Dielectric relaxation molecular basis

Davidson and Ripmeester (1984) discuss the mobility of water molecules in the host lattices, on the basis of NMR and dielectric experiments. Water mobility comes from molecular reorientation and diffusion, with the former being substantially faster than the water mobility in ice. Dielectric relaxation data suggest that Bjerrum defects in the hydrate lattice, caused by guest dipoles, may enhance water diffusion rates. 

Thus, specific interactions directly determine the spectroscopic features due to hydrogen bonding of the water molecules, while unspecific interactions arise in all or many polar liquids and are not directly related to the H-bonds. Now it became clear that the basis of four different processes (terms) used in Ref. [17] and mentioned above could rationally be explained on a molecular basis. One may say that specific interactions are more or less cooperative in their nature. They reveal some features of a solid state, while unspecific interactions could be understood in terms of a liquid state of matter, if we consider chaotic gas-like motions of a single polar molecule, namely, rotational motions of a dipole in a dense surroundings of other molecules. The modem aspect of the spectroscopic studies leads us to a conclusion that both gas-like and solid-state-like effects are the characteristic features of water. In this section we will first distinguish between the following two mechanisms of dielectric relaxation ... 

Coming to the present volume, one aim has been to provide a basis on which the student and researcher in molecular science can build a sound appreciation of the present and future developments. Accordingly, the chapters do not presume too much previous knowledge of their subjects. Professor Scaife is concerned, inter alia, to make clear what is the character of those aspects of the macroscopic dielectric behaviour which can be precisely delineated in the theoretical representations which rest on Maxwell s analysis, and he relates these to some of the general microscopic features. The time-dependent aspects of these features are the particular concern of Chapter 2 in which Dr. Wyllie gives an exposition of the essentials of molecular correlation functions. As dielectric relaxation methods provided one of the clearest models of relaxation studies, there is reason to suggest that dipole reorientation provides one of the clearest examples of the correlational treatment. If only for this reason, Dr. Wyllie s chapter could well provide valuable insights for many whose primary interest is not in dielectrics. 

The earliest treatment of dielectric relaxation, on a molecular basis, is that of Debye,12 He treated it as a diffusional process, assuming that spherical molecules were rotating in a continuous medium. Applying the Stokes law., he found... 

For further study of DMA see, for example Ferry JD (1980) Viscoelastic Properties in Polymers, 3 edition. J. Wiley, New York Ward IM (1983) Mechanical Properties of Solid Polymers, 2 edn. Wdey, New York Meier DJ (1978) Molecular Basis of Transitions and Relaxations. Gordon and Breach, New York McCrum NG, Read BE, Williams G (1967) Anelastic and Dielectric Effects in Polymeric Solids. Wiley, New York Aklonis JJ, MacKnightWJ(1967) Introduction to Polymer Viscoelasticity. Wiley, New York Matsuoka S (1992) Relaxation Phenomena in Polymers. Hanser Publ, Munich. 

Dielectric analysis is used most frequently with thermoplastics to provide information on and an understanding of the molecular basis for relaxations. This is a more traditional context for DEA and ample information with munerous interesting examples is available in the literature, particularly in the text by McCrum (71) and the review by Chartoff (77). The reader is referred to these references for further insights. More recently, DEA has had extensive appUcations in the characterization and in situ process monitoring of the ciu-e of thermosets, particularly in the manufacturing of composites. Since this topic is not as well known, we summarize in a subsequent section some of the more salient aspects of the use of DEA for the cure characterization of thermosets. 

N.G. McCmm, B.E. Read, G. Williams Anelastic and Dielectric Effects in Polymeric Solids, John WUey Sons, 1967 N.G. McGmm, G.P. Buckley, G.B. Bucknall Principles of Polymer Engineering, Oxford University Press, 1997 D.J. Meier (ICd.) Molecular Basis of Transitions and Relaxations, Gordon and Breach, 1978... 

The effective correlation times for an approximately isotropic motion, tr, ranged from 40.3 ps in methanol to 100.7 ps in acetic acid for 5a, and from 61.6 ps to 180.1 ps for 5b in the same solvents. Neither solvent viscosity nor dielectric constant bore any direct relationship to the correlation times found from the overall motion, and attempts to correlate relaxation data with parameters (other than dielectric constant) that reflect solvent polarity, such as Kosover Z-values, Win-stein y-values, and the like, were unsuccessful.90 Based on the maximum allowed error of 13% in the tr values derived from the propagation of the experimental error in the measured T, values, the rate of the overall motion for either 5a or 5b in these solvents followed the order methanol N,N-dimethylformamide d2o < pyridine < dimethyl sulfoxide. This sequence appears to reflect both the solvent viscosity and the molecular weight of the solvated species. On this basis, and assuming that each hydroxyl group is hydrogen-bonded to two molecules of the solvent,137 the molecular weights of the solvated species are as follows in methanol 256, N,N-dimethylformamide 364, water 144, pyridine 496, and dimethyl sulfoxide 312. 

Photophysical and photochemical processes in polymer solids are extremely important in that they relate directly to the functions of photoresists and other molecular functional devices. These processes are influenced significantly by the molecular structure of the polymer matrix and its motion. As already discussed in Section 2.1.3, the reactivity of functional groups in polymer solids changes markedly at the glass transition temperature (Tg) of the matrix. Their reactivity is also affected by the / transition temperature, Tp, which corresponds to the relaxation of local motion modes of the main chain and by Ty, the temperature corresponding to the onset of side chain rotation. These transition temperatures can be detected also by other experimental techniques, such as dynamic viscoelasticity measurements, dielectric dispersion, and NMR spectroscopy. The values obtained depend on the frequency of the measurement. Since photochemical and photophysical parameters are measures of the motion of a polymer chain, they provide means to estimate experimentally the values of Tp and Tr. In homogeneous solids, reactions are related to the free volume distribution. This important theoretical parameter can be discussed on the basis of photophysical processes. 

Assumption b) is known to be a good approximation for small molecules. However studies of polymers and glass forming materials by dielectric and mechanical loss methods have frequently been interpreted by assuming that molecular motions are best described by a distribution of correlation times. This has resulted in the formulation of a number of well-known distribution functions such as the Cole-Cole (symmetric) and Cole-Davidson (asymmetric) functions, which have been used to fit dielectric data. It is reasonable to suppose that magnetic relaxation times are also subject to the possible presence of distributions, and a number of modifications of Eq, (4) have been made [16 —i 9] on this basis. 

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