Broadband dielectric spectroscopy

Additional information on ferroelectric and electroclinic switching can be obtained with broadband dielectric spectroscopy. It appears that the molecular dynamics of FLCPs are comparable to those of low molecular weight compounds [67]. However, the experimental observations are made more difficult for FLCPs than for low molecular weight SmC liquid crystals due to the conductivity contribution which takes place at frequencies below 10 Hz and to the difficulty to get a macroscopically well-aligned sample. 

Nevertheless, several authors, in studying SmC polyacrylates [22] or SmC poly-siloxanes [ 14,41,67], have observed the two expected collective relaxations in ferroelectric liquid crystals, namely the Goldstone mode and the soft mode. These two relaxations occur at frequencies lower than 10 Hz. 

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. 

Dielectric techniques for moleculrir dynamics studies 2,1, Broadband dielectric spectroscopy 

Dielectric techniques are a powerful tool for studying molecular dynamics in various materials. Their main advantage over other techniques of measuring molecular dynamics is the extremely broad frequency range covered, which extends from about 10 to about 10 Hz [14-16]. Obviously, this broad frequency range cannot be covered by a single technique. 

In most cases, the measurements are carried out isothermally in the frequency domain and the terms dielectric spectroscopy (DS) and dielectric relaxation spectroscopy (DRS) are then used. Other terms frequently used for DRS are impedance spectroscopy and admittance spectroscopy. Impedance spectroscopy is usually used in connection with electrolytes and electrochemical studies, whereas admittance spectroscopy often refers to semiconductors and devices. Isothermal measurements in the time domain are often used, either as a convenient tool for extending the range of measurements to low frequencies (slow time-domain spectroscopy, dc transient current method, isothermal charging-discharging current measurements) or for fast measurements corresponding to the frequency range of about 10 MHz - 10 GHz (time-domain spectroscopy or time-domain reflectometry). Finally, TSDC is a special dielectric technique in the temperature domain, which will be discussed in Section 2.2. 

In slow time domain spectroscopy, a voltage step is applied to the sample and the polarization or depolarization current /(t) is measured as a function of time. The time-dependent dielectric permittivity e(t) is then given by 

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Kremer F. and Schonhals, A., Eds., Broadband Dielectric Spectroscopy, Springer, Berlin, 2003. 

Arbe A, Colmenero J, Richter D (2002) In Kremer F, Schonhals A (eds) Polymer dynamics by dielectric spectroscopy and neutron scattering - a comparison in broadband dielectric spectroscopy... 

L. Trakhtenberg, G. Gerasimov, E. Axelrod, Yu. Feldman, 3rd International Conference on Broadband Dielectric Spectroscopy and its Applications, BDS 2004, Delft, Netherland, August 23-26, 2004, p. 199. 

Broadband Dielectric Spectroscopy provides a direct experimental access to the molecular relaxations of polymers over a broad frequency and temperature range. It is also especially suitable for the investigation of thin polymer films, because it does not suffer sensitivity loses with decreasing sample amount. This technique does require a special sample preparation for thin films, because of the need to have metal electrodes and good electrical contacts at both interfaces. Spin-coating, one of the most commonly employed methods for the preparation of... 

For thin polystyrene films annealed for 12 hours at 150 °C in high vacuum (10-6 mbar) and measured in a pure nitrogen atmosphere the dynamic glass transition was characterized using two experimental techniques capacitive scanning dilatometry and Broadband Dielectric Spectroscopy. Data from the first method are presented in Fig. 15a, showing the real part of the complex capacity at 1 MHz as a function of temperature for a thin PS film of 33 nm. 

Similar results were obtained by Broadband Dielectric Spectroscopy (Fig. 17) no shifts in the relaxation time of the dynamic glass transition were detected, even for PS films as thin as 20 nm. 

This chapter concentrates on the results of DS study of the structure, dynamics, and macroscopic behavior of complex materials. First, we present an introduction to the basic concepts of dielectric polarization in static and time-dependent fields, before the dielectric spectroscopy technique itself is reviewed for both frequency and time domains. This part has three sections, namely, broadband dielectric spectroscopy, time-domain dielectric spectroscopy, and a section where different aspects of data treatment and fitting routines are discussed in detail. Then, some examples of dielectric responses observed in various disordered materials are presented. Finally, we will consider the experimental evidence of non-Debye dielectric responses in several complex disordered systems such as microemulsions, porous glasses, porous silicon, H-bonding liquids, aqueous solutions of polymers, and composite materials. 

The successful development of the time-domain dielectric spectroscopy method (generally called time-domain spectroscopy, TDS) [79-86] and broadband dielectric spectroscopy (BDS) [3,87-90] have radically changed the attitude towards DS, making it an effective tool for investigation of solids and liquids on the macroscopic, mesoscopic, and, to some extent, microscopic levels. 

The characterization of the physical and chemical changes that occur in montmorillonite/PDMS nanocomposite elastomers as they are thermally aged is reported. Broadband Dielectric Spectroscopy (BDS) was used to track changes in the physical interaction between the polymer and clay associated with increases in non-oxidative thermal stability (as determined by TGA). The evolution of volatile siloxane species from the elastomers was characterized with Thermal Volatilization Analysis (TVA). Results suggest that the improved thermal stability and the increases in polymer/clay association are a result of significant re-structuring of the polymer network. 

T. Blochowicz, Broadband Dielectric Spectroscopy in Neat and Binary Molecular Glass Formers, ISBN 3-8325-0320-X, Logos Verlag, Berlin, 2003. 

Does a shift of the glass transition temperature reflect a change in the relaxation time distribution or in its mean value only This makes a fundamental difference underlining the power of microscopic techniques like Broadband Dielectric Spectroscopy. 

Measurements by broadband dielectric spectroscopy also reveal no shifts of the dynamic glass transition (inset, Fig. 39). In consequence, the average relaxation rate of the dynamic glass transition remains unchanged for all thicknesses investigated in the present study (Fig. 39). 

In relation to the questions addressed in the introduction, the following answers are given, based on experimental results obtained for three polymer systems investigated in thin layers using broadband dielectric spectroscopy, capacitive scanning dilatometry, and calorimetry ... 

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