Electric birefringence of AOT/water solutions

The variation of the Kerr constant versus surfactant concentration in a solution of AOT in water was studied. The observed behavior shows changes when aggregates begin to form. allowing to measure the critical micelle concentration, with results coincident with those obtained through static light scattering and electric conductivity measurements. For concentrations higher than the critical micellar concentration the birefringence decreases proportional to the aggregates concentration The micelle contribution to birefringence is estimated to be negative.     

Light transmission, linear dichroism and birefringence of nematic/polymer dispersions

A theoretical study of light transmission, linear dichroism and birefringence of partially ordered dispersions of bipolar nematic droplets in a polymer matrix is presented. The treatment rests on the single scattering approach to an ensemble of uncorrelated and noninteracting anisotropic particles. Theoretical evaluations of the extinction cross sections and phase functions are performed in the anomalous diffraction approximation. Four basic model systems are analysed: PDLC and NCAP films in an external electric field, and PDLC and NCAP films under uniaxial mechanical deformation. The calculated dependences of the generalized parameters characterizing the light transmission, dichroism and birefringence on the angle of incidence of the probing light, droplet size and shape, refractive indices of the LC and polymer, and parameters of the external field are presented graphically and discussed in detail.     

Electric birefringence and streaming-electric birefringence of synthesized imogolite: the anisotropy of electric polarizability

Imogolite synthesized from sodium orthosilicate and aluminum trichloride was fractionated into four fractions by centrifuging at 12,000 x g (1h). The supernatant, which did not deposit by three times centrifugations, was used for all measurements. The signal of birefringence under a reversing electric pulse showed that the permanent dipole moment is negligibly small at low fields. The electric birefringence under a rectangular electric pulse is positive. The saturated value is proportional to the concentration of imogolite in the range of 0-0.1mg/ml and decreases rapidly with an increase of added salt concentration for NaCl and AgNO(3). It slightly depends on the pH of solution and is biggest in pure water. Then we have determined the anisotropy of electric polarizability (Deltaalpha) for imogolite in pure water at 0.05 mg/ml. Deltaalpha we obtained from the method decreases rapidly at low fields and slowly at high fields as shown in references [M. Matsumoto, Colloids Surf. A 148 (1999) 75, M. Matsumoto, Biophys. Chem. 58 (1996) 173]. It is approximately shown by the following expression, Deltaalpha=Deltaalpha(infinity)+(Deltaalpha(0)-Deltaalpha(infinity))/(1+KE), (Deltaalpha(0):Deltaalpha at E=0, Deltaalpha(infinity):Deltaalpha at E=infinity). Using this relation we can draw the curve of electric birefringence as a function of electric field and compare it with experimental values. The results, when Deltaalpha(0)=1.17x10(-28)Fm(2), Deltaalpha(infinity)=0.005x10(-28)Fm(2) and K=0.00031 m/V, are in good agreement with each other. In order to explain the reason why the anisotropy of electric polarizability rapidly decreases with an increase of electric field we propose that the difference of electrophoretic mobility between parts of colloidal particle causes the orientation of a rod like particle. The theoretical electric birefringence as a function of electric field we obtained is considerably in good agreement with the experimental values.     

Electric birefringence and conformation of polychlorohexylisocyanate in solutions

Electric birefringence was investigated for solutions of polychlorohexylisocyanate fractions for molecular weights 30·6 × 10⁴–1·2 × 10⁴ in tetrachloromethane.Experimentally found dispersion of the Kerr effect is used for estimating the coefficients of rotatory diffusion D_r of molecules. A comparison of rotatory diffusion D_r values with molecular weights M and intrinsic viscosities [η] of fractions shows that the value of D_rM[η] decreases with M. This illustrates the change in the conformation of molecules from a random coil to a rod.On the basis of experimental dependences of D_r and the Kerr constants K on M, the main structural parameters of the polymer investigated were determined: the number of monomer units in a segment, the projection of the length of the monomer unit on the axis of the molecule, the value of the dipole moment μ₀ of the monomer unit and the angle formed by μ₀ and the chain direction.     

