Imine-Based Reactive Mesogen and Its Corresponding Exchangeable Liquid Crystal Elastomer

Abstract

To date, exchangeable liquid crystalline elastomers (xLCEs) have been mainly fabricated by combining conventional LCEs with additional exchangeable functional groups in their networks. While conventional LCEs are frequently made from commercially available aromatic–ester reacting mesogens or from mesogens based on a biphenyl core, such reacting monomers are not optimized to fabricating xLCEs whose bond-exchange reaction is fast and clean cut. Here, we develop a fast synthesis route to produce a new type of reactive mesogen based on an aromatic–imine structure that intrinsically enables a fast and stable bond-exchange reaction in the resulting imine-based xLCE. This new xLCE displays vitrimer plastic-flow behavior, and its bond-exchange activation energy is calculated to be 54 kJ/mol. We also demonstrate that this xLCE is thermally stable to withstand many recycling cycles without visible decay, and its liquid crystallinity is preserved. Finally, we demonstrate the reprogramming and realignment of the mesogen orientation in this xLCE with the realigned xLCE capable of reversible thermal actuation.

Manipulation of Liquid Crystalline Properties by Dynamic Covalent Chemistry─En Route to Adaptive Materials

“…In recent years, the concept of adaptivity has gained broad attention for the development of new functional materials as it allows the controlled manipulation of materials properties. − With respect to this, dynamic covalent chemistry (DCC) bears great potential as it combines the error-correction capability and adaptivity of supramolecular chemistry with the robustness of the covalent bond. − Regarding the formation of functional systems, such as molecular cages and macrocycles, significant progress has been achieved, − and also first steps toward the application in materials were reported. ,− So far, DCC was mainly used to manipulate the properties of liquid crystalline elastomers (LCEs). − An interesting example was presented by Mulder et al, who reported the fabrication of nanoporous liquid crystalline networks using the dynamics of imine bonds to manipulate the pore size and functionality of their materials . However, reports on low-molecular-weight liquid crystals based on DCC are rare. − An interesting example was reported in 2006 by Giuseppone and Lehn.…”

Functional Liquid Crystal Elastomers Based on Dynamic Covalent Chemistry

“…For example, a slow creep under load was performed to align an LCE film in the nematic phase because it exhibited a fast imine metathesis reaction. [74] The introduction of permanent crosslinking moieties allowed controlling plasticity in the nematic phase while avoiding a large creep caused by the presence of fast boronic ester reactions. [75] Although the programming of DCB-based LCEs in the LC phase appears to be detrimental due to loss of thermal and actuation stability, this is advantageous when thermally activated DCBs are incorporated into phototropic LCEs, as it allows orthogonal stimuli to induce reprogramming of deformation modes and retention of actuated states via bond exchange.…”

“…Although the xLCE film was shown to exhibit thermally reversible action, the rapid dynamic exchange hindered proper isotropic-genesis programming and thus limited thermal stability upon heating/cooling cycles above T i (70 °C). [74] 3.6. Urethane and thiourea bond exchange…”

“…Therefore, carefully implemented “LC phase‐genesis” programming is also necessary for DCB‐based LCEs. For example, a slow creep under load was performed to align an LCE film in the nematic phase because it exhibited a fast imine metathesis reaction [74] . The introduction of permanent crosslinking moieties allowed controlling plasticity in the nematic phase while avoiding a large creep caused by the presence of fast boronic ester reactions [75] .…”

“…In addition, the xLCE samples became rigid after a period of time attributed to a very low degree of crystallinity of the long aliphatic spacer chains, a feature that was confirmed by an unexpectedly high nematic‐isotropic transition enthalpy of about 20 J g −1 . Although the xLCE film was shown to exhibit thermally reversible action, the rapid dynamic exchange hindered proper isotropic‐genesis programming and thus limited thermal stability upon heating/cooling cycles above T i (70 °C) [74] …”