97 research outputs found
Structure and relaxation processes of an anisotropic molecular fluid confined into 1D nanochannels
Structural order parameters of a smectic liquid crystal confined into the
columnar form of porous silicon are studied using neutron scattering and
optical spectroscopic techniques. It is shown that both the translational and
orientational anisotropic properties of the confined phase strongly couple to
the one-dimensional character of the porous silicon matrix. The influence of
this confinement induced anisotropic local structure on the molecular
reorientations occuring in the picosecond timescale is discussed
Criticality of an isotropic-to-smectic transition induced by anisotropic quenched disorder
We report combined optical birefringence and neutron scattering measurements
on the liquid crystal 12CB nanoconfined in mesoporous silicon layers. This
liquid crystal exhibits strong nematic-smectic coupling responsible for a
discontinuous isotropic-to-smectic phase transition in the bulk state. Confined
in porous silicon, 12CB is subjected to strong anisotropic quenched disorder: a
short-ranged smectic state evolves out of a paranematic phase. This
transformation appears continuous, losing its bulk first order character. This
contrasts with previously reported observations on liquid crystals under
isotropic quenched disorder. In the low temperature phase, both orientational
and translational order parameters obey the same power-law
Propriétés physiques des cristaux liquides discotiques nanoconfinés
L'objectif de cette thèse est de mener une étude fondamentale et expérimentale des propriétés physiques des cristaux liquides discotiques colonnaires (CLDCs) confinés dans des matrices poreuses templates hautement ordonnées à l'échelle nanométrique. Les molécules des CLDCs de forme plane, composées de noyaux polyaromatiques rigides entourées de chaînes aliphatiques flexibles fonctionnalisables, sont susceptibles de s'auto-assembler dans des colonnes favorisant ainsi le recouvrement de leurs orbitales électroniques p. Ce qui fait de ces matériaux de véritables candidats pour des applications dans l'électronique moléculaire et la photovoltaïque grâce à la possibilité de migration des porteurs de charges le long de leurs colonnes. Cependant, ces applications nécessitent une bonne maîtrise des paramètres influant sur les mécanismes d'alignement dans les phases colonnaires, sur de grands monodomaines, et de préférence à température ambiante. Une méthode très prometteuse visant à optimiser les longueurs de diffusion des porteurs de charge a été récemment proposée, basée sur la formation de nanofils orientés de CLDCs par auto-assemblage dans des matrices dites templates (de moulage). Toutefois, les propriétés structurales, dynamiques et les effets de confinement sur ces technologies restent aujourd'hui mal connus et morcelés et pourraient constituer un véritable verrou scientifique pour leur réalisation. Notre étude s'est portée sur les CLDCs commerciaux (HPT) et le Py4CEH (moins connus) qui sont confinés dans des alumines poreuses (AAO) et du silicium poreux (Sip) de diamètres de pores de quelques dizaines de nm. Les diagrammes de phase ont été d'abord étudiés par DSC puis les effets structuraux ont été approfondis grâce à la diffusion de neutrons. Dans les géométries confinées, nous observons une dépression des températures de transition, un élargissement du domaine de stabilité de la phase colonnaire et l'ouverture d'une hystérèse amplifiée dans les pores de plus petite taille. Un ordre orientationnel très élevé a été trouvé dans les phases colonnaires bulk par la RMN du solide et la structure des systèmes confinés colonnaires, dominée par une distribution radiale avec un ancrage homéotrope a été déterminée. La dynamique moléculaire a été étudiée par diffusion quasiélastique de neutrons. Elle est affectée par le confinement : la dynamique de grande amplitude est fortement ralentie, tandis que la dynamique rapide locale devient régie par une distribution très large de temps caractéristiques.The aim of this work is to conduct fundamental and experimental studies of the physical properties of columnar discotic liquid crystal (CDLCs) confined in highly ordered porous templates at the nanoscale. CDLC molecule of planar shape, consist in rigid polyaromatic nuclei surrounded by functionalizable flexible aliphatic chains, and are capable of self-assembly in columns, thereby promoting overlap of their p electron orbitals. This makes these materials real candidates for applications in molecular electronics and photovoltaics due to the possibility of migration of the charge carriers along their columns. However, these applications require a good control of the parameters affecting the alignment mechanisms in the columnar phases of large single domains, preferably at room temperature. A very promising approach to optimize the diffusion lengths of charge carriers has been recently proposed, based on the formation of oriented CDLC nanowires by self-assembly in so-called "templates". However, structural and dynamical proprieties and confinement effects are still scarce, and could be a real scientific lock to their implementation. Our study is focused on commercial CDLCs (HPT) and Py4CEH which are confined in porous alumina and porous silicon membranes with pore diameters of c.a. tens of nm. The phase diagram was first studied by DSC and more deeply characterized by neutron scattering. In confined geometries, we observe a depression of the phase transition temperatures, a broadening of the columnar phase stability domain and an opening of hysteresis loops amplified by smaller pore size. A high orientational order was found in the bulk columnar phases by solid-state NMR, and the structure of confined columnar systems, dominated by a radial distribution with homeotropic anchoring was observed. The molecular dynamics was studied by quasielastic neutron scattering. It is affected by confinement: large lengthscale motions are massively slowed down, whereas the rapid and local dynamics becomes submitted to large distributions of correlation times.RENNES1-Bibl. électronique (352382106) / SudocSudocFranceF
Exploring conformational energy landscape of glassy disaccharides by CPMAS 13C NMR and DFT/GIAO simulations. I. Methodological aspects
