Relaxation real-time experiments

For the measurement of cross-correlated relaxation rates, there are mainly three methods that have been used in practice. In the /-resolved constant time experiment, the multiplet Hnes exhibiting differential relaxation are resolved by the f couplings, and the line width is translated into intensity in a constant time experiment (Fig. 7.19a,d). In the J-resolved real time experiment the line width of each multiplet line is measured instead (Fig. 7.19b, d). This experiment has been applied so far only for the measurement of... 

Fig. 2. Time (x) and scattering vector (Q) raqge for various scattering experiments on dynamics SANS ILL spectrometers Dll and DI7 relaxation after stepstrain, cyclic experiments, steady couette shear. Elastic neutron scattering ILL spectrometer D20, also real time experiment. Neutron Spin Echo (NSE) ILL spectrometer, inelastic measurement and classical quasielastic light scattering (QELS) (from Ref. )...

The interest of studies on relaxation mechanisms of van der Waals complexes is general they may be considered as models for a larger class of unimolecular reactions. The specificity of van der Waals systems relies on the exceptionally weak coupling between intra- and inter-molecular modes, much weaker than typical intra-state coupling in molecules. The time scales for IVR and VP processes are therefore significantly longer and more convenient for real-time experiments. 

The excitation of a molecule or cluster by an ultrashort laser pulse - as is shown for silver aggregates in Chap. 5 - induces time-dependent changes of their electronic and atomic structure. These changes can be nicely studied by real-time experiments inducing charge reversal processes (see the NeNePo scheme in Fig. 1.1). Very recently, several rather different theoretical approaches have been published or presented at conferences dealing with the relaxation mechanisms induced by ultrafast excitation processes. A brief summary of these theories is given here. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Ultrafast vibrational spectroscopy offers a variety of techniques for unraveling the microsopic dynamics of hydrogen bonds occurring in the femto- to picosecond time domain. In particular, different vibrational couplings can be separated in nonlinear experiments by measuring vibrational dynamics in real-time. Both coherent vibrational polarizations and processes of population and energy relaxation have been studied for a number of hydrogen bonded systems in liquids [1],... 

The sample must have reached steady state before cessation of the test or the application of a second step. Steady state in a creep test is seen as a constant slope in the strain curve. A constant slope in the stress curve may also be seen in a stress relaxation test, but often the signal is lost in the noise. A material that is liquid-like in real time will need a test period of 5 to 10 min. A stress relaxation test is likely to be somewhat shorter than a creep test since the signal inevitably decays into the noise at some point. A creep test will last indefinitely but will probably reach steady state within an hour. For a material that is a solid in real time, all experiments should be longer as molecular motion is, by definition, slower. Viscoelastic materials will lie in between these extremes. Polymer melts can take 1 hr or more to respond in a creep test, but somewhat less time in a stress relaxation test. 

Wide-line and 2H NMR spectra and T2 relaxation experiments have been used to determine the composition of the phases in semi-crystalline polymers [133, 136, 138-144]. The experiments were also used to obtain real-time information on the kinetics of crystallisation and melting [143-148]. The use of high-resolution NMR methods to characterise semi-crystalline polymers is reviewed elsewhere [17, 18, 30, 34, 149]. 

Network structures have been quantitatively determined by means of real-time XH NMR T2 relaxation experiments for several polymers [174-178]. The effect of the curing conditions on sol and gel fractions and the spatial heterogeneity of the network structure has been studied for polyethylene [174], polyacrylamide [175], PDMS [176], BR [177], epoxy resins [178] and EPDM [179]. 

The time it takes for the temperature of a sample to stabilise in an NMR probe can also be determined in real-time XH NMR Tj relaxation experiments, because at vulcanisation temperatures Tj is only slightly affected by a moderate crosslink density [22, 180, 181]. The T2 data obtained in the aforementioned experiments were used for determining an increase in the density of chemical crosslinks upon vulcanisation time (Figure 10.18) [179]. The method can also be used in kinetic studies of the vulcanisation of filled and oil-extended rubbers. 

Since the pulse time is so short (see Sec. 3.6.2.2.3) one can coherently excite many vibrational modes at a time and monitor relaxation processes in real time. The first reported femtosecond time-resolved CARS experiments (Leonhardt et al., 1987 Zinth et al., 1988) showed beautiful beating patterns and fast decays of the coherent signal for several molecular liquids. The existence of an intermolecular coherence transfer effect was suggested from the analysis of the beating patterns (Rosker et al., 1986). Subsequent studies by Okamoto and Yoshihara (1990) include the vibrational dephasing of the 992 cm benzene mode. A fast dephasing process was found that is possibly related to... 

These circuits naturally have a frequency-dependent impedance, and it is this that is measured in impedance spectroscopy experiments. The components of the circuit also determine the response of the reaction in the real time domain to any dc perturbation, for example, an electrical pulse or termination of a prior steady current (potential-relaxation experiment). 

Elucidation of the kinetics and mechanisms of mineral-fluid interactions requires high-resolution X-ray scattering measurements on rapid time scales. Time series analyses are desired for addressing the evolution of structure and composition at the interface, on time scales as small as milliseconds or less. The high brilliance of the third-generation synchrotron sources affords new opportunities for such time-resolved studies, because we can observe in real time the processes of adsorption/desorption and complex formation at mineral-fluid interfaces. For example, experiments using a pressure-jump relaxation techniques yield rates of adsorption and desorption of protons and hydroxide at the surface of metal oxides in the range of milliseconds to seconds (reviewed by Casey and... 

As pointed out in the preceding paragraph the recording equipment used for a continuous flow experiment may be simpler and slower in response than that required by relaxation methods operating in real time. As a result, if it is necessary to make a few kinetic measurements, and if the supply of material will permit, it may be much cheaper to set up an unambitious flow apparatus than to attempt other procedures. 

The objective of this contribution is to review the improvements in the combined use of real time X-ray scattering and dielectric relaxation techniques experiments for a better understanding of polymer crystallization. 

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