【作者】 陈云；

【导师】 孙德军；

【作者基本信息】 山东大学 ， 物理化学， 2017， 硕士

【摘要】 胶体颗粒的自组装是现代纳米科技的一个研究热点,并且已经在合成及制造领域有了重要的应用。各向异性的胶体颗粒能够自组装形成无机胶态液晶,在化学工业、电子工业、医学、环境保护事业等方面都有着广泛的应用前景。由于胶体颗粒的体积较大,重力能够对分散体系的液晶相转变产生显著的影响。目前关于胶体颗粒液晶相转变的实验研究大都在重力场中完成,重力的存在严重限制了人们对各向异性颗粒自组装过程的了解。层状双金属氢氧化物（LDH）是一类具有水滑石层状结构的无机非金属材料,易于在实验室合成并具有良好的光和磁性能。由于其较高的各向异性,LDH颗粒已被作为一种典型的无机液晶材料。合成的LDH颗粒具有较宽的多分散度,但是针对多分散的LDH胶体体系的液晶相行为的研究则较少。Onsager理论表明液晶相转变仅仅基于颗粒的各向异性,已有大量的计算机模拟验证了其正确性。然而,目前关于胶体分散体系的液晶相转变的实验却都是在重力场中进行的,不能够严格符合Onsager理论的模型,因此需要相关微重力实验对Onsager理论来进行验证。因此,本文研究了重力场中多分散的Mg₂Al LDH胶体盘状颗粒水分散体系的液晶相行为,并借助实践十号卫星进行了相关微重力实验,首次探索了没有重力场时胶体颗粒的液晶相转变过程。通过对比有无重力时的宏观相行为,分析了重力对多分散的胶体体系液晶相转变的影响。论文主要分为以下两个部分:第一部分,重力场中高度多分散的Mg₃Al LDH胶体盘状颗粒水分散体系的液晶相行为。我们首先使用多种表征手段分析了通过非稳态共沉淀法制备的Mg₂A1 LDH颗粒的形貌和晶体结构,然后通过偏光镜片观察其在重力场中的宏观液晶相转变。发现随着浓度从低到高,分散体系主要表现出三种不同的相行为。当颗粒浓度低于18 wt%,分散体系表现为各向同性相和沉淀层共存;当浓度位于18 wt%和30 wt%之间,体系发生液晶相转变,为各向同性相、向列相和沉淀三者共存;当浓度高于30 wt%,体系形成凝胶相。体系的宏观相转变特征与其流变学性质相吻合。在液晶相转变过程中,我们还发现了大量的向列相微滴,认为向列相的形成起始于这些液晶微滴的出现,在重力的作用下,微滴沉降并且聚并,逐渐生长扩大最后形成向列相层。通过分散体系的分级实验,我们发现重力的沉降诱导颗粒的粒径从上到下显著增加,且体系的局部多分散度也显著降低。第二部分,微重力下多分散的盘状Mg₂A1 LDH胶体颗粒水分散体系的液晶相行为。在本章中,我们首次进行了胶体颗粒液晶相转变的微重力实验,观察到了没有重力场时无机胶态液晶的形成,在更接近理论模型的情况下为Onsager理论的正确性提供了直接的实验证据。其中,22、23、25 wt%三个浓度的样品在微重力场中没有液晶相形成,而在重力场中则有,可以得出结论重力能够显著降低液晶相形成的临界浓度。对于27、28、30 wt%三个浓度的样品,在微重力场中有新的双折射的液晶出现,且液晶沿着颗粒浓度较高的区域向低浓度区域生长,最终在整个比色池都有所分布;而在重力场中的这三个样品,尽管也有液晶相形成,但是液晶只能沿着竖直方向生长,并且只能形成在比色池的下方。同时,我们还发现磁粉搅拌诱导的双折射条纹在重力场中松弛的要比微重力场中快得多。我们的研究填补了微重力场中的胶体颗粒液晶相转变的实验空白,通过对比有无重力时的液晶相行为直观地表明了重力在液晶相形成过程中的影响,并促进了对微重力下颗粒自组装过程的理解。更多还原

【Abstract】 Self-assembly of colloidal particles is a central issue in modern nanotechnology that has been widely applied in synthesis and fabrication.The ability of non-spherical colloidal particles to self-assemble into mineral colloidal liquid crystals（MCLCs）is of great interest as a simple and low-cost approach for producing artificial colloidal.structures.Due to the large volume of colloidal particles,gravity plays an important role on the phase behavior of mineral colloidal liquid crystals.However,most of the findings of liquid crystal phase transition were based on sedimentation tests carried out on the ground,gravity limited the understanding of the self-assembly of colloidal particles.The layered double hydroxides（LDH）are a kind of inorganic nonmetallic materials with layered structure.LDH particles have interesting optical and magnetic properties.Based on the high aspect ratio,colloidal LDH platelets have been used as a typical kind of inorganic liquid crystal materials.The synthesized LDH particles possess a wide distribution of diameter.However,few studies were focused on the polydisperse LDH suspensions.According to the Onsager theory,the liquid crystal phase transition is based on the anisotropy of particles.The Onsager theory does not contain gravity,therefore,strictly speaking the Onsager theory only applies to zero gravity.Due to the limitation of experiment condition,few study focused on the MCLCs in microgravity.As a result,in this thesis,we studied the liquid crystal phase transition of highly polydisperse Mg₂Al LDH particles in gravity,and conducted a microgravity experiment based on the SJ-10 satellite,firstly observing the process of the formation of MCLCs without gravity.Furthermore,we studied the influence of gravity by direct comparison of samples with and without the earth gravitational field.The whole paper contains two sections:Section 1:Liquid crystal phase transition of polydisperse Mg₂Al LDH particles in gravity.We firstly analyzed the morphology and crystal structure of Mg₂Al LDH particles prepared by nonsteady coprecipitation method,and then observed the macroscopic liquid crystal phase transition in gravity.We found that the colloidal suspension showed three different phase behavior with the particle concentration:（a）the samples were still isotropic（1）when particle concentration<23 wt%;（b）when 23 wt%<particle concentration<30 wt%,a coexistence of three phases was reached,including an opaque isotropic top phase,a birefringent nematic phase,and a sediment layer of larger platelets;（c）when particle concentration>30 wt%,the suspensions were in a gel state.During the formation of MCLCs,lots of nematic droplets were observed.Hence we considered that the isotropic-nematic phase separation process is initiated by