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- Author or Editor: C. Lacabanne x
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Abstract
Thermally Stimulated Current (TSC) spectrometry has been applied to the characterization of polymeric materials. The study of a series of amorphous polymers having different physical structures has shown that the compensation parameters are independent of physical aging; contrarily, the activation enthalpy distribution reflects the evolution of the heterogeneity of the amorphous phase. In copolymers, TSC allows us to identify segregated amorphous phases. In semi-crystalline polymers, with semi-rigid chains, we have shown the existence of an amorphous crystalline interphase characterized by a plateau in the temperature distribution of activation enthalpy.
Abstract
Dielectric relaxation and thermal transitions in β-PVDF have been investigated by Thermo-Stimulated Current spectroscopy and Differential Scanning Calorimetry respectively. A comparative study of spectra and thermograms has been performed. The relaxation mode associated with the glass transition of the true amorphous phase is characterized by relaxation times obeying a compensation law due to cooperative molecular movements. A conformationally disordered structure is proposed for β-PVDF to explain thermal events occurring around 60°C. Ageing of ferroelectric properties of β-PVDF has been associated with cooperative molecular movements liberated largely below the melting point.
Abstract
Thermally Stimulated Creepe and Current (TSCr and TSC) have been used for investigating the interface/interphase in DGEBA-DDA matrix and glass beads composites. In all samples, a complex α retardation/relaxation mode is observed near the glass transition of the matrix. A study of the fine structure has shown that, for the elastic processes, the activation enthalpies are ranging from 2 and 8 eV, while for the dielectric ones, they are restricted between 0.5 and 1.5 eV. Moreover, for a given activation enthalpy, the preexponential factor τo and the activation entropy ΔS are characteristic of matrix filler interface.
Abstract
Thermally Stimulated Current (TSC) spectroscopy and Differential Scanning calorimetry (DSC) have been applied to the characterization
of the microstructure of Poly (Ether Ether Ketone)/PEEK. the dielectric relaxation spectra show two modes, dependent upon
crystallinity:
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* for the mode stuated in the vicinity of the glass transition temperature, two components have been distinguished and attributed
to the molecular mobility in the ‘true amorphous phase’ and in the ‘rigid amorphous region’.
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* below 0°C, two sub-modes appear, situated around-110°C and-75°C, due to the two different crystal entities, beads and laths.
Abstract
Thermally Stimulated Current (TSC) has been used for investigating the interface and interphase in a natural composite, bone. Young adult male femoral diaphysis has been studied at different stages of demineralization by EDTA. Analysis of the fine structure of complex TSC spectra has shown that in the early stages of demineralization, dielectric relaxations are due to the hydroxyl reorientations in the mineral apatite structure. After a certain stage of demineralization, the mineral response disappears and organic matrix mobility is observed. A dissociative buffer has been used to identify collagen relaxations. The contribution of the mineral-organic interface/interphase has been isolated.
The correlation between structure/microstructure and thermomechanical properties has been investigated by the Thermally Stimulated Creep (TSC) technique in a high performance thermostable thermoset matrix composite. The high resolving power of this technique allows us to analyse the α retardation mode. The kinetics of molecular movements liberated at the glass transition has been investigated by the technique of fractional loading: the analysis of each elementary process gives the real compliance and the retardation time as a function of temperature. The values of the activation parameters show the existence of a compensation phenomenon which characterizes the microstructure. It also gives access to the loss compliance of the composite material as a function of temperature and frequency. The predictive calculation of loss compliance has been validated by the results obtained by dynamic mechanical analysis (DMA).
During the past two decades, Thermally Stimulated Creep Recovery (TSCR) has been used successfully to study molecular mobility in polymers. The main feature of this technique is its quite good sensitiveness: the sample undergoes a shear stress, so that the subsequent strain is linear in a range larger than in elongation or compression experiments. Furthermore, the low equivalent frequency of TSCR permits the resolution of normally overlapping peaks.
Abstract
The analysis of thermostimulated currents by the fractional polarization procedure has been used to establish the existence of a coherent vibration in 
Linseed oil/mastic varnish
mixtures. This excitation is seen in the oil component when its proportion in the system is inferior to its percolation threshold. The observed phenomenon complies with compensation laws and is interpreted in the framework of the formalism developed by Fröhlich to understand collective phenomena in biological systems.
Relaxation phenomena in polycarbonate have been explored by Thermally Stimulated Current (TSC) spectroscopy. A comparative study of transitions by differential scanning calorimetry has also been undertaken. In the sub-T g region, the observed relaxations have been associated with the diffusion of local defects along the chains. In the glass transition region, the annealing induces the segregation of two relaxation modes: the lower temperature component associated with the unstressed amorphous phase, the higher temperature component attributed to stressed amorphous phase responsible for the modules observed in electron microscopy and X-ray diffraction.
