Anežka Lengálová

The effect of dielectric properties on the electrorheology of suspensions of silica particles coated with polyaniline

The flow behaviour of silicone-oil suspensions of five types of silica particles coated with a polyaniline base in a DC electric field has been linked to their dielectric properties. The relaxation frequencies corresponding to the position of the dielectric-loss maxima in the frequency spectra identify the interfacial polarization of suspension particles as a controlling factor for a strong electrorheological effect. The yield stresses of suspensions under the influence of electric field and critical shear rates, at which the chains of polarized particles were broken by shear forces, is correlated with the difference between the limit values of dielectric constants above and below the relaxation frequency. The analysis of particle dipole coefficient β showed that particle polarizability is the main factor affecting rigidity of the electrorheological structure. In contrast with this, particle shape and size, controlling the field-off suspension viscosity, become unimportant after the electric field has been applied. The plots of the relative viscosity of studied suspensions vs. Mason number characterizing the relation between shear and polarization forces have been discussed. While the results obtained at different shear rates and field strengths were reduced to a single dependence, for various particle suspensions these dependences differed.

Influence of particle concentration on the electrorheological efficiency of polyaniline suspensions

The dependence of the efficiency of positive electrorheological (ER) phenomenon, expressed as a ratio of the low-shear electroviscosity to the field-off viscosity, on the particle concentration of polyaniline base suspensions in silicone oil has been studied. A simple analysis reveals that the dependence of the efficiency on the volume fraction of suspended particles may have a maximum at which the optimum ER performance of material is expected. Experimental results confirmed this expectation. It is demonstrated that with higher temperature the maximum is shifted to higher particle concentration.

The effect of dispersed particle size and shape on the electrorheological behaviour of suspensions

Abstract Electrorheological (ER) and dielectric properties of suspensions of four inorganic powders (talc, magnesium hydroxide, aluminium hydroxide, and titanium dioxide) of different particle size and shape and polyaniline particles in silicone oil has been compared. It has been shown that the efficiency of ER effect expressed as a ratio of electroviscosity Δη (an increase in suspension viscosity on the application of electric field) to the field-off viscosity, η0, in addition to dielectric phenomena of dispersed and continuous phase may be strongly influenced by the hydrodynamic properties of individual particles. The non-spherical or irregular particle shape can considerably increase rigidity of the structure formed in the electric field and enhance the ER performance. The main factor controlling the ER effect, however, is particle polarisability. Thus, the electroviscosity of suspensions of highly polarisable polyaniline particles proved to be much higher than that of suspension of the most ER-efficient inorganic powder.

Electrorheology of polyaniline-coated inorganic particles in silicone oil

Abstract The electrorheological behavior of suspensions of two types of inorganic particles, with different size and shape—granular aluminum hydroxide and flaky talc—coated with a polyaniline (PANI) base overlayer, is presented. In suspensions of uncoated particles in silicone oil, the particle size and shape affects the character of the organized structure produced in an electric field and, consequently, also the flow properties. After the particles are coated with PANI base, the electroviscosity for both aluminum hydroxide and talc at high electric-field strengths becomes comparable. This means that the effect of the surface PANI layer has become dominant and the inherent particle nature is less important. The electrorheological efficiency, expressed as relative low-shear electroviscosity of the PANI-coated inorganic particles, is compared with that of PANI particles alone. In spite of the higher electroviscosity of suspensions of PANI particles, their ER efficiency merged with the PANI-coated inorganic powders due to high field-off viscosity of PANI suspensions.

Effect of compressive strain on electric resistance of multi-wall carbon nanotube networks

The network of entangled multi-wall carbon nanotubes is shown as a conductor whose resistance is sensitive to compressive strain, both in the course of strain growth and when loading/unloading cycles are imposed. If the compression is applied, the resistance decrease is up to 25% at the maximum applied deformation. The experimental data are analysed using the Weibull distribution model and a contact network model to get an estimate of the contact resistance between carbon nanotubes and the formation of contacts in the course of compression.

Volume and enthalpy relaxation in a-PMMA after temperature up-jumps

Abstractthe volume and enthalpy relaxation in a-PMMA subjected to temperature jumps in tg region has been analysed. The measured H and V data were compared with respect to aging time and proportionality between them as a slope of (∂H/∂V)T dependencies has been found. According to previous works the slope was identified as an apparent bulk modulus, Ka. This method is applied to aging following temperature up-jumps after consolidation periods of varying lengths. the main finding is a marked increase of Ka with consolidation time, approaching a limiting value in an asymptotic fashion.

Rheology and fiber degradation during shear flow of carbon-fiber-reinforced polypropylenes

The processing of fiber-reinforced thermoplastics is often accompanied by a significant fiber fracture. Therefore, it is important to assess the effect of processing variables on the extent of fiber damage occurring during product fabrication, such as extrusion or injection molding. The present paper discusses fiber damage caused by shear forces exerted on the composite by a molten matrix in both experimental and theoretical terms. The degradation process in carbon fiber-polypropylene composites is studied in a broad range of shear rates, although it occurs significantly only under high shearing in a capillary. Changes in fiber length and its distribution during the multi-flow through a capillary, as well as the materials’ rheological properties found in research after shearing, are discussed and the results are compared with a model of fiber-length analysis for the mixing-regrinding process.

Pressure and Temperature Dependence of LDPE Viscosity and Free Volume: The Effect of Molecular Structure

Abstract Temperature and pressure dependencies of shear and elongational viscosities were examined using rotational and capillary rheometers. Two different batches of the same polymer grade have shown that the molecular structure diversion significantly influences the magnitudes of pressure coefficients, which vary more than the temperature coefficients. The pressure effect on viscosity notably depends on the amount of long-chain branching in polymer. Further, the paper shows that pVT data analysis via the Simha-Somcynsky equation of state can be employed to reveal differences in temperature and pressure viscosity dependencies through the free volume fraction.

INCREASING ELECTRORHEOLOGICAL RESPONSE OF PARTICLES: THE EFFECT OF CONDUCTIVE POLYMER

The paper deals with the effect of two conductive polymers (polyaniline, PANI, and polypyrrole, PPy) in combination with inorganic particles on the response of silicone-oil suspensions. Conductive polymers cause a significant increase in electrorheological response. PANI combined with silica is much more efficient when in the form of a thin layer on the particle surface than if the two components are simply mixed. All combinations are followed from the viewpoint of viscosity and yield stress.

A Pressure Sensing Conductive Polymer Composite with Carbon Nanotubes for Biomechanical Applications

A sensing element made of conductive composite created by an entangled network of electrically conductive carbon nanotubes embedded in polyurethane was used for simultaneous measurements of the pressure between the shoe and floor as well as the extension of the leg at the knee joint during marching. The results recorded as sensor resistance change show reasonable reversibility of the basic sensor characteristics, which gives potential for practical applications.