Monodomains of smectic C liquid crystalline elastomers were prepared by controlled stretching of a polydomain thin film. Specimens were prepared from this film for stress–strain study. A Poisson's ratio of zero was found at small strains for uniaxial stretching perpendicular to the director. For uniaxial strain parallel to the director, Poisson's ratios of approximately 0.4–0.5 were found (by extrapolation) for very small strains. A modulus anisotropy of 8 was found for stretching parallel versus perpendicular to the director.magnified image
Stress–strain results are reported for two series of smectic C main chain liquid crystalline elastomers (MCLCEs). The structural variable in one series is crosslink density; in the other, transverse rod content. With increasing crosslinker content, the LCEs exhibit increased modulus, a shorter polydomain‐to‐monodomain (P–M) plateau, and a lower elongation at break. Increasing transverse rod content leads to lower clearing temperatures and lower modulus. For each series, stress–strain measurement was made simultaneously with Poisson's ratio determination. For the variable crosslinker series, typical behavior showed the Poisson's ratio increased at small strain, rose to a maximum about 50% strain, then decreased with greater strain. For the transverse rod series, typical behavior also showed the Poisson's ratio increased at small strain, rose to a maximum about 50% strain, then decreased with greater strain. In both series, the maximum in the Poisson's ratio corresponded to the onset of the P–M transition. Possible deformation mechanisms are presented.
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.
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