Aerosil Dispersions Research Articles

Effect of hydrophobic aerosil dispersions on the Goldstone mode dynamics in ferroelectric liquid crystal near the point of polarisation sign reversal

ABSTRACTBroadband dielectric spectroscopy in the frequency range from 10−2 Hz to 107 Hz was used to study the effect of hydrophobic aerosil nanoparticles on the dynamics of the Goldstone mode in a ferroelectric liquid crystal near the point of spontaneous polarisation sign reversal. The obtained results show that an important role in the modification of the dynamics of the Goldstone mode and the phenomena at the point of spontaneous polarisation sign reversal is played not only by the presence of the nanoparticles themselves in the ferroelectric liquid crystal structure, but also the state of the surface of these nanoparticles. Similarly to the hydrophilic aerosil, the polarisation sign reversal temperature slightly decreases with the increase of the hydrophobic aerosil concentration. For small concentrations of hydrophobic aerosil, only slight changes in the dynamics of the Goldstone mode over the entire temperature range are detected. The observed deviations are due to the different influence of aerosil nanoparticles on the structure and dielectric properties of the tested composite caused by the specific surface treatment. The presence of the Goldstone mode above and below the point of polarisation sign reversal seems to confirm the model of two conformers with different structure and opposite dipole moments.

ANALYSIS OF THE INFLUENCE OF THE NATURE OF THE FILLER SURFACE ON THE PROPERTIES OF HYBRID ORGANIC-INORGANIC NANOCOMPOSITES BASED ON EPOXY OLIGOMER

The aim of this work was to investigate the influence of the chemical nature of the filler surface on the properties of hybrid organo-inorganic nanocomposites based on epoxy oligomer ED-20 in the presence of nanoscale functional filler of inorganic origin - aerosil, with different surface nature. The influence of the chemical nature of the surface of highly dispersed aerosil on the thermal properties of nanocomposites based on epoxy oligomer ED-20 has been studied. The peculiarities of the process of thermooxidative destruction have been studied. It is shown that the introduction of highly dispersed aerosil in the amount of 0.5% in the epoxy matrix does not lead to changes in the thermal properties of composites, and the nanofiller in the amount of 5% improves thermal stability of composites. The kinetics of the curing process of epoxy nanocomposites was studied by IR-spectroscopy. The influence of the presence of functional groups on the nanofiller surface on rate and the degree of conversion of epoxy groups was determined. The presence of hydroxyl groups on the surface of A-300 contributes to the rate of conversion of epoxy groups to a greater extent, compared with aerosil with a modified surface, which contains on the surface methyl groups capable of blocking reactive groups. It is established that the rate of conversion of epoxy groups in the presence of aerosil decreases in the range of ED-20 > ED-20 + A-300 > ED-20 + AM-300. The sorption properties of epoxy nanocomposites have been studied. It is established that the sorption process proceeds at a higher rate when the matrix is filled with unmodified aerosil. The mechanism of influence of the chemical nature of the filler surface and content on formation and properties of epoxy nanocomposites is discussed.

Bulk-boundary correspondence in soft matter.

Bulk-boundary correspondence is the emergence of features at the boundary of a material that are dependent on and yet distinct from the properties of the bulk of the material. The diverse applications of this idea in topological insulators as well as high energy physics prove its universality. However, whether a form of bulk-boundary correspondence holds also in soft matter such as gels, polymers, lipids, and other biomaterials is thus far unknown. Aerosil-dispersed liquid crystal gels provide a good testing ground to explore the relation between the controlled variations of the aerosil density within the liquid crystal bulk and the surface topography of the sample. Here we report on a direct observation of such a correspondence where the controlled strength of random disorder created by aerosil dispersion in the bulk liquid crystal is correlated with the fractal dimension of the surface. We obtained the surface topography of our gel samples with different quenched random disorder strengths by using atomic force microscope techniques, and computed the fractal dimension for each sample. We found that an increase of the aerosil gel density in the bulk corresponds to an increase in the fractal dimension at the surface. From our results emerges a method to acquire the bulk properties of soft matter such as density, randomness, and phase merely from the fractal dimension of the surface.

