Subject relaxation-resolved

The stress—relaxation process is governed by a number of different molecular motions. To resolve them, the thermally stimulated creep (TSCr) method was developed, which consists of the following steps. (/) The specimen is subjected to a given stress at a temperature T for a time /, both chosen to allow complete orientation of the mobile units that one wishes to consider. (2) The temperature is then lowered to Tq T, where any molecular motion is completely hindered then the stress is removed. (3) The specimen is subsequendy heated at a controlled rate. The mobile units reorient according to the available relaxation modes. The strain, its time derivative, and the temperature are recorded versus time. By mnning a series of experiments at different orientation temperatures and plotting the time derivative of the strain rate observed on heating versus the temperature, various relaxational processes are revealed as peaks (243). 

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

Since an ion is subject to a resuitant or net force, its drift velocity also must be a net drift veiocity resolvable into components. Furthermore, since each component force shouid produce a component of the overaii drift velocity, there must be three components of the net drift veiocity. The first component, which will be designated v°, is the direct result of the externally applied field only and excludes the influence of interactions between the ion and the ionic cloud the second is the electrophoretic component Vg and arises from the participation of the ion in the electrophoretic motion of its cioud finaiiy, the third component is the reiaxation field component originating from the reiaxation force that retards the drift of the ion. Since the electrophoretic and reiaxation forces act in a sense opposite to the externally applied eiectric field, it follows that the electtophoretic and relaxation components must diminish the overall drift velocity (Fig. 4.91), i.e.,... 

Although creep-compliance (Kawabata, 1977 Dahme, 1985) and stress-relaxation techniques (Comby et al., 1986) have been used to study the viscoelestic properties of pectin solutions and gels, the most common technique is small-deformation dynamic measurement, in which the sample is subjected to a low-amplitude, sinusoidal shear deformation. The resultant stress response may be resolved into an in-phase and 90° out-of-phase components the ratio of these stress components to applied strain gives the storage and loss moduli (G and G"), which can be related by the following expression ... 

Carotenoids are still highly topical systems for research. Both Sj Sq and S2 Sq electronic relaxation process in carotenoids with 7 to 11 conjugated double bonds have been subjected to very comprehensive study . The lifetime of the S2 state of P-carotene in CS2. measured by a femtosecond absorption method, is found to be 200-250 fs at room temperature . Fs time resolved CARS from p-carotene in solution shows the occurrence of ultra-high frequency (llTHz) beating phenomena and sub-ps vibrational relaxation. The same technique has been used to observe solvent effects on the a C=C stretching mode in the 2 Ag excited state of P-carotene and two derivatives . A similar study has been made with several derivatives of P-carotene. ... 

A major application of solid state NMR is the study of polymer morphology. Information potentially available includes the amount and orientation of crystalline phases in semi-crystalline polymers and the domain sizes in phase-separated polymeric systems. For the determination of crystallinity, a common method is to measure Ti relaxation in NMR (or NMR for deuterated polymers). The relaxation data can often be resolved into two (or more) components, which may correspond to magnetization arising from crystalline and amorphous phases (11-15,130-134). The development of the maximum entropy regularization method has permitted more facile and less subjective analysis of the data (143). In optimal cases, multiple components can be identified. 

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