Mechanical Orientability of the Mesogens

The mechanical orientability is the most prominent property of liquid-crystalline elastomers. Above a threshold stress (Sec. 3.3.1), small strains (about 20%-40%) are 

The strain-indueed orientation is analyzed with the help of birefringence measurements [107, 115, 120, 123] (realized in the isotropic state, close to the transition), measurements of the IR dichroism [124, 126], and X-ray experiments [93, 108,120, 122, 125]. It was observed that the orientational order parameter, labeled P2 on Fig. 32, obtained from X-ray experiments [122], quiekly inereases with strain at the beginning, then saturates at a P2 value of about 0.4-0.6 for a sample synthesized in an isotropic state [99, 107, 121, 122]. On condition that they were realized above the gel point [122], the networks display reproducible and reversible behavior independent of the sample history. 

IV Behavior and Properties of Side Group Thermotropic Liquid Crystal Polymers 

For networks that exhibit a smectic phase, mechanical elongation always causes orientation of the director perpendicular to the axis of the stress [122, 125], as shown on Fig. 33. The mesogenic groups therefore become perpendicular to the polymer main chain if we make the reasonable assumption that the polymer chain is extended preferentially in the extension direction. Such an arrangement allows the polymer backbone to occupy the space between the layers. 

For samples that only show a nematic phase, both parallel and perpendicular 

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The difference in the character of the orientation of the mesogenic groups is apparently determined by a number of factors, including the length of the aliphatic spacer and the flexibility of the main chain, and the ratio between them determines the degree of their correlation under the effect of a mechanical field. The type of deformation of a LC polymer in the mesophase (shear, extension) should also significantly affect the process of destruction of LC domains and the formation of an oriented structure. In contrast to a mechanical... 

The obtaining of NEs is particularly interesting because of the coupling between the director, which defines the macroscopic orientation of the mesogens, and the polymer network leads to unique mechanical properties, such as an elastic anisotropy, a spontaneous contraction or elongation of the NE induced by a temperature change, or the presence of an elastic plateau in the stress-strain curve when the NE is stretched perpendicularly to the initial direction of the director. The elasticity of these materials has been the subject of numerous theories, which will be discussed below. 

Discontinuities in the main chain can be obtained by different means. Example 4-6 describes the incorporation of flexible aliphatic segments. Of special interest is the overall orientation of incorporated mesogens by mechanically or electrically applied external forces. 

Quite recently, Finkelmann and co-work-ers [232, 233] showed that an appropriate mechanical deformation of an SmC elastomer 50 yields a permanent macroscopically uniform orientation. This process also unwinds the helicoidal superstructure, and accordingly, frequency doubling is observed where the intensity of the SHG is directly related to the perfection of the uniform smectic layer orientation. The 22 < 23= 34 coefficients for a highly oriented sample were reported to be 0.1 pm/V and 0.15 pm/V, respectively. Taking into account that only 50% of the mesogenic units in the LC elastomer are active groups, these values are of the same order as those reported for low molar mass LCs containing similar chromophores [222]. 

FTIR polarisation spectroscopy was used to study the orientational behaviour of the different constituents of a liquid crystalline block copolymer (copolyester-imide) during uniaxial elongation at different temperatures. Differences in the degree of alignment (as well as in the response to the application of the mechanical load) were observed. With increasing temperature, the level of the applied stress and the induced degree of orientation decreased, while the differences in the orientational behaviour of the mesogen and the flexible spacer were retained (243). 

It is well known that human muscles are made of many bundles of muscle fibers and their anisotropic contractions are induced by electric stimulus. To construct artificial muscles, crosslinked PLCP fibers were fabricated due to their high mechanical flexibility [40]. As shown in Figure 7.18, crosslinked PLCP fibers wifh high orientational order of the mesogens along the fiber axis were fabricated. 

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