A New Probe: AFM Measurements for Random Disorder Systems**Supported by TUBITAK under Grant No 115F315.

We study the quenched random disorder (QRD) effects created by aerosil dispersion in the octylcyanobiphenyl (8CB) liquid crystal (LC) using atomic force microscopy technique. Gelation process in the 8CB+aerosil gels yields a QRD network which also changes the surface topography. By increasing the aerosil concentration, the original smooth pattern of LC sample surfaces is suppressed by the emergence of a fractal aerosil surface effect and these surfaces become more porous, rougher and they have more and larger crevices. The dispersed aerosil also serves as pinning centers for the liquid crystal molecules. It is observed that via the diffusion-limited-aggregation process, aerosil nano-particles yield a fractal-like surface pattern for the less disordered samples. As the aerosil dispersion increases, the surface can be described by more aggregated regions, which also introduces more roughness. Using this fact, we show that there is a net correlation between the short-range ordered x-ray peak widths (the results of previous x-ray diffraction experiments) and the calculated surface roughness. In other words, we show that these QRD gels can also be characterized by their surface roughness values.

Effect of hydrophobic aerosil nanoparticles on the dielectric properties of ferroelectric liquid crystal

ABSTRACTThe effect of hydrophobic aerosil nanoparticles on the dynamics of the soft mode and the Goldstone mode near the SmA*–SmC* phase transition in the ferroelectric liquid crystal (S)-2-methylbutyl 4-n-nonanoyloxybiphenyl-4′-carboxylate has been investigated by means of broadband dielectric spectroscopy. The obtained results for hydrophobic aerosil composites exhibit some differences in the dynamics of the collective processes as compared with dielectric measurements for hydrophilic aerosil dispersions. The observed changes in dynamics of the collective modes near the SmA*–SmC* phase transition were explained taking into account differences in surface treatment of the aerosil nanoparticles and effect of random disorder.

Preparation and investigation of ion-conducting nanocomposite materials based on the aerosil-silver iodide system

The phase composition of samples in the aerosil-silver iodide system has been determined using X-ray diffractometry. It has been demonstrated that the sizes of crystals of the initial silver iodide, silver iodide in the initial aerosil-silver iodide powder, and silver iodide in the aerosil-silver iodide powder after heat treatment are approximately identical to each other and equal to about 55 nm. The average size and specific surface area of particles of the initial dispersed aerosil have been determined by the adsorption method (Brunauer-Emmett-Teller method). The average particle size is approximately equal to 12 nm, and the specific surface area is ∼220 m2/g. The electrical properties of ion-conducting nanocomposite materials based on the aerosil-silver iodide system have been studied by impedance spectroscopy in the temperature range 20–160°C. An increase in the silver halide content in the mixture with the aerosil leads to an increase in the electrical conductivity. The ionic component of the electrical conductivity is dominant for glasses with a high silver halide content.

Dispersions of pyrogenic alumina in pentylcyanobiphenyl studied by deuteron NMR

Dispersions of hydrophilic (Aeroxide Alu C) and hydrophobic (Aeroxide Alu C 805) pyrogenic alumina (Al2O3) in liquid crystal 4;{'} -n-pentyl-4-cyanobiphenyl (5CB) were investigated with deuteron nuclear magnetic resonance. The disorder effects of Al2O3 particles on the orientational order of liquid-crystal media and on the field-induced director configuration were studied as a function of alumina density in samples prepared by zero-field cooling and in-field cooling procedures. The order parameters and their variation with alumina density suggest a stronger disordering effect from the nonpolar surface of Alu C 805 particles. For dispersions of hydrophobic Alu C 805 experiments involving in-field cooling from the isotropic phase indicate that the director of "disordered" domains can be aligned, though not perfectly, by the field-aided annealing process. But the same in-field cooling procedure has shown rather limited alignment effects for hydrophilic Alu C/5CB samples. The more robust network of hydrophilic gel possibly coupled with weak liquid-crystal-network interactions could be responsible for the observed behavior. Spectra recorded during in-field cooling and within the isotropic-nematic coexistence region reveal the augmentation of the disorder strength during the transition and illustrate the effect of field-aided annealing. The stability of the aligned states as revealed by deuteron NMR is described. The results are discussed in comparison with previous studies of aerosil dispersions in alkylcyanobiphenyl.

