Rate-dependent stress-order coupling in main-chain liquid crystal elastomers
Rate-dependent stress-order coupling in main-chain liquid crystal elastomers
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主链液晶弹性体中速率相关的应力阶耦合
DOI：
10.1039/d3sm00770g
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发表时间：
2023
期刊：
影响因子：
3.4
通讯作者：
Jin, Lihua
中科院分区：
文献类型：
--
作者：
Wei, Chen;Cao, Scott;Zhou, Yu;Lin, Dehao;Jin, Lihua
液晶弹性体（LCE）表现出显着的粘弹性。尽管 LCE 的速率依赖性应力-应变关系已被广泛观察到，但导向轮旋转和网络延伸的复杂相互作用对主链 LCE 粘弹性行为的影响仍不清楚。在这项研究中，我们报告了在不同的加载速率和松弛测试下，在平行于和倾斜于初始导向器的单轴张力下，主链向列LCE中的应力、导向器旋转和所有应变分量的实时测量。我们发现网络延伸和导向器旋转都对粘弹性起作用，并且网络延伸的特征弛豫时间比导向器旋转的特征弛豫时间大得多。有趣的是，指向矢在长时间弛豫中的逐渐变化表明指向矢重定向延迟不仅是由于液晶的粘性旋转引起的，而且还源于其与高粘性网络的耦合。此外，在单轴拉伸下，LCE 中会出现显着的速率相关剪切应变，当拉伸与初始指向矢之间的角度足够大时，显示出非单调变化。最后，通过引入变形梯度的乘法分解，仅考虑网络的粘度的粘弹性本构模型被用来体现LCE中依赖于速率的宏观变形和微观指向矢旋转之间的关系。
Liquid crystal elastomers (LCEs) exhibit significant viscoelasticity. Although the rate-dependent stress–strain relation of LCEs has already been widely observed, the effect of the intricate interplay of director rotation and network extension on the viscoelastic behavior of main-chain LCEs remains inadequately understood. In this study, we report real-time measurements of the stress, director rotation, and all strain components in main-chain nematic LCEs subjected to uniaxial tension both parallel and tilted to the initial directors at different loading rates and relaxation tests. We find that both network extension and director rotation play roles in viscoelasticity, and the characteristic relaxation time of the network extension is much larger than that of the director rotation. Interestingly, the gradual change of the director in a long-time relaxation indicates the director reorientation delay is not solely due to the viscous rotation of liquid crystals but also arises from its coupling with the highly viscous network. Additionally, significant rate-dependent shear strain occurs in LCEs under uniaxial tension, showing non-monotonic changes when the angle between the stretching and the initial director is large enough. Finally, a viscoelastic constitutive model, only considering the viscosity of the network by introducing multiplicative decomposition of the deformation gradient, is utilized to manifest the relation between rate-dependent macroscopic deformation and microscopic director rotation in LCEs.