Martin Linares Cristina P, Traugutt Nicholas A, Saed Mohand O, Martin Linares Alejandro, Yakacki Christopher M, Nguyen Thao D
Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD, USA.
Department of Mechanical Engineering, University of Colorado Denver, Denver, CO, USA.
Soft Matter. 2020 Oct 7;16(38):8782-8798. doi: 10.1039/d0sm00125b.
This study investigated the effect of alignment on the rate-dependent behavior of a main-chain liquid crystal elastomer (LCE). Polydomain nematic LCE networks were synthesized from a thiol-acrylate Michael addition reaction in the isotropic state. The polydomain networks were stretched to different strain levels to induce alignment then crosslinked in a second stage photopolymerization process. The LCE networks were subjected to dynamic mechanical tests to measure the temperature-dependent storage modulus and uniaxial tension load-unload tests to measure the rate-dependence of the Young's modulus, mechanical dissipation, and characteristics of the soft stress response. Three-dimensional (3D) digital image correlation (DIC) was used to characterize the effect of domain/mesogen relaxation on the strain fields. All LCE networks exhibited a highly rate-dependent stress response with significant inelastic strains after unloading. The Young's modulus of the loading curve and hysteresis of the load-unload curves showed a power-law dependence on the strain rate. The Young's modulus increased with alignment and larger anisotropy and a smaller power-law exponent was measured for the Young's modulus and hysteresis for the highly aligned monodomains. The polydomain and pre-stretched networks loaded perpendicular to the alignment direction exhibited a soft stress response that featured a rate-dependent peak stress, strain-softening, and strain-stiffening. The 3D-DIC strain fields for the polydomain network and programmed networks stretched in the perpendicular direction were highly heterogeneous, showing regions of alternating higher and lower strains. The strain variations increased initially with loading, peaked during the strain softening part of the stress response, then decreased during the strain stiffening part of the stress response. Greater variability was measured for lower strain rates. These observations suggest that local domain/mesogen relaxation led to the development of the heterogeneous strain patterns and strain softening in stress response. These findings improved understanding of the kinetics of mesogen relaxation and its contributions to the rate-dependent stress response and mechanical dissipation.
本研究调查了取向对主链液晶弹性体（LCE）速率依赖性行为的影响。通过硫醇 - 丙烯酸酯迈克尔加成反应在各向同性状态下合成了多畴向列型LCE网络。将多畴网络拉伸至不同应变水平以诱导取向，然后在第二阶段光聚合过程中进行交联。对LCE网络进行动态力学测试以测量温度依赖性储能模量，并进行单轴拉伸加载 - 卸载测试以测量杨氏模量的速率依赖性、机械耗散以及软应力响应特性。使用三维（3D）数字图像相关（DIC）来表征畴/液晶元弛豫对应变场的影响。所有LCE网络均表现出高度的速率依赖性应力响应，卸载后存在显著的非弹性应变。加载曲线的杨氏模量和加载 - 卸载曲线的滞后表现出对应变速率的幂律依赖性。杨氏模量随取向增加，对于高度取向的单畴，杨氏模量和滞后的幂律指数较小。垂直于取向方向加载的多畴和预拉伸网络表现出软应力响应，其特征为速率依赖性峰值应力、应变软化和应变硬化。多畴网络和沿垂直方向拉伸的程序网络的3D - DIC应变场高度不均匀，显示出高低应变交替的区域。应变变化最初随加载增加，在应力响应的应变软化部分达到峰值，然后在应力响应的应变硬化部分减小。较低应变速率下的变异性更大。这些观察结果表明，局部畴/液晶元弛豫导致了非均匀应变模式的发展以及应力响应中的应变软化。这些发现增进了对液晶元弛豫动力学及其对速率依赖性应力响应和机械耗散贡献的理解。