Monitoring the stability of nanosized silica dispersions in presence of polycations by a novel centrifugal sedimentation method

AbstractThe stability of Aerosil‐silica dispersions is analyzed in dependence on the addition of polycations with different charge density and hydrophobic modification using the analytical centrifuge LUMiSizer. This novel centrifugal sedimentation method allows the direct calculation of the stability parameters like sedimentation velocity and prediction of the shelf life. The highly charged polycation poly (diallyl‐dimethylammonium chloride) (PDADMAC) is compared with tailored cationic polyelectrolytes of equal degree of polymerization but lower and different charge density as well as various hydrophobic substitutions. The used technique allows to accelerate demixing and to quantify the stability of dispersions in a direct way. All polymers under study are able to stabilize aerosil dispersions, but the shelf life strongly depends on the polymer concentration. Typically for polycations, the stability increases with the polymer concentration whereas at low‐polymer content flocculation can be detected. In this context, the behavior of PDADMAC and three cationic PVB derivatives is similar, but, whereas the stability of PDADMAC containing dispersions decreases at higher polymer concentrations, PVB derivatives are effective stabilizers at higher concentrations too. The highest increase of the stability with increasing polymer concentration is obtained with the ampholytic terpolymer TP that has the lowest cationic charge. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Kinetics of nanocolloids in the aligned domain of octylcyanobiphenyl and aerosil dispersion

In this study we explore the interesting kinetics of nanocolloids formed by the solvent dispersion method (SDM) in an aligned matrix of octylcyanobiphenyl (8CB) liquid crystals and aerosil nanoparticles. The presence of alignment changes the behaviour of the induced crystallisation (IC), and melting transition (K–SmA) observed in the aligned nanocolloidal liquid crystal samples when compared with unaligned samples. Heating‐rate‐dependent experiments were performed for the aligned and unaligned samples at various heating ramp rates varied from 20 to 1 K min−1 using a calorimetric technique. In the aligned samples, the IC transition peak shifts towards higher temperature, whereas the melting transition peak shifts towards lower temperature. The IC peak shows an increase in enthalpy whereas the melting transition shows a decrease following an Arrhenius behaviour. The presence of alignment increases the activated kinetics of the system. This behaviour can be explained in terms of molecular interaction between aligned domains of nanocolloids in the 8CB matrix which makes the material stiffer.

Preparation of microspherical granules from aerosilogels

The influence of the conditions of aerosil dispersion spray drying process in the medium of overheated water steam on the formation of microspheric granules and their porous structure was studied. Water and ammonium solutions as aerosil particles dispersing agents and water steam and air as heat carriers were used. It was stated that the effects of spray drying depend on the kind of dispersing medium and on the conditions of atmosphere and temperature at which drying and forming of aerosilogel granules takes place.

The detection of the effect of viscosity growth in nanodisperse systems during their shearing with ultralow rates

With a precision Haake RS1 rheometer, it is first found that at extremely low rates of shear (\( \dot \varepsilon \) ≤ 10−3 to 10−4 s−1 or lower), in nanodisperse systems based on highly dispersed aerosil or aerosil-bentonite compositions, with an increase in the shear rate, a non-Newtonian flow is observed. It is accompanied with a continuous growth of the effective viscosity of the dispersions ηeff, till the yield point P ≈ Pk is reached. At P ≥ Pk, the effective viscosity decreases abruptly; the structure decomposes via an aggregate mechanism. Earlier it was believed, that each viscosity level there corresponds to a combination of the full destruction of structural bonds and their partial restoring up to the equilibrium level determined by ηeff. In addition, the structured dispersed system flow remains Newtonian unless the static yield point has been overcome; it corresponds to the largest viscosity of a virtually undestructed system. Using a special procedure, SEM-imaging of the structure of the bentonite-suspension, as well as the bentonite added with minor amounts of aerosil, was carried out in water-ethanol medium. Photomicrographs of structure of the studied nanodisperse systems subject to shearing were obtained. The active filler (aerosil) particles distribution in the model system structure is studied. A procedure of studying the thixotropic properties of the structured nanodisperse systems in dynamic conditions is developed. The complex modeling of the structured nanodisperse system spreading over the scaled physical model, as well as special conditions of rheological tests in the Haake RS1 rotational rheometer cell, made us detecting a correlation between the spreading macroscopic factors and microscopic factors of the rheological behavior of the nanodisperse systems structure in dynamic conditions.

