Liquid crystal elastomers (LCEs) combine the orientational order of liquid crystals with the rubber-like elasticity of polymer networks. This thesis investigates the relaxation dynamics and mechanical dissipation of main-chain and side-chain LCEs, and proposes a reprogrammable 3D-printed LCE actuator concept.
The relaxation dynamics of main-chain LCEs were characterised using dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and broadband dielectric spectroscopy (BDS). Temperature ramps and frequency sweeps revealed two distinct features in tanδ, Gʺ, and Gʹ. Whilst the lower-temperature feature is well attributed to segmental α-relaxation and Rouse modes, the origin of the higher-temperature relaxation has remained uncertain. Time-temperature superposition (TTS) analysis revealed Arrhenius behaviour, with an activation enthalpy of approximately 165 kJ/mol for the slow relaxation that is invariant with crosslink density. Combined with data from BDS, the results suggest that the relaxation arises from a collective structural process rather than local segmental motion.
The alignment dependence of mechanical dissipation in the LCEs was investigated in a series of chemically identical side-chain LCEs (SCLCEs) in isotropic, polydomain nematic, homeotropic nematic, and planar nematic orientations. DMA revealed that the primary relaxation (T₁), due to segmental α-relaxation and Rouse modes, shifts systematically with alignment, reflecting differences in mesogenic order, a finding corroborated by BDS. A secondary slow relaxation (T₂) was observed in all samples under temperature ramps but was strongly suppressed in frequency sweeps for nematic alignments. This discrepancy was attributed to distinct underlying mechanisms: temperature-driven changes in orientational order in nematic samples, and pendant-mesogen rotational mobility in isotropic ones. These results establish that energy dissipation in SCLCEs is governed primarily by alignment-dependent molecular mobility.
Finally, the feasibility of a reprogrammable 3D-printed LCE actuator was investigated. The proposed bilayer design couples an amorphous, printed LCE, which actuates reversibly above its nematic-isotropic transition temperature (Tni), with a semi-crystalline LCE that softens above its melting temperature (Tm), enabling thermal reprogramming of actuator geometry. Reversible actuation was demonstrated upon thermal cycling. Full reprogrammability was ultimately constrained by the requirement Tni < Tm, with efforts to satisfy this through material modification limited by competing demands on printability, viscosity, and crosslinking. These findings define the key material design criteria for future realisation of this actuator concept.
