Main-Chain Liquid Crystalline Elastomers: Monomer and Cross-Linker Molecular Control of the Thermotropic and Elastic Properties

Abstract

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 roomtemperature 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.

Modeling the polydomain-monodomain transition of liquid crystal elastomers

“…The model is based on a representation of LC molecules as ellipsoidal particles [24, 25]. Experimentally, crosslinking molecules in an elastomeric network may be either flexible or rigid [26]. Rigid crosslinkers enhance the anisotropy in the LCE and provide stability to the elastomeric network.…”

Synthesis, liquid crystalline mesophases and morphologies of diblock copolymers composed of a poly(dimethylsiloxane) block and a nematic liquid crystalline block

“…Depending on the mesogenic units in the polymer, liquid crystalline polymers are divided into main-and side-chain liquid crystalline polymers [21][22][23][24][25]. The side-chain liquid crystalline polymers are further divided into 'side-on' and 'end-on' types depending on whether the mesogenic groups are attached laterally or terminally to the polymer backbone via a flexible spacer [26,27].…”

Thermo - optical studies of nematic liquid crystal elastomer

“…These investigations confirm that understandings of structure-property relationship of the LCEs are extremely important for optimizing properties and achieving real applications. [7,8,9,10].…”

“…They are soft, elastic materials that have wide range of applications. [11,12,13]. In recent years, their applications as micropumps, microvalves for microfluidic devices and opto-mechanical shutters have been developed.…”