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Liquid crystal elastomers (LCEs) are composed of rod-like liquid crystal (LC) molecules (mesogens) linked into elastomeric polymer networks. They present a nematic phase with directionally ordered mesogens at room temperature and an isotropic phase with no order at high temperatures, enabling large thermal-induced deformation. As a result, LCEs have become promising candidates for new applications in soft robotics and shape morphing. LCEs are being actively studied in both experiment and theory in recent years. However, the fundamental relationship among synthesis, processing, and thermomechanical behaviors of modern LCEs are still largely unclear. This knowledge gap is further complicated by the various LCE types, including polydomain, monodomain, nematic-genesis, and isotropic-genesis, each fabricated and used under different experimental conditions and applications. Here we explore synthesis-processing-property relationships in thermomechanics of various LCEs, by combining fabrication, characterization, and theoretical modeling. We adapt the widely used two-stage method to fabricate isotropic-genesis polydomain LCEs and nematic-genesis LCEs with varying pre-stretches during polymerization. We characterize the thermal-induced spontaneous deformation and the temperature-dependent uniaxial stress-stretch responses of the LCEs. We identify a new relationship among the soft elasticity, the thermal-induced spontaneous deformation, and the pre-stretch during polymerization, in the LCEs under study. Building on classical theories and our experimental results, we develop a constitutive model to describe the uniaxial behaviors of various LCEs. The theoretical predictions agree well with the experimental results on uniaxial stress-stretch responses at different temperatures. Finally, we discuss the remaining challenges and future opportunities in synthesis-processing-property relationships of LCEs.
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Modelling the Deformation of Polydomain Liquid Crystal Elastomers as a State of Hyperelasticity
Abstract A hyperelasticity modelling approach is employed for capturing various and complex mechanical behaviours exhibited by macroscopically isotropic polydomain liquid crystal elastomers (LCEs). These include the highly non-linear behaviour of nematic-genesis polydomain LCEs, and the soft elasticity plateau in isotropic-genesis polydomain LCEs, under finite multimodal deformations (uniaxial and pure shear) using in-house synthesised acrylate-based LCE samples. Examples of application to capturing continuous softening (i.e., in the primary loading path), discontinuous softening (i.e., in the unloading path) and auxetic behaviours are also demonstrated on using extant datasets. It is shown that our comparatively simple model, which breaks away from the neo-classical theory of liquid crystal elastomers, captures the foregoing behaviours favourably, simply as states of hyperelasticity. Improved modelling results obtained by our approach compared with the existing models are also discussed. Given the success of the considered model in application to these datasets and deformations, the simplicity of its functional form (and thereby its implementation), and comparatively low(er) number of parameters, the presented isotropic hyperelastic strain energy function here is suggested for: (i) modelling the general mechanical behaviour of LCEs, (ii) the backbone in the neo-classical theory, and/or (iii) the basic hyperelastic model in other frameworks where the incorporation of the director, anisotropy, viscoelasticity, temperature, softening etc parameters may be required.
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- Award ID(s):
- 2146409
- PAR ID:
- 10492554
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Journal of Elasticity
- Volume:
- 156
- Issue:
- 2
- ISSN:
- 0374-3535
- Format(s):
- Medium: X Size: p. 387-413
- Size(s):
- p. 387-413
- Sponsoring Org:
- National Science Foundation
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