Poly dielectric loss

The time-temperature superpositioning principle was applied f to the maximum in dielectric loss factors measured on poly(vinyl acetate). Data collected at different temperatures were shifted to match at Tg = 28 C. The shift factors for the frequency (in hertz) at the maximum were found to obey the WLF equation in the following form log co + 6.9 = [ 19.6(T -28)]/[42 (T - 28)]. Estimate the fractional free volume at Tg and a. for the free volume from these data. Recalling from Chap. 3 that the loss factor for the mechanical properties occurs at cor = 1, estimate the relaxation time for poly(vinyl acetate) at 40 and 28.5 C. 

The equimolar copolymer of ethylene and tetrafluoroethylene is isomeric with poly(vinyhdene fluoride) but has a higher melting point (16,17) and a lower dielectric loss (18,19) (see Fluorine compounds, organic-poly(VINYLIDENE fluoride)). A copolymer with the degree of alternation of about 0.88 was used to study the stmcture (20). Its unit cell was determined by x-ray diffraction. Despite irregularities in the chain stmcture and low crystallinity, a unit cell and stmcture was derived that gave a calculated crystalline density of 1.9 g/cm. The unit cell is befleved to be orthorhombic or monoclinic (a = 0.96 nm, b = 0.925 nm, c = 0.50 nm 7 = 96%. 

Electrical Properties. AH polyolefins have low dielectric constants and can be used as insulators in particular, PMP has the lowest dielectric constant among all synthetic resins. As a result, PMP has excellent dielectric properties and alow dielectric loss factor, surpassing those of other polyolefin resins and polytetrafluoroethylene (Teflon). These properties remain nearly constant over a wide temperature range. The dielectric characteristics of poly(vinylcyclohexane) are especially attractive its dielectric loss remains constant between —180 and 160°C, which makes it a prospective high frequency dielectric material of high thermal stabiUty. 

F ure 6.37 Mechanical and dielectric loss tangents for poly (chlorotrillnoroethylene). Reprinted, by permission, from F. Rodrignez, Principles of Polymer Systems, 2nd ed., p. 271. Copyright 1982 by Hemisphere Publishing Corporation. 

Boyd, R. H. Dielectric loss in linear poly(hexamethylene adipamide.) J. Chem. Phys. 34, 1276 (1959). 

FIG. 11.3 Complex dielectric functions of poly(vinyl acetate). (A) Dielectric loss s"[T) as a function of temperature for three frequencies. (B) Temperature dependence of the dielectric constant s (v) (top panel) and the dielectric loss s"(v) (bottom panel) of the complex dielectric function curves from right to left in the temperature range from 377 to 313 K with steps of 4 K and 312.5,311.5,310.5,310 K. (C) 3D plot of the dielectric loss s"[ ,T). The author is much indebted to Prof. M. Wubbenhorst (KU Leuven) for his illustrative measurements on PVAC, especially for the benefit of this book. 

Fig. 3.11 The plasticising effect of diphenyl in poly(vinylchloride) dielectric loss curves at 60 Hz for (right to left) 0, 1, 3, 6, 9, 12, 15, 20% diphenyl. Reproduced from Fuoss (1941). Copyright of the American Chemical Society.

Fig. 5.1 Photographs of relief models showing the variation of relative permittivity and dielectric loss of poly(ethyleneterephthalate) with temperature and frequency of measurement. From Reddish (1950) with permission of the Royal Society of Chemistry.

The electical properties of amorphous poly-DSP are characterized by a small temperature dependence of the dielectric constant measured between room temperature and 100 °C. The dielectric loss tangent is small and, in addition, the dc conductivity is extremely low. 

Figure 12.4 The dielectric loss in the frequency domain, at different temperatures, for poly(methyl acrylate). (From Ref. 6.)...

Here we consider a series of new poly(ester ether carbonate) (PEEC) multiblock terpolymers with varying amount of ether and carbonate soft-segment content. Dielectric relaxation experiments on the same PEECs revealed the existence of two relaxation processes (Roslaniec et al, 1995). The dielectric loss values show the existence of a relaxation maximum appearing at about 0 °C for 10 kHz relaxation) accompanied by a lower temperature relaxation (y relaxation) which appears at about —50 °C. 

Figure 13. Dielectric loss data at various combinations of temperature and pressure as indicated to demonstrate the invariance of the dispersion of the a-relaxation at constant a-loss peak frequency va or equivalently at constant a-relaxation time for (a) poly(vinylacetate) (PVAc), (b) poly(methyltolylsiloxane) (PMTS), and (c) polyfphenyl glycidyl ether)-co-formaldehyde (PPGE) d)poly(oxy butylene) (POB). In all cases, spectra obtained at higher P are normalized to the value of the maximum of the loss peak obtained at the same frequency at atmospheric pressure.

