Tahar Othman

Thermal hysteresis at the phase transition in liquid crystalline materials

Experimental results on antiferroelectric liquid crystalline materials are presented using techniques such as dielectric and electro-optical measurements. A demixing phenomenon has been observed for compounds exhibiting the ferroelectric SmC* phase in their phase sequence. This phenomenon is interpreted within the framework of a thermodynamic study. Large temperature hysteresis during cooling and heating has been observed and is discussed. The results of dielectric spectroscopy and electro-optical properties suggest that thermal hysteresis is related to the demixing phenomenon. The effect of cell thickness and spontaneous polarisation on the temperature range of this phenomenon has also been investigated.

Investigation of critical concentrations of poly(vinyl pyrrolidone) in N, N-dimethylformamide by a viscosity technique

The viscosity of solutions of poly(vinyl pyrrolidone) (PVP) in N,N-dimethylformamide over a wide range of concentrations and at different temperatures was measured. The polymer solutions exhibited a critical concentration c**, separating dilute solutions into extremely dilute solutions and dilute solutions. The viscosity–temperature dependence is discussed on the basis of a formula of the correlation between the true and the experimental relative viscosity. The results revealed that an abnormal viscosity of neutral PVP solutions in the dilute solution region (c › c**) and extremely dilute solutions (c ‹ c**) can be ascribed to interface effects. The polymeric solutes were readily adsorbed on the wall surface of the cone-plate rheometer, which greatly influenced the viscosity measurements of the PVP solutions and resulted in the apparent abnormal viscosity behaviour. The intrinsic viscosity [η] and the Huggins constant kH, were calculated. The solute adsorption behaviours and structural information of the polymer were discussed in depth.

Dielectric and thermodynamic studies of Smectic-A–Smectic-C* phase transition in antiferroelectric liquid crystal: thermal hysteresis

We present the experimental evidence and theoretical studies of the para–ferroelectric phase transition in the chiral liquid crystal C12HH. The investigated compound presents a large SmC* range. Both microscopic observation and dielectric measurements show thermal hysteresis and coexistence phases at paraelectric (SmA)–ferroelectric (SmC*) phase transition. The thermal hysteresis is analysed in samples with different planar cell thickness. The SmA–SmC* phase transition under electric field is also investigated. Finally, the Landau free energy density is used to analyse the experimental measurements, showing a satisfactory agreement with experimental data.

Complex formation between ovalbumin and strong polyanion PSSNa: Study of structure and properties

The mixture system of long-chain polyelectrolyte complexed with a globular protein was investigated based on dynamic light scattering and turbidimetric measurements. We have discussed at different pH values the influence of high salt concentration and mass ratio (protein:PSSNa) on the behavior of the mixture. In dilute concentration regime, the PSSNa chain contracts at pHc by patch binding. We found two critical values of mass ratio: The first corresponds to the maximum shrinking of PSSNa. The second indicates the system that became more stable where the number of proteins attached to the PSSNa chain was constant. The screen of electrostatic interaction shows a high contribution of hydrophobic interaction at large salt concentration to form the coacervates. By building phase diagram, the continuity of pHφ1 in over whole range of salt concentrations and the widening of pH window (pHφ1-pHφ2) were observed. At certain salt concentrations, we can obtain the coexistence of two types of complex particles formed by electrostatic and hydrophobic interactions.

Impact of macromolecular crowding on structure and properties of pepsin and trypsin

The change of conformation of pepsin and trypsin in absence and presence of a high molecular crowding agent has been characterized using dynamic light scattering (DLS). Structural properties were investigated as a function of chemical denaturant concentrations, the guanidine hydrochloride (GdmCl). The results showed that Ficoll 400, macromolecular crowding, has a strong effect on the chemical denaturation process of these two proteins. The changes of measured hydrodynamic radius are attributed to the enhancement effect of the crowder agent due to the excluded volume effects. The data proved that the large size of a macromolecular crowder plays a crucial role on the conformation of a protein in its unfolded states. The values of interactions Parameter kd of complex particles and a number of proteins npr attached on the Ficoll 400 measured in different GdmCl concentrations. The effect of aging on the structure of complex are studied by small angle light scattering (SALS).

