Cooperative reorientational dynamics

Taken together, the relaxation data demonstrate that there is a hierarchy of dynamic processes. The spin probe undergoes fast intramolecular librations on the time scale of a few picoseconds, experiences local rearrangement of the cage on the time scale of about hundreds of nanoseconds, and performs cooperative reorientation over time scales comparable to or longer than several microseconds in the vicinity of the glass transition. 

The evolution of the many-molecule dynamics, with more and more units participating in the motion with increasing time, is mirrored directly in colloidal suspensions of particles using confocal microscopy [213]. The correlation function of the dynamically heterogeneous a-relaxation is stretched over more decades of time than the linear exponential Debye relaxation function as a consequence of the intermolecularly cooperative dynamics. Other multidimensional NMR experiments [226] have shown that molecular reorientation in the heterogeneous a-relaxation occurs by relatively small jump angles, conceptually simlar to the primitive relaxation or as found experimentally for the JG relaxation [227]. 

The broad width of the JG relaxation can be accounted for by the broad transition from the cage dynamics (revealed by the nearly constant losses) to the fully cooperative Kohlrausch relaxation [36]. In fact, it is important to recall that the time scale where JG relaxation takes place usually exceeds tc, especially in molecular glass-formers, whereas the onset of many-molecule dynamics starts at tr 2 ps. So, when tc short time and then a gradual development of cooperativity occurs when increasing numbers of molecules are ready to reorient independently. In the region tc [Pg.550]

Liquid crystals are generally characterized by the strong correlation between molecules, which respond cooperatively to external perturbations. That strong molecular reorientation (or director reorientation) can be easily induced by a static electric or magnetic field is a well-known phenomenon. The same effect induced by optical fields was, however, only studied recently. " Unusually large nonlinear optical effects based on the optical-field-induced molecular reorientation have been observed in nematic liquid-crystal films under the illumination of one or more cw laser beams. In these cases, both the static and dynamical properties of this field-induced molecular motion are found to obey the Ericksen-Leslie continuum theory, which describe the collective molecular reorientation by the rotation of a director (average molecular orientation). 

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