Mechanical energy dissipation in polydomain nematic liquid crystal elastomers in response to oscillating loading
Mechanical energy dissipation in polydomain nematic liquid crystal elastomers in response to oscillating loading
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DOI：
10.1016/j.polymer.2019.01.042
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发表时间：
2019-03-12
期刊：
影响因子：
4.6
通讯作者：
Frick, Carl P.
中科院分区：
文献类型：
--
作者：
Merkel, Daniel R.;Shaha, Rajib K.;Frick, Carl P.
液晶弹性体（LCEs）表现出奇异的机械行为，如可逆驱动，玻璃化转变以上的损耗角正切升高，和软弹性;然而，LCEs尚未得到彻底的研究，在高应变循环载荷。本研究探讨了一种主链型多结构域聚氨酯弹性体的合成方法，该弹性体是由硫醇-丙烯酸酯迈克尔加成反应合成的。低应变（即，< 0.2%）的动态力学分析和蠕变行为来帮助将粘弹性和介晶再取向与高应变循环载荷联系起来。具体而言，在低于玻璃化转变的离散温度下，在向列型橡胶态制度，和在各向同性橡胶态制度的行为进行了比较。使用两个特征弛豫时间来模拟向列相橡胶态范围中的蠕变行为，所述特征弛豫时间随着温度的升高而降低（τ（1），τ（2）（19 ℃）= 140 s，16 s，以及τ（1），τ（2）（62 ℃）= 2.4 s，0.6 s）。在每个温度下以5次循环/分钟将样品重复加载至50 kPa，持续350次循环。在62摄氏度的橡胶态下，低阈值应力和短弛豫时间允许每个循环之间可逆的域重新取向，导致具有高滞后（220 kJ/m（3））的可重复的应力-应变曲线。相比之下，在19 ° C和39 ° C的条件下，在橡胶状的热塑性状态下表现出响应于循环载荷的棘轮行为，这降低了滞后值与增加的循环。这里提出的研究结果代表了第一次调查的机械能耗散在大的振荡应力条件下的主链APLCEs。总的来说，这项研究有助于更好地理解LCE在具有高应变值和不同温度的能量耗散应用中的作用。
Liquid crystal elastomers (LCEs) exhibit exotic mechanical behaviors such as reversible actuation, elevated loss tangent above the glass transition, and soft elasticity; however, LCEs have yet to be thoroughly investigated under high-strain cyclic loading. This study explores a main-chain polydomain nematic elastomer synthesized from a thiol-acrylate Michael-addition reaction in the nematic state. Low-strain (i.e., < 0.2%) dynamic mechanical analysis and creep behavior was used to help link viscoelasticity and mesogen reorientation to high-strain cyclic loading. Specifically, the behavior was compared at discrete temperatures below the glass transition, in the nematic-rubbery regime, and in the isotropic-rubbery regime. Creep behavior in the nematic-rubbery regime was modeled using two characteristic relaxation times, which decreased with increasing temperature (tau(1), tau(2) (19 degrees C) = 140 s, 16 s, and tau(1), tau(2) (62 degrees C) = 2.4 s, 0.6 s). Samples were repeatedly loaded to 50 kPa at 5 cycles/min at each temperature for 350 cycles. In the rubbery nematic state at 62 degrees C, low threshold stress and short relaxation times allowed reversible domain reorientation between each cycle, resulting in a repeatable stress-strain curve with high hysteresis (220 kJ/m(3)). In contrast, the 19 degrees C and 39 degrees C conditions in the rubbery nematic state demonstrated ratcheting behavior in response to cyclic loading, which lowered the hysteresis values with increased cycling. The findings presented here represent the first investigation into mechanical energy dissipation in main-chain nematic LCEs under large oscillating stress conditions. Overall, this study helps to better understand the role LCEs can play in energy dissipating applications with high values of strain and varying temperature.