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Nematic networks with three different concentrations of polar and nonpolar mesogens and the same concentration of a novel cross-linking agent give rise to unusual liquid single crystal elastomers (LSCEs) that are transparent monodomain nematic networks with smectic clusters. The largest spontaneous length change is observed in the sample with 70 mol% of the polar mesogen which also has the highest glass transition temperature and smectic clusters with a slowly increasing but nearly constant layer spacing on cooling from 90° C to 25°C. X-ray scattering intensity from smectic clusters with layer spacings that monotonically increase on cooling first increases to a maximum at T* ~ 60°C corresponding to clusters of about 30 layers. Below T*, the scattering intensity decreases as the number of layers in a cluster decreases. To account for this surprising nonlinear behavior that correlates with nonlinear features of the networks' macroscopic spontaneous shape change and birefringence, a model is proposed where dislocations form in the layers at T*. Below T*, more dislocations form to break down the layer structure. The possibility of dislocation formation at T* independent of mesogenic concentrations is attributed to a conformational change in the crosslinker which is present at the same concentration in the three LSCEs. [ABSTRACT FROM AUTHOR]

A review of the mechanisms that influence the critical behaviour of the nematic monodomain main-chain and side-chain liquid-crystalline elastomers is presented. The crosslinking density, the applied mechanical load and the temperature at which the second crosslinking step takes place are examples of such mechanisms. This knowledge is a powerful tool when the tuning of the critical behaviour and, thus, of the thermo-mechanical response of the elastomers is needed. [ABSTRACT FROM AUTHOR]

Swelling behavior and deformations of main chain liquid crystal elastomers (MCLCEs) have been reported. The swelling agent used here is a low molecular weight liquid crystal (LMWLC), 4-n-pentyl-4-cyanobiphenyl (5CB). Monodomain MCLCE swells anisotropically with the swelling degree qmono = 11.9 and polydomain MCLCE swell isotropically in 3D with qpoly = 12.1. These values are much larger than observed in swelling side-chain LCEs (SCLCEs). In the stress-strain measurement, the plateau area is found among two steep areas. The electromechanical effects of monodomain MCLCE is also observed, and the results show that the maximum contraction in ∥[image omitted] is about 7%. The threshold for the onset of the electromechanical effect is about 10 V, which is larger than observed in SCLCEs. [ABSTRACT FROM AUTHOR]

Abstract: We summarize investigations on four different nematic main-chain elastomer systems. In the first part, the synthetic approaches towards nematic elastomers with clearing temperatures suitable for mechanical investigations are described. In the second part, the coupling between the liquid crystalline order and the chain conformation is quantified by the Landau-de Gennes coefficient U. Comparing different types of elastomers shows that the coupling depends on the type of mesogens forming the polymer backbone, and on the elastic modulus of the polymer network. In the third part, we compare the orientation behavior of the different systems under strain. [Copyright &y& Elsevier]

A multifunctional main chain liquid crystal elastomer (MCLCE) with large mechanical and optical effects in applied electric fields is investigated, when MCLCE is swollen in a low molecular weight liquid crystal, 4-n-pentyl-4-cyanobiphenyl, a nematic solvent. The size change by the field effects is linearly proportional to the transmittancy change. This suggests the possibility of broad application as a field-induced-optical actuator device. [ABSTRACT FROM AUTHOR]