Daniel Chatain

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.

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>>