Rheology of Smectic Liquid Crystalline Elastomers with Dynamic Covalent Bonds

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 beta 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.

Automated summary

Liquid crystal elastomer networks (LCEs) are highly versatile materials with unique mechanical and optical properties. The addition of dynamic covalent bonds allows for network renewal and further opportunities. Through increasing the content of liquid crystal mesogens (LCM), elastomeric vitrimer epoxies (EV, m-EV, and vLCE) were synthesized with varying rheological properties. The specific contributions of the smectic structure in vLCE were discussed, and the underlying mechanisms were explored.