Nematic elastomers

We will discuss some preliminary results, which have been performed recently l01). In Fig. 39a the results for polymer No. 2d of Table 10 are shown, which were obtained by torsional vibration experiments. At low temperatures the step in the G (T) curve and the maximum in the G"(T) curve indicate a p-relaxation process at about 120-130 K. Accordingly the glass transition is detected at about 260 K. At 277 K the nematic elastomer becomes isotropic. This phase transformation can be seen only by a very small step in G and G" in the tail of glass transition region, which is shown in more detail in Fig. 39 b. From these measurements we can conclude, that the visco-elastic properties are largely dominated by the properties of the polymer backbone the change of the mesogenic side chains from isotropic to liquid crystalline acts only as a small disturbance and in principle the visco-elastic behavior of the elastomer... 

Naciri J, Srinivasan A, Joen H, Nikolov N, Keller P, Ratna BR (2003) Nematic elastomer fiber actuators. Macromolecules 36 8499... 

De Gennes [81, 82] first suggested that remarkable effects, such as mechanical critical points, shifts in the phase transition temperature, jumps in the stress-strain relation, and a spontaneous shape change, would take place in nematic elastomers. His proposal was based on symmetry arguments independent of any microscopic consideration. 

Figure 29. Nominal stre.ss

For an experimental check-up of the theoretical considerations about liquid-crystalline elastomers in a mechanical field, Fin-kelmann and coworkers [107, 123] studied, in nematic networks, the evolution of the order parameter and of the transition temperature as a function of the stress. The observed results are in full agreement with the predictions of the Landau-de Gennes theory, since an increasing clearing temperature as well as an increasing order parameter are observed with increasing stress. From their results, it was possible to estimate the crosscoupling coefficient U (see Sec. 3.1.1) between the order parameter and the strain of a nematic elastomer [123]. 

Figure 4. Temperature dependence of log( ) and tan (S) for nematic elastomers measured at 30 Hz (reproduced with permission from [8]).

Figure 7. Birefringence An versus true stress cr of a nematic elastomer for different temperatures above clearing temperature Tn i as indicated (reproduced with permission from [2]).

Figure 8. Reciprocal birefringence An vs. reduced temperature T cii nematic elastomer for different loads ( = 1.0 X 10", + = 4.5 X10 N mm" ) (reproduced with permission from [3]).

Deformations of nematic elastomers in electric fields have been described [44, 57, 58]. Reversible shape variations of a nematic sidechain LCE swollen with low molecular weight (LMW) materials in an electric field were reported, provided the elastomer is freely suspended [57]. If it is compressed between glass slides, no field effect is observed. Shape changes in mainchain LCEs swollen with a LMW material have been investigated in an electric field [44]. Similarly, reversible shape changes were observed for freely suspended LCEs on a time scale of one second and no shape changes were obtained for the unswollen elastomers [57]. More recently further experiments of the same type were done on swollen sidechain LCEs [58]. In addition it was checked that there was no response to an electric field if both, LMW material and LCE, were in their isotropic phase [58]. 

No reports on deformations and shape changes of nematic elastomers in external magnetic fields appear to exist yet. 

Macroscopic properties of nematic elastomers have been discussed [56, 60]. De Gennes focused on the static properties, emphasizing especially the importance of coupling terms associated with relative rotations between the network and the director field [60]. The electrohydrodynamics of nematic elastomers has been considered generalizing earlier work by the same authors [54,55] on the macroscopic properties of nematic sidechain polymers [56]. The static considerations of earlier work [60] were extended to incorporate electric effects in addition a systematic overview of all terms necessary for linear irreversible thermodynamics was given [56]. 

An extension of rubber elasticity (i.e. of the description of large, static and incompressible deformations) to nematic elastomers has been given in a large number of papers [52, 61-66]. Abrupt transitions between different orientations of the director under external mechanical stress have been predicted in a model without spatial nonuniformities in the strain field [52,63]. The effect of electric fields on rubber elasticity of nematics has been incorporated [65]. Finally the approach of rubber elasticity was also applied recently to smectic A [67] and to smectic C [68] elastomers. Comparisons with experiments on smectic elastomers do not appear to exist at this time. Recently a rather detailed review of the model of an-... 

