Hidden solidlike properties in the isotropic phase of the 8CB liquid crystal.

Novel dynamic experiments have enabled the identification of a macroscopic solidlike response in the isotropic phase of a low molecular weight liquid crystal, 4,4'-n-octylcyanobiphenyl (8CB). This unknown property indicates that the low frequency shear elasticity identified in the isotropic phase of liquid crystal polymers is not reminiscent from the glass transition but reveals likely a generic property of the liquid state. The comparison to high molecular weight liquid crystals indicates, however, that the shear modulus is much enhanced when the liquid crystal moieties are attached to a polymer chain. The macroscopic length scales probed (0.050-0.100 mm) exclude wall-induced effects.

Critical behavior of nanoparticle-containing binary liquid mixtures.

Simultaneous measurements of small-angle neutron scattering and dynamic light scattering have been performed on a binary mixture of partially miscible liquids, 2,6-dimethylpyridine and water. At critical composition the temperature dependence of the correlation length of fluctuations in composition is strongly affected by the addition of nanoparticles of a triblock copolymer polyethylene oxide-polypropylene oxide-polyethylene oxide. A crossover between Ising-type critical behavior and mean-field critical behavior is observed when the correlation length is equal to the size of the nanoparticles.

[Respiratory bronchiolitis and respiratory bronchiolitis-associated interstitial lung disease]. / Bronchiolite respiratoire et bronchiolite respiratoire avec pneumopathie interstitielle.

The increasing use of chest CT imaging in medical practice rises the likelihood of the general practitioner to be confronted with cases of interstitial lung disease. Respiratory bronchiolitis (RB) and respiratory bronchiolitis-associated interstitial lung disease (RB-ILD) are two smoking-related lung damages that may have important implications for the patient's management. The authors present in this paper a review of current knowledge of the epidemiology, clinical features, prognosis, and treatment options of RB and RB-ILD.

Self-assembly of PEGylated peptide conjugates containing a modified amyloid beta-peptide fragment.

The self-assembly of PEGylated peptides containing a modified sequence from the amyloid beta peptide, FFKLVFF, has been studied in aqueous solution. PEG molar masses PEG1k, PEG2k, and PEG10k were used in the conjugates. It is shown that the three FFKLVFF-PEG hybrids form fibrils comprising a FFKLVFF core and a PEG corona. The beta-sheet secondary structure of the peptide is retained in the FFKLVFF fibril core. At sufficiently high concentrations, FFKLVFF-PEG1k and FFKLVFF-PEG2k form a nematic phase, while PEG10k-FFKLVFF exhibits a hexagonal columnar phase. Simultaneous small angle neutron scattering/shear flow experiments were performed to study the shear flow alignment of the nematic and hexagonal liquid crystal phases. On drying, PEG crystallization occurs without disruption of the FFKLVFF beta-sheet structure leading to characteristic peaks in the X-ray diffraction pattern and FTIR spectra. The stability of beta-sheet structures was also studied in blends of FFKLVFF-PEG conjugates with poly(acrylic acid) (PAA). While PEG crystallization is only observed up to 25% PAA content in the blends, the FFKLVFF beta-sheet structure is retained up to 75% PAA.

Pressure effects revealed by small angle neutron scattering on block copolymer gels.

We study the effects of hydrostatic pressure (P) on aqueous solutions and gels of the block copolymer B(20)E(610) (E, oxyethylene; B, oxybutylene; subscripts, number of repeats), by performing simultaneous small angle neutron scattering/pressure experiments. Micellar cubic gels were studied for 9.5 and 4.5 wt % B(20)E(610) at T = 20-80 and 35-55 degrees C, respectively, while micellar isotropic solutions where studied for 4.5 wt % B(20)E(610) at T > 55 degrees C. We observed that the interplanar distance d 110 (cubic unit cell parameter a = [see text for formula]) decreases while the correlation length of the cubic order (delta) increases, upon increasing P at a fixed T for 9.5 wt % B(20)E(610). The construction of master curves for d(110) and delta corresponding to 9.5 wt % B(20)E(610) proved the correlation between changes in T and P. Neither d(110) and delta nor the cubic-isotropic phase transition temperature was affected by the applied pressure for 4.5 wt % B(20)E(610). The dramatic contrast between the pressure-induced behavior observed for 9.5 and 4.5 wt % B(20)E(610) suggests that pressure induced effects might be more effectively transmitted through samples that present wider domains of cubic structure order (9.5 wt % compared to 4.5 wt % B(20)E(610)).

Rich polymorphism of a rod-like liquid crystal (8CB) confined in two types of unidirectional nanopores.

We present a neutron and X-rays scattering study of the phase transitions of 4-n-octyl-4' -cyanobiphenyl (8CB) confined in unidirectional nanopores of porous alumina and porous silicon (PSi) membranes with an average diameter of 30 nm. Spatial confinement reveals a rich polymorphism, with at least four different low temperature phases in addition to the smectic A phase. The structural study as a function of thermal treatments and conditions of spatial confinement allows us to get insights into the formation of these phases and their relative stability. It gives the first description of the complete phase behavior of 8CB confined in PSi and provides a direct comparison with results obtained in bulk conditions and in similar geometric conditions of confinement but with reduced quenched disorder effects using alumina anopore membranes.

Micelles of a diblock copolymer of styrene and ethylene oxide in mixtures of 2,6-lutidine and water.