The aim of this article is to assess the ability of chemical shift surfaces
to provide structural information on conformational distributions of
disaccharides in glassy solid state. The validity of the general method leading
to a simulation of inhomogeneous 13C chemical shift distributions is discussed
in detail. In particular, a proper consideration of extrema and saddle points
of the chemical shift map correctly accounts for the observed discontinuities
in the experimental CPMAS spectra. Provided that these basic requirements are
met, DFT/GIAO chemical shift maps calculated on relaxed conformations lead to a
very satisfactory description of the experimental lineshapes. On solid-state
trehalose as a model of amorphous disaccharide, this simulation approach
defines unambiguously the most populated sugar conformation in the glass, and
can help in discriminating the validity of different models of intramolecular
energy landscape. Application to other molecular systems with broad
conformational populations is foreseen to produce a larger dependence of the
calculated chemical shift distribution on the conformational map
Relation between static short-range order and dynamic heterogeneities in a nanoconfined liquid crystal
International audienceWe analyze the molecular dynamics heterogeneity of the liquid crystal 4-n-octyl-4'-cyanobiphenyl nanoconfined in porous silicon. We show that the temperature dependence of the dynamic correlation length wall, which measures the distance over which a memory of the interfacial slowing down of the molecular dynamics persists, is closely related to the growth of the short-range static order arising from quenched random fields. More generally, this result may also shed some light on the connection between static and dynamic heterogeneities in a wide class of condensed and soft matter systems
Exploring conformational energy landscape of glassy disaccharides by CPMAS 13C NMR and DFT/GIAO simulations. II. Enhanced molecular flexibility in amorphous trehalose
This paper deals with the comparative use of the chemical shift surfaces to
simulate experimental 13C CPMAS data on amorphous solid state disaccharides,
paying particular attention to -1-1 linkage of trehalose, to
-1,4 linkage between pyranose rings (lactose) and to linkage implying a
furanose ring (sucrose). The combination of molecular mechanics with DFT/GIAO
ab-initio methods provides reliable structural information on the
conformational distribution in the glass. The results are interpreted in terms
of an enhanced flexibility that trehalose experiences in amorphous solid state
compared to the other sugars. An attempt to relate this property to the balance
between intra- and inter-molecular hydrogen bonding network in the glass is
presented
Sliding and translational diffusion of molecular phases confined into nanotubes
The remaining dynamical degrees of freedom of molecular fluids confined into
capillaries of nano to sub-nanometer diameter are of fundamental relevance for
future developments in the field of nanofluidics. These properties cannot be
simply deduced from the bulk one since the derivation of macroscopic
hydrodynamics most usually breaks down in nanoporous channels and additional
effects have to be considered. In the present contribution, we review some
general phenomena, which are expected to occur when manipulating fluids under
confinement and ultraconfinement conditions.Comment: 17 pages, 8 fig
Mutarotational Kinetics and Glass Transition of Lactose
We report for the first time real time in situ and quantitative measurements
of the mutarotation reaction of lactose in the solid state. The experiments
have been performed by 13C NMR. We show that mutarotation is initiated on
heating the amorphous state, and reaches chemical equilibrium close above the
glass transition temperature Tg. We do not observe this transformation when
starting from stable crystalline states. The final ratio of and
anomers is 1:1, which suggests that the energy profile of the mutarotation
reaction pathway in the solid state is actually different from the mechanism
proposed for aqueous solution. This chemical equipartition is reached before
the crystallization into the corresponding 1:1 molecular compound. These new
data clearly illustrate the interrelation between the chemical molecular
properties, the physical state of the material, and the relaxational dynamics
of the glass
High-resolution dielectric study reveals pore-size-dependent orientational order of a discotic liquid crystal confined in tubular nanopores
International audienceWe report a high-resolution dielectric study on a pyrene-based discotic liquid crystal (DLC) in the bulk state and confined in parallel tubular nanopores of monolithic silica and alumina membranes. The positive dielectric anisotropy of the DLC molecule at low frequencies (in the quasistatic case) allows us to explore the thermotropic collective orientational order. A face-on arrangement of the molecular discs on the pore walls and a corresponding radial arrangement of the molecules is found. In contrast to the bulk, the isotropic-to-columnar transition of the confined DLC is continuous, shifts with decreasing pore diameter to lower temperatures, and exhibits a pronounced hysteresis between cooling and heating. These findings corroborate conclusions from previous neutron and x-ray-scattering experiments as well as optical birefringence measurements. Our study also indicates that the relative simple dielectric technique presented here is a quite efficient method in order to study the thermotropic orientational order of DLC-based nanocomposites
Bioprotectant glassforming solutions confined in porous silicon nanocapillaries
Glycerol and trehalose-glycerol binary solutions are glass-forming liquids
with remarkable bioprotectant properties. In this paper, we address the effects
of confining of these solutions in straight channels of diameter D=8 nm formed
by porous silicon. Neutron diffraction and incoherent quasielastic neutron
scattering are used to reveal the different effects of nanoconfinement and
addition of trehalose on the intermolecular structure and molecular dynamics of
the liquid and glassy phases, on a nanosecond timescale.Comment: Accepted for the 2008 MRS Fall Meeting Symposium TT proceeding
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