Colette Lacabanne

Differential scanning calorimetry and thermostimulated current spectroscopy for the study of molecular orientation in polymers

Abstract The influence of uniaxial and biaxial orientation on the transition/relaxation of poly(ethylene) terephthalate (PET) has been investigated. The transitions were studied by differential scanning calorimetry; the corresponding relaxations were studied by thermostimulated current (TSC) spectroscopy. At around 50°C, a particularly intense TCS peak was observed in the uniaxially oriented sample; this mode is a precursor of the glass transition Tg. The relaxation mode associated with the glass transition is observed at 82°C in unoriented PET, at 85°C in uniaxially oriented PET and at 100°C in biaxially oriented PET. An analysis of the fine structure has shown that it is made up of elementary processes that obey a compensation law: at the compensation temperature Tc, all the relaxation times become equivalent. It is important to note that for amorphous films (unoriented and uniaxially oriented) Tc - Tg ≈ 5°C, whereas for the semi-crystalline film (biaxially oriented) Tc - Tg ≈ 15°C. This result shows that crystallites are strongly coupled to the amorphous phase.

Thermally stimulated creep: A new method for the study of molecular movements in solids

Abstract We propose a new method for the investigation of orientational movements: in a mechanical step-function experiment, we thermally stimulate the response to a constant stress. The Thermally Stimulated Creep allows to compute the complex xompliance in the frequency range (10−12–104Hz) for a temperature range (500 K-LNT). The high resolving power of this technique has been used to resolve experimentally the retardation-time spectrum of polyamide 66, as example.

Transient Method of Measuring Very Low Conductivities without Contacting Electrodes

An experimental procedure is proposed, which amounts to Fourier transforming the frequency depending torque in the Ogawas torsion pendulum method. This reduces by a factor of the order 100 the fairly long times needed for measuring the very low conductivities of insulating materials.

Dielectric relaxation phenomena in PEEK

Abstract Molecular movements in poly(ether-ether-ketone) have been investigated by thermally stimulated currents (TSC). The TSC spectra have been analyzed as a function of crystallinity. Around −80°C, two sub-modes are observed: the lower temperature sub-mode has been attributed to localized cooperative movements in the crystallizable amorphous phase; and the upper temperature sub-mode has been located in the crystalline phase. The magnitude of the corresponding compensation line increases with the ratio of crystallinity. Around the glass transition temperature (145°C), the existence of two sub-modes has been shown: the lower temperature sub-mode has been attributed to the dielectric manifestation of the glass transition and the cooperative movements have been assigned to the “true amorphous region” of PEEK; the upper temperature sub-mode is dependant upon crystallinity. Regardless of the preceding sub-mode, the activation enthalpies of the constituting elementary processes are practically constant. This sub-mode has been attributed to the “rigid amorphous region”, constrained by the crystallites.

Influence of uniaxial and biaxial orientation on the dielectric properties of PET

A comparative study of amorphous, uniaxially and biaxially oriented films was performed using thermally stimulated current (TSC) spectroscopy. The TSC peak around the glass temperature (T/sub g/) was investigated. In amorphous polyethylene terephthalate (PET) a TSC peak was observed at 82 degrees C, while under uniaxial orientation, it was shifted to 88 degrees C, and under biaxial orientation, it reaches 100 degrees C. The shift of this T/sub g/ peak under stretching indicates a stiffening of the amorphous chains. The analysis of the fine structure of these complex peaks shows the existence of compensation phenomena. For uniaxially and biaxially oriented PET this analysis reveals a lower and upper temperature component for the glass transition. This phenomenon indicates the existence of a segregated phase.<<ETX>>

Influence of interface/interphase on the dielectric properties of composites

Thermally stimulated current spectroscopy (TSC) has been applied to the study of interfaces/interphases in composites. Three DGEBA-DDA matrix model samples have been considered: matrix, matrix plus elastomer-coated glass beads, and matrix plus silane-treated glass beads. Below room temperature, the gamma and beta relaxations have been observed together with the mode associated with the glass transition of the elastomeric interphase that only represents less than 1% in weight of the composite. Above room temperature, a complex TSC alpha peak has been found with the same characteristics regardless of the nature of the interface/interphase in composites. However, the analysis of the fine structure of the alpha mode shows the existence of a compensation phenomenon characteristic of the matrix-filler interface/interphase. Surface treatment by silane decreases the number of accessible sites while the elastomeric coating increases the number of accessible configurations.<<ETX>>

Conformation and Physical Structure of Tropoelastin from Human Vascular Cells: Influence of Cells Lipid Loading

Aggregated low density lipoproteins (agLDL) contribute to massive intracellular cholesteryl ester (CE) accumulation in human vascular smooth muscle cells (VSMC). Our aim was to determine the conformational and physical structure of agLDL and elastic material produced either by control human VSMC or by agLDL-loaded human VSMC (agLDL-VSMC). At the conformational level scanned by FTIR spectroscopy, a new undefined, probably non-H-bonded, structure for tropoelastin produced by agLDL-VSMC is revealed. By differential scanning calorimetry, a decrease of water affinity and a drop of the glass transition associated with aggregated tropoelastin (from 200°C to 159°C) in the supernatant from agLDL VSMC are evidenced. This second phenomenon is due to an interaction between agLDL and tropoelastin as detected by the weak specific FTIR absorption band of agLDL in supernatant from agLDL-loaded VSMC.

Influence of Vacuum and Temperature During Simulated Aging on Properties of a Polyepoxy Adhesive Used in Spacecraft Applications

Adhesively bonded joints used on spacecraft are subject to thermal cycling in orbit. Effect of this ageing is investigated in terms of polyepoxy adhesive properties and corresponding assemblies. A simplified isothermal ageing model is used to understand effect of temperature and secondary vacuum. Vacuum involves the same ageing phenomena than an ageing under neutral gaseous environment at atmospheric pressure. Temperature induces a post-curing of the adhesive. For the highest ageing temperature, a plasticization phenomenon is observed (glass transition temperature decrease). Thermal cycling leads to adhesive’s properties modifications similar to those monitored after an isothermal ageing. The effect of thermal cycle mainly depends on maximum temperature reached during a thermal cycle.