Self-organization of N<Superscript>*</Superscript> inclusions in SmC<Superscript>*</Superscript> free-standing films

The behaviour of freely suspended smectic-C* ( SmC^*) films at the bulk SmC^*-cholesteric ( N^*) phase transition has been investigated using polarized-reflected-light microscopy. Our experimental observations show that above the bulk SmC^*- N^* phase transition the N^* order appears in different ways according to the film thickness. In thin films, the conventional layer-by-layer thinning occurs. In films of intermediate thickness N^* inclusions nucleate inside the SmC^* film. The distortions of the in-plane orientational order of the SmC^* host phase induce elastic interactions between the inclusions and lead to their self-organization in chain-like structures. Both the dynamic of the chaining and the parameters driving the equilibrium distance between the inclusions in the chain are investigated. In thick films, N^* fingers grow inside the film. The influence of the experimental conditions on the various processes is analysed. Received 1 July 2002 / Published online: 15 April 2003 RID="a" ID="a"e-mail: Philippe.cluzeau@univ-lille1.fr     

A biaxial nematic gel phase in aqueous vanadium pentoxide suspensions

Aqueous suspensions of V₂O₅ ribbons are one of the very few examples of mineral liquid crystals. In the concentrated regime, we show that these ribbons organize in a biaxial nematic gel phase. A Couette shear cell was used to produce a well oriented sample for in situ synchrotron X-ray scattering studies. We observed two perpendicular anisotropic sections of reciprocal space, which proves the biaxial symmetry of the nematic order. The thermodynamic and flow properties of the biaxial nematic are well described by hard-core theories. We suggest the use of a shear geometry to produce and investigate single domains of other biaxial nematics, reported but still questioned in the literature. Received 26 May 1999     

Chain length dependence of anomalous swelling in multilamellar lipid vesicles

Using small-angle X-ray scattering, the repeat distance vs. temperature is measured for a homologous series of multilamellar vesicles of lecithins with varying acyl chain length in excess water condition around the lipid main transition. A systematic chain length dependence is found which is in accordance with a bending rigidity renormalization and critical unbinding of the lamellae close to the transition, as previously suggested in H?nger et al. [Phys. Rev. Lett. 72, 3911 (1994)]. Received 13 January 1999 and Received in final form 6 September 1999     

Phase behavior of binary blends of symmetric polystyrene-polybutadiene diblock copolymers studied using SANS

Binary blends of compositionally symmetric diblock copolymers are investigated using small-angle neutron scattering. The study focuses on the miscibility of blends of polystyrene-polybutadiene diblock copolymers as a function of chain length ratio and blend composition, and the results are related to the theoretical phase diagram put forward by M.W. Matsen (J. Chem. Phys. 103, 3268 (1995)). Three different low molar mass copolymers were blended with a high molar mass copolymer. We find very good coincidence with the theoretical phase diagram obtained. Only for blends having a chain length ratio of 0.06, theory predicts that a larger amount of short copolymers can be dissolved in the matrix of long copolymers, and vice versa. With the latter blends and volume fractions of short chains between 0.11 and 0.70, the second-order Bragg-peaks do not vanish, which indicates that the lamellae are asymmetric. Received: 9 February 1998 / Revised: 20 April 1998 / Accepted: 24 April 1998     

Experimental determination of the translational kinetic energy of liquid and solid hydrogen

TOSCA is a novel inelastic spectrometer operating on the pulsed neutron source ISIS (UK). It covers a wide momentum and energy range, even though its kinematic region is close to a line in the (k, E) plane. Its use is mainly intended for vibrational spectroscopy. However, taking advantage of its good resolving power, we have carried out a test experiment aimed to use this instrument to measure the centre of mass kinetic energy of molecular hydrogen. The experiment was successful and we have obtained the translational kinetic energy of liquid and solid para-hydrogen improving the overall accuracy by almost an order of magnitude with respect to previous determinations. The data are compared with the results of a Path Integral Monte-Carlo simulations with almost perfect agreement. We have demonstrated that TOSCA can be used for measuring the translational kinetic energy of small molecular systems, taking advantage of the intrinsic incoherence that is introduced in the scattering process by the intra-molecular transitions. Received 9 June 1999     

Collective dynamics and self-diffusion in a diblock copolymer melt in the body-centered cubic phase

The structure and dynamics of a strongly asymmetric poly(ethylene propylene)-poly(dimethylsiloxane) (PEP-PDMS) diblock copolymer in the melt have been studied over a wide temperature range. Small-angle neutron scattering reveals that the sample exhibits two stable phases in this temperature range: Above the order-to-disorder transition temperature, it is disordered, whereas the domain structure is body-centered cubic (bcc) below, being stable down to the lowest temperatures measured. In the disordered state, dynamic light scattering (DLS) in the polarized geometry reveals the heterogeneity mode and the cluster mode. In the bcc phase, the PEP and the PDMS blocks form the micellar cores and the matrix, respectively. Here, two modes are observed in DLS, and the diffusion coefficients measured using pulsed field gradient (PFG) NMR are broadly distributed with the most probable diffusion coefficient coinciding with the slow DLS mode. We attribute the fast process in the bcc state to concentration fluctuations of the micellar cores (PEP), relaxing by mutual diffusion of the micelles with copolymers dissolved in the PDMS matrix. The slower process in the bcc state is ascribed to activated long-range self-diffusion of single copolymers from micelle to micelle through the PDMS matrix. This assignment is corroborated by the good coincidence of the reduced diffusivities with the ones from the literature. However, this mode may also be assigned to the rearrangement of entire micelles.This paper is dedicated to the memory of G. Fleischer     

