Viscous Behavior and Shear-Induced Structural Changes in Perfectly Oriented Liquid Crystals

The anisotropy of the viscosity and shear-induced structural changes are studied via nonequilibrium molecular dynamics (NEMD) simulations for two types of model liquid crystals which possess both isotropic and smectic phases. These models are (a) perfectly oriented Gay–Berne particles and (b) r ^–12-soft-spheres plus an r ^–6-interaction with a P ₂-anisotropy. Results are presented for the Miesowicz viscosity coefficients in the nematic phase. Presmectic effects are observed. Structural changes are revealed by snapshots of configurations and by the static structure factor, presented in analogy to scattering experiments. The shear-induced transition from the smectic to the nematic phase is analyzed. Similarities between magnetorheological fluids and discotic systems which can form columnar phases are discussed.     

Organoclay/thermotropic liquid crystalline polymer nanocomposites. Part II: shear-induced phase separation

Experimental studies on a kind of thermotropic liquid crystalline polymer (TLCP) containing 30% p-hydroxybenzoic acid (HBA), 35% hydroquinone (HQ), and 35% sebacic acid (SA) in mole fractions and its nanocomposite (TC3) containing 3.0 wt% organoclay are reported. The structures and dynamics of shear-induced phase separation and the effects of these structures on the macroscopic rheological properties of the nanocomposite are characterized under different shear conditions at 190 °C, which is in the nematic transition region of TLCP. The molecular level interactions between organoclay and TLCP molecules form a percolated-network structure in the composite, causing the composite to display complex viscosity with more than two orders of magnitude greater than that of TLCP in linear regions. However, such a network structure is easily destroyed in steady shear deformation, and it does not recover. Polarized optical microscopy (POM) equipped with a Cambridge shear system and transmission electron microscopy (TEM) confirm a shear-induced phase separation phenomenon during steady shear deformation. Two phases are observed in POM and TEM, with TLCP-rich and organoclay-rich phases. Steady shear at a small shear rate is effective to separate the two phases for characterizations.     

Molecular motions of banana-shaped liquid crystals studied by NMR spectroscopy

Recent studies based on NMR spectroscopy put in evidence the occurrence of a slow motion regime in bent-shaped liquid crystals having the aromatic core formed by five phenyl rings linked by ester groups both in the isotropic and in the nematic phases. In this paper a brief overview of these NMR studies is presented and additional new relaxation data recorded on a banana-shaped mesogen are reported and discussed. In particular, proton spin-lattice relaxation times, T₁, were acquired at different Larmor frequencies from 8 MHz to 5 kHz by means of NMR relaxometry in a wide range of temperatures from the isotropic to the crystalline phases. These data confirm that NMR relaxation in bent-core liquid crystals is affected by much slower dynamics than that observed in common rod-like liquid crystals and that overall molecular reorientations are responsible of this slow motion regime. This finding is discussed in the frame of the results obtained by means of several NMR methods, such as ²H NMR T₂ analysis and ¹H self-diffusion NMR measurements, on the same bent-core molecule.     

A-B Phase Conversion and Coexistence of Superfluid 3He in Aerogel

We have studied phase transition of superfluid ³He at 2.4 MPa in cylindrical aerogel by NMR method. When the liquid is cooled down from the normal state, the A-like phase appears below superfluid transition temperature T _c ^a which is suppressed in comparison with the transition temperature of the bulk liquid. With further cooling below the certain temperature T _ab,c ^a , the A-like phase is converted into the B-like phase gradually. Both phases stably coexist within about 90 μK. When you keep the temperature constant in which both phases coexist, the A-B phase conversion stops. With furthermore cooling, the whole liquid becomes the B-like phase. The cwNMR spectra at the coexistence state suggest that the B-like phase is not uniformly distributed in the A-like phase like a large number of small bubbles in a liquid, but separated as a whole from the A-like phase. By applying a field gradient which changes as a function of square of radius, we found that the A-like phase is in the edge part with a cylindrical shape and the B-like phase is in the central part with a columnar shape.      

