Smart Materials and Biomechanics Lab, Mechanical Engineering, University of Colorado Denver, Denver, CO 80217, USA.
Soft Matter. 2017 Oct 11;13(39):7013-7025. doi: 10.1039/c7sm01405h.
This study presents the first direct comparison of the influence of liquid-crystal order during synthesis on the thermo-mechanical behaviors of main-chain liquid-crystal elastomers (LCEs) in thiol-acrylate networks. Six polydomain nematic elastomer (PNE) chemistries were compared directly by synthesizing with the mesogens in either an isotropic state (i-PNE) or a nematic state (n-PNE). The i-PNE networks were created in the presence of solvent, which disrupted any liquid-crystal order during network formation. Conversely, the n-PNE networks were created without the presence of solvent below the isotropic transition (T). Differential scanning calorimetry (DSC) was first performed, and it showed that i-PNE networks experienced a clearly defined nematic-to-isotropic transition upon heating, whereas the transition in n-PNE networks was unable to be identified, which may be the result of a nematic-to-paranematic phase transition. Dynamic mechanical analysis (DMA) tests revealed that while both networks maintained elevated loss tangent in the nematic region, only i-PNE networks prominently displayed dynamic soft elasticity behavior. The two-way shape switching behaviors of LCE networks were examined using actuation tests under a 100 kPa bias stress. It showed that the strain amplitude strongly depends on synthesis history; it ranges from 66% to 126% in i-PNE samples and 3% to 61% in n-PNE samples. To help interpret the different actuation strain behaviors between i-PNEs and n-PNEs, wide-angle X-ray scattering (WAXS) was then performed where the LCE samples were strained to 40%. The results showed that order parameter (S) in n-PNE samples (ranging from 0.37 to 0.50) is lower than that in i-PNE samples (0.54 for all cases), and the parameter decreased as the cross-linking density increased. The stress-strain behaviors of the LCE networks measured from uniaxial tension tests revealed that all i-PNE samples had a lower soft-elasticity plateau during loading compared to the n-PNE samples. Finally, free-standing strain recovery of LCE samples after being strained to 100% was investigated. Immediately after removing stress on the samples, i-PNE and n-PNE samples recovered 14% to 38% and 27% to 73% of strain, respectively. We discuss the advantages and disadvantages of the different synthetic histories on LCE design.
本研究首次直接比较了在硫醇-丙烯酸酯网络中合成过程中液晶有序性对主链液晶弹性体（LCE）的热机械性能的影响。通过将介晶分别处于各向同性状态（i-PNE）或向列态（n-PNE）来直接比较六种多畴向列型弹性体（PNE）化学。i-PNE 网络在存在溶剂的情况下形成，这在网络形成过程中破坏了任何液晶有序性。相反，n-PNE 网络在低于各向同性转变温度（T）的情况下在不存在溶剂的情况下形成。首先进行差示扫描量热法（DSC），结果表明，i-PNE 网络在加热时经历了明显的向列-各向同性转变，而 n-PNE 网络的转变无法被识别，这可能是向列-准向列相转变的结果。动态力学分析（DMA）测试表明，尽管两个网络在向列区都保持着升高的损耗角正切，但只有 i-PNE 网络显著显示出动态软弹性行为。通过在 100kPa 偏置应力下的致动测试来检查 LCE 网络的双向形状切换行为。结果表明，应变幅度强烈依赖于合成历史；在 i-PNE 样品中范围为 66%至 126%，而在 n-PNE 样品中范围为 3%至 61%。为了帮助解释 i-PNEs 和 n-PNEs 之间不同的致动应变行为，然后进行广角 X 射线散射（WAXS），将 LCE 样品应变至 40%。结果表明，n-PNE 样品中的有序参数（S）（范围为 0.37 至 0.50）低于 i-PNE 样品（所有情况下均为 0.54），并且该参数随着交联密度的增加而降低。从单轴拉伸测试中测量的 LCE 网络的应力-应变行为表明，与 n-PNE 样品相比，所有 i-PNE 样品在加载过程中具有较低的软弹性平台。最后，研究了应变至 100%后的 LCE 样品的自由站立应变恢复。在样品上的应力立即消除后，i-PNE 和 n-PNE 样品分别恢复了 14%至 38%和 27%至 73%的应变。我们讨论了不同合成历史对 LCE 设计的优缺点。