Reentrant isotropic-calamitic nematic phase transition in potassium laurate-decanol- D<Subscript>2</Subscript>O mixtures

The phase diagram is an interesting field of research, particularly in lyotropic liquid crystals (LLC). In this way, one of the most important phase diagrams of this LLC system was reported by Yu and Saupe. Two uniaxial (calamitic--N(C) and discotic--N(D)) and one biaxial nematic (N(B)) phases were determined by these authors. Furthermore, in this phase diagram the classical isotropic phase (I was observed at high temperature as well as a reentrant isotropic phase (I(RE)) which takes place at lower temperature. Later, this phase diagram was also studied by several authors and in all cases the I(RE)-N(C)-I phase transitions were not observed. In this work, we present a study of this phase diagram through digital image processing and refractometry optical techniques. The occurrence of these phase transitions is investigated and characterized. In addition, the order parameter is obtained based on the Vuks hypothesis from a particular point, in the range of the N(C) phase, where the absolute value of the optical birefringence (Deltan is maximum.     

Dynamic anisotropy and heterogeneity of polystyrene thin films as studied by inelastic neutron scattering

We studied the dynamic anisotropy and heterogeniety of polystyrene thin films in glassy state with inelastic neutorn scattering. Adjusting the scattering vector to the directions parallel and perpendicular to the film surface, we observed the elastic scattering intensities as a function of temperature. It was found for the 200 A film that the elastic intesity decreased with increasing temperature more rapidly in the perpendicular direction than in the pararell direction, showing the higher mobility in the perpendicular direction. However, such dynamical anisotropy was not observed in the 1000 A film. The decrease in the mobility was observed with the film thickness in both the directions. These results were explained in terms of an interface hard layer. We also evelauated the dynamical heterogeniety from the non-Gaussian parameter A0, which increased with decreasing the film thickness, showing the increase in the dynamical heterogeneity. Assuming a simple bi-layer model consisting of the interface hard layer and the bulk-like layer, we analyzed the thickness dependence of the non-Gaussian parameter A0 and the mean square displacement (u2) to find that the hard layer has a thickness of approximately 130 A and a mean square displacement of approximately 0.018 A2 at 230 K.     

Dynamical molecular disorder and diffuse scattering in an alkane/urea incommensurate inclusion compound

The diffraction pattern of n-alkane/urea inclusion compounds is known to contain a broad diffuse scattering related to the molecular form factor of the alkane species, indicating individual rotational and/or translational disorder of these molecules. Inelastic coherent neutron scattering reveals for the first time the entirely dynamical character of the alkane molecular disorder around room temperature. This observation fully agrees with a recent model of the coherent scattering cross section of totally uncorrelated motions. In the ordered phase this diffuse scattering remains, but elastic, and very low energy molecular vibrational modes are evidenced. These observations are discussed in relation with previous incoherent neutron scattering results. Received 9 February 2001 and Received in final form 1st August 2001     

Influence of the nematic order on the rheology and conformation of stretched comb-like liquid crystalline polymers

We have studied the rheology and the conformation of stretched comb-like liquid-crystalline polymers. Both the influence of the comb-like structure and the specific effect of the nematic interaction on the dynamics are investigated. For this purpose, two isomers of a comb-like polymetacrylate polymer, of well-defined molecular weights, were synthesized: one displays a nematic phase over a wide range of temperature, the other one has only an isotropic phase. Even with high degrees of polymerization N, between 40 and 1000, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nematic phase, the chain dynamics is more cooperative than for a usual linear polymer. Small-angle neutron scattering has been used in order to determine the evolution of the chain conformation after stretching, as a function of the duration of relaxation t _r. The conformation can be described with two parameters only: , the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine: is always smaller than (the deformation ratio of the whole sample). In the isotropic phase, has a constant value, while pincreases as tr. This latter behavior is not that expected for non-entangled chains, in which p varies as t _r ^1/2 (Rouse model). In the nematic phase, decreases as a stretched exponential function of t _r, while p remains constant. The dynamics of the comb-like polymers is discussed in terms of living clusters from which junctions are produced by interactions between side chains. The nematic interaction increases the lifetime of these junctions and, strikingly, the relaxation is the same at all scales of the whole polymer chain. Received 5 May 1999 and Received in final form 18 October 1999     

Extraction of G<Subscript>M</Subscript><Superscript>n</Superscript> from inclusive electron scattering on D, <Superscript>4</Superscript>He

We use the relation between Structure Functions (SFs) of nuclei A and nucleons N in order to fomulate a criterion which isolates the QE part out of the total inclusive cross-section. From data points around the QEP we extract the reduced neutron magnetic form factor 〈α _n = G _M ⁿ/μ _n G _d〉. The latter shows an unexpected decrease up to Q ² = 10GeV^2, the largest measured.     

