A new homologous series of achiral five-ring bent-core mesogens is presented. The mesophase behaviour has been studied by polarizing microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical measurements. The homologues with shorter terminal chains (C8, C12) form an SmCPA phase, homologues with longer chains (C14, C16, C18) show liquid-crystalline tetramorphism with a sequence SmAâSmCSPAâColobâSmCSPA. The SmCPA phases are structurally identical and differ only in the mechanism of polar switching. The switching of the high-temperature SmCSPA phase takes place through the collective rotation of the molecules around their long axes whereas in the reentrant SmCSPA phase the switching is based on the director rotation around the tilt cone. By the application of an electric field the Colob phase can be irreversibly transformed to an SmCPA phase. In a limited temperature range the SmA phase shows a reversible field-induced transition into the SmCSPF phase. The electro-optical response of the mesophases is discussed on the basis of the structural features of these phases.
本文介绍了一种新的非手性五环弯核介质同源系列。研究人员通过偏振显微镜、差示扫描量热法、X 射线衍射和电光测量等方法对介相行为进行了研究。具有较短末端链(C8、C12)的同系物形成 Sm
CPA 相,而具有较长末端链(C14、C16、C18)的同系物则显示出
液晶四态性,其序列为 SmAâSmC
SPAâColobâSmC
SPA。Sm
CPA 相的结构完全相同,只是在极性切换机制上有所不同。高温 SmC
SPA 相的切换是通过分子绕其长轴的集体旋转实现的,而在再入射 SmC
SPA 相中,切换则是基于绕倾斜锥的定向旋转。通过施加电场,Colob 相可以不可逆地转变为 Sm
CPA 相。在有限的温度范围内,SmA 相在电场的诱导下可逆地转变为 SmC
SPF 相。根据这些相的结构特征讨论了介相的电光响应。