Figure 15. Dielectric loss data of poly(ethylene-co-vinyl acetate) (EVA, with 70 wt% vinyl acetate) at various combinations of temperature and pressure as indicated to demonstrate the invariance of the dispersion of the a-relaxation at constant a-loss peak frequency va or equivalently at constant a-relaxation time tx.

Polymers that have bulky repeat units can have multiple secondary relaxations. If more than one secondary relaxation is found, then the slowest one has to be the JG relaxation, assuming that the latter is resolved. Excellent illustrations of this scenario are found by dielectric relaxation studies of aromatic backbone polymers such as poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) [43]. The calculated To from the parameters, n and xa, of the a-relaxation are in good agreement with the experimental value of %jq obtained either directly from the dielectric loss spectra or from the Arrhenius temperature dependence of xjg in the glassy state extrapolated to Tg. The example of PET is shown in Fig. 46. 

Dielectric Behavior The dielectric loss behavior of poly-sulfone samples was measured below 23 C as a function of unasso-... 

The dielectric loss behavior of PVAc was similar to that of the other polymers. An Increase in dielectric Intensity of the polymer s S mechanism was directly proportional to the amount of unclustered water. In addition when clustered water was present two separate low temperature peaks occurred as shown In the frequency dependent data of Figure 8. The higher frequency peaks were the result of clustered water. This is confirmed by the similarity between poly(vinyl acetate) and the clustered water peaks of other polymers as plotted in Figure 7. 

Water absorbed in a polymer can exist in an unassociated state or as a separate phase (cluster). In this investigation the DSC technique of water cluster analysis was used in conjunction with coulometric water content measurements to characterize the water sorption behavior of polysulfone and poly(vinyl acetate) The polysulfone had to be saturated above its Tg (190°C) and quenched to 23°C for cluster formation to occur while cluster formation occurred isothermally at 23°C in the poly(vinyl acetate) Both polymers showed an enchancement of their low temperature 3-loss transitions in proportion to the amount of unclustered water present. Frozen clustered water produced an additional low-temperature dielectric loss maximum in PVAc and polysulfone common to polyethylene and polycarbonate as well. Dielectric data obtained on a thin film of water between polyethylene sheets was in quantitative agreement with the clustered water data. 

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). 

Chiu (116) used the apparatus previously described to study the thermal decomposition of selected polymers such as polyethylene terephthalate), po y(vinyl fluoride), po y(vinylidene fluoride), and others. The dielectric constant curves of a group of fluorocarbon polymers are shown in Figure 11.33. As illustrated, the more polar polymers such as poly(vinylidinefiuoride) (PVDF) and poly(vinyl fluoride) (PVF) show characteristic dielectric loss peaks that are distinguishable from the relatively featureless and low-loss curves of the other polymers. For PVF, the low-temperature process is due... 

The frequency and temperature dependent dielectric losses in lightly doped poly-3 methylthiophene have been studied by Pameix [44b]. The frequency dependence (S) of ac conductivity (UacCxw ) was found to decrease linearly with temperature in agreement with a hopping model. 

FIGURE 13.26 (a) Frequency dependence of e" for the P-relaxation in poly(vinyl acetate) measured at different temperatures, (b) Master dielectric loss curve for the data in (a) (O) compared with similar data for the P-relaxation of poly(vinyl benzoate) ( ). (From Ishida, Y. et al., Roll. Z. 180, 108, 1962. With permission from Dr. Dietrich Steinkopff Verlag, Darmstadt.)... 

Figure 5. Dielectric loss tan8 as a function of temperature at IkHz for poly(propylene oxide) in the imaged (o) and aged ( ) states. The aged sample was held for 5 hr at 5.3 C below Tg. Inset shows the tan8 peak intensity for the P relaxation as a function of aging time at 10.3 C below Tg. (Adapted from ref. 49.)...

Dielectric loss e" of miscible blends of poly(2-chlorostyrene) (P2CS450 Mp2cs = 4.5 x 10 ) and poly (vinyl methyl ether) (PVME96 Mpvme = 9.6 x 10 ) with various P2CS volume fractions ( )p2cs measured at 1 kHz at various temperatures. (Data taken, with permission, from Urakawa, O., Y. Fuse, H. Hori, Q. Tran-Cong, and O. Yano. 2001. A dielectric study on the local dynamics of miscible polymer blends Poly(2-chlorostyrene)/poly(vinyl methyl ether). Polymer 42 765-773.)... 

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