Stretching-induced director rotation in thin films of liquid crystal elastomers with homeotropic alignment
Stretching-induced director rotation in thin films of liquid crystal elastomers with homeotropic alignment
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DOI：
10.1021/ma071104y
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发表时间：
2007-10-16
期刊：
影响因子：
5.5
通讯作者：
Takigawa, Toshikazu
中科院分区：
文献类型：
--
作者：
Urayama, Kenji;Mashita, Ryo;Takigawa, Toshikazu
我们研究了液晶弹性体（LCEs）薄膜在垂直于其初始同向取向（x轴）的单轴伸长（z轴）下的介观取向行为。应力-应变关系表现为三个区域：(I)小应变区呈线性关系，（II）中等应变区具有准高原应力，（III）大应变区应力再次增加。泊松比mu（yz）是应变相关的：mu（yz）在区域II近似于0.17，而在区域I和III， mu（yz）近似于0.5，与不可压缩各向同性弹性体的泊松比mu（yz）一致。II区相当小的mu（yz）是在伸长期间抑制y方向（对应于指导者旋转轴）横向收缩的结果。这种变形模式比单轴拉伸更接近于纯剪切。红外二色性测量表明，在II区介质主要在x-z平面内向z方向旋转，而在III区没有进一步的介质重新排列。这些结果表明，导向旋转过程具有准高原应力和纯剪切变形的特征。在完全重定向状态下，介元的取向顺序与未加载状态相当。完全卸载后，即使拉伸到III区，也恢复了原来的同向取向。
We investigate the mesogen reorientation behavior of thin films of liquid crystal elastomers (LCEs) under uniaxial elongation (z-axis) normal to their initial homeotropic alignment (x-axis). The stress-strain relation is characterized by three regions: (I) a small-strain regime exhibiting a linear relationship, (II) a moderate-strain region with a quasi-plateau stress, and (III) a large-strain region where the stress again increases. The Poisson's ratio mu(yz) is strain-dependent: mu(yz)approximate to 0.17 in region II, whereas in regions I and III, mu(yz) approximate to 0.5 in accordance with that of incompressible isotropic elastomers. The considerably small mu(yz) in region II is a consequence of the suppression of the transverse shrinkage in the y-direction (corresponding to the director rotation axis) during elongation. Such a deformation mode is considerably closer to pure shear than uniaxial stretching. The infrared dichroism measurement reveals that the mesogen primarily rotates within the x-z plane toward the z-direction in region II, whereas no further mesogen realignment takes place in region III. These results indicate that the director-rotation process is featured by a quasi-plateau stress and a pure shearlike deformation. The orientation order of the mesogens in the fully reoriented state is comparable to that in the unloaded state. The complete unloading, even after stretching up to region III, recovers the original homeotropic alignment.