Dielectric mechanical measurements

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. 

Although the intermediate peak and the low-temperature (-80°C) peak have similar magnitudes in a mechanical measurement, in a dielectric experiment, the intermediate peak is over twice as large (Figure 2). This seems to be due to moisture, since heating briefly to 300 C decreases the peak loss tangent substantially. 

Hinrichs and Thuen [28] used ultrasonic attenuation to determine the proper time for pressure application during an otherwise traditional pre-established cure cycle. Because dielectric is an electrical property, it is influenced by moisture content and temperature as well as viscosity, so it may vary quantitatively. Ultrasonic measurements are also affected by other parameters (i.e., void content), but they are a mechanical measurement rather than an electric one. The ultrasonic sensors used by Hinrichs unfortunately were less reliable than the dielectric sensors. 

By combining the results of several methods (dynamic mechanical, dielectric, NMR, etc.), it is usually possible to determine quite reliably the structural units whose motions give rise to secondary relaxations. If dynamic mechanical measurements alone are employed, the usual procedure is that the chemical constitution is systematically altered and correlated with the dynamic mechanical response spectra, i.e. with the temperature-dependence of the G" and G moduli. If the presence of a certain group in polymers is marked by the formation of a loss peak characterized by a certain temperature position, size and shape etc., then the conclusion may be drawn that the motional units responsible for the secondary relaxation are identical or related with that group. Naturally, the relations obtained in this way are empirical and qualitative. 

It is worth pointing out that the relaxation map is quite useful for comparing results obtained by techniques operating at various frequencies, like dynamic mechanical measurements, dielectric relaxation, NMR, etc. 

The associated activation energy (Table 1) is 56=bl0kJ, a value significantly lower than the one derived from mechanical measurements (Table 1). In the same way, the activation entropy (Table 2) of 53 d= 10 J K-1 mol-1 corresponding to the centre of the relaxation peak is lower that the value obtained from mechanical measurements. The symmetrical character of the dielectric P peak is also reflected in the constant value of ASa over the temperature range (Table 2). 

It is worth noting that the relaxation observed from dielectric measurements (Fig. 77) occurs at the same temperature as the one from dynamic mechanical measurements. In addition, the activation energies derived from the maximum of the mechanical /3 peak (Table 7) are close to those obtained from dielectric relaxation (Table 6). 

In another paper in this issue [1], the molecular motions involved in secondary transitions of many amorphous polymers of quite different chemical structures have been analysed in detail by using a large set of experimental techniques (dynamic mechanical measurements, dielectric relaxation, H, 2H and 13C solid state NMR), as well as atomistic modelling. 

The efficient mechanism for translative motion of a stem through an orthorhombic crystal discovered by dielectric relaxation measurements has... 

The results corresponding to dielectric relaxation measurements are more explicit than the mechanical ones. Figure 2.68 shows the data corresponding to s and tan 8 for the four first members of the series of poly(itaconate)s. 

Assuming that the subglass dielectric activity observed in these poly(diitaconate)s have a similar origin than that observed in mechanical measurements by Heijboer [28] the results for these systems are in good agreement with those previously reported. 

Dielectric Measurements. The dielectric loss (c") curves at different frequencies for samples containing 100, 80, 40, and 0% PVC, respectively, are shown in Figures 4, 5, 6, and 7. Figure 8 is a composite of the dielectric loss data at 1 kHz for each sample. The general characteristics of a and p relaxation peaks of the component polymers and their mixtures parallel the results of dynamic mechanical measurements. For each... 

FIG. 13.24 Frequency-temperature correlation map for dynamic mechanical, dielectric and NMR measurements on Polyisobutylene. The a and P relaxation regions are shown. Measurements ( ) NMR (O) Dielectric ( ) Mechanical. The full lines are the WLF equation (curved) and the Eyring Equation (straight). From Schlichter (1966). Courtesy John Wiley Sons, Inc. 

FIG. 13.28 Frequency-temperature correlation map for polyfmethyl methacrylate) (PMMA). Dynamic mechanical measurements are indicated by filled points and dielectric measurements by open points ... 

Dielectric relaxation measurement in similar to dynamic mechanical measurements, except that it exploits the dipole electrical properties of the blend. It is, therefore. 

Dynamic mechanical and NMR investigations of crystals grown from dilute solutions for polymers other than linear polyethylene have been much less extensive. Studies have been reported for the linear polymers polyoxy methylene (3, 40, 94), poly (ethylene oxide) (3, 78), and nylon 6 (42), and the branched polymers polypropylene (40), poly-l-butene (19, 95), poly(4-methyl-l-pentene) (33), poly (vinyl alcohol) (78), and branched polyethylene (78). In addition, dielectric loss measurements have been made on crystal aggregates of poly (ethylene oxide) (23), poly (vinyl alcohol) (68), and polyoxymethylene (3) and mechanical loss measurements have been carried out on polyoxymethylene formed by solid state polymerization (94). 

The molecular motions of the PNF (i.e., primary and secondary molecular relaxations) were documented by dielectric and dynamic (Rheovibron) mechanical measurements (14) (Figure 5). The -160°C relaxation has been ascribed to the combined onset of the trifluoroethoxy, (3, and fluoroalkoxy, 3", side chain motion (15,16). The — 50°C relaxation has been ascribed (15) to the glass transition (segmental backbone) of the PNF. 

In contrast with the dielectric constant, the tangent delta/temperature relation between 0°C and 240°C is not (detectably) influenced by these moisture concentrations. The tangent delta/temperature curves of both wet systems show, however, a clear relaxation effect with maxima between -60eC and -70°C. These effects disappear after drying i.e. they stem from the water phase. Such a low-temperature, low-frequency loss process was also detected in dynamic mechanical measurements. Banhegyi et al. reported such an effect due to absorbed water for CaC03 filled polyethylene samples [21]. 

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