Field-induced phase transitions and reversible field-induced inversion of chirality in tilted smectic phases of bent-core mesogens

Three homologous achiral five-ring bent-core mesogens are presented where 4-chlororesorcinol is the central core and the aromatic rings are linked by ester groups. These compounds form smectic phases with a tilted arrangement of the molecules (tilt angle ≈ 45^°). On cooling the isotropic liquid this phase adopts a fan-like texture which shows for two homologues at relatively high electric fields ( 25-35V μm^-1) an antiferroelectric electro-optical response based on the collective rotation of the molecules around their long axes. At lower temperature the application of a sufficiently high electric field leads to a continuous transition into a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. These domains can be reversibly switched into a state of opposite chirality by reversal of the field polarity. This switching is bistable and shows a current response typical for a ferroelectric ground state. The possible mechanism of the field-induced phase transition, of the ferroelectric switching and of the field-induced inversion of the chirality is discussed on the base of XRD, ¹³C- and ¹H-NMR investigations, dielectric and electro-optical measurements.     

How and when can one identify hadronic molecules in the baryon spectrum<Superscript>⋆</Superscript>

A method to identify hadronic molecules in the particle spectrum is reviewed and the conditions for its applicability discussed. Special emphasis is put on the discussion of molecule candidates in the baryon spectrum.     

Observation of a re-entrant kinetic glass transition in a micellar system with temperature-dependent attractive interaction

We detect in a tri-block co-polymer micellar system an ergodic-to-nonergodic-to-ergodic transition, as a function of temperature, in a range of concentrations, by photon correlation measurements. The shear viscosity is also shown to jump two order of magnitude at these transition temperatures. Surprisingly, the structure factor as measured by small angle neutron scattering shows a marked narrowing at the structural arrest state. Rationalization of these results with the existence of an attractive branch in the phase diagram of an attractive colloid system predicted by mode coupling theory is made. Received 9 Aprile 2002     

Temperature and concentration dependence of SANS spectra of aqueous solutions of short-chain amphiphiles

The small-angle neutron scattering (SANS) of some aqueous solutions of short-chain amphiphiles (glycols, diglycols, diols) has been measured as a function of concentration and temperature. The analysis of the spectra in terms of the Teubner-Strey phenomelogical formula indicates that, on increasing the concentration of the amphiphile, the structure of all these systems evolves in a similar way, i.e. a transition from disordered structures toward correlated aggregates (microstructures). The transition is depressed by increasing the temperature.     

Magnetic properties of LaNi <Emphasis Type="Bold">1-</Emphasis><Emphasis Type="Bold">x</Emphasis>Mn <Emphasis Type="Bold">x</Emphasis>O <Emphasis Type="Bold">3+δ</Emphasis> perovskites

Magnetic measurements have been carried out in different LaNi_1-xMn_xO _{3 + δ} samples with 0.1 ⩽ x ⩽ 0.9. All these samples show two magnetic anomalies, one at relatively high temperature characteristic of a ferromagnetic ordering and the other at low temperature, typical of magnetic relaxation phenomena. Neutron diffraction patterns indicate that long-range ferromagnetic ordering is only achieved for x ≥ 0.5. Neutron patterns of LaNi_0.5Mn_0.5O _{3 + δ} samples show an ordered arrangement of Ni and Mn atoms in the perovskite lattice. LaNi_0.5Mn_0.5O _{3 + δ} is then, a double perovskite A₂BB'O₆ whereas Ni and Mn atoms are randomly distributed for the rest of the samples. X-ray magnetic circular dichroism experiments confirm the presence of collinear ferromagnetism in LaNi_0.5Mn_0.5O _{3 + δ} . The role of competitive magnetic interactions, structural disorder, magnetic anisotropy and magnetic disaccommodation is also discussed Received 19 July 2002 / Received in final form 23 October 2002 Published online 31 December 2002     

Prospect of triggering the <Superscript>178m2</Superscript>Hf isomer and the role of resonance conversion

A mechanism of triggering the 12.7keV E3 transition, based on the new decay mode of the 31y isomer via resonance internal conversion and emission of a 1.4keV X-ray quantum, is considered. Actually, this decay mode was observed previously in the decay of 45- and 46-fold ions of ¹²⁵Te . For the purpose of triggering, the atomic radiative vertex has to be induced by resonance radiation. This mechanism makes triggering by an order of magnitude more efficient than triggering a bare nucleus, and is achieved at a lower combination frequency. An experiment is proposed for the direct observation of the new decay mode. This also offers a new way of resonance scattering of these X-rays. Triggering through higher-lying 2573 and 2805keV states is also considered. The results are extended to the general problem of triggering. The main obstacle for enhancing the efficiency is a high internal conversion rate. For this reason, shape isomers with low multipole order --E1 , M1 , and with a high enough energy of triggering transition are of interest for triggering. The partial ionization of the outer electrons will also help. The same recommendations hold for triggering isomers in laser-produced plasma.