Electric birefringence of discotic benzene-hexa-n-heptanoate in isotropic solution phase

Abstract

The electric birefringence of solutions of oblate discogen and prolate nematogen in dioxane has been examined. Discotic benzene-hexa-n-heptanoate (BH7) exhibits negative birefringence, while nematic 5CB and MBBA exhibit positive birefringences at low concentrations. Possible modes of solute orientation, compatible with the observed birefringence sign, are discussed. For BH7, it is suggested that the in-plane axes with larger optical polarizabilities align perpendicular to the field direction. The dependence of the specific Kerr constant on the solution concentration is also reported.     

Electric birefringence anomaly of solutions of ionically charged anisometric particles

The term “electric birefringence anomaly” is known as the electric birefringence (EB) signal that occurs in solutions of ionically charged anisometric particles in a narrow concentration region. The signal is of opposite sign to the normal birefringence that occurs below and above this narrow concentration region. The normal electric birefringence signals in the dilute and more concentrated regions are due to the orientation of the particles in the direction of the applied electric field. The origin for the anomalous signal was not completely understood until now.     

Electric field induced birefringence in a ferroelectric liquid crystal

We have measured the d.c. electric field dependence of the birefringence and conoscopic images for the smectic C* phase of a partially racemized ferroelectric chiral smectic liquid crystal CE-8. The experiments were performed using 50mu m thick homeotropic cells with lateral electrodes which created a d.c. electric field parallel to the smectic layers. The observed field induced birefringence shows a characteristic step-like behaviour which is due to the stepby-step unwinding of the helical structure in a sample with finite dimensions along the helical axis. In conoscopic observations we observe that these steps are associated with moving disclination lines that traverse the sample in the direction of the smectic layers. The observed electric field dependence of the birefringence is discussed in terms of the soliton-like unwinding of helical smectic structures and compared with the predictions of the Landau theory. A qualitatively good agreement is obtained.     

Unusual polymer dispersions—polypyrrole suspensions made of rings, frames, and platelets

Experimental results show that the polymerization of pyrrole in the presence of β-naphthalenesulfonic acid and different fluorosurfactants like perfluorooctanesulfonic acid, perfluorooctyldiethanolamide, and ammonium perfluorooctanoate leads to polypyrrole with special morphologies, such as rings or disks and rectangular frames or plates. The formation of these unusually shaped particles of polymer dispersions is explained by the chemical and colloidal peculiarities of the oxidative pyrrole polymerization with ammonium peroxodisulfate in aqueous medium.     

Electric birefringence in solutions of cellulose carbanilate as a function of molecular weight

Electro-optical, dynamo-optical and hydrodynamic properties of solutions of some fractions of cellulose carbanilate (CC) in dioxan have been investigated. In a variable electric field, strong dispersion of the Kerr effect is observed, indicating the dipole-orientational mechanism of electrical birefringence and its relaxation. A comparison of relaxation times of fractions with their molecular weights and intrinsic viscosities indicates that the mechanism responsible for the Kerr effect is the rotation of the molecule as a whole in an electric field (a kinetically rigid molecule). The dependence of relaxation time on molecular weight (M) shows that, with increase in M, the conformation of the CC molecule changes from a slightly curved rod to a rigid Gaussian coil. The same conclusion may be drawn from a study on the dependence of the equilibrium value of the Kerr constant on M. In the Gaussian range (high M), the Kerr effect depends on the longitudinal (with respect to the chain) component of the dipole moment formed by the CO bonds in the glucoside ring. At low M, the transverse components of the monomer dipoles begin to play an important part in birefringence.     

Electric birefringence of solutions of aromatic poly(amide hydrazide) in dimethyl sulphoxide

Birefringence in a pulsed electric field has been investigated for solutions of para-aromatic polyamid hydrazide (PAH) in dimethyl sulphoxide. The values of the specific Kerr constant K extrapolated to zero concentration have been determined. The experimental data are adequately described by the theoretical dependence of K on the contour length of the PAH molecules for kinetically rigid wormlike chains. The angle formed by the dipole moment of the monomer unit and the direction of the PAH chain was found to be 61.5 ± 1.5°. Comparison between relaxation times obtained from the curves of the decay of birefringence after the end of the electric pulse and the intrinsic viscosities of some PAH samples confirm the conclusion about high kinetic rigidity of PAH macromolecules.     