Confinement effects on strongly polar alkylcyanobiphenyl liquid crystals probed bydielectric spectroscopy

Dielectric spectroscopy has often been used to study confinement effects in alkylcyanobiphenylliquid crystals. In this paper, we highlight some of the effects that have been discoveredpreviously and add new data and interpretation. Aerosil nanoparticles form a hydrogenbonded random porous network. In dispersions of alkylcyanobiphenyls with aerosils, anadditional slow process arises, that we ascribe to the relaxation of liquid crystal moleculesin close interaction with these nanoparticles. Their relaxation is retarded by a hydrogenbond interaction between the cyano group of the liquid crystals and an aerosil surfacehydroxyl group. A similar surface process is also observed in Vycor porous glass,a random rigid structure with small pores. A comparison of the temperaturedependence of the relaxation times of the surface processes in decylcyanobiphenyland isopentylcyanobiphenyl is made, both for Vycor and aerosil confinement. Indecylcyanobiphenyl, the temperature dependence for the bulk and surface processes isArrhenius (in a limited temperature range above the melting point), except in Vycor,where it is a Vogel–Fulcher–Tamman dependence (over a much broader temperaturerange). In bulk and confined isopentylcyanobiphenyl, the molecular processes have aVogel–Fulcher–Tamman dependence, whereas the surface processes have an Arrhenius one.Another effect is the acceleration of the rotation around the short molecular axis inconfinement, and particularly in aerosil dispersions. This is a consequence of the disorderintroduced in the liquid crystalline phase. The disorder drives the relaxation timetowards a more isotropic value, resulting in an acceleration for the short axisrotation.

High-resolution x-ray study of nematic–smectic-Aand smectic-A–reentrant-nematic transitions in liquid-crystal–aerosil gels

We have studied the effects of quenched random disorder created by dispersed aerosil nanoparticle gels on the nematic to smectic- A (N- SmA ) and smectic- A to reentrant nematic ( SmA -RN) phase transitions of thermotropic liquid-crystal mixtures of hexyloxycyanobiphenyl (6OCB) and octyloxycyanobiphenyl (8OCB). These effects are probed using high-resolution synchrotron x-ray diffraction techniques. We find that the reentrant characteristics of the system are largely unchanged by the presence of the aerosil gel network. By comparing measurements of the smectic static structure amplitude for this 8OCB- 6OCB+aerosil system with those for butyloxybenzilidene-octylaniline (4O.8)+aerosil gels, we find that the short-range smectic order in the smectic- A phase is significantly weaker in the reentrant system. This result is consistent with the behavior seen in pure 8OCB-6OCB mixtures. The strength of the smectic ordering decreases progressively as the 6OCB concentration is increased. Detailed line shape analysis shows that the high- and low-temperature nematic phases (N and RN) are similar to each other.

Dynamics of alcohols dispersed with aerosils

We investigated the dynamics of three alcohols dispersed with aerosils in the frequency range from 1MHz to 1GHz with dielectric relaxation spectroscopy. In comparison with the pure systems, no new relaxation processes were observed in this frequency range. The main relaxation time in octanol decreased with increasing aerosil concentration. In ethanol and glycerol there was no significant change in the relaxation times found. These results are different from those well-known for liquid crystal–aerosil dispersions. We think that the absence of the retarded relaxation at the surface of the aerosils can be explained by the fact that the character of the hydrogen bond is not too much different for an intermolecular one as compared to an alcohol–aerosil bond. Also it seems that the (shorter) alcohols are already disordered to an extent that the addition of an extra disordering agent does not change the dynamics.