Rheological characterization and thermal degradation of PTFE

PTFE (¯M _n=5×10⁶), when heated near the melting temperature (335 to 337 C) while in contact with carbon black, is characterized by an effective viscosity and a thermal stability which are orders of magnitude lower than those found in the absence of the contacting high surface area material. The penetration of the PTFE into the porous carbon black occurs by the spreading of a very thin polymer film followed by a thickening of this film with time at temperature until a limiting concentration is reached. The lower the average molecular weight of the PTFE, the more rapidly it penetrates into the porous material. Similar phenomena have been observed with high molecular weight PTFE heated near the melting temperature while contacting high surface area metal blacks or porous sintered metals.     

Structure and mechanical properties of polyethylene-fullerene composites

The microhardness of films of fullerene-polyethylene composites prepared by gelation from semidilute solution, using ultrahigh molecular weight polyethylene (PE) (6×10⁶), has been determined. The composite materials were characterized by optical microscopy and X-ray diffraction techniques. The microhardness of the films is shown to increase notably with the concentration of fullerene particles within the films. In addition, a substantial hardening of the composites is obtained after annealing the materials at high temperatures (T _a=130 °C) and long annealing times (t _a=10⁵s). The hardening of the composites with annealing temperature has been identified with the thickening of the PE crystalline lamellae. Comparison of X-ray scattering data and the microhardness values upon annealing leads to the conclusion of phase separation of C₆₀ molecules from the polyethylene crystals within the material. The temperature dependence is discussed in terms of the independent contribution of the PE matrix of the C₆₀ aggregates to the hardness value.     

Influence of sintering atmosphere on the microstructure and water durability of SnO–MgO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glass

Effects of sintering atmosphere (Ar, air, and O₂) on the sinterability and crystallization at 380–470 °C of 60SnO, 10MgO, 30P₂O₅ (mol%) glass powder, and the water durability of the sintered glass were investigated. Increasing the oxygen partial pressure (P_\textO2 ) (P_{{{\text{O}}_{2} }} ) in the sintering atmosphere enhanced the oxidation tendency of Sn²⁺ to Sn⁴⁺ near the surface region of the glass particles. Therefore, the glass viscosity was increased, resulting in the increase in both the temperature of densification and the temperature at which crystalline phases developed. Phase assemblage and the amounts of crystalline phases were also affected by P_\textO2 . P_{{{\text{O}}_{2} }} . The water durability of the sintered glasses is discussed in terms of the above microstructural parameters.     

Nonequilibrium Molecular Dynamics Simulation of the Rheology of Linear and Branched Alkanes

Liquid alkanes in the molecular weight range of C₂₀–C₄₀ are the main constituents of lubricant basestocks, and their rheological properties are therefore of great concern in industrial lubricant applications. Using massively parallel supercomputers and an efficient parallel algorithm, we have carried out systematic studies of the rheological properties of a variety of model liquid alkanes ranging from linear to singly branched and multiply branched alkanes. We aim to elucidate the relationship between the molecular architecture and the viscous behavior. Nonequilibrium molecular dynamics simulations have been carried out for n-decane (C₁₀H₂₂), n-hexadecane (C₁₆H₃₄), n-tetracosane (C₂₄H₅₀), 10-n-hexylnonadecane (C₂₅H₅₂), and squalane (2, 6, 10, 15, 19, 23-hexamethyltetracosane, C₃₀H₆₂). At a high strain rate, the viscosity shows a power-law shear thinning behavior over several orders of magnitude in strain rate, with exponents ranging from –0.33 to –0.59. This power-law shear thinning is shown to be closely related to the ordering of the molecules. The molecular architecture is shown to have a significant influence on the power-law exponent. At a low strain rate, the viscosity behavior changes to a Newtonian plateau, whose accurate determination has been elusive in previous studies. The molecular order in this regime is essentially that of the equilibrium system, a signature of the linear response. The Newtonian plateau is verified by independent equilibrium molecular dynamics simulations using the Green–Kubo method. The reliable determination of the Newtonian viscosity from non-equilibrium molecular simulation permits us to calculate the viscosity index for squalane. The viscosity index is a widely used property to characterize the lubricant's temperature performance, and our studies represent the first approach towards its determination by molecular simulation.     

