Long-time stress relaxation in polyacrylate nematic liquid crystalline elastomers

We study the slow relaxation of stress in polydomain acrylate liquid crystalline elastomers undergoing the alignment transition under an imposed extension. We analyse the long-time stress relaxation, the slow approach to the mechanical equilibrium and the role of time-temperature superposition. By building the master curves, we investigate extrapolated time intervals and show the presence of two distinct relaxation regimes. At the first stage, the fast power-law relaxation of stress, with the exponent 0.67, means that directional changes in nematic domains are dominant. At very long times, we find that a different, slow power law (with the exponent 0.15) becomes the dominant mode, similar to the classical results in isotropic rubbers. Model equilibrium stress-strain curves have been obtained by extrapolating the master curves. It appears that, at a true mechanical equilibrium, one finds no mesogenic effects in stress-strain, meaning that the non-trivial nematic effects could be transient, locked by network entanglements, but capable of completely relaxing by (very slow) rearrangement of network chains.