Effect of aerosil dispersions on the photoinduced nematic–isotropic transition

We report differential scanning calorimetric (DSC) and dielectric measurements on thenematic–isotropic transition in the bulk and aerosil composites of a liquid-crystal mixturehaving a photoactive guest azobenzene compound in a non-photoactive host, 4-n-heptylcyanobiphenyl (7CB). The DSC scans taken at different cooling rates show that, at slowerrates, the bulk displays a single peak across the transition, whereas the composites in thesoft gel regime exhibit a double-peak profile. Such a double-peak profile, although seen inhigh-resolution ac calorimetric studies, has been observed for the first time inDSC experiments. The temperature range of the region between the two peaks iscomparable to that seen in ac calorimetric experiments and has similar features.This observation is significant since the appearance of the low-temperature peakin ac calorimetric data has been explained to be due to a crossover from therandom-dilution to the random-field limits. This work also constitutes the firstexperiments on the photoisomerization driven isothermal phase transitions inliquid-crystal–aerosil composites. The studies carried out in the absence and presence of alow-magnitude UV radiation not only bring out the standard features now establishedfor such photostimulated phase transitions, but display a few surprises. Notableamong them are that (i) the photoinduced shift in the transition temperature is anon-monotonic function of the aerosil composition and appears qualitatively similar to thedependence of the transition temperature itself, and (ii) the thermal anomaly mentionedabove characterizing the crossover is also seen in the temperature-dependentas well as the temporal variation of the sample capacitance for a composite inthe soft gel regime. We have also evaluated, using the temporal variation of thecapacitance, the different response times associated with the UV-on photochemicalprocess as well as the UV-off thermal back-relaxation process; the response timesappear to have a similar dependence on the aerosil concentration as the transitiontemperature.

Induced thermal dynamics in the melt of glycerol and aerosil dispersions

A high-resolution calorimetric spectroscopy study has been performed on pure glycerol and colloidal dispersions of an aerosil gel in glycerol covering a wide range of temperatures from 300 to 380 K, deep in the liquid phase of glycerol. The colloidal glycerol+aerosil samples with 0.07, 0.14, and 0.32 g of silica per cm3 of glycerol reveal activated energy (thermal) dynamics at temperatures well above the Tg of the pure glycerol. The onset of these dynamics appears to be due to the frustration or pinning imposed by the silica gel on the glycerol liquid and is apparently a long-range, cooperative phenomena. Since this behavior begins to manifest itself at relatively low silica densities (large mean void length compared to the size of a glycerol molecule) and speeds up with increasing density, these induced dynamics are likely due to a coupling between the flexible aerosil gel and large groups of glycerol molecules mediated by mutual hydrogen bonding. This is supported by the lack of such thermal dynamics in pure aerosil gels, pure glycerol, or aerosil gels dispersed in a non-glass-forming, non-hydrogen-bonding, liquid crystal under nearly identical experimental conditions. The study of such frustrated colloids may provide a unique avenue for illuminating the physics of glasses.

