【作者】 陈敏智；

【导师】 薛奇； Shi Jin；

【作者基本信息】 南京大学 ， 高分子物理与化学， 2011， 博士

【摘要】 在液晶材料的研究中,材料的相结构与分子构象对于器件性能的影响有着非常重要的影响。尤其是对于被视为潜在的或已经可以用于发光二极管、光电响应器件、显示元件等领域的有机半导材料,对其液晶相结构的调控与其用途息息相关。即使是同样化学组成的材料,随着在基底表面排列方式(站立或平躺在基底上)相结构(层状结构或柱状结构)等的不同,其应用也有着很大的区别。由于液晶材料往往处于热力学的亚稳态,对于常见的由刚性液晶基元与柔性侧链组成的液晶材料,可以通过化学基团改性、分子大小形状的调控、基底修饰、温度控制、热历史调节等方法,获得不同的相结构。本论文中,我们主要以二萘嵌苯的二酰亚胺类衍生物(PDIs)为代表,展示了通过取代基修饰、改变柔性链长等方法,获得具有各种特异性结构的PDIs1,前人有人利用在液晶基元上直接引入氢键官能团,得到过π-π堆积距离只有0.32nm的碟状液晶的柱状相。我们借鉴这种方法,通过在分子的侧链β位引入酯基,并减小a位的位阻,我们利用相邻PDI分子酯基间的极性相互作用与π-π堆积作用相互增强的效应,第一次获得了具有紧密π堆积层状相结构PDIs,其层内相邻PDI分子之间的π-π堆积距离同样只有0.33nm。2, P. G. de Gennes在其The physics of liquid crystals中早已指出：液晶分子的长径比由小到大,会使其对应的相结构由柱状相向层状相变化。并且早已为大量的实验所证实。我们合成了侧链a位位阻不对称取代的PDIs衍生物,通过调节侧链烷烃链的长度,也获得了六方柱状相向层状结构的变化。然而a,我们得到的六方柱状相结构中,具有两套相互独立的点阵结构,是一种自为主客体的非公度结构。并且这种结构在高温(115℃)退火可以得到常规的六方柱状相结构。这是第一次在常压下,也是第一次对于有机材料,获得自为主客体的非公度结构。b,我们所得到的层状结构液晶具有光学各向同性。通常这种各向同性液晶相只在具有立方对称性的蓝相Ⅲ中有过报道,可以用于快速光电响应器件;但是这种蓝相Ⅲ稳定存在的温度范围(<60℃)通常很窄。我们得到的各向同性液晶具有层状结构,可以在非常宽的温度范围(>150℃)稳定获得,并且热力学可逆。除了通过化学修饰获得了各种特异相结构以外,我们还研究了小分子液晶辛氧基联苯氰(80CB)快速升降温下的动力学行为。通过合适的升降温的过程我们第一次通过非溶液的方法获得了80CB的亚稳态square plate型结晶,并观察到结晶熔融过程中亚稳态结晶向稳态结晶的快速重排过程。此外,我们还捕捉到由于重排不完全而造成的复杂N-I转变。更多还原

【Abstract】 In the studies of liquid crystal, phase structure and molecular conformation is of great importance to device performance. Manipulating of the phase structures of liquid crystals would benefit its application in light emitting diodes, photovoltaic modulators, display devices, and etc.. Even with the same chemical component, difference in molecular alignment on bases(edge-on or face-on), or in phase strcture(lamella or columnar), would also lead to huge diference in applications.As liquid crystals, mostly composed of rigid mesogen and flexible side-chains, are usually in a thermodynamically metastable state, various phase structure can be obtained by introducing functional groups, changing the size of the molecules, alignment of the base, controling temperature, applying special thermal history, and etc..In this thesis, we mainly present some special phase structures of perylenetetracarboxylic acid diimides (PDIs), by introducing functional group or changing the length of side-chains:1, Closely packed columnar phase, withπ-πstacking of 0.32nm, was reported before by introducing hydrogen bonding group directly to mesogens. Buying the idea, we introduce an ester group on theβ-position of side-chain, and minimize the steric hindrance of the a-position. As a result, the dipolar interaction andπ-πinteraction enhance each other, and tightπ-stacking of only 0.33nm is obtained, firstly for lamella PDIs.2, P. G. de Gennes claimed in his book "The physics of liquid crystals" that:phase structure of liquid crystals would change from columnar to lamella with increasing the length diameter ratio, which has been proved by plenty of observations. In our experiments, PDIs derivatives, with asymmetric steric hindrance at the a-position of the side-chain, was sucessfully synthesized. By modifying the side-chain length, changing of phase structure from hexagonal columnar to lamella is also observed. However:A, the hexagonal columnar phase we obtained two sets of independent crystal lattices, and is proved of a self-hosting incommensurate structure. After annealing at high temperature(115℃), the incommensurate hexagonal column would reorganize into traditional hexagonal column. This is the first time a self-hosting incommensurate structure being obtained at normal pressure, and also the first time for organic compounds.B, the lamella structured liquid crystal obtained is optically isotropic. The optically isotropic liquid crystals was only reported for blue phase III with cubic symmetry before, and was considered as a potential material for fast light modulator. However, the narrow temperature range (smaller than 60℃) limits its application. For our isotropic lamella liquid crystal, much wider temperature range (larger than 150℃) is obtained, and the structure is thermoreverible.Besides of various phase structure obtained by chemically modification, we also studies the dynamic behaviors of small molecular liquid crystal 4-Cyano-4’-octylbiphenyl (8OCB) under fast cooling and heating. By appropriate heating and cooling, we firstly get the square-plate crystal, a metastable crystal, of 8OCB by non-solution method, and observed the fast reorganization from metastable crystal to stable crystal during melting. We also captured the complex N-I transition due to the imcomplete reorganization during melting.更多还原