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.

No abstract

No abstract

Liquid crystalline networks (LCNs) are described in which there is anelasticity in strain recovery response (under zero load) after uniaxial tensile loading. This strain retention is shown as a function of time after release of load and is further characterized by thermal, X‐ray, and stress/strain experiments. It was found that, at temperatures in the smectic phase far below the isotropization temperature, this LCN film retains significant levels of strain when in the monodomain state. On free recovery (zero load) of the LCN film there is a rapid elastic response followed by a slow anelastic response for those films that had undergone a polydomain‐to‐monodomain transition during the initial imposed strain regimen. It is postulated that the mechanism leading to the strain retention involves nanosegregation‐driven pinning of unfolded hairpins in shallow energy wells and that this effect is responsible for the thermally activated recovery of strain (shape memory) at elevated temperatures. Stress–strain curves of pre‐strained C11(MeHQ)Si8XL10 LC networks having various initial strains: (a) 350%, (b) 300%, (c) 250%, (d) 200%, (e) 150%, (f) 100%, and (g) 50%.

An LCE with a p‐pentaphenyl transverse rod in the main chain was synthesized, in which the rod can be oriented parallel or normal to the main chain under uniaxial tension. DSC and WAXD studies indicate a highly ordered lamellar structure typical of a smectic A phase. Stress‐strain curves showed a large Young modulus at small strains, followed by a yield point, at which necking occurred and the specimen became transparent. An interesting rigid‐soft‐rigid phenomenon was observed in the yield section, which is likely to indicate a self‐assembly‐driven reconstruction process. Two possible arrangements of transverse rods and chain extenders are proposed for the network structure.magnified image

A series of smectic liquid crystalline network polymers was subjected to a large uniaxial stress at temperatures far below the clearing temperature. Their dimensional recovery is anelastic showing substantial retained strain. This process produced a temporally stable monodomain state. Upon heating this monodomain from room temperature, recovery of the original film dimensions occurs. The strain recovery (length) curves show a pronounced curvature as the temperature approaches the smectic-isotropic temperature. It is proposed that nanosegregation of netpoints in the smectic structure is responsible for the anelasticity and that the temperature dependence of the shape recovery is consistent with a balance between enthalpic and entropic forces with temperature. An interesting mechanically induced strains ratio (MISR) was observed on heating these prestressed films. Loss of the monodomain structure near the isotropization temperature is postulated to rationalize the shape of the MISR curves.