Local reorientation processes

The local reorientation processes observed in FAD and the macroscopic relaxation have the same temperature behavior. Similar observations have already been made using NMR or Fluorescence polarization under continuous excitation However,... 

In the present set of experiments, we have first demonstrated that the model used to interpret the data accounted precisely for the orientation relaxation in a rather wide range of temperatures. Thus, our results strongly support the idea that the local reorientation processes observed in FAD are indeed the elementary processes of macroscopic viscoelasticity. 

The wavenumber-dependent orientation rates of individual transition dipoles observed for dynamically stimulated polymer systems poses several intriguing questions What makes these transition dipoles reorient at different rates Why do some of the transition dipoles seem to reorient at a rate similar to each other Is there any underlying mechanism responsible for synchronization, or lack thereof, in the local reorientation processes of submolecular structures To answer such questions, one must introduce an effective way of representing a measure of the similarity or difference of the reorientation rates of transition dipoles. 

The high quantum yield of 18% for the flip of a single water molecule becomes obvious by a comparison of the number of absorbed IR photons (typically lO s ) with the total number of water molecules (about 10 ) in the sample volume. Since a strong induced spectral diffusion is observed within several minutes, a local reorientation process with a high quantum yield must be involved. 

Coming back to the model discussed above, one should expect some local reorientation processes which need a rela-... 

The local reorientation processes observed in FAD and the macroscopic relaxation have the same temperature behavior. Similar observa-tinuous excitation (40). However, in this latter technique, the slopes of the curves depend on the choice of the model, so that the confidence one could put in the agreement (or discrepancy) between spectroscopic and mechanical results relies directly on the confidence one put in the model arbitrarily chosen to treat the fluorescence polarization data. ... 

Anew experimental method based on the polarization-selective photochromic reactions is proposed to monitor extremely slow reorientation dynamics of molecular tracers in glassy polymer matrix. The correlations between the local relaxation processes of polymers and the reorientation dynamics of the tracers with different sizes are found from the experimental results obtained by this method. 

Reorientation dynamics of molecular tracers in polymers is not only important for the understanding of slow relaxation phenomena in glassy polymers but plays also a critical role in practical problems such as molecular design of nonlinear optical materials with long-term stability based on dyes/polymers complexes. We show here the reorientation dynamics of molecular tracers in glassy polymers obtained by the armealing-after-irradiation method described below. These experimental results are compared to the local relaxation processes of glassy polymers obtained by the already established measurement techniques such as dielectric relaxation and solid state NMR. Finally, the molecular interpretation of the relaxation of free-volume distribution in polymers will be discussed. 

Picosecond laser spectroscopy offers direct access to molecular vibrational (T,) and phase (Tj) relaxation as well as orientational dynamics of molecules, t,< . In contrast to experiments on vibrational relaxation in liquids, all those on the reorientational process have been confined to large polyatomic molecules, particularly dye molecules in probe solvents for pragmatic reasons. Since the slip boundary conditions are also a sensitive function of the shape of the molecules and solute-solvent interactions, there is some uncertainty in deciding whether or not it is the local interaction in terms of the solvation volume, or the boundary condition, or both, that varies for a given molecule in a range of liquids such as the... 

While according to the BPP model, the minimum in the T., vs. temperature plot is expected to occur at a lower temperature than that of the Ti minimum, in fact in AFC the behaviour is opposite as shown in Fig. 22.3. The minimum in appears to occur above room temperature while the minima of Ti occur well below 300 K. This clearly indicates that the two relaxation processes have different origins. It is concluded that while translational diffusion causes relaxation, it is the local reorientational dynamics that determines relaxation. 

As noted above, the in-phase and quadrature spectra represent components of dynamic optical anisotropy caused by the re-orientational behaviour characteristic of the type and local environment of each group. Reorientation processes tend to synchronize if there is a specific chemical interaction or connectivity between them, and herein lies the value of correlation analysis, in that it provides a valuable method for studying the time dependent variation of infrared dichro-ism signals. 

The volume strain does not increase continuously with deformation. There is a plateau in volume strain for applied local strains between 1.0 and 4.0, where the change of internal crystalline structure - orientation and fragmentation of lamellae - leads to a change in shape of voids from elongated perpendicular to elongated in the deformation direction. A reorientation process sets in for local strains of 0.8-1.0. 

The most intuitively acceptable explanation for the breakdown of the theory relating IR dichroism and the uniaxial molecular orientation is that some of the molecular structural parameters, such as i and Aq, are indeed affected by dynamic orientation processes. In other words, molecules undergoing dynamic reorientation processes do not always rotate as rigid and independent entities. Changes in local molecular environments and molecular conformations induced by the macroscopic perturbations imposed on the system significantly affect the submolecular spatial relationship between the individual electric dipole transition moments arising from the vibrations of the molecular constituents and the principal orientation axis of the molecule. 

In Section 2.30.3 of this chapter, attention was focused on the rheo-optical study of polymers at the submolecular and segmental scale. Optical spectroscopy, especially infrared dichroism spectroscopy, was utilized in conjunction with the application of a small-amplitude oscillatory dynamic strain to probe the local dynamics of molecular constituents of polymers undergoing reorientation process. The insights obtained by such measurements were somewhat unexpected, such as the observation that macromolecules exhibit considerable local flexibility and individual rotational freedom with respect to the dynamics of various mesoscopic-scale supramolecular structures. 

Reorientations produce characteristic maxima in the relaxation rate, which may be different for the various symmetry species of CD4. The measured relaxation rates exhibit dependence on two time constants at low temperatures, but also double maxima for both relaxation rates. We assume that molecules may move over some places (adsorption sites) on the cage walls and experience different local potentials. Under the assumption of large tunnelling splittings the T and (A+E) sub-systems relax at different rates. In the first step of calculation the effect of exchange between the different places was considered. Comparison with experimental data led to the conclusion that we have to include also a new relaxation process, namely the contribution from an external electric field gradient. It is finally quite understandable to expect that such effect appears when CD4 moves in the vicinity of a Na+ ion. 

In the excited state, the redistribution of electrons can lead to localized states with distinct fluorescence spectra that are known as intramolecular charge transfer (ICT) states. This process is dynamic and coupled with dielectric relaxations in the environment [16]. This and other solvent-controlled adiabatic excited-state reactions are discussed in [17], As shown in Fig. 1, the locally excited (LE) state is populated initially upon excitation, and the ICT state appears with time in a process coupled with the reorientation of surrounding dipoles. 

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