Elasticity of polydomain liquid crystal elastomers
Elasticity of polydomain liquid crystal elastomers
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DOI：
10.1016/j.jmps.2012.01.008
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发表时间：
2012-04-01
影响因子：
5.3
通讯作者：
Bhattacharya, K.
中科院分区：
文献类型：
--
作者：
Biggins, J. S.;Warner, M.;Bhattacharya, K.
液晶弹性体是由熵占主导地位的聚合物链组成的橡胶网络，具有可移动的液晶秩序。这些材料最近对在单畴试样中观察到的软行为（在相对较小的应力下发生大变形）很感兴趣，在单畴试样中，松弛弹性体的每个点的方向都是相同的。本文研究了在松驰弹性体的不同点上，指向不同方向的多域试样的软行为。我们发现在均匀高对称状态下交联然后冷却到低对称多域状态的多域与在低对称多域状态下直接交联的多域之间存在显著差异。具体来说，在各向同性状态下交联的弹性体，然后冷却到向列多畴，在理想极限下，将是完全柔软的，并且随着非理想性的引入，将在非常低的应力下变形，直到它们在宏观上对齐。事实上，正如Urayama等人最近观察到的那样，我们期望这些样品比单畴样品表现出明显更软的弹性。此外，指示图案将是宏观上各向同性的精细尺度结构，而不是纹影纹理。相反，在向列多畴状态下交联的多畴将在机械上更硬，并且包含特征纹影指示图案。我们用于多域弹性体的模型是新古典软和半软自由能的空间非均匀扩展，已成功地用于描述单域样品。我们通过使用Taylor-like（兼容测试应变场）和Sachs-like（平衡应力场）边界的能量来阐明有效行为，两者都适用于大变形。与实验结果吻合较好。我们还分析了sm晶型多域弹性体，并提出在SmA单畴态交联的多域SmC*弹性体是低场电驱动的有希望的候选材料。(C) 2012 Elsevier Ltd.版权所有。
Liquid crystal elastomers are rubbery networks of entropically dominated polymer chains that exhibit mobile liquid crystalline order. These materials have been of recent interest for the soft behavior (large deformations at relatively small stresses) observed in monodomain specimens where the director is the same at every point in the relaxed elastomer. This paper concerns the soft behavior of polydomain specimens where the director points in different directions at different points in the relaxed elastomer. We show that there is a significant difference between polydomains cross-linked in homogeneous high symmetry states then cooled to low symmetry polydomain states and those cross-linked directly in the low symmetry polydomain state. Specifically, elastomers cross-linked in the isotropic state then cooled to a nematic polydomain will, in the ideal limit, be perfectly soft, and with the introduction of non-ideality, will deform at very low stress until they are macroscopically aligned. In fact, we expect these samples to exhibit elasticity significantly softer than monodomain samples, as has recently been observed by Urayama et al. Further, the director patterns will be fine-scale structures that are macroscopically isotropic and not schlieren textures. In contrast, polydomains cross-linked in the nematic polydomain state will be mechanically harder and contain characteristic schlieren director patterns. The models we use for polydomain elastomers are spatially heterogeneous extensions of the neo-classical soft and semi-soft free energies used successfully to describe monodomain samples. We elucidate the effective behavior by bounding the energies using Taylor-like (compatible test strain fields) and Sachs-like (equilibrated stress field) bounds, both valid to large deformations. Good agreement is found with experiments. We also analyze smectic polydomain elastomers and propose that polydomain SmC* elastomers cross-linked in the SmA monodomain state are promising candidates for low field electrical actuation. (C) 2012 Elsevier Ltd. All rights reserved.