Loss tangent maximum

The position of the loss tangent maximum is quite different for pure B and pure I, indicating distinctly different Tg values for the two. 

In practice the computerized data reduction procedure consists of the series of steps listed in Table I. For both multimodal resonant frequency data and constant frequency dynamic taken at several different frequencies, the correct value of the reference temperature (Tq) is that which reduces the scatter of the modulus and loss tangent data to fit a smooth curve. The initially assumed value of T0 is taken as the temperature corresponding to the loss tangent maximum or Tg. If this value of T0 does not give satisfactory superposition, the assumed value of T0 is adjusted to a slightly higher value and the procedure is repeated until a satisfactory superposition is achieved. Previous... 

Therefore, in all the polymer crystal specimens studied to date the principal loss tangent maximum found for melt-formed samples is greatly reduced in size if present at all, and a maximum at higher temperatures either appears or is more in evidence. The exact mechanism (or mechanisms) responsible for this crystalline loss process is still in doubt. The principal loss mechanism for melt-formed samples is believed to be associated with segmental motion in disordered regions which could possibly be those regions containing tie molecules between lamellae, the surface folds of the crystals, or both. 

Direct differentiation on the contribution of the Debye or the Wagner-Maxwell polarization to the ER effect was carried out by Hao [35]. The strategy employed in Hao s paper is to compare the temperature dependence difference of the dielectric loss tangent maximum values of commonly encountered thrcc-lypc polarizations, the ionic polarization, the Debye polarization, and the interfacial polarization. As shown in Eq.(147) in Chapter 7, for the Maxwell-Wagner polarization (also called the interfacial polarization) the dielectric loss tangent can be expressed as follows if the particle conductivity Op is much larger than that of the medium... 

For the microwave-assisted experiments, both solvents were replaced by 1,2-dichlorobenzene, as it couples very effectively with microwaves (loss-tangent (tan 5) at 20 °C 1,2-dichlorobenzene 0.280 as compared to 0.101 for chlorobenzene). Diels-Alder reactions of 3-methoxy or 3-phenyl pyrazinones with DMAD were performed at a pre-selected maximum temperature of 200 °C, whereas the intramolecular reaction of alkyne tethered pyrazinone required a higher temperature (220 °C). The yields obtained under microwave irradiation are comparable with those obtained under conventional conditions, while for the dihydrofuropyridinone the yield was improved from... 

There are also some far-fetched proposals for the LST a maximum in tan S [151] or a maximum in G" [152] at LST. However, these expectations are not consistent with the observed behavior. The G" maximum seems to occur much beyond the gel point. It also has been proposed that the gel point may be reached when the storage modulus equals the loss modulus, G = G" [153,154], but this is contradicted by the observation that the G — G" crossover depends on the specific choice of frequency [154], Obviously, the gel point cannot depend on the probing frequency. Chambon and Winter [5, 6], however, showed that there is one exception for the special group of materials with a relaxation exponent value n = 0.5, the loss tangent becomes unity, tan Sc = 1, and the G — G" crossover coincides with the gel point. This shows that the crossover G = G" does not in general coincide with the LST. 

In addition to high breakdown strength, the electrical insulators for superconducting magnets must have excellent dielectric properties at cryogenic temperatures. Chant reported the results of measurements on dielectric constant and loss tangent (tan 5) for several polymers over the temperature range from 4.2 to 300 K [83], The variation of dielectric constant of samples as a function of temperature is shown in Fig. 15. The dielectric constants of nonpolar polymers, such as polyethylene, polypropylene and polytetrafluoroethylene, are substantially independent of temperature, whereas those of polar polymers except polyimide decrease by a maximum of 20% as the temperature is reduced. The values of tan 8 at the frequency of 75 cps for nonpolar polymers decreased by... 

Figure 11. Maximum low temperature loss tangent (35 Hz) measured normal to mold flow vs. styrene block length heterogeneity. Circles— branched polymers, triangles—linear polymers, solid symbols—electron micrographs displayed.

The characteristic measure of damping is the ratio of energy dissipated per cycle to the maximum potential energy stored during a cycle. It is related to the called dissipation factor or loss tangent, which is defined to be... 

Figure 1 - The breadth of the glass to liquid transition in blends of PIP and 85% 1,2-PBD as reflected by the temperature range over which the loss tangent had a magnitude greater than 50% of its maximum value [1].

Figure 4. The large broad "maximum" in the loss tangent isotherm of 73-19 polyurethane adhesive at 40 C is attributed to hard segment development. This peak is not observed above 50 C because the hard segment glass transition temperature has been exceeded.

As noted previously by Senich et al. (8), the usefulness of the DSA technique in following the cure of epoxy curing systems stems from the fact that a relative maximum Is produced in the composite loss tangent when the viscosity of the resin reaches a predetermined value. In a previous study (23) which considered a series of polystyrenes of varying molecular weights characterized... 

An analysis of the dielectric data similar to that for the dynamic mechanical data was undertaken. The natural logarithm of the time to peak maximum vs. 1/T was plotted for the two peak maxima observed in the dielectric loss tangent as shown in Figure 11. The activation energies derived from linear least squares fit of the data in these plots are listed in Table II. 

Temperature dependence of the mechanical tensile storage modulus, loss modulus, and loss tangent the precursor measured with a vibrating reed. The peak maximum frequencies in Hz are indicated. Moduli are in dyn cm". Reproduced from Ref. 45. Copyright 1978 American Chemical Society. 

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