Françoise Ehrburger-Dolle

Electrical conductivity of carbon black-polyethylene composites : Experimental evidence of the change of cluster connectivity in the PTC effect

Abstract The electrical resistivity of carbon black filled high density polyethylene increases significantly when the composite is heated to the melting temperature of the matrix. The paper aims to provide new insights on this feature, known as the Positive Temperature Coefficient (PTC) effect for resistivity. The influence of the volume fraction φ of carbon black and of the temperature on the DC conductivity is analyzed and compared to their influence on matrix expansion. Measurements of the dependence on frequency of the AC conductivity of carbon black–polyethylene composites, at different loadings and at different temperatures, are performed. They yield experimental evidence of a change of connectivity of the conducting network during heating near the melting temperature of polyethylene. It is shown that the initial volume fraction of carbon black strongly affects the PTC mechanism.

Strengthening of silica gels and aerogels by washing and aging processes

Gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. During washing in water solution a significant increase in the permeability of the gels was observed, showing that dissolution-reprecipitation occurs. After washing, the gels were further soaked in a solution of polyethoxydisiloxane precursor to strengthen and stiffen the gel. As expected, a significant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability does not decrease below the value for the as-prepared gels. Hence, a promising process has been developed where both the stiffness and the strength have been increased as well as the permeability. The increase in permeability is of importance to facilitate the supercritical drying process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Optical properties have been measured on laboratory scale aerogels. The corresponding results have been correlated with structural characteristics measured by small-angle X-ray scattering (SAXS).

Small-angle X-ray scattering of a new series of organic aerogels

Abstract During the last decade, several new precursors have been proposed for the synthesis of organic aerogels. In a large majority of cases, the sol–gel reaction is base catalyzed and water is used as the solvent. Because of the poor solubility of liquid CO 2 in water, a time consuming two-step exchange is necessary (exchange of water by acetone followed by exchange of acetone by liquid CO 2 ). To eliminate this step, a new process with the solvent acetone was developed. Also in order to reduce gelation time, acid catalysis is used. The aim of the present work is to compare the solid and the porous texture of resorcinol–formaldehyde (RF) aerogels prepared by the conventional method (in water and base catalysis) to that of samples prepared by the new method (in acetone and acid catalysis), all other parameters (mass ratio, catalyst concentration R/C, supercritical extraction, pyrolysis conditions) remaining the same. Determination of pore size distribution (PSD) in different series of organic and pyrolyzed aerogels by thermoporometry and characterization of the solid structure by small-angle X-ray scattering (SAXS) measurements over a wide range of length are reported. The main difference between the two series of aerogels lies in the aggregation of the primary particles: the new series prepared in acetone and acid catalysis displays fractal scaling over more than a decade in length, while the conventional one does not. It is also shown that thermoporometry yields reliable information for both series of aerogels.

DLS and SAXS investigations of organic gels and aerogels

Abstract Recent investigations have shown that the structure of organic aerogels can be significantly modified by changing the precursors, the solvent and the nature of the catalyst involved in the sol–gel reaction. It is therefore highly desirable to investigate the sol–gel mechanism. For this purpose, dynamic light scattering (DLS) measurements have been performed at different stages of the reaction for base- or acid-catalyzed gelation of resorcinol–formaldehyde (RF) using water or acetone as solvents. The structure of aged gels was investigated by small-angle X-ray scattering (SAXS) and compared to that of the aerogels obtained after exchange of solvent by supercritical CO2 and drying of the aged gels. It is shown that acid-catalyzed gelation of RF in acetone can be described by percolation, which explains that this series of aerogels consists of mass fractal aggregates (Dm=2.5). The partial collapse of this polymeric gel yielding colloidal particles in the aerogel can be attributed to deswelling in supercritical CO2. DLS indicates that gelation of RF with a base catalyst yields a colloidal gel whose structure remains practically unchanged in the aerogel, as shown by SAXS.

Percolation in carbon black powders

Abstract The variation of conductivity and pressure transmitted by several carbon black powders is measured as a function of the solid volume fraction during low-speed uniaxial compaction. For all samples made of aggregates, the experimental results of both pressure and conductivity can be fitted with a percolation law. The percolation thresholds are, however, much smaller than the theoretical value φ ∗ = 0.15. It is shown that the percolation threshold depends on the mass fractal dimension and on a length R max characteristic of the aggregates of a given carbon black.

X-ray intensity fluctuation spectroscopy by heterodyne detection.

A straightforward way of measuring X-ray intensity fluctuation spectroscopy in a small-angle X-ray scattering configuration is demonstrated using heterodyne techniques. Two examples are presented: the Brownian motion of latex spheres in glycerol, and a Doppler velocity experiment demonstrating the motion and the relaxation of carbon-black-filled elastomers after uniaxial stretching. In the latter case the effects of mechanical relaxation can be separated from those of aggregate diffusion. The results suggest that the dynamics of these filled elastomers are similar to the universal features observed in disordered jammed systems.

Characterization of the aggregate void structure of carbon blacks by thermoporometry

Abstract The intra-aggregate void size distribution of carbon blacks dispersed in water or in undecane was measured by thermoporometry. It appears that the interpenetration of the carbon black aggregates is deeper when dispersed in undecane than in water. This effect is attributed to the presence of an adsorbed film of alkane on the surface of carbon black which leads to a smaller sticking coefficient between the primary aggregates in the alkane than in water. The fractal dimension, measured from the void size distribution curves, is in good agreement with that obtained by STEM analysis on isolated aggregates. However, due to the interpenetration of the aggregates, the fractal character of the aggregates is limited to a very narrow range of lengths.

Scattering from dilute ferrofluid suspensions in soft polymer gels

Small angle neutron and x-ray scattering methods are used to investigate the structure of dilute suspensions of two different ferrofluid systems dispersed in soft polyacrylamide hydrogels. It is found that the particles in the fluid are fractal aggregates composed of smaller particles of radius ca. 5 nm. The fractal dimension is strongly dependent on sample, taking the value 1.7 in the first sample and 2.9 in the second sample. In the presence of a magnetic field the aggregates orient, but are restricted in both their translational and rotational freedom. The effect of the gel elasticity is treated as a hindrance to the orientation process.

Relations between the texture of silica aerogels and their preparation

Abstract The effect of the preparation conditions on the morphology (Brunauer, Emmett and Teller surface area, micro-, meso- and macroporosity, fractal dimension) of silica aerogels is presented. Solutions of tetramethyloxysilane (TMOS) in methanol were used for the sol-gel reactions. The parameters were volume fraction of TMOS, pH of the medium and hypercritical drying method (methanol or CO2). Adsorption of different gases was used for the characterization of the size and arrangement of primary particles at the molecular scale and thermoporometry gave information on the arrangement of primary particles into aggregates.

Characterization of the porous texture of aerogel, precipitated and pyrogenic silicas

Two base-catalyzed silica aerogels were characterized by gas adsorption and thermoporometry, which led to description of the microporous texture and pore size distribution between about 1 and 300 nm. The results are compared with those obtained on precipitated and pyrogenic silicas. The fractal dimension was determined at different length scales. It is shown that the surface of the secondary particles of both aerogels is fractal. A mass fractal dimension ( D m = 1.8) was determined for the low density aerogel.