-crystalline network

A cholesteric imprinted elastomer was obtained by cross-linking a nematic side-chain polysiloxane around a chiral template. The template was first linked to some functionalised groups of the polymer via hydrogen-bound interactions, then was removed by washing. The sample was macroscopically oriented during the synthesis; so, both a molecular chirality and a supramolecular phase chirality were topologically imprinted inside the network. Batch rebinding experiments, performed in the presence of the template or of the other enantiomer, showed that the imprinted polymer has a pronounced stereo-selectivity towards the template enantiomer. The rebinding capacity appeared to be greater than an unimprinted mesogenic network as well as than an imprinted non mesogenic one

Using polydimethylsiloxane as a thermocurable resist for a soft imprint lithography process

International audienceNanoimprint lithography has been investigated using polydimethylsiloxane as a thermocurable resist. This novel process allowed us to reduce both pressure (<10 bars) and temperature (80 °C) when compared to a conventional imprinting process with a thermoplastic polymer resist such as polymethylmethacrylate. Using a new formulation of the elastomeric material, we have demonstrated high quality imprinting of both micronic and nanometric structures with no evidence of any viscous flow problems. The excellent etching resistance of the polydimethylsiloxane structures to a reactive ion etching silicon process and the compatibility with a lift-off procedure for pattern transfer are also presented

Acoustical properties of the polymer liquid crystal (P41)35 in the nematic, smectic A and glass phases

The longitudinal acoustical velocity and the absorption were measured in the polymer liquid crystal polymethylsiloxane (P41)35 from 106 °C down to - 6°C. The nematic phase is characterized by a viscoelastic behavior and a relatively large relaxation time. We do not observe any critical absorption nor at the isotropic-nematic transition neither at the nematic-smectic A transition. In the smectic A phase, we observe a regular increase of the velocity anisotropy and of the elastic coefficients C11, C13 and C 33. The transition toward the glass phase is marked by a strong maximum in the absorption around 40 °C. The four elastic constants C 11, C13, C33 and C44 were measured : C11, C33 and C44 increase if the temperature decreases but C 13 decreases.Nous avons mesuré la vitesse et l'absorption des ondes acoustiques longitudinales dans le polymère cristal liquide polymethylsiloxane (P41)35 depuis 106 °C jusqu'à - 6 °C. La phase nématique est caractérisée par un comportement viscoélastique et un temps de relaxation relativement grand. Nous n'avons pas observé d'absorption critique ni à la transition isotrope-nématique ni à la transition nématique-smectique A. Dans la phase smectique A, nous avons observé une croissance régulière de l'anisotropie des vitesses et des coefficients élastiques C 11, C13, et C33. La transition vers l'état vitreux est accompagnée par un maximum de l'absorption, aux environs de 40 °C. Les quatre constantes élastiques ont été mesurées : C 11, C33 et C44 croissent quand la température décroît, mais C13 décroît

Small angle neutron scattering experiments in smectic A and smectic B phases of diluted liquid crystal copolysiloxanes

Small Angle Neutron Scattering experiments were carried out on liquid crystal “side-end fixed” polysiloxanes : we observe that the polymer backbone keeps an oblate shape in the smectic A phase when the proportion of mesogens is reduced. This behavior is different from “side-on fixed” polymers since their anisotropic conformation vanishes at low mesogenic rate. Finally, we show for the first time an oblate anisotropic conformation in the smectic B phase.Par diffusion des neutrons aux petits angles, nous observons que la chaîne de polysiloxanes “en peigne” conserve une conformation de type oblate en phase smectique A lorsqu'on diminue le nombre de mésogènes par unité répétée. Ce comportement est différent de celui des polymères “en haltère” puisqu'à taux de mésogènes équivalent, ces derniers perdent toute anisotropie. Enfin, nous montrons pour la première fois l'existence d'une anisotropie de type oblate en phase smectique B

PDLC-like patterns at the isotropic to cholesteric transition entrapped by in situ photopolymerization

International audienceWhen investigated by optical microscopy between crossed polarizers, the isotropic to cholesteric transition may appear like fingerprint-patterned droplets embedded in a black isotropic matrix. In the present work, such PDLC-like (polymer dispersed liquid crystal) patterns, only occurring over 0.7 C, have been entrapped and stored at ambient temperature in a polymer film. We used a UV polymerization process with different sequences in which illumination time and UV power progressively vary. From a conceptual viewpoint, these PDLC-like patterns come solely from liquid crystalline material, whereas all the conventional PDLCs are binary mixtures of a macromolecular compound or 'prepolymer' with a conventional low molecular mass liquid crystal. The fact that isotropic matrix and cholesteric droplets differ only from the viewpoint of molecular order and not in their chemical nature, permits comparisons with the usual case for which the choice of polymer-forming material is crucial and the polymer/liquid crystal interface is an important factor for controlling PDLC electro-optic properties. The present system gives an opportunity to investigate by scanning electron microscopy (SEM) the droplet microstructure (isotropic-cholesteric interface, fingerprint patterns or defects), whereas previous SEM studies were focused on the shape and size of empty cavities, since the fluid liquid crystal was inevitably removed from the PDLC system

Mechanical Behavior of Side-Chain Liquid Crystalline Networks

The mechanical properties of a homologue series of side-chain mesomorphic networks were studied with a piezo-rheometer over frequencies ranging from 10$^{-2}$ Hz to 10$^4$ Hz. The results show that the compound's response is governed essentially by the dynamic glass transition. It is sensitive to the N-SmA transition, but insensitive to the N-I transition, with the result that the empirical principle of time-temperature superposition can be applied throughout the N-I transition. The influence of the crosslinking density and the amount of mesogenic side groups was also studied. For each of the samples, the static rigidity modulus $G_0$, the infinite-frequency dynamic rigidity modulus $G_\infty$, and the characteristic frequencies respectively associated with the longest visco-elastic mode and the glass transition, were determined.Les propriétés mécaniques d'une série homologue de réseaux mésomorphes à chaînes latérales ont été étudiées avec un piézo-rhéomètre entre 10$^{-2}$ Hz et 10$^4$ Hz. Les résultas obtenus montrent que la réponse du matériau est essentiellement gouvernée par la transition vitreuse dynamique; elle est sensible à la transition N-SmA mais insensible à la transition N-I de sorte qu'il est possible d'appliquer le principe empirique de superposition temps-température au travers de la transition N-I. L'influence du taux de réticulation et du taux de mésogènes a également été étudiée. Le module de rigidité statique $G_0$, le module de rigidité dynamique à fréquence infinie $G_\infty$ et les fréquences caractéristiques associées respectivement au mode viscoélastique le plus long et à la transition vitreuse, ont été déterminées pour chaque échantillon

Elaboration d'enzymes artificielles à base de matériau à empreinte moléculaire

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