Rheology of Smectic Liquid Crystalline Elastomers with Dynamic Covalent Bonds

Abstract

Liquid crystal elastomer networks (LCEs) have found a wide range of applications due to their unusual mechanical and optical properties. The introduction of dynamic covalent bonds with their possibilities for network renewal opens up further opportunities. Elastomeric vitrimer epoxies (EV, m-EV, and vLCE) were synthesized with increased liquid crystal mesogens (LCM), in which the rheological properties vary with the fraction of LCM in the backbone. Differing from EV and m-EV, the distinctive contributions arising from the smectic structure in the vLCE were manifested in several viscoelastic responses. The underlying mechanisms were discussed for the first time. The smectic-isotropic transition of LCM is expressed, and the residual partially ordered structure in vLCEs contributes to the relatively high storage modulus in heating scans and seems to be the cause of thermal memory effects in the closed-loop cooling and heating experiment. Time−temperature superposition (tTS) is applicable to the EV and m-EV. vLCE becomes�thermorheologically complex in the vicinity of the order−disorder transition temperature of LCM domains. The temperature dependence of the shift factors in tTS can be divided into two regions, showing Williams−Landel−Ferry (WLF)-type at lower temperatures and Arrhenius-type at higher temperatures. At lower temperatures, the flow activation energy (E WLF ) increases in the sequence of EV, m-EV, and vLCE, reflecting the increase in backbone stiffness and its control over chain mobility. The activation energy, associated with the release of the transient cross-link from a smectic layer, also exists in vLCE. Above the LC transition temperature, a stretched exponential Kohlrausch−Williams−Watts (KWW) function is introduced to quantitatively describe the network rearrangement kinetics in the stress relaxation process; a low exponent β indicates marked structural heterogeneity in vLCE. Increased LCM causes stress relaxation processes to slow down significantly in a highly elastic state. The activation energy (E a ) obtained from the Arrhenius dependence of characteristic relaxation times declines as the LCM content is increased. This could be attributed to the locally higher concentration of the ester group and catalyst in vLCE.

Advancements in The Cross-Linking and Morphology of Liquid Crystals

“…The first of them is liquid crystal elastomers, which are characterized by long, often interwoven polymer chains, which make them highly elastic. These materials are the subject of many studies due to their flexibility, which results in a wide range of applications including soft robotics, and the production of sensors and actuators [12][13][14]. Problematic, however, is the fact that the arrangement of mesogens in elastomers must be established before the cross-linking reaction stops the molecular movement, as well as the fact that they cannot be processed or recycled like thermoplastics due to the permanent covalently cross-linked network [15].…”