We studied the micelle formation of a diblock copolymer of styrene and ethylene oxide in mixtures of 2,6-dimethylpyridine (2,6-lutidine) and water. Micelles are formed in a broad solvent composition range with a volume fraction of water ranging from 0.05 to 0.85, where neither polystyrene nor polyethylene oxide homopolymers are soluble. The diffusion behavior of pure solvent mixtures and in solutions of copolymer micelles is reported. In LTD/water mixtures, two diffusive processes corresponding to self-difusion and two modes belonging to mutual diffusion and diffusion of solvent clusters have been found. In copolymer solutions, the mode of copolymer micelle diffusion replaces the mode of solvent cluster diffusion. Quasielastic light scattering, small-angle neutron scattering, and pulsed-field gradient NMR have been employed in our study.

The frozen state in the liquid phase of side-chain liquid-crystal polymers.

Quenched isotropic melts of side-chain liquid-crystal polymers reveal surprisingly an anisotropic polymer conformation. This small-angle neutron-scattering (SANS) result is consistent with the identification of a macroscopic, solidlike response in the isotropic phase. Both experiments (rheology and SANS) indicate that the polymer system appears frozen on millimeter length scales and at the time scales of the observation. This result implies that the flow behavior is not the terminal behavior and that cross-links or entanglements are not a necessary condition to provide elasticity in melts.

Solid-like rheological response of non-entangled polymers in the molten state.

We show that non-entangled polymers display an elastic-like behaviour at a macroscopic scale (probed at some 0.100 mm thickness) up to at least hundred degrees above the glass transition temperature. This observation, found under non-slippage conditions, both for side-chain liquid crystalline polymers and ordinary polymers, is in contradiction with the typically found flow behaviour of polymer melt. Our measurements were carried out with a conventional rheometer at thicknesses of several tenths millimetres. Thus, we were probing bulk properties. The observed elasticity supposedly implies that even in the melt the chains experience a cohesive effect of macroscopic distances, involving collective motions over time scales longer than the individual relaxation time of an individual polymer chain. The detection of such a solid-like property of molten non-entangled polymers is of considerable importance for a better understanding of the polymer dynamics.

Structure and texture of anisotropic nematic gels.

Anisotropic nematic gels are prepared via in situ polymerization of diacrylate monomers in an oriented nematic liquid crystal (LC) matrix. The structure of the gels is studied from micrometer to nanometer scales by optical microscopy, small angle neutron scattering, and theta/2theta light scattering. A strong anisotropy is evidenced at all scales without electric field for both mesogenic and nonmesogenic monomers. The gel network can be pictured as an ordered but strongly distorted and polydisperse structure with two characteristic sizes: the mean size of the polymer objects and a correlation length between these objects, corresponding to the mean-size of the LC domains, which are estimated from neutron and light scattering results to be of the order of some tens of nanometers and some micrometers, respectively. Moreover, a sheet-like structure of the polymer network is evidenced. When an electric field is applied, one part of the LC switches while the other part remains anchored to the polymer network. The electric field dependence of the volume fraction of anchored LC is estimated from the analysis of the light scattering data. We emphasize systematic correlations between structure and electro-optical properties of the gels.

Pressure-induced reduction of the Landau-Peierls instabilities in a side-chain polymer liquid crystal with reentrant polymorphism.

Cyanobiphenyl mesogens are known to exhibit partially bilayered smectic A (S(Ad)) and also reentrant nematic (N(re)) phases. Nematic and smectic orders are coupled parameters which depend both on temperature and pressure. We report the first structural study of the influence of a hydrostatic pressure on the smectic phase. This study was carried out on a side-chain liquid crystalline polymer, by neutron diffraction using two specifically designed pressure cells. These results concluded first that the pressure acts on the phase elastic constants via a reduction of the layer fluctuations giving rise to a hardening of the phase together with an extension of the smectic domain towards higher temperatures. Second, the S(Ad)-N(re) phase transition temperature remains unchanged in the studied pressure range revealing that the polymer component plays an important role which allows us to subtract the associated packing interactions from the pressure-induced volume reduction.

Observation of shear-induced nematic-isotropic transition in side-chain liquid crystal polymers.

Flow-induced phase transitions are a fundamental (but poorly understood) property of non-equilibrium systems, and are also of practical importance for tuning the processing conditions for plastics, petroleum products, and other related materials. Recognition that polymers may exhibit liquid crystal properties has led to the discovery of the first tailored side-chain liquid crystal polymers (SCLCPs), which are formed by inserting a spacer between the main polymer chain and the lateral mesogen liquid-crystalline graftings. Subsequent research has sought to understand the nature of the coupling between the main polymer chain and the mesogens, with a view to obtaining better control of the properties of these tailored structures. We show here that the parallel or perpendicular orientation of the mesogens with respect to the main chain can be reversed by the application of an external field produced by a shear flow, demonstrating the existence of an isotropic nematic phase transition in SCLCPs. Such a transition, which was theoretically predicted for low-molecular-weight liquid crystals but never observed, is shown to be a general property of SCLCPs too. We expect that these SCLCPs will prove to be good candidate systems for the experimental study of these non-equilibrium phenomena.

Shear induced smectic- A-smectic- C transition in side-chain liquid-crystalline polymers

We show that the smectic layer orientation in side-chain liquid-crystalline polymers depends on the molecular weight and the shear rate. Parallel orientation is obtained with the low molecular weight polymer and reveals a shear induced decrease of the smectic layer thickness. In contrast with usual liquid crystals, the shear torque acts at the mesogen scale through an efficient coupling between the polymer main chain and the mesogens resulting in a smectic- A-smectic- C transition.