Clarke SM, Hotta A, Tajbakhsh AR, Terentjev EM, 2001. Effect of cross linker geometry on equilibrium thermal and mechanical properties of nematic elastomers. Phys Rev E 64 061702. 

Courty S, Mine J, Tajbakhsh AR, Terentjev EM. 2003. Nematic elastomers with aligned carbon nanotubes new electromechanical actuators. Europhys Lett 64 654 660. 

Cviklinski J, Tajbakhsh AR, Terentjev EM. 2002. UV isomerisation in nematic elastomers as a route to photo mechanical transducer. Eur Phys J E 9 427 434. 

Kundler I, Finkelmann H. 1998. Director reorientation via stripe domains in nematic elastomers influence of cross link density, anisotropy of the network and smectic clusters. Macromol Chem Phys 199 677 686. 

Li MH, Keller P, Li B, Wang X, Brunet M. 2003. Light driven side on nematic elastomer actuators. Adv Mater 15 569 572. 

Tammer M, Li J, Komp A, Finkelmann H, Kremer F. 2005. FTIR spectroscopy on segmental reorientation of a nematic elastomer under external mechanical fields. Macromol Chem Phys 206 709 714. 

Warner M, Terentjev EM. 1996. Nematic elastomers a new state of mater. Prog Polym Sci... 

Fig. 3.13. Relative temperature dependences for the BC nematic lODClPBCP fluid monomer, for a BC nematic swollen in a calamitic liquid crystal elastomer (BCN-LCE) and a BC nematic elastomer (BCLCE). The flexoelectric coefficient of an LCE is also shown (note that it is not distinguishable from the horizontal axis at the present scale due to the three orders of magnitude difference).

The frequency dependences of the bend fiexoelectric coefficients were also measured for the same BC nematic fluid monomer, BC nematic swollen in a calamitic liquid crystal elastomer (BCN-LCE) and for the bent-core nematic elastomer (BCLCE) as shown in Fig. 3.14. One can see that for each material the fiexoelectric effect was found to be zero below 1 Hz, then the response increases abruptly up to 2 Hz and then decreases slightly. The apparent absence of the response below 1 Hz is probably due to screening by free ions. The slow decrease of the fiexoelectric coefficient at higher / is not yet clear. We assume, however, that it is not a measurement error, because 5CB showed a constant value in this frequency range. 

Disch, S. Schmidt, C. Finkelmann, H., Nematic Elastomers Beyond the Critical Point. Makromol. Rapid Commun. 1994,15, 303-310. 

Tammer, M. Li, J. Komp, A. Finkelmann, H. Kremer, F., FTIR-Spectroscopy on Segmental Reorientation of a Nematic Elastomer Under External Mechanical Fields. Makromol. Chem. Phys. 2005,206,709-714. 

MarshaU, J. E. Ji, Y Torras, N. Zinovievb, K. Terentjev, E. M. Carbon-nanotube sensitized nematic elastomer composites for IR-visible photo-actuation. Soft Matter 2012, 8, 1570-1574. 

Different types of LC systems are found in elastomers. In nematic liquid crystals, the molecules have orientational but no positional order, their center of mass positions being randomly distributed. Most nematic elastomers are employed in uniaxial deformation. If the LC elements contain chiral groups, they are termed as cholesteric elastomers. Discotic nematic LC elastomers contain disk-shaped molecules that can be oriented in layers. Smectic LC elastomers form well-defined layers. 

If LC monomers are used as starting materials, it is very important to crmsider that monomer and polymer networks can differ in their phase behavior as previously mentioned. This is a particular issue for nematic elastomers. Only very few examples are known in which the temperature regime of a nematic phase of a monomer overlaps with the nematic temperature regime of the polymer. The systematic that was hereby obtained for the LC phase behavior of linear polymers also holds for LC polymer networks because for elastomers the cmicentration of the mesogens is much higher than that of the crosslink. The chemistry that can be used for the crosslink is determined by the chemistry of the polymerization technique. 

When LCEs are synthesized in the absence of external fields, so-called polydomain LC elastomers are obtained, which show macroscopically isotropic properties similar to polycrystalline materials. This resembles, e.g., bulk material of a low molar mass liquid crystal, where thermal fluctuations prevent a uniform director orientation over the whole sample. For nematic elastomers the overall isotropic behavior also indicates an overall isotropic conformation of the polymer chains, which is the consequence of the maximization of the chain entropy. 

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]. 

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