Microstructure and boundary phases of Lu–Al-doped silicon nitride by pressureless sintering

Various silicon nitride materials were fabricated by pressureless sintering using lutetia and alumina as sintering additives. Densification behavior, microstructure, strength and formation of secondary crystalline phases were investigated. The combination of Lu₂O₃/Al₂O₃ sintering aids can promote the densification and evolution of a fine grain microstructure of Lu–Al-doped silicon nitride because of the low viscosity of the liquid. The J′ phase given by Lu₄Si_2−xAl_xO_7+xN_2−x was considered to be secondary crystalline phase at the grain pockets. The composition with a Lu₂O₃/Al₂O₃ weight ratio 10/10 had highest strength of 690 ± 50 MPa.     

Peptide Washout and Permeability from Glyceryl Monooleate Buccal Delivery Systems

ABSTRACT

Simultaneous evaluation of the permeation and washout of a peptide from the mucoadhesive liquid crystalline phases of glyceryl monooleate (GMO) has been investigated using a donor compartment flow-through diffusion cell. [d-Ala², d-Leu⁵]enkephalin (DADLE) was incorporated into the cubic and lamellar liquid crystalline phases of GMO and applied to excised porcine buccal mucosa mounted in the donor compartment flow-through cell. Phosphate-buffered saline pH 7.4 (PBS) was pumped across the upper surface of the liquid crystalline phases to mimic salivary flow. The steady-state fluxes of DADLE and GMO from the cubic phase were significantly greater than that from the lamellar phase (P<0.01). There was no statistical difference between the amounts of DADLE and GMO washed out from the lamellar and cubic phases (P>0.05). The donor compartment flow-through diffusion cell was found to be a useful tool to evaluate the impact of salivary washout on mucoadhesive oral mucosal delivery systems.     

Sorbitan Ester Organogels for Transdermal Delivery of Sumatriptan

ABSTRACT

The partial phase behavior, rheological, and drug release characteristics of an organogel (OG) composed of water, isooctane and sorbitan esters, sorbitan monopalmitate (Span-40) and poly(oxyethylene)sorbitan monostearate (Polysorbate-60) were studied. Phase diagrams showed decreasing areas of optically isotropic organogel region depending on the surfactant ratio, K_w and drug incorporation. The nonbirefringent, clear isotropic solution suggested the reverse micellar/microemulsion nature of the organogel without any molecular ordering. The increase in drug concentration in OGs leads to increase in the viscosity and sol-gel transition temperature (T_g). Fractal dimension (d_f) values calculated for different compositions suggested that the density of the tubular network increases with increasing drug concentration in OGs. The release rate of the drug from OGs was found to be non-Fickian through the dialysis membrane. The permeation rate of sumatriptan from pig skin was 0.231 mg/h/cm² (781.9 nmol/h/cm²). The study indicates potential of OG as a reservoir system for transdermal drug delivery.     

Determination of viscosities for alumina–polyethylene blends

The rheology of different proportions of low-density polyethylene (LDPE) and low-density polyethylene wax (LDWAX) without and with alumina was studied and correlated to a model. The viscosities of the polymer blend (LDPE + LDWAX) were studied for various compositions of LDWAX at different temperatures. The Arrhenius plot of logarithmic viscosity and reciprocal temperature for the polymer blends is linear. The enthalpy of viscous flow, determined from the slope of the Arrhenius Plot, varies linearly as a function of the average molecular weight. It is thus possible to predict the viscosity of any intermediate composition of the polymer blend. The viscosities of alumina-polymer blends (AP blend) with 50 vol% of alumina, prepared by solvent method, were also studied as a function of temperature, at a shear rate of 1333.33 s^–1. The AP blends show a lower enthalpy of viscous flow compared to pure polymer blend because of the presence of stearic acid used as surfactant for alumina. The model developed in this study enables the prediction of viscosities of AP blends at any weight average molecular weight (M _w) of the main binder and temperature.     

X-ray diffraction study of polyphilic smectic liquid crystals

4-substituted n-5,5,6,6,7,7,8,8,9,9,10,10,10-tridecafluorodecan-1-yloxybenzene and n-5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-pentadecafluorododecan-1-yloxybenzene, the former with CO₂CH₃ and the latter with CO₂CH₃ and CO₂H substituents in para position, are polyphilic compounds exhibiting smectic liquid crystalline phases. The structure of these phases was studied by X-ray diffraction and modelling. The method consisted of reconstructing all possible one-dimensional electron density profiles from diffraction intensities and matching them with the profiles calculated from structural models. The method is shown to be useful in both determining molecular arrangement in the smectic phase and in phase identification itself. An S_Ad phase is found in the two esters, having interdigitated perfluorinated alkyl segments. In contrast, a double-layer S_C phase, also known as S_C2, is found in the carboxylic acid. The method used here is particularly applicable to polyphilic liquid crystals such as the present ones, which are characterized by a complex electron density profile and a high translational order parameter.     