Electric properties of macromolecules. XIII. Transient electric birefringence of sodium polyethylenesulfonate solutions

The electric birefringence of sodium polyethylenesulfonate in water has been studied. For a low molecular weight fraction, the electric field dependence followed the Kerr law quite closely. For a higher molecular weight fraction, the specific Kerr constant was much larger, and pronounced saturation of the electric birefringence was observed in fields of 15 kv./cm. The specific Kerr constant increased with decreasing polyelectrolyte concentration in pure water. On the other hand, it decreased on isoionic dilution. The saturation effect was the more marked, the lower the polyelectrolyte concentration. The saturation behavior resembled that of permanent dipole orientation, but this mechanism was not supported by the build-up of the birefringence. The magnitudes of specific Kerr constants of various types of macromolecules in solution are tabulated and discussed.     

Electric conductance of dispersions of metal oxides in solutions of weak acids in mixed dioxane-water solvents

The electric conductance of solutions of sulfuric, oxalic, benzoic, and salicylic acid (up to 0.02 M) in dioxane-water mixed solvents (90% and 93% dioxane by mass) has been studied in the presence and absence of TiO(2) and Al(2)O(3) (0.5-5% by mass). TiO(2) and Al(2)O(3) enhanced the conductance of solutions of organic acids in aqueous dioxane. The conductance is interpreted in terms of adsorption of acid in molecular form, dissolution of ceramic oxides in form of anionic complexes, and leaching of acidic impurities from ceramic oxides.     

Electric field-induced birefringence, optical rotatory power and conoscopic measurements of a chiral antiferroelectric smectic liquid crystal

Using a photoelastic modulator-based novel set-up, the electric field-induced in-plane birefringence and the optical rotatory power (ORP) were measured of an antiferroelectric liquid crystalline compound (12OF1M7) in its various phases using 30 µm homeotropic cells. Some specific signatures of the in-plane birefringence and of the ORP for the various phases are being established. A relatively small threshold field is needed for the unwinding process of the antiferroelectric phase with a unit cell of four layers [SmC_A*(1/2)] compared with that for two layers [SmC_A*(0)]. On application of the electric field on the high temperature side of the SmC_A*(1/2) phase (80.1-81.5°C), a field-induced phase transition is shown to occur directly to the SmC* phase, whereas on the lower temperature side (79.4-80.1°C) the transition takes place to SmC* via the SmC_A*(1/3) phase. The in-plane birefringence exhibits a critical power law dependence for the SmC*-SmA transition. The ORP changes sign within the temperature range of the phase with a unit cell of three layers, reflecting a change in the handedness during this phase. Using tilted conoscopy, the results for the biaxiality and the apparent tilt angle for a smectic liquid crystal with a tilt angle greater than 18° in the ferroelectric phase are reported. The biaxiality implies the difference in the refractive indices between the two minor axes of the refractive index ellipsoid. The optical transmittance at visible and IR wavelengths for free-standing films reveal characteristic reflection bands for these phases. The modulated structures of the reflected bands appear just above the SmC_A* phase and below SmC_A*(1/3); these are possibly due to an easy deformation of the phase by the surfaces.     

Graphene dispersions

Aqueous dispersions of graphene are of interest to afford environmentally safe handing of graphene for coating, composite, and other material applications. The dispersion of graphene in water and some other solvents using surfactants, polymers, and other dispersants is reviewed and results show that nearly completely exfoliated graphene may be obtained at concentrations from 0.001 to 5% by weight in water. The molecular features promoting good dispersion are reviewed. A critical review of optical extinction shows that the visible absorption coefficients of graphene have been reported over the ranges of 12 to 66 cm²/mg at various wavelengths. The practice of energetically activating graphene in various solvents with various stabilizers followed by centrifugation to isolate the “good” dispersion components is fine for producing samples amenable to TEM analysis and quantification, but cannot be expected to drive value added production of products on the kg or higher scale. Such approaches lack practical application and often involve 90–99% wasted graphene. However, alternative approaches omitting centrifugation are yielding dispersions 0.5 to 5% by weight graphene, with higher yields likely in the near future. These dispersions yield effective extinctions of about 49 cm²/mg, in conformity with macroscopic optical analysis of single and few layer graphene.     

