Full-field strain reveals relaxation behaviors of main-chain liquid crystal elastomers in the polydomain-monodomain transition

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Abstract

Liquid crystal elastomers (LCEs) are polymers with nematic ordering that exhibit unique mechanical behaviours attributed to the interplay of network stretching and mesogen reorientation under loading. These competing viscoelastic mechanisms can lead to complex and heterogeneous local deformation response when exposed to an external stimulus such as light, thermal gradients, or applied stress. In this study, three-dimensional stereo digital image correlation (stereo-DIC) was employed to measure strain fields under uniaxial extension. Two deformation modes were used: quasi-static uniaxial extension and stepped stress-relaxation. Polydomain LCE specimens were stretched through the polydomain-monodomain (P-M) transition. Strain maps from stereo-DIC of the specimen surface during the constant applied displacement period showed that the strain heterogeneity increased with time. A clustering algorithm was used to identify coherent strain clusters in the strain field at the maximum stress of the step loading. The strains in the majority of the clusters either increased asymptotically or decreased asymptotically with time, leading to the heterogeneity in the strain field. A stretched exponential model was fit to the time-evolving strain response of the clusters and to the stress relaxation response to quantify differences in the viscoelastic nature of the domains involved in the P-M transition. The characteristic relaxation times varied between the clusters and were different than the characteristic relaxation time of the macroscopic stress response. These findings suggest that the kinetics of viscous mesogen rotation depends on the local strain state.

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Polymers and Materials

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