Study of the effect of Na/Cu substitution in Bi4Sr3Ca3Cu4O16 glass-ceramic superconductor

The composition Bi₄Sr₃Ca₃Cu_4–x Na _x O₁₆ for x=0.2, 0.4, 0.6, 0.8 and 1.0 was examined, in order to study the effect of Na/Cu substitution on both the glass-forming ability as well as the superconducting properties of the glass-ceramic (GC) phase. Because the GC phase of the composition Bi₄Sr₃Ca₃Cu₄O₁₆ (4334) showed superconducting properties below 78.5 K, the crystalline phases formed after heat treatment were identified by X-ray diffraction. This suggests that Na⁺ substituted the Cu⁺ cations. The GC phases were studied by X-ray diffraction, differential thermal analysis, infrared absorption, d.c. electrical conductivity and low-temperature a.c. magnetic susceptibility in the temperature range 77–300 K. The present results support the considerations that the addition of sodium enhances the crystallization of the 4334 phase while sacrificing the glass-forming ability. The crystalline phases precipitated from the rapidly quenched glasses in the Bi-Sr-Ca-Cu-Na-O system were greatly dependent on the heat-treatment time as well as the treatment temperature. The stability and crystallization process of the glass differ greatly depending on the sodium content which acted as fluxing agent. Considering that the formation of the 4334 phase is largely enhanced in sodium-doped samples, it is concluded that the lowering of the partial melting temperature is very important for the enhancement of the formation of the 4334 phase, as well as in raising the critical transition temperature. Infrared spectroscopy measurements indicate that part of the ceramic phase is non-metallic. The two-probe method and the standard four-point probe method electrical resistivity indicate that Bi-Sr-Ca-Cu-Na-O superconductor formation was greatly dependent on heat-treatment time as well as treatment temperature. The superconducting crystalline phase, which grew upon heat treatment, was identified as a quasi-tetragonal phase 4334. Electrical resistance measurements together with thermopower results indicate that the electrical properties move from a metal region to a semiconducting region according to the magnetic phase diagram of oxide superconductors. Superconducting 4334 phase with T _c=84 K could be successfully prepared by the GC techniques within 1–2 h thermal cycling, which renders a great saving in processing costs and is a simple method of moulding superconducting articles.     

Electron transport along molecular stacks in discotic liquid crystals

We have previously shown that a new class of quasi-one-dimensional p-type semiconductors can be created by dissolving small amounts of oxidising agents into the hydrocarbon chain matrix of discotic liquid crystals. This paper reports the elucidation of the mechanism of conduction in these new materials. In particular, the ac conductivity of 2,3,6,7,10,11 -hexahexyloxytriphenylene (HAT6) doped with the Lewis acid AlCl₃, has been measured as a function of frequency (10^?3–10⁷ Hz), and temperature in its crystalline solid (K), hexagonal discotic liquid crystal (D_ho) and isotropic liquid (I) phases. In macroscopically aligned K and D_ho phases, the conductivity measured along the column axes is approximately 10³ greater than that in the perpendicular direction. Both components of the conductivity are found to be independent of frequency at low frequencies, but show a power law dependence on frequency (Σ(Ω)～Ω^s, s～0.8) at higher frequencies. This behaviour is characteristic of charge carrier transport by a hopping mechanism. The conductivity data have been analysed in terms of the Scher and Lax theory of hopping transport to obtain the parameters describing this process. The conduction along the columns is identified with a single charge transport process in which the carriers hop between localized states (radical cations) associated with counterions (perceived as AlCl₄ ^?) linearly distributed off-axis along the columns.     

