Ph. Demont

Combined dielectric spectroscopy and thermally stimulated currents studies of the secondary relaxation process in amorphous poly(ethylene terephthalate)

The dielectric secondary β relaxation process of poly(ethylene terephthalate) has been investigated by dielectric spectroscopy (DS) and thermally stimulated currents (TSC). To analyze the broad and asymmetric DS spectra, without previous assumptions on the shape and the number of overlapped loss components constituting the β relaxation, a description in terms of distribution of relaxation times (DRT) has been used. The Tikhonov regularization method has been used to extract the DRT from DS data. At lower frequency, the TSC technique revealed two components in the β relaxation process. The TSC spectra are experimentally resolved into elementary Debye processes by using the fractional polarization method. A set of elementary relaxation times with relaxation strength Δe i as statistical weight of TSC-DRT have been calculated. The temperature shifts of the DRT obtained from both dielectric techniques are analyzed. Whereas the maximum and the lower time wing of the DRT obtained from DS correspond to the β 2 process resolved by TSC and associated to the noncooperative motions of carbonyl groups, the upper time wing of DRT extracted from DS is in accordance with the β 1 process resolved by TSC and assigned to the cooperative motions of phenyl rings.

Study of molecular mobility at the secondary relaxations range in polyamide 66 and polyamide 66/EPR blends by thermally stimulated creep and current

Abstract Thermally stimulated creep with fractional loading technique and thermally stimulated current have been used to investigate the relaxation processes of polyamide 66 and polyamide 66/EPR blends over temperature range covering γ, β and α EPR . Absorbed water decreases the thermally stimulated creep and the thermally stimulated current height of γ-relaxation. The thermally stimulated creep mechanical β-relaxation in PA66 is characterized by a broad peak, while the thermally stimulated current dielectric one is composed of two separate processes β 1 and β 2 according to the maximum temperature decrease. The peak height of the β 1 component is increased by moisture while the β 2 component is not modified. According to the criteria of Starkweather for sample and complex relaxations, the γ- and low temperature part of β-relaxations have been assigned to localized non-cooperative motions. The high temperature part of β-process and α EPR -relaxation have positive activation entropies and result from high cooperative motions. These interpretations from thermal sampling analysis is in close aggrement with the predictions of Ngais coupling model.

Molecular mobility associated with the Tg in DGEBAn̄-DDM networks

Abstract Thermally stimulated creep (TSCr) spectroscopy has been used to investigate the molecular mobility associated with the glass transition in epoxy networks. A series of samples resulting from the crosslinking of DGEBA prepolymer (average degrees of condensation n = 0.03 and 0.15) with the primary amine DDM was studied. The amino-hydrogen-to-epoxy ratio r was systematically varied below the stoichiometric composition. For all resins, a significant shift of the high temperature retardation mode is clearly observed as r is increased. The shift is about 100°C between the formulations with r = 0.5 and r = 1. The complex TSCr spectra resolved into elementary peaks display a compensation law characteristic of the network structure and show that the shift of the peak temperature has an essentially entropie origin.

Thermally stimulated creep (TSCr) study of polyamide 66 and polyamide 66/EPR blends

Abstract Thermally Stimulated Creep (TSCr) spectrometry with fractional loading (FL) has been used to investigate the relaxation processes of polyamide 66/EPR blends. According to the criteria of Starkweather, the γ- and β-retardation modes have been assigned to localized non-cooperative movements. No significant modifications of their fine structure are observed. The α E -process, occuring in PA 66/EPR blends, and associated with the EPR glass transition, exhibits a cooperative feature. The scanning nature of the fractional loading method for the α-process has revealed two distributions of activation enthalpies corresponding to wet ambient and partially dry PA 66, respectively. The same behaviour is observed in PA 66/EPR blends.

Thermally stimulated creep study of bismaleimide/carbon fiber composite

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).

Chain flow in thermo-stimulated creep experiments: application to poly(methyl methacrylate)

Abstract Experimental data from thermo-stimulated creep (TSCr) spectrometry display a retardation mode in poly(methyl methacrylate) (PMMA) at T > T g (glass transition temperature). In this work an attempt is made to relate the restoring force involved during TSCr measurements beyond the glass transition zone to the viscoelastic behaviour of the polymeric chains flowing in the entangled network. In order to determine the temperature dependence of the molecular relaxation time, or lifetime for monomer diffusion, τ mol , three different sources of viscoelastic measurements are used. The TSCr data are then mapped onto the Arrhenius diagram of log τ mol to deduce the corresponding terminal relaxation time, or flow time, τ flow . Comparison between the terminal relaxation time and the characteristic time of TSCr spectrometry shows that, during the temperature scan, the recovery of the frozen-in strain can be effectively well described by the long-range diffusion of macromolecular chains within the surrounding entanglement lattice.

Enhanced Raman signal of CH 3 on carbon nanotubes

ABSTRACTWe find that functionalized SWCNT and DWCNTs (mainly double wall carbon nanotubes) in composites, DWCNTs under hydrostatic pressure and blue illuminated DWCNTs in methanol show the same up shift of the Raman G band and the appearance of a new band at 1455cm−1. This is attributed to the interaction of the CH3 group of the amphiphilic molecule in composites or the CH3 group of alcohol with the outer tube of DWCNTs and indicates that laser heating of DWCNTs in methanol can induce the chemical adsorption of CH3 onto the CNT (carbon nanotube) surface.

Influence of Tacticity on the Retardation Mode Associated with Tg in PMMA

The anelastic properties of PMMA in the temperature range of the main retardation mode are strongly dependent upon tacticity. The thermally stimulated creep (TSCr) technique was used to analyse the distribution function of retardation times. A series of PMMA, with various tacticities, synthesized by living polymerization, were investigated.One of the most important findings was the observation of a sub-mode, at 150°C, with a magnitude increasing with syndiotactic triads. The existence of physical interactions might explain this behaviour. On the low-temperature tail of the a retardation mode, a sub-mode was observed at 60°C: it was associated with remaining isotactic triads.

Combined Effect of Thermal and Mechanical Stresses on the Viscoelastic Properties of a Composite Material for Space Structures

Thermal ageing of polymer-matrix composite materials of any long-life spacecraft in LEO involves evolution of mechanical, thermophysical and morphological properties, especially if the structure operates under external mechanical stress like bending. Behavior of KMU-4l carbon/epoxy composite material under combined effect of thermal profiles and static mechanical loading has been studied in a simulated LEO space environment. Thermal cycling was performed on specimens under bending stress reaching 30.4 % of the material ultimate strength. Specimens subjected to combined thermal and mechanical fatigue were examined in a series of 4 intermediate points with a maximum of 120 cycles.