Effects of mechanical constraint on director configuration of monodomain nematic elastomers under temperature variation
Effects of mechanical constraint on director configuration of monodomain nematic elastomers under temperature variation
复制标题
DOI：
10.1678/rheology.44.17
复制
发表时间：
2016-03
影响因子：
1.3
通讯作者：
K. Tamashima;T. Takigawa;K. Urayama
中科院分区：
文献类型：
--
作者：
K. Tamashima;T. Takigawa;K. Urayama
液晶弹性体是液晶与弹性体的结合体。lce对应于松散交联的LC聚合物。由于液体结晶度和橡胶弹性的耦合作用，LCEs的一个独特特征是宏观形状和分子取向之间有很强的相关性。这一特性提供了丰富多样的刺激响应特性，因为弹性体可以受到各种影响LC分子取向顺序的外部场（如温度变化、电场和磁场、光照射）的变形。相反，施加的变形可以驱动LC分子沿着应变场重新取向。因此，lce作为软执行器和传感器材料具有很大的潜力。也有报道说，一个机械约束，例如，在一个方向上禁止应变，显著地抑制了沿施加的外场方向的定向重新排列。我们发现，在足够高的电场条件下，无约束向列相凝胶中的定向器几乎可以完全旋转，而在凝胶被刚性电极牢牢夹住的几何形状中，定向器的旋转程度受到相当大的限制。此外，受约束的几何结构显著提高了定向器旋转开始时的阈值场强。仿真和实验研究表明，根据向列性和膜厚的不同，在这种约束几何中会出现几种定向器的特征构型。Verduzco等人证明，当受到冷却时，在细胞内的软向列相凝胶中出现了指导性的周期性调制。这种屈曲失稳是指聚合物网络的变形与定向取向耦合时发生的一种低能变形。对于胆甾体，选择性反射带的位置（对应于螺旋构型的螺距）在加热时由于底层向列层的顺序减少而移动到更长的波长，但在禁止热诱导膜增厚的约束几何中没有显示出移动。因此，放置在机械约束几何中的lce在受到外场作用时表现出各种类型的定向配置，这一现象为液晶和聚合物的物理学提供了一个有趣的问题。然而，在温度变化下，力学约束对单域向列弹性体定向位形的实验表征影响
Liquid crystal elastomers (LCEs) are the combinations of liquid crystals (LCs) and elastomers. LCEs correspond to loosely cross-linked LC polymers. A unique feature of LCEs is a strong correlation between macroscopic shape and molecular orientation due to the coupling of liquid crystallinity and rubber elasticity. This feature provides a rich variety of stimulus-response properties, because the elastomers can be deformed by various types of external field that influence the orientational order of LC molecules such as temperature variation, electric and magnetic field, light irradiation. Conversely, the imposed deformation can drive the reorientation of LC molecules along the strain field. Thus LCEs have great potential as the materials of soft actuators and sensors. It has also been reported that a mechanical constraint, for example, the prohibition of the strain in one direction, significantly suppresses the director realignment along the imposed external field. We revealed that almost full rotation of the director in unconstrained nematic gels occurred under sufficiently high electric field, while the degree of director rotation was considerably limited in the geometry where the gel was firmly sandwiched by rigid electrodes. Furthermore, the constrained geometry significantly increases the threshold field strength for the onset of the director rotation. A simulation and experimental studies showed several characteristic configurations of director could appear in such constrained geometries depending on the nematicity and film thickness. Verduzco et al. demonstrated that a periodic modulation of the director appeared in the soft nematic gels confined in the cells when subjected to cooling. This buckling instability is a kind of low-energy deformation that occurs when the deformations of polymer networks are coupled to the director alignment. For a cholesteric elastomer, the location of the selective reflection band, which corresponds to the pitch of helical configuration, shifted to longer wavelengths upon heating as a result of a decrease in order of underlying nematic layer, but it showed no shift in the constrained geometry where the thermally induced film-thickening is prohibited. Thus LCEs placed in mechanically constrained geometries exhibit various types of director configuration when subjected to external fields, and this phenomenon provides an interesting issue in the physics of liquid crystals and polymers. However, the experimental characterization Effects of Mechanical Constraint on Director Configuration of Monodomain Nematic Elastomers under Temperature Variation