Cooling dielectric relaxation

Optical and electro-optical behavior of side-chain liquid crystalline polymers are described 350-351>. The effect of flexible siloxane spacers on the phase properties and electric field effects were determined. Rheological properties of siloxane containing liquid crystalline side-chain polymers were studied as a function of shear rate and temperature 352). The effect of cooling rate on the alignment of a siloxane based side-chain liquid crystalline copolymer was investigated 353). It was shown that the dielectric relaxation behavior of the polymers varied in a systematic manner with the rate at which the material was cooled from its isotropic phase. 

Ca(N03)2-KN03 (CKN) is a well-known molten salt that easily vitrifies upon cooling. An attempt to ascertain the fragility of this system was made on a CKN sample with a glass transition temperature of350 K. This sample was heated up to 390 K and its dielectric relaxation time measured by an impedance bridge as 10 s. Classify this ionic liquid. (Xu)... 

Thermally stimulated depolarization currents are detected in a sample first cooled to low temperature in a capacitor with shorted electrodes, then warmed slowly with the electrodes connected to a sensitive d.c. electrometer. In this way the dielectric relaxation processes occurring in the sample are displayed separately, according to their activation energies and barrier heights, during the scan over temperature. 

Figure 35. Time evolution of the secondary dielectric relaxation loss spectrum of DPGDB on isothermal annealing at 173.15 K after rapid cooling from 300 K. From top to bottom, the data were obtained after the sample has been annealed isothermally at 173.15 K for times, ta, equal to 93 s, 745 s, 1353 s, 3752 s, and 7272 s elapsed after the thermal stabilization. Solid circles represent the spectrum obtained by slowly cooling the sample at 0.05 K/min. Vertical arrows show the frequencies of the maximum loss for the JG P- and the y-processes.

Figure 2. The log-log plot of dc-conductivity versus dielectric relaxation time in the isotropic phase of 5CB. The line shows the validity of the FDSE relation (1). The inset shows results of the derivative analysis of data from the main plot. It shows the temperature evolution of the apparent value the exponent S on cooling towards the clearing temperature.

As shown in Figure 11.1, to obtain a complex TSC spectrum, a static electric field E is applied to the sample at a polarization temperature labeled Tp for a time tp, which is necessary to obtain polarization saturation, i.e., the equilibrium polarization. Afterward, the sample is cooled down to a temperature T0 in such a way that the dielectric relaxation proceeds extremely slowly, so that after removal of the field the sample retains a frozen-in polarization. The depolarization current, Id, caused by the return to equilibrium of dipolar units, is then recorded by increasing the temperature at a constant rate from T0 up to the final temperature Tf, where Tf > Tp. The plot of Id as a function of temperature is a complex TSC spectrum. 

PVC commercial sample Solvay Cie, type Solvic 229) 346.5 11.2 34.6 338.7 from dilatometry at a cooling rate of 3K/h) [56] aj s of the softening dispersion from creep, J(f), data. It has a much weaker temperature dependence compared with ar, from dynamic mechanical and dielectric relaxation given above. This discrepancy between the shift factors of the mechanical data of Schwarzl with the other sets of data may be due to the much lower Tg of the sample used. 

Transitions. Samples containing 50 mol % tetrafluoroethylene with ca 92% alternation were quenched in ice water or cooled slowly from the melt to minimise or maximize crystallinity, respectively (19). Internal motions were studied by dynamic mechanical and dielectric measurements, and by nuclear magnetic resonance. The dynamic mechanical behavior showed that the CC relaxation occurs at 110°C in the quenched sample in the slowly cooled sample it is shifted to 135°C. The P relaxation appears near —25°C. The y relaxation at — 120°C in the quenched sample is reduced in peak height in the slowly cooled sample and shifted to a slightly higher temperature. The CC and y relaxations reflect motions in the amorphous regions, whereas the P relaxation occurs in the crystalline regions. The y relaxation at — 120°C in dynamic mechanical measurements at 1 H2 appears at —35°C in dielectric measurements at 10 H2. The temperature of the CC relaxation varies from 145°C at 100 H2 to 170°C at 10 H2. In the mechanical measurement, it is 110°C. There is no evidence for relaxation in the dielectric data. 

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