Thermal depolarization current measurement

A detailed discussion of the statistical thermodynamic aspects of thermally stimulated dielectric relaxation is not provided here. It should suffice to state that kinetics of most of the processes are again complicated and that the phenomenological kinetic theories used to described thermally stimulated currents make use of assumptions that, being necessary to simplify the formalism, may not always be justified. Just as in the general case, TSL and TSC, the spectroscopic information may in principle be available from the measurement of thermally stimulated depolarization current (TSDC). However, it is frequently impossible to extract it unambiguously from such experiments. 

When the film is short-circuited and heated to high temperatures at which the molecules attain a sufficiently high mobility, a current is observed in the external circuit. This phenomenon is called pyroelectric effect, thermally stimulated current, or, when the film has been polarized by a static field prior to measurement, depolarization current. The conventional definition of pyroelectricity is the temperature dependence of spontaneous polarization Ps, and the pyroelectric constant is defined as dPJdd (6 = temperature). In this review, however, the term will be used in a broader definition than usual. The pyroelectric current results from the motion of true charge and/or polarization charge in the film. Since the piezoelectricity of a polymer film is in some cases caused by these charges, the relation between piezoelectricity and pyroelectricity is an important clue to the origin of piezoelectricity. 

The electrical properties, i.e, volume resistivity, dielectric permittivity and dielectric loss factor, as well as thermally stimulated depolarization current, were measured on polypropylene-polycarbonate (PP-PC) blends. The results confirm the existence of some interactions between the non--compatible components of PP-PC blends. 

Figure 1 Thermally stimulated depolarization current curves in reduced current density units, measured on PP homopolymer samples in a wide temperature range. Wl, reference W2, annealed W3, oxidized (data taken from [3]) Jl, nondrawn film J2, drawn film (data taken from [4]).

Figure 6.26. Comparative data from DMA (at/= 1 Hz),TSC q= l°C/min) and DSC measurements q = 10°C/min) for a polyurethane based on 2,4-toluene diisocyanate (NCO/OH = 1.2).The thermally stimulated currents spectrum is the only one to present the current signatures of both the soft-segment and hard-segment glass transitions. [Adapted from plots presented by Hsu and coworkers (1999), with permission of Elsevier. Tliis article was pubhshed in Thermochimica Acta, Volume 333, by J.-M. Hsu, D.-L. Yang, and S.-K. Huang, TSC/RMA study on the depolarization transitions of TDI-based polyurethane elastomers with the variation in NCO/OH content, pp. 73-86, Elsevier (1999).]...

In series of publications [25,27,29,35-40] several methods were used for eharaeterization of the microphase structure of the semi-IPNs studied. Small-angle X-ray seattering (SAXS), differential scanning calorimetry (DSC) [27, 35-37], dynamic mechanical thermal analysis (DMTA) [27, 30-32], dielectric relaxation spectroseopy (DRS), and thermally stimulated depolarization currents (TSDC) [25, 39, 40] measurements have shown that pure PCN is characterized by a typical homogeneous structure, but for segmented LPU the microphase separation on the level of hard and soft domains due to their thermodynamic immiscibUity was denoted. As for semi-IPNs, the destruction of the microphase separated morphology of LPU was observed and the microphase separation between PCN and LPU phases, expected from the difference of solubility parameters, was not found. 

An alternative method to observe dielectric properties is termed thermal stimulated currents (TSC). This method involves polarization of a sample at high temperature (relative to Tg) and quenching to a temperature where depolarization is kineticaUy prevented in the time scale of the experiment. The temperature is then increased and the depolarization current is measured, yielding peak values associated with polymer transitions analogous to t", E" and tan S values obtained by conventional dielectric and dynamic mechanical measurements. The TSC spectra can reveal secondary relaxations, glass transitions and liquid or crystalline phase transitions and hquid crystalhne phase transitions. TSC has been applied to PBT/PC and PA6/ABS blends to study the intermixing of the components of the respective blends [58]. The TSC method is described in several references [59-61]. 

Some secondary relaxations of the components in thermoplastic AIPNs have been investigated by thermally stimulated depolarization current (TSDC) techniques and thermally stimulated conductivity (TSC) measurements [10,11]. It was found that upon addition of S-co-AA to CPU, the secondary and 3 CPU peaks (at ca. -140 °C and ca. -100 °C, respectively) shift slightly to lower temperatures, i.e., the corresponding relaxations become faster, these shifts being more pronounced at low S-co-AA contents. The shifts can be related to physical interactions between the IPN components and to their partial miscibility. Rizos et al. [15] have shown that as a result of such interactions, changes in the local free volume may occur, affecting the secondary relaxation times. The same changes in the [3 relaxation of PU have been found in polyurethane/polystyrene IPNs by Pandit and Nadkarni [16]. 

The organization of the present chapter is as follows. Dielectric techniques for molecular dynamics studies, in particular broadband dielectric spectroscopy (DS) and thermally stimulated depolarization currents (TSDC) techniques are shortly presented in the next section. Section 3, devoted to ionic conductivity measurements and analysis, focuses mostly on analysis, as the measuring techniques and equipment are often similar to those used for DS. The microphase separation and morphology of segmented PUs is discussed in the following Section 4, which completes the first introductory part of the chapter. Results obtained with selected PTE are presented in Section 5, followed by a larger Section 6 devoted to PU ionomers with ionic moieties in either of the HS and SS. PU ionomers of the latter type are often based on poly(ethylene oxide) (PEO) as the SS component and, for this reason. Section 6 includes a discussion of telechelics based on PEO, which may serve as model systems for PU ionomers. In Section 7, we discuss recent results obtained with nanocomposites based on PTE, a topic attracting much current interest, before we conclude with Section 8. 

Thermally stimulated depolarization (TSD) current was also measured using an Ekco type 616B electrometer. The samples were polarized at the following conditions polarization temperature 150 0, polarization time 2 min, electric field 8 kV/cm. The heating rate was 1°C/min. 

The thermally stimulated relaxation in heterogeneous dielectrics, consisting of relaxing components and exhibiting interfacial relaxation, presents a special problem, which has been solved exactly only for bilayer systems [2]. Here, the accumulation of space charge at the interfaces may cause anomalous depolarization effects (currents of reversed polarity). The same is true for samples measured with air gap or with one-sided electrodes (this latter is frequently used for corona charged samples). 

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