Capillary-induced giant elastic dipoles in thin nematic films

Significance The coupling of capillarity and elasticity at liquid crystal interfaces is an emerging topic driven by the ability of both mechanisms to guide the self-assembly of microparticles into ordered structures. In this research, we explore fundamental questions about how capillary deformations at liquid crystal interfaces can affect the texture around particles captured in thin nematic films. We report for the first time, to our knowledge, the formation of giant elastic dipoles and develop a fuller understanding of their creation. The understanding of such elastocapillary effects could lead to new opportunities to control the spatial organization of colloidal particles and develop novel classes of reconfigurable materials that are highly interesting for technological applications. Directed and true self-assembly mechanisms in nematic liquid crystal colloids rely on specific interactions between microparticles and the topological defects of the matrix. Most ordered structures formed in thin nematic cells are thus based on elastic multipoles consisting of a particle and nearby defects. Here, we report, for the first time to our knowledge, the existence of giant elastic dipoles arising from particles dispersed in free nematic liquid crystal films. We discuss the role of capillarity and film thickness on the dimensions of the dipoles and explain their main features with a simple 2D model. Coupling of capillarity with nematic elasticity could offer ways to tune finely the spatial organization of complex colloidal systems.

Electrically controllable multicolor cholesteric laser

A new strategy to obtain multicolor lasing from cholesteric liquid crystals is presented. A four layer cell is prepared with three different cholesteric layers and a layer containing a photoluminescent dye. The three cholesteric mixtures are prepared so that their photonic band gaps are partially overlapped. Through this combination, two laser lines are obtained in the same spot under the pumping beam irradiation. Eventually, one of the laser lines can be switched off if an electric field is applied to the first or the last cholesteric layer.

Thermodynamic properties of N,N-dimethylformamide + water

The excess viscosities, ηE, and excess energy of activation (ΔΕη)Ε of dynamic viscosity have been investigated by using dynamic viscosity measurements for N,N-dimethylformamide + water (DMFW) mixtures over the entire range of mole fractions at five different temperatures. The results were also fitted with the Redlich–Kister equation. This system exhibited very large positive values of ηE and (ΔΕη)Ε due to the increased dipole–dipole interactions and correlation length between unlike molecules. The activation parameters ΔΗσ and ΔSσ have been also calculated, and they show that the critical region has an important effect on the dynamic viscosity properties. The results obtained are discussed from the viewpoint of the existence of interactions between the components.

Theoretical and experimental study of the effect of fluorine substitution on stability and viscoelastic properties of smectic phases

Some chiral series with benzoate cores (CnHH, CnF3 and CnF2) are reported. These series (hydrogenous, monofluoro-substituted in positions 2 and 3 of the first phenyl ring near the chiral chain) display very rich polymorphism, including SmC*, SmC*α, SmC*FI1, SmC*FI2 and SmC*A phases. Based on the theoretical Hammaneh–Taylor model, we analyze phase diagrams and physical properties, including spontaneous polarization, transverse dipole moment and torsion of the studied compounds. This description offers an insight into the factors that affect the stability of the intermediate smectic phases. The effect of fluoro substitution on the visco-elastic properties is investigated.

Effect of enantiomeric excess on the smectic-C* phases under electric field

A series of enantiomeric mixtures of 4-(1-methylheptyloxycarbonyl) phenol 4-octyloxybiphenyl-4-carbonate (MHPOBC) is reported to understand the structures and stability conditions of emerging liquid crystal phases. The mesomorphic properties are studied by microscopic observation, electro-optic and dielectric measurements. The electric field–concentration (E–x) phase diagram for each phase at constant temperature is constructed. The complete electric field–temperature (E–T) phase diagrams for several enantiomeric mixtures of the antiferroelectric MHPOBC are established. These phase diagrams show the influence of optical purity on the phase stability and the unwinding threshold.