Dielectric spectroscopy molecular reorientation

Microwave spectroscopy and dielectric relaxation studies probe the autocorrelation function of the total electrical polarization of the system and thereby also provide information about molecular reorientation. This information is difficult to interpret. 

For si and sll, Davidson et al. (1977a, 1981) performed NMR spectroscopy and dielectric relaxation measurements where applicable, in order to estimate the barriers to molecular reorientation for simple hydrates of natural gas components, except carbon dioxide. Substantial barriers to rotation should also affect such properties as hydrate heat capacity. 

The applied electric field perturbs the orientational distribution function of the dipolar molecules. Dielectric relaxation due to classical molecular reorientational motions is a form of pure absorption spectroscopy whose frequency range of interest for materials, including polymers, is between 10 6 and 1011 Hz. 

Since the late nineteenth century, dielectric spectroscopy has been used to monitor dynamical properties of solid and liquid materials. At that time, dielectric measurements were performed either at a single frequency or in a very limited frequency range now, however, measurement technique and instrumentation have developed to such an extent that dielectric spectroscopy is today a well-established method to probe molecular dynamics over a broad range in frequency or time (cf. reviews by Johari [1], Bottcher and Bordewijk [34], Williams [35,36], and Kremer and Schonhals [37]), even with commercially available equipment. Including the latest developments, one can even say that nowadays dielectric spectroscopy is the only method that is fully able to realize the idea of 0- to 1-THz spectroscopy. In data sets that cover the range of up to 10 6—1013 Hz—that is, from ultra-low frequencies up to the far infrared—the full range of reorientational dynamics in... 

For an understanding of the phase behavior of liquid-crystalline systems in general knowledge of the dynamics is necessary. Moreover, in these materials molecular, dynamics is contemporary and important for their application in memory devices because the storage of information is directly connected with a reorientation of molecules or parts of it. Dielectric relaxation spectroscopy has proven to be a very suitable tool to study the molecular motion in these compounds for several reasons. Firstly, mesogens are usually polar. If different molecular reorientations involve... 

Dielectric relaxation thus resembles self-diffusion. Both processes observe the motion of single macromolecules through a uniform albeit fluctuating background. In a two-component polymer-solvent system, dielectric spectroscopy reveals the effect of intermacromolecular interactions on single-molecule size and reorientation. Dielectric measurements on a three-component polymer-polymer-solvent mixture, in which a tracer polymer has a nonzero type-A dipole and a potentially nondilute matrix polymer has none, can be used for example to separate the effects of probe and matrix molecular weights on dielectric relaxation. This motif in the comparative study of binary and ternary solutions appears repeatedly below. Finally, dielectric measurements on block copolymers in which some copolymer subchains have been inverted end-to-end or have no dipole moment allow one to observe internal motions and dynamic cross-correlations of subchains. 

A useful and common way of describing the reorientation dynamics of molecules in the condensed phase is to use single molecule reorientation correlation functions. These will be described later when we discuss solute molecular reorientational dynamics. Indirect experimental probes of the reorientation dynamics of molecules in neat bulk liquids include techniques such as IR, Raman, and NMR spectroscopy. More direct probes involve a variety of time-resolved methods such as dielectric relaxation, time-resolved absorption and emission spectroscopy, and the optical Kerr effect. The basic idea of time-resolved spectroscopic techniques is that a short polarized laser pulse removes a subset of molecular orientations from the equifibrium orientational distribution. The relaxation of the perturbed distribution is monitored by the absorption of a second time-delayed pulse or by the time-dependent change in the fluorescence depolarization. 

Reports on the rate of spontaneous fluctuations in confinement are relatively recent. With regards to thin polymer films, pioneering studies in this sense have been performed by broadband dielectric spectroscopy (BDS) on Al-capped thin PS films more than one decade ago by Fukao and Miyamoto [60]. This technique probes the reorientation of dipoles under the application of electric fields in the linear regime [88], and therefore is ideal to characterize the intrinsic molecular mobility in glass formers. Fukao and Miyamoto found that the typical relaxation time remains bulk-like down to thicknesses of 20 nm. For smaller thicknesses, acceleration of the rate of spontaneous fluctuations was observed. Similar results were later reported by others [73, 165]. 

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