Dynamic optical birefringence

The measurements of dynamic optical birefringence is described, together with instrumentation for this purpose. Complex birefringence, stress- and strain-optical coefficients are determined simultaneously with complex mechanical properties (E and tan δ_m). Results are shown for poly(vinyl chloride) comparing data obtained in this study with earlier measurements. Optical birefringence is shown to be a very sensitive probe for molecular motions and is therefore a useful structural elucidation tool.     

Magnetic birefringence of minerals

The earliest reports of magnetically induced optical birefringence included data for liquids, magnetic fluids and colloidal suspensions. Recent work has shown that with relatively straightforward apparatus, when carefully designed and aligned, measurable effects can be recorded even for suspensions of relatively weak diamagnetic materials, including mineral particles. By recording the magnitude of the birefringence induced in magnetic fields of up to two Tesla, a method for the analysis of the magnetic and optical characteristics of these diamagnetic colloids is evidenced. The principles, apparatus and methodology involved are described and novel data reported for the minerals attapulgite, bentonite, hectorite, kaolinite, montmorillonite and vermiculite. Preliminary experiments using pulsed fields on vermiculite sols show that, in favourable circumstances, estimates of particle size can be made by analysing signal response rates.     

Electro-optical studies on synthetic polyelectrolytes—II: Electric birefringence of poly-N-butyl-4-vinylpyridinium salts in aqueous solution

The field strength dependence of the electric birefringence of poly- N-butyl-4-vinylpyridinium bromide in aqueous solution changes markedly with polymer concentration. Analysis of the orientation curves reveals that the mean electric polarizability ?Δα sharply increases when the concentration decreases, as a result of the extension of the chain, while the negative optical anisotropy factor remains unchanged. An empirical relationship, similar to that found for the reduced viscosity, is derived for the variation of electric polarizability with concentration.Increasing the ionic strength produces a coiling of the polyelectrolytic chain and a decrease of ?Δx. The replacement of the bromide ions by sulphate ions causes a conformational change evidenced by a decrease of ?Δα and of the relaxation time. This effect is interpreted by the formation of electrostatic bonding between the pyridinium rings on non-adjacent segments of the chain, and to a limited extend, by intermolecular bonding.The dependence of the relaxation time, extrapolated to infinite dilution, on the molecular weight of the samples is analysed on the basis of a worm-like chain model. The persistent length obtained is found of the same order of magnitude as in the case of other semi-rigid polymers. The predominant influence of the segmental orientation on the optical, electric or conformational parameters is discussed in detail.     

Electro-optical studies on synthetic polyelectrolytes—III: Electric birefringence of poly-n-alkyl-4-vinylpyridinium bromides in various solvents

The field strength dependences of the electric birefrigence of poly-N-butyl-4-vinylpyridinium (4-PVP · BuBr) samples have been measured in water and in various organic polar solvents. The observed change of sign of the birefringence, negative in water and positive in nitromethane, is explained by considering the respective influences of the intrinsic and form optical anisotropies. The lower degree of extension of the polyelectrolyte in some organic solvents is attributed to a lower degree of dissociation of the ionic sites. The influence of the dielectric constant and of the functional character of the solvent on the dissociation and solvation processes of the ionic sites is taken into account. Relaxation measurements in dimethylsulphoxide as a function of the molecular weight allow quantitative expression of the degrees of extension and of rigidity of the polyelectrolyte; the presence of aggregates has also been detected in DMSO solutions. A long alkyl chain, such as octyl, attached to the quaternary pyridinium groups strongly modifies the conformation of the polyelectrolyte and a polyelectrolyte-polysoap transition has been observed as a function of the solvent composition in water-ethanol mixtures.