Calamitic Liquid‐Crystalline Elastomers Swollen in Bent‐Core Liquid‐Crystal Solvents

Abstract

2009 WILEY-VCH Verlag Gmb Liquid-crystal elastomers (LCEs) combine the properties of liquid crystals (LCs) and rubber elasticity. They are unique materials in their ability to change shape spontaneously by large degrees as a consequence of their global orientational (nematic) or combined orientational/positional (smectic) order. LCE systems are thermotropic, and may also be responsive to light, magnetic fields, or electric fields. They have possible uses as artificial muscles or sensors in addition to being intriguing materials for basic research. Bent-core nematic LCs (BCNs) are a relatively new class of mesogenic materials (compared to their calamitic—or rodshaped—cousins). They exhibit both rich phase behavior and novel properties; they also hold the potential of fundamentally new technological applications. In particular, some BCNs have recently been shown to exhibit a giant flexoelectric effect (electric polarization induced by orientational distortion of the average direction of molecular alignment). Since the reported flexoelectric coefficients are some three orders of magnitude larger than standard calamitics, these BCNs have exciting potential for applications ranging from power generation to sensing. Additionally, the phase behavior of mixtures of calamitic (rod-shaped) nematics and BCNs show that the range of the nematic phase can be extended to temperatures approaching ambient, presumably by frustrating the formation of the crystal phase. The flexoelectric response of such mixed BCN and calamitic LCs is still sufficiently large to be promising for applications, even at fairly low unit concentrations of the BCN. However, to realize practical materials that take advantage of this response, one needs a method of encapsulating the BCN in a mechanically robust but flexible matrix that stabilizes (or ruggedizes) the BCN orientational order against variations in environmental conditions. A well-known property of common isotropic elastomeric systems is their ability to swell in suitable solvents. Applied to LCEs in both isotropic and anisotropic solvents of calamitic LCs, this property has proven an effective means to manipulate viscoelastic properties and phase behavior. In this study, we demonstrate, to our knowledge for the first time, the swelling of a calamitic nematic monodomain LCE with large amounts (greater than 30mol%) of two types of bent-core mesogens, both of which show large flexoelectric response in their pure nematic states. LCEs swollen with the two mesogens are additionally investigated with respect to the degree of saturation of the latter’s hydrocarbon chains. We report on the swelling degree (amount of bent-core material imbibed), characteristic swelling times, the resulting phases and transition temperatures, and the orientational order parameter of the swollen LCEs. This study demonstrates a potential method for creating a robust liquid-crystalline device with enhanced flexoelectricity. The chemical structures of the various bent-core materials utilized in our study are presented in Figure 1a and b. The bent-core compound 4-chloro-1,3-phenylene bis[4-(10-decenyloxy) benzoyloxy]benzoate (BCa) was synthesized and purified following a previously described procedure. Versions with symmetric saturated and unsaturated hydrocarbon chains (designated ‘‘uu’’ and ‘‘ss,’’ respectively) were prepared. In BCa, the arms are relatively flexible, as the outer benzene rings are separated by ester groups. The other bent-core mesogen, 4,6-dichloro-1,3-phenylene-bis 4-[{40-(9-octenyloxy)}biphenyl] carboxylate (denoted BCb), features two chlorine units in its central phenyl ring, and the aromatic rings of the arms are directly linked, making themmuchmore rigid. This material and in variants featuring unsaturated (uu) and saturated (ss) chains as well as an asymmetric combination of the two (designated ‘‘us’’) were prepared by a synthetic route previously published. The purity of the samples was checked by high-performance liquid chromatography using a Merck–Hitachi chromatograph equipped with a Merck RP18 column (Cat. No. 16051). Various properties of the nematic and isotropic phases of the neat BCa and BCb mesogens are described in Refs.[19,20]. Unsaturation of the hydrocarbon chain generally results in lowering of the transition temperatures and increasing the flow viscosity, which is, strikingly, several orders of magnitude higher in both nematic and isotropic phases than for typical rod-shaped mesogens. The transition temperatures and associated enthalpy values of the BCa and BCb compounds are shown in Table 1. All mesogens exhibit nematic phases; BCa-ss also exhibits a smectic phase whose detailed in-layer structure is unknown, but is believed to be of the tilted (smectic-C) type. A conventional poly[oxy(methylsilylene)]-based side-chain monodomain calamitic LCE, whose components are shown in