Role of Aerosil Dispersion on the Activated Kinetics of the LC1-xSilx System

This study explores the role of aerosil dispersion on activated phase transitions of bulk octylcyanobiphenyl (8CB) liquid crystals by performing heating rate-dependent experiments. Differential scanning calorimetry (DSC) was used at various heating ramp rates in order to probe the activated phase dynamics of the system. The system, LC1-xSilx, was prepared by mixing aerosil nanoparticles (7 nm in diameter) in the bulk 8CB by the solvent dispersion method (SDM). LC represents bulk 8CB, and Sil represents aerosil nanoparticles with concentration x in percent. The concentration of the aerosil nanoparticles (x) varied from 0 to 0.2 g/cm3 in the bulk 8CB. Well-defined, endothermic peaks were found on a heating scan at melting and at the smectic-A to nematic (SmA-N) and nematic to isotropic (N-I) transitions. These peaks show a temperature shift and a change in their shapes and sizes in the presence of aerosil nanoparticles. In addition, an exothermic peak also appeared before the melting peak during the heating scan in the presence of aerosil nanoparticles. All transitions shifted significantly with different heating ramp rates, following an Arrhenius behavior, showing activated kinetics. The presence of aerosil nanoparticles caused a significant increase in the enthalpy and a decrease in the activation energy compared to the results found in bulk 8CB. This behavior can be explained by aerosil dispersion in the LC1-xSilx, inducing a disorder in the bulk 8CB. Infrared (IR) spectroscopy shows a shift to higher frequency for the broad peak at 1082 cm-1, corresponding to an Si-O bond as the density of the aerosil increases, and can be explained in terms of surface and molecular interactions between aerosil nanoparticles and 8CB liquid crystal molecules.

Production of microspherical aerosilogels by spray drying in a superheated steam

The effect of spray drying of aerosil dispersions in a superheated steam on the formation of microspherical grains and their pore structure was studied.

Dielectric spectroscopy of aerosil-dispersed liquid crystal embedded in Anopore membranes

The complex dielectric permittivity epsilon* values are presented for aerosil-dispersed 4-pentyl-4-cyanobiphenyl (5CB) confined in Anopore membranes. The dielectric permittivities are measured in the frequency range from 10(-2) Hz to 1 GHz at temperatures ranging from 50 degrees C down to -20 degrees C. In bulk 5CB, which has only a nematic phase, there exist two main relaxation processes: one due to the rotation of molecules around their short axes for parallel orientation of the director to the probing field and another fast relaxation process due to the librational motion of molecules for perpendicular orientation. Inside Anopore membranes both these main relaxation processes can be observed, but with subtle differences. The relaxation process due to the rotation of molecules around the short axis is faster in Anopores at all temperatures in comparison with the bulk process. Hydrophilic aerosil particles, when dispersed in the liquid-crystal (LC) phase, attach to each other via hydrogen bonds and form a three-dimensional interconnecting aerosil network, thus dividing the LC phase into small domains. Dispersing 5CB with different concentrations of hydrophilic aerosils leads to a decrease in relaxation time with aerosil concentration. In these dispersed systems a slow additional relaxation process emerges. This slow process becomes stronger with higher concentrations of aerosil. From our experiments we conclude that this process is the relaxation of 5CB molecules homeotropically aligned to the surface of the aerosil particles. In the case of 5CB-aerosil dispersions enclosed in Anopore membranes this slow process still exists and increases also with aerosil concentration. The relaxation time of the rotation of the 5CB molecules around their short axis systematically increases in these 5CB-aerosil samples in Anopore membranes with aerosil concentration from the 5CB-Anopore behavior towards the behavior observed for 5CB-aerosil dispersions. We explain the evolution as resulting from opposing tendencies from size effects (in the Anopore membranes) and disorder effects (in the aerosil dispersions).

Effect of aerosil dispersions on the nematic-to-isotropic interface

The effect of silica aerosils on the kinetics of the first-order nematic-isotropic (NI) phase transition is phenomenologically described in the framework of the time-dependent Landau-Ginzburg equation. A steady-state solution to the equation is presented such that the NI interface may propagate with a solitary-like wave profile under constant quenching. The results provide a plausible basis for the interpretation of the dynamical effects of quenched disorder in the liquid-crystal systems, caused by randomly interconnected porous media, such as aerosils. In the low silica aerosil rho(s) ( < or =0.1 g/cm;3) regime, the calculated values of the interface velocity v(T,rho(s)), the interface thickness kappa(T,rho(s)), and the critical radius of a spherical nucleus of new nematic phase in a bulk isotropic environment, composed of polar molecules, such as 4-n-octyl- 4(')- cyanobiphenyl and 4-n-heptyl- 4(')- cyanobiphenyl shows that the effect of silica aerosils on the kinetics is reflected in a shifting of the set of temperature-dependent curves to lower temperature values.-1.