Molecular topology design in liquid crystal elastomers: substitutive versus insertive incorporation of flexible chains

In this study, a base liquid crystal elastomer (LCE) network was first constructed via a thiol-ene ‘click’ reaction, using pentaerythritol tetra(3-mercaptopropionate) (P’) as the crosslinker and 1,4-phenylene bis(4-(allyloxy)benzoate) (R) as the mesogenic unit. Two contrasting series, P’xRyCz and P’xRyBz, were then developed by respectively incorporating dodecane-1,12-diyl bis(undec-10-enoate) (C) or 3,6-dioxa-1,8-octanedithiol (B) through distinct ‘substitution’ and ‘insertion’ strategies, respectively. In substitution, C competes with R for reaction with P’, replacing mesogens while maintaining crosslink density. In insertion, B inserts between R units, with P’ content reduced to preserve stoichiometry, altering network topology. All R-containing samples were confirmed to be nematic LCEs. Both approaches effectively lowered the glass transition temperature and promoted ‘confined ordering’ within a P’-derived periodic nanoscale framework, creating reversible melting domains that provided the thermodynamic basis for material functionality. The two strategies yield different shape-memory behaviours: substitution allows synergistic optimisation at moderate compositions (recovery >75% with balanced strength-ductility), but excessive C leads to high crystallinity and loss of recovery. Insertion generally enhances strength and actuation, but recovery depends on network integrity: moderate insertion enables full recovery, while excessive insertion disrupts the network and reduces recovery.