Liquid crystalline network

Liquid crystalline polymers can be crosslinked to form a network, or an elastomer, while retaining liquid crystallinity. This section is devoted to both the liquid crystallinity and the rubber elasticity of the crosslinked polymers. 

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Covalently crosslinked siloxane containing liquid crystalline networks with elastic properties were prepared 349). In all of the networks liquid crystalline phases of the linear precursors were retained. For low degrees of crosslinking the phase transition temperatures remained nearly unchanged, whereas higher degrees of crosslinking reduced the phase transition temperatures. 

In a system based largely on self-assembly through mesogenic interactions such as the above example by Percec, it can also be beneficial to incorporate hydrogen bonding as well for an added element of organizational control. For instance, Frechet and coworkers demonstrated sequential, hierarchical self-assembly of various levels of order in a small molecule-based system which ultimately resulted in a hydrogen-bonded liquid crystalline network (Fig. 7.9) [53]. 

Fig. 7.9 Self-assembly arising from both hydrogen bonding and mesogenic behavior leading to a smectic liquid crystalline network.

Stoddart et al.l<,lb have reported the self-assembly of branched [njrotaxanes in an investigation aimed at the preparation of larger, dendritic rotaxanes. Kato et al. 9lcl have reported the preparation of supramolecular liquid crystalline networks based on self-assembly of carboxylic acid-based, trigonally branched, //-bonding donors and bipyridine-type //-bonding acceptors. 

Usually the surfactant concentration in ointments and creams is significantly lower than in surfactant gels. Ointments are non-aqueous preparations, whereas creams result from ointments by adding water. The microstructure of both ointments and creams may consist of liquid crystals, as long as a liquid crystalline network or matrix is formed by amphiphilic molecules. In a liquid crystalline matrix, it is easier to deform the system by shear such formulations show plastic and thixotropic flow behavior on shear. In comparison to systems with a crystalline matrix which are usually destroyed irreversibly by shear, those with a liquid crystalline matrix exhibit a short regeneration time of... 

The field of thermoset polymers is an area of polymer science that has not yet been widely integrated with liquid crystal polymer research. It is interesting to note that Finkelmann et al. (5) reported on the formation of crosslinked elastomeric liquid crystalline networks in 1981. Although the general concept of a... 

Yu, Y., Nakano, M., Shishido, A., Shiono, T. and Ikeda, T. (2004) Effect of cross-linking density on photoinduced bending behavior of oriented liquid-crystalline network films containing azobenzene. Chem. Mater., 16, 1637-1643. 

Small molecular mass liquid crystals do not respond to extension and shear stress. Liquid crystalline polymers may exhibit a high elastic state at some temperature due to the entanglements. However, the liquid crystalline network itself is an elastomer, showing rubber elasticity. In the presence of external stress, liquid crystalline networks deform remarkably and then relax back after the release of stress. The elasticity of liquid crystalline networks is more complicated than the conventional network, such as the stress induced phase transition, the discontinuous stress-strain relationship and the non-linear stress optical effect, etc. 

We have only dealt with the main chain nematic networks so far. Actually many liquid crystalline networks are formed by crosslinking the backbones of side chain liquid crystalline polymers. The side chain nematic polymers have three nematic phases and their backbones have either prolate or oblate conformations, depending on their phase. It is expected that the rubber elasticity of a side chain nematic polymer network is more complex. For instance, the stress-induced Ni-Nm phase transition is predicted as the network shape transforms from oblate to prolate. Liquid crystalline networks have a bright potential in industry. 

The use of hydrogen bonding for mesogenic network formation has been expected for the formation of dynamic mesomorphic states due to the induction of dynamic properties, e.g. reversibility and fast exchange of the H-bonding [108-111]. Supramolecular liquid-crystalline networks have been prepared by the self-assembly of a polyacrylate with a benzoic acid moiety and 4,4 -bipyridine [108]. Complexes 46 form dynamically cross-linked structures obtained by the complexation of a carboxyl-functionalized polyacrylate and a mixture of a stilbazole, which has the single H-bond acceptor, and a bipyridine, which has the two acceptors. 

Fig. 15. Reversible phase transition of supramolecular liquid-crystalline network 46...

Cholesteric liquid-crystalline networks were obtained when chiral building block 49 was complexed with compound 4 [111]. 

Side Chain Liquid Crystalline Networks and Mechanical Properties... 

Liquid-crystalline networks can be obtained in several ways, either from an isotropic middle or from materials in which the mes-ogenic groups were first macroscopically aligned prior to the final crosslinking (see Chap. V of this Volume). The latter networks should keep a complete memory of the original orientation and recover it even after been held well above the clearing temperature for extended periods [92, 93]. 

Swelling experiments and stress-strain measurements are widely used to estimate the crosslinking density in liquid-crystalline networks [101,106-108], For ideal rubbers with tetrafunctional crosslinks, these techniques allow the evaluation of the number average molecular weight between crosslinks [104, 105]. 

Figure 28. Elastic modulus of liquid crystalline networks obtained from the polymerization of monomers a and b as the function of the composition. The solid line corresponds to the value predicted by Eq. (21) [99].

Mechanical Field Effects on Liquid-Crystalline Networks... 

The potential of the mesogenic units for alignment has a marked effect on the stress-strain behavior [85, 111, 112]. Consider a uniaxial stress applied to a liquid-crystalline network synthesized in an isotropic state this means without any macroscopic orientation. 

Experimentally, piezoelectricity in cholesteric and chiral smectic C phases was reported for liquid-crystalline networks [140-147]. Multidomain lightly cross-linked systems were synthesized, then the orientation is obtained by mechanical strain [140] or by poling [147]. In other samples this orientation is performed prior to the crosslinking process [144, 146]. Macro-scopically oriented samples were subjected to either a static or a periodically varying strain. Open circuit voltages across the samples were measured that are linear functions of the applied strain [140-142, 144, 145],... 

Kihara, H., Kato, T., Utyu, T., Frechet, J.M.J., 1996. Supramolecular liquid-crystalline networks built by self-assembly of multifunctional hydrogen-bonding molecules. Chem. Mater. 8, 961-968. 

It is worth mentioning that some side-chain liquid crystal polymers and liquid-crystalline networks (M = Cu, Zn, Co, Ni, Pb) have also been prepared, usually preserving the properties of the monomeric discotic units. They may find some interesting applications in the field of onedimensional conductors as well as applications using flexo- and piezo-electricity. 

Anisotropic PLC gels are blends of a PLC network and a miscible MLC, and the first reports appeared in the early 1990s [138-140]. Systems of this kind are also referred to as liquid crystal-liquid crystalline network composite systems. 

Figure 9.12 Chemical route to a macroscopically oriented liquid crystalline network (adapted from Reference [29]).

Fischer, P. Schmidt, C. Finkelmann, H., Amphiphilic Liquid-Crystalline Networks—Phase Behavior and Alignment by Mechanical Fields. Makromol. Rapid Common. 1995,16, 435-447. 

Figure 10 Subunits used by Frechet and coworkers to construct liquid-crystalline, network polymers [54].

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