Papers by Keyword: Thermally Stimulated Depolarization Current

Abstract: In this work, we investigated the influence of physical aging on polylactic acid (PLA) films using thermal and optical techniques; Differential Scanning Calorimetry (DSC), Thermally Stimulated Depolarization Current (TSDC), and Attenuated Total Reflection Spectroscopy (ATR). The PLA films were aged for different periods: 60, 90, and 120 minutes at a temperature T_a = 43 °C. The result obtained by DSC showed that the effect of physical aging appeared as an endothermic peak, which increased with increasing aging time and evolved towards higher temperatures. TSDC results showed a thermal current peak located between 30 and 80 ° C, which represented the main relaxation mode (α relaxation) of the dielectric manifestation of the glass transition. The intensity of this peak decreased and was shifted to higher temperatures when aging time increased, this result can be explained by a decrease in the molecular mobility of macromolecular chains due to the decrease in the free volume. The effect of physical aging on the PLA by the ATR technique showed a gradual decrease in all absorption bands during the aging period. In particular, the wide absorption band between 3000 and 3700 cm^-1 attributed to the hydroxyl group (OH), which disappeared after two hours of aging

Abstract: Compensation has been reported for the relaxation parameters: the activation energy W and the pre-exponential factor τ0, determined from the Thermal Sampling of Thermally Stimulated Depolarization Current technique. Below the glass transition it is assumed that the relaxation time follows an Arrhenius equation. In the vicinity of glass transition temperature an experimental thermogram may be analyzed using the Vogel-Fulcher-Tamman-Hesse (VFTH) or the Williams- Landel -Ferry equation. In this article we use the VFTH relationship to study the compensation effect in the range of glass transition. For an elementary peak obtained by TS there is a relationship between the activation energy W, the temperature of the maximum current Tm, the VFTH temperature, the compensation temperature Tc and the compensation time τ c. We employ this relationship for a basic analysis of the compensation effect in the temperature range around Tg. By numerical simulations, and assuming parameters similar to those measured experimentally, we show that it is possible to observe a compensation point in some well defined conditions

Abstract: The apparent activation energy W, the temperature Ta at which the molecular movement is assumed frozen (the relaxation time is infinitely large) and the preexponential factor t0 were determined for Nylon 11 using the data obtained by fractional polarization thermally stimulated discharge current technique. The aim was to investigate the temperature dependence of the relaxation time in the range of the glass transition temperature. An experimental thermogram can be analyzed equally well using the Vogel-Fulcher-Tammann-Hess relationship or the Arrhenius equation. Experimental results and simulations, for nearby peaks, demonstrate that the relaxation time at peak temperature changes from about 100 to 500 s revealing a very narrow distribution in relaxation times. On contrary, W and especially t0 varies in large limits and the values for t0 are in many situations physically meaningless. It was found that the temperature at which the molecular movement can be assumed frozen for electric effects is about 150 K below the glass transition temperature in Nylon 11. A small change in W (in the limit ± kTm which represents the minimum uncertainty interval in W) induces a change in t0 of about two decades, indicating (i) that any analysis assuming t0 constant should be avoided and (ii) the selectivity limit of the method.