Main-chain liquid crystalline elastomers: Monomer and cross-linker molecular control of the thermotropic and elastic properties

In this study, we report on the design and synthesis of new main-chain liquid crystal elastomers (MC-LCEs), i.e., LCEs with the mesogenic groups inserted in the main-chain, and on their mesomorphic and mechanical properties. We proposed and tested various approaches to produce in a simple way MC-LCEs exhibiting systematically the nematic phase, preferably within an accessible temperature range and ideally with a room-temperature activity, and for which systematic smectic mesophases formation and stability would be reduced. The first approach consisted in the straight modification of the nematic monomer core by methylation of the central ring to reduce the side-by-side molecular interactions, but still preserving the molecular anisotropy to promote nematic materials. As for the second approach, it implied the incorporation within the reticulated network of bulky and anisotropic cross-linkers, with the expectation to disfavor the tendency for lamellar phase formation by limiting the microsegregation between the plastifying segments and rigid parts. All the elastomeric systems reported here exhibited a room-temperature mesomorphic behavior; in all cases, both SmC and N phases were systematically observed. Particularly relevant, a low-temperature nematic behavior was strongly promoted over the formation of smectic phases with elastomers containing discotic cross-linkers. In contrast, the degree of methylation appeared to have almost no influence on the mesophase nature, but contributed to a substantial reduction of the transition temperatures. The impacts of these structural modifications on the mechanical properties were also evaluated. For instance, elastomers, with anisotropic cross-linkers have a more reduced stretching ability and are slightly more fragile than those with flexible (and deformable) ones and do not form as stable monodomains either. The results of these investigations are reported, the mesomorphic and elastic properties are discussed in correlation with the molecular structures of the components, and also some aspects connected to the synthesis are evoked.