Mechanical monodomains

Alternatively, if one dilates a smectic stack by increasing its thickness by an amount Sh > 27t X, then the sample will prefer to bend the layers in an undulational instability (Rosenblatt et al. 1977 Ostwald and Allain 1985) in order to restore the lamellar spacing to its preferred value (see Fig. 10-29d). Note that the increase in thickness 8h required to produce this instability is independent of the initial thickness h of the stack. Hence for a macroscopic sample of thickness, say, h = 60 p,m, the strain Sh/h required to induce the undulational instability is extremely small, 8h/h IzrXfh 10-4. Thus smectic monodomains are extremely delicate and can easily be disrupted by mechanical deformation. 

In recent years considerable effort has been directed toward the study of the structures of liquid-crystalline polymers and copolymers. Using x-ray diffraction techniques developed for the examination of fibers (1,2). detailed structural information can be obtained provided the polymers are aligned in a strong magnetic field or by mechanical means. As reported more recently (3,), quite similar x-ray intensity distributions have been obtained from nematic monodomains of dimer molecules aligned in relatively weak magnetic fields. 

X-ray measurements on LC elastomers have shown [6-8] that the reversible transition between a chiral smectic C phase with and without a helical superstructure can be induced mechanically. The helix untwisted state corresponds in this case to a polar ferroelectric monodomain. The piezoelectricity arising from this deformation of the helical superstructure (which does not require a complete untwisting) has been demonstrated [9] for polymers cross-linked by polymerization of pendant acrylate groups (Figure 15). 

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

To account for the mechanical reorientation properties of monodomains (LSCE), which differ depending on whether the crosslinking has been done in the nematic or the isotropic phase, the concept of frozen order has been suggested [22]. The essen-... 

Mechanical Properties of Monodomains Liquid Single Crystal Elastomers... 

Static mechanical measurements to evaluate the stress-strain relationship in cholesteric sidechain LCEs have been described [71, 72]. In [72] it has been found, for example, thatfor0.94nominal stress Cn is nearly zero as the poly domain structure must be converted first into a monodomain structure. For deformations A < 0.94, the nominal stress increases steeply. Similar results have also been reported elsewhere [71]. The nominal mechanical stress as a function of temperature for fixed compression has also been studied for cholesteric sidechain elastomers [71]. It turns out that the thermoelastic behavior is rather similar as that of the corresponding nematic LCE [2, 5]. 

Static measurements of the change in sample thickness, ALILq (with Lq the thickness in the mechanically loaded state, but without external electric field), as a function of an applied electric field (parallel to the helical axis of a cholesteric monodomain sample) have been carried out [72]. To eliminate effects that are quadratic in the electric... 

A discotic LCE containing triphenylene group was also synthesized by the two-step process (Disch et al., 1995). Samples with a chemically fixed macroscopic alignment of the director (monodomains) was prepared by application of a uniaxial mechanical field during the synthesis of the LCEs. 

More recently, it has been theoretically predicted by Brand [81] that elastomeric networks that have chiral nematic or smectic C mesophases should have piezoelectric properties. The non-centro-symmetric material responds to the deformation via a piezoelectric response. Following this prediction, both Finkelmann and Zental have reported the observation of piezoelectricity. In one case, a nematic network was converted to the cholesteric form with the addition of CB15, 2 -(2-methylbutyl)biphenyl-4-carbonitrile [82]. By producing a monodomain, it is possible to measure the electro-mechanical or piezoelectric response. Compression leads to a piezoelectric coefficient parallel to the helical axis. Elongation leads to the perpendicular piezoelectric response. As another example, a network with a chiral smectic C phase that possesses ferroelectric properties can also act as a piezoelectric element [83]. Larger values of this response might be observed if crosslinked in the Sc state. 

FIGURE 7.9 Photoresponsive properties of monodomain homogeneously aligned films of crosslinked PLCPs and their plausible mechanism of photoinduced bending and unbending behaviors. Reproduced with permission from Reference 28. Copyright 2003 John WUey Sons, Inc. 

The application of the previously discussed techniques to induce monodomain structures in side-chain liquid-crystalline polymers by the application of electric or electromagnetic fields, by shearing or on anisotropic surfaces, frequently leads to comparatively low, macroscopically uniform orientation. Additionally, the methods are limited to a sample thickness of about 100 pm. Liquid-crystalline side-chain elastomers do not have this restriction, because a high macroscopic orientation can be induced in polymeric networks by mechanical deformation up to a sample thickness of about a centimeter [103, 109]. The synthesis of such systems can be performed by crosslinking linear, side-chain liquid-crystalline polymers to networks [llOj. The inherent combination of rubber elasticity and liquid-crystalline phase behavior, may then be exploited for the induction of a macroscopic mesogen orientation by mechanical deformation. 

LC elastomers based on the side-chain and main-chain LC polymers containing rather small concentrations of the chiral mesogens can form SmC mesophase-possessing domains with permanent electric dipole moment, which exhibit piezoelectric properties. The application of an uniaxial mechanical field (shear) produces a centrosymmetric morphology, where the piezoelectric effeas are observed. The piezoelectric coefficient reaches its maximum at a certain shear angle that corresponds to the completion of polydomain to monodomain transformation. The piezoelectric module of different types of such LC elastomers can be higher than those... 

However, it is well known that a mechanical deformation of a conventional, isotropic polymer network causes anisotropy. Under deformation the chain segments become oriented according to the symmetry of the external field and the state of order of the network can be characterized by an order parameter similar to that of nematic liquid crystals. Very early mechanical experiments on nematic polydomain elastomers actually demonstrate that a uniaxial deformation of a nematic elastomer converts the polydomain structure into a macroscopically xmi-formly ordered monodomain network [44]. This is shown in Fig. 2, where the opaque polydomain becomes optically transparent and converts into a monodomain... 

The concept of mechanical field induced orientation can easily be transferred to nematic elastomers with oblate chain conformation, i.e., side chain end-on elastomers with an even number of spacer atoms. In order to achieve a monodomain structure, a globally oblate chain conformation has to be established. This can be achieved by uniaxial compression or biaxial stretching of the polydomain elastomer which induces a uniform homeotropic alignment of the nematic director perpendicular to the film plane. Up to now, this orientaticMi technique has only been realized experimentally for chiral nematic elastomers [72]. 

Fig. 9 Formation of monodomains from LC elastomers by using mechanical fields for the case of locally prolate (left) and locally oblate (right) chain conformation of the polymers...

If LSCEs are prepared using one of the methods described in Sect. 4.1, a monodomain may be obtained with respect to the main director but not necessarily for the whole phase structure. In the case of Sc elastomers an orientation of the director by mechanical stretching or by external fields yields a polydomain with respect to the layer normal. The additional orientation steps that are necessary for a full orientation of this phase are outlined in Sect. 4.2.2. 

In a first step a monodomain sample with respect to the director is produced. This can be done completely analogous to Kiipfer s procedure described for nematic elastomers in Sect. 4.1.1. For this a lightly crosslinked elastomer gel that is swollen with solvent is slowly deswoUen under uniaxial mechanical load for several hours. Then in the second orientation step a mechanical field has to be applied that induces a reorientation of the smectic layer structure in order to produce a uniform orientation of both the director and the layer normal. 

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