Thermally Stimulated Creep: a Theoretical Understanding of the Compensation Law

Experimental data from thermally stimulated techniques (either mechanical or electrical measurements) are often used to characterize polymeric materials. The very high values obtained for the apparent activation energy, associated with very short pre-exponential times, as well as the so-called “compensation law” have not yet been properly explained on a physical basis. The purpose of this work is to propose such an interpretation on the basis of a theoretical model already developed for the molecular mobility in amorphous polymers below and around $T_{\rm g}$

Future trends in health monitoring of materials

New nanocomposites materials based on electrically conductive fillers with high aspect ratio (whiskers) dispersed in a film forming polymeric matrix were prepared with various amount of fillers. The transport properties are directly linked with the macroscopic mechanical strain on the composites during uniaxial tensile test and with time under relaxation, meaning that the method is suitable for monitoring microstructural evolution of such composites. This evolution has been related to microstructural evolution of the network formed by the fillers (whiskers) when the material is stretched. The decrease of the conductivity real part indicates the breaking of the percolating network, while the imaginary part gives information on the possible “spatial correlation” of the damage events. The relationships between microstructure and macroscopic properties were simulated with the help of a RC model for the electrical properties and with finite element technique for the mechanical properties. The use of such material as sensor coated on a material to be monitored is discussed. Strain, strain rate and failure can be easily characterized through the electrical properties

Shape effect in the magnetostriction of ferromagnetic composite

cited By 36International audienceThe magnetostriction of composite consisting of a soft matrix, non-magnetic, randomly filled by ferromagnetic particles is measured. The measured elongation on cylinder-shaped samples displays shape dependence. A model based on the demagnetizing field and the effective Young's modulus is provided. Both calculation and measurement show a positive magnetostriction with larger values as the samples are flatter. The model is derived to have the behavior of the elongation as a function of the filling factor. An expression of the optimal filling factor, providing a maximal strain, is also expressed. © 2010 Elsevier B.V. All rights reserved

New experimental features and revisiting the α and β mechanical relaxation in glasses and glass-forming liquids

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Dependence of the magnetostriction of magnetic rheological elastomers on temperature

cited By 1International audienceThe magnetostriction of composites consisting of a non-magnetic matrix, randomly filled by ferromagnetic particles is measured as a function of temperature. The magnetic property of the particles and the mechanical property of the matrix are both dependent on temperature. Extracting the thermal expression of the magnetic stress and recording the composite mechanical stressstrain response allows us to predict the magnetostriction thermal behavior of the composites. © 2012 IOP Publishing Ltd

Improvement of dispersion and electromechanical properties of polyurethanes nanocomposites by grafting CNT

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Isoconfigurational state dependent molecular mobility in the glass temperature range

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Quantitative Analysis of grafted CNT dispersion and of their stiffening of polyurethane (PU)

International audienceElectroactive devices are developed for energy conversion purposes. In particular, polyurethanes (PU) are lightweight and flexible materials, which have demonstrated their ability to convert electrical energy into mechanical energy (actuation by electrostriction) and vice-versa (energy harvesting). It has been shown that energy conversion efficiency can be increased by incorporating carbon nanotubes (CNTs) into a PU matrix. The counterpart of this 2 improvement is the stiffness increase, which in turn limits the electrostriction efficiency. On the other hand, it is well known that CNTs are hardly dispersed in a polymeric matrix, and that the interfacial adhesion strength is generally poor. One solution to improve both dispersion and adhesion consists in grafting polymeric chains onto the CNT surfaces. As most of the works dedicated to improve material electroactivity are mainly empirical, this work aims to (i) better characterize these material microstructures by electron tomography, through the measurement of the CNT tortuosity, the CNT-CNT minimum distance and the number of their contacts, and (ii) and to predict their mechanical stiffness from these microstructural data. From electron microscopy observations of the studied materials, CNTs can be assumed to be composed of successive stiff rods of measured length and orientation, linked together by flexible kinks. Their mechanical stiffening effect in PU is, simply and in an original way, evaluated using the classical analytical equations derived by Halpin and Kardos, accounting for the microstructural parameters determined by electron tomography. It appears clearly that, due to their tortuosity and despite their ultra-high longitudinal stiffness, CNTs only poorly stiffen soft matrices. Fully stretching 10 m long nanotubes increases the composite modulus by almost 10 for a fraction of only 2 vol.%