Wang Zhenming, Raistrick Thomas, Street Aidan, Reynolds Matthew, Liu Yanjun, Gleeson Helen F
School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK.
Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China.
Materials (Basel). 2022 Dec 31;16(1):393. doi: 10.3390/ma16010393.
Auxetic materials exhibit a negative Poisson's ratio, i.e., they become thicker rather than thinner in at least one dimension when strained. Recently, a nematic liquid crystal elastomer (LCE) was shown to be the first synthetic auxetic material at a molecular level. Understanding the mechanism of the auxetic response in LCEs is clearly important, and it has been suggested through detailed Raman scattering studies that it is related to the reduction of uniaxial order and emergence of biaxial order on strain. In this paper, we demonstrate direct observation of the biaxial order in an auxetic LCE under strain. We fabricated ~100 μm thick LCE strips with complementary geometries, exhibiting either planar or homeotropic alignment, in which the auxetic response is seen in the thickness or width of the sample, respectively. Polarized Raman scattering measurements on the planar sample show directly the reduction in the uniaxial order parameters on strain and suggest the emergence of biaxial order to mediate the auxetic response in the sample thickness. The homeotropic sample is studied via conoscopy, allowing direct observation of both the auxetic response in the width of the sample and increasing biaxiality in the LCE as it is strained. We verified that the mechanism of the auxetic response in auxetic LCEs is due to the emergence of the biaxial order and conclude such materials can be added to the small number of biaxial nematic systems that have been observed. Importantly, we also show that the mechanical Frèedericksz transition seen in some LCEs is consistent with a strain-induced transition from an optically positive to an optically negative biaxial system under strain, rather than a director rotation in a uniaxial system.
拉胀材料呈现负泊松比，即当受力时，它们在至少一个维度上会变厚而非变薄。最近，一种向列型液晶弹性体（LCE）被证明是分子水平上的首个合成拉胀材料。清楚地理解LCE中拉胀响应的机制显然很重要，并且通过详细的拉曼散射研究表明，它与单轴有序度的降低以及受力时双轴有序度的出现有关。在本文中，我们展示了对拉胀LCE在受力状态下双轴有序度的直接观测。我们制备了具有互补几何形状、厚度约为100μm的LCE条带，分别呈现平面取向或垂面取向，在其中拉胀响应分别出现在样品的厚度或宽度上。对平面样品进行的偏振拉曼散射测量直接显示了受力时单轴有序参数的降低，并表明双轴有序度的出现介导了样品厚度上的拉胀响应。通过锥光观察法研究垂面样品，从而能够直接观测样品宽度上的拉胀响应以及LCE在受力时双轴性的增加。我们证实了拉胀LCE中拉胀响应的机制是由于双轴有序度的出现，并得出结论，此类材料可被添加到已观察到的少数双轴向列体系中。重要的是，我们还表明，在一些LCE中观察到的机械弗雷德里克斯转变与受力状态下从光学正性双轴体系到光学负性双轴体系的应变诱导转变一致，而非单轴体系中指向矢的旋转。
