The Effects of Mesogen Spacer and Linker on the Actuation of Liquid Crystal Elastomers

Experiments have shown that the actuation temperature of thermo-responsive main-chain liquid crystal elastomers (LCE) can be controlled by adjusting the spacer length (N_L) of reactive mesogen (i.e., LC monomer). Actuation occurs when the order parameter of LC mesogens changes at the nematic-isotropic transition temperature (T_NI). The precursor LC oligomers, which are connected LC units via linking thru an alkyl diamine chain extender, exhibits an increasing T_NI as N_L is increased. This behavior is opposite to what is observed in LC monomers. To elucidate this behavior, we performed isothermal-isobaric (NPT) ensemble coarse-grained molecular dynamics simulation of stiff-chains that are connected to flexible spacers at different temperatures, where T_NI is determined when the smoothly decaying orientational correlation function, g₂(r), transitions to an oscillating decay function. Simulations show that increasing N_L decreases T_NI for both monomers and oligomers. However, the dangling beads (N_D), representing the alkyl length of the diamine linkers in the experiments, amplifies the decrease of T_NI when the linker length (N_L) is short. We infer that the combination of N_L and N_D changes the shape anisotropy of the LC mesogen, affecting its ability to transition to the nematic phase.