Abstract
Main chain liquid crystal elastomers exhibit several interesting phenomena, such as three different regimes of elastic response, unconventional stress-strain relationship in one of these regimes, and the shape memory effect. Investigations are beginning to reveal relationships between their macroscopic behavior and the nature of domain structure, microscopic smectic phase structure, relaxation mechanism, and sample history. These aspects of liquid crystal elastomers are briefly reviewed followed by a summary of the results of recent elastic and high-resolution X-ray diffraction studies of the shape memory effect and the dynamics of the formation of the smectic-C chevron-like layer structure. A possible route to realizing auxetic effect at molecular level is also discussed.
Highlights
Liquid crystal elastomers (LCEs) are complex soft materials where somewhat rigid anisotropic liquid crystalline (LC) moieties are incorporated into flexible polymer chains and randomly cross-linked forming an initially isotropic or anisotropic rubbery structure depending on temperature and the presence of external fields
LCE1 was chosen as the parent elastomer because its cross linking concentration is optimum for efficient liquid crystalline and rubbery network properties, and presence of the plateau [60] in the stress-strain curve might correspond to the soft elastic behavior
The corresponding values of the angle α and S are lower for LCE2 than LCE1 which appears to be related to the introduction of the transverse rod group TR3; (2) with increasing global orientational order of the mesogens, the chevron domains are rotated towards the stretch direction; and (3) the chevron angle and the orientational order of the mesogens remain practically unchanged after the monodomain state is achieved (Figure 11)
Summary
Liquid crystal elastomers (LCEs) are complex soft materials where somewhat rigid anisotropic liquid crystalline (LC) moieties are incorporated into flexible polymer chains and randomly cross-linked forming an initially isotropic or anisotropic rubbery structure depending on temperature and the presence of external fields. These materials embody a unique competition between the anisotropic liquid crystalline and the isotropic polymeric tendencies. The trace of the product of the deformation tensor with its transpose decides the final value of the elastic free energy density and the characteristic rubber modulus μ= nskBT, where, ns is the average strand count per unit volume, kB is the Boltzmann constant, and T is the absolute temperature. From the expression for FE, one can calculate the nominal (or, engineering) σn and the true σt stresses for the rubbery network: σn σt f
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