Effects of network structure of main-chain liquid crystal elastomer on its thermal actuation performance

The effect of the network structure on the thermal actuation of the main-chain liquid crystal elastomers (LCEs) was studied by controlling the strand length and crosslinking density of the network. The LCEs were prepared with the thiol-terminated oligomers synthesized via a thiol-acrylate Michael addition reaction between dithiol and diacrylate monomers. The LCE network structure was formed by crosslinking the oligomers using the pentaerythritol tetraacylate crosslinkers and coupling between the thiol end groups in the oligomer to form disulfide linkages. The monodomain LCE (MLCE) films were fixed by the dynamic exchange reaction between disulfide linkages through ultraviolet irradiation after drawing the polydomain LCE films. The maximum degree of actuation was obtained when 50% of the available thiol groups were crosslinked with the acrylate groups of the crosslinkers. The degree of actuation increased with the oligomer length for sufficiently high crosslinking density. However, the degree of actuation decreased with the oligomer length after reaching the maximum especially for low crosslinker density because crosslinking with long oligomers did not provide enough crosslinking sites and disulfide linkages. Thus, the oligomer length and crosslinking density of the MLCE are important control factors during LCE preparation to obtain the best actuation performance of the MLCE film.