Stress-induced reorientation

An applied stress lowers the symmetry of the crystal and can make defects with different orientations inequivalent. A review of stress techniques has been written by Davies (1988). The degeneracy of the ground state and also of the spectroscopic transition energies can be lifted. In this section we suppose that the defects cannot reorient and consider only the splitting of the transition energies. The stress-induced reorientation of defects is discussed in the next section. 

Figure 7.11 Irreversible stress-induced phase reorientation in 2,10-undecandione-urea. (a) Crystal before stress, (b) Schematic of crystal before stress arrows represent carbonyl dipoles for guests in one layer, (c) Crystal under uniform stress from bottom left of picture, (d) Schematic of stressed crystal. (Reproduced by permission from Macmillan Publishers Ltd).

Brown, M. E., Hollingsworth, M. D., Stress-induced domain reorientation in urea inclusion-compounds. Nature 1995, 376, 323-327. 

Surface nano-morphology changes of photoreactive molecular crystals are an attractive area of research, because the phenomena could potentially be applied to photodriven nanometer-scale devices and provide important information on crystal-line-state reaction mechanisms and dynamics [2a, 21]. As described in Section 25.3.2, the single crystal of lEt, in which the CpEt rings have no reorientation freedom in the crystal, tends to collapse and degrade as the reaction proceeds. This observation for the crystal of lEt can be explained by the local stress induced by the photoreaction that is not suitably released by the crystal lattice. In such a crystal, does the surface morphology of the crystal change ... 

Figure 2-52. Possible mesogen orientations during the reorientation process at S=0 Oi-direction of internal stress induced during the second crosslinking step, Oci direction of external stress applied during the rheo-optical experiment), (a) Homeotropic orientation perpendicular to the film plane (b) isotropic orientation (c) planar biaxial orientation.

Reece, M.J. and Guiu, F., 2002, Toughening produced by crack tip stress induced domain reorientation in ferroelectric and/or ferroelastic materials. Philosophical Magazine A82 29 38. 

Interestingly, IR dichroism studies on liquid crystalline polymers can reveal some insight into the mechanism of mechanical stress-induced orientation processes. For example, in studies with polyurethanes carrying mesogenic side chains (Chart 2.7), it was found that an initially stress-induced orientation system would be broken up and reoriented when the strain surpassed a critical value [77]. 

If an isotropic polymer is subjected to an imposed external stress it undergoes a structural rearrangement called orientation. In amorphous polymers this is simply a rearrangement of the randomly coiled chain molecules (molecular orientation). In crystalline polymers the phenomenon is more complex. Crystallites may be reoriented or even completely rearranged and oriented recrystallisation may be induced by the stresses applied. The rearrangements in the crystalline material may be read from the X-ray diffraction patterns. 

A reorientation or depolarization of the domain is also effected by mechanical stress (e.g. 20... 50 N/mm for PZT). Influencing factors besides the stress magnitude are its direction and frequency as well as the kind of electrical circuit (e.g. open circuit, load or short circuit). If the electrical field induced by a force is in the polarization direction, the nonlinearities are essentially smaller than those of a generated field in the opposite direction or in the case of short circuit. 

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