Breiby Dag W, Hansteen Fredrik, Pisula Wojciech, Bunk Oliver, Kolb Ute, Andreasen Jens W, Müllen Klaus, Nielsen Martin M
Danish Polymer Centre, Risø National Laboratory, P.O. Box 49, 4000 Roskilde, Denmark.
J Phys Chem B. 2005 Dec 1;109(47):22319-25. doi: 10.1021/jp054949p.
The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-peri-hexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C12H25) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42 degrees C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90 degrees C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C12H25 was refined to unprecedented detail.
利用掠入射X射线衍射（GID）、电子衍射（ED）和可变角度光谱椭偏仪（VASE），对十二烷基侧链六重取代的六并苯（HBC-C12H25）区域浇铸薄膜中的晶态到液晶态（Cr-LC）相变进行了详细研究。加热该材料时，在约42℃观察到第一个较小的转变。这种变化归因于结晶烷基链堆积的改变，这只会轻微改变材料的电子性质。在约90℃的较高温度下，但仍显著低于先前报道的本体转变温度时，观察到Cr-LC转变。伴随的光学各向异性的大幅增加与X射线数据相符，表明从铸态的人字形结晶态转变为高度有序的盘状六方柱状液晶相。结构转变具有增加膜厚度的宏观效应。铸态低温相的高结构有序性在冷却后仅部分恢复，并且相变表现出较大的滞后现象。根据椭偏仪数据，HBC-C12H25的介电张量被细化到前所未有的详细程度。