Shear effects in a micellar solution near the critical point

This paper is concerned with theological properties of a phase-separating mixture in the presence of macroscopic shear flow. Specifically, we have investigated the shear viscosity of a nonionic micellar solution in shear-induced quasi-stationary states, where a dynamical balance between the growing concentration domains and the suppression by shear is established. During the phase separation, when concentration domains of the two phases with viscosity difference are formed, the measured viscosity of all mixtures as a function of temperature is found to satisfy to a good approximation a novel theological equation proposed by Onuki. In addition, we have determined that the observed characteristic temperature related to the crossover from the shear-affected regime to the shear-unaffected regime depends on the shear rate asS ^p , withp=1:vz≃0.51, wherev andz are universal critical exponents associated with the range and decay rate of the critical fluctuations. This power law depdendence onS. expected for a mixture at the critical composition, appears to be valid also for all off-critical mixtures. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Shear viscosity of the B phase of superfluid3He. III

We report the calculation of the shear viscosity in superfluid³He-B at 6, 21, and 30 bar. We have used the variational solution of the Boltzmann equation for quasiparticles. The transition probabilities are obtained within thes-p-d-wave approximation by fitting the normal state transport coefficients. The old and new Landau parameters are used to estimate the scattering amplitudes. The reduced shear viscosity does not depend very much on the choice of Landau parameters and agrees very well with experiment in the temperature range 0.5<t (=T/T _c )1.0, whereT _c is the transition temperature. We have also investigated the strong coupling corrections to the shear viscosity. Although the strong coupling corrections reduce the value of shear viscosity, the overall feature does not change from the weak coupling result. The pressure dependence of the reduced shear viscosity is found to be very small.     

Thermal stability and melt rheology of poly(<Emphasis Type="Italic">p</Emphasis>-dioxanone)

Melt viscosities of poly(p-dioxanone) (PPDO) samples having different molecular weights were studied using a controlled-strain rotational rheometer under a nitrogen atmosphere. First, PPDO’s thermal stability was evaluated by recording changes in its viscosity with time. The result, that samples’ viscosities decreased with time when heated, demonstrated that PPDO is thermally unstable: its degradation activation energy, obtained by using a modified MacCallum equation, was a relatively low 71.8 kJ/mol K. Next, viscoelastic information was acquired through dynamic frequency measurements, which showed a shear thinning behavior among high molecular weight PPDOs, but a Newtonian flow behavior in a low molecular weight polymer (M _w = 18 kDa). Dynamic viscosity values were transferred to steady shear viscosities according to the Cox–Merz rule, and zero shear viscosities were derived according to the Cross model with a shear thinning index of 0.80. Then flow activation energy (48 kJ/mol K) was extrapolated for PPDO melts using an Arrhenius type equation. This activation energy is independent of polymer molecular weight. A linear relationship between zero shear viscosity and molecular weight was obtained using a double-logarithmic plot with a slope of 4.0, which is near the usually observed value of 3.4 for entangled linear polymers. Finally, the rheological behaviors of PPDO polymer blends having bimodal molecular weight distributions were investigated, with the results indicating that the relationship between zero shear viscosity and low molecular weight composition fraction can be described with a Christov model.     

Icosahedral and decagonal quasicrystals,crystalline phases,and multiple twins in rapidly solidified Al13Cr4Si4

Both the icosahedral and decagonal phases have been found in Al₁₃Cr₄Si₄ melt-spun ribbons. A definite orientation relationship existing between these two phases is determined by SAD which is the same as that obtained previously by Schaefer and Bendersky. During the icosahedral quasicrystal-crystal transformation in this Al−Cr−Si alloy, the Al₁₃Cr₄Si₄ crystalline phase forms at low temperature and an unknown phase at high temperature. Both crystalline phases have definite orientation relationships with the icosahedral phase. The five-fold rotational twins of Al₁₃Cr₄Si₄ around the [110] direction formed from the icosahedral phase follow the group-subgroup relationship. The unknown phase has a hexagonal lattice,a=0.73 andc=1.62nm.     

Antifungal Activity of Undecylenic Acid Emulsions by Microbiological Methods

Abstract

Two nonionic surfactants (Simulsol 98 and Simulsol OL 50) alone and I:I mixtures, corn oil-undecylenic acid and water formed emulsions, oily isotropic liquid phases, lamellar and hexagonal liquid crystal phases. The optimum release of undecylenic acid from these phases were controlled microbiologically. The active ingredient undecylenic acid, is released more from emulsion systems containing liquid crystals than only liquid crystalline and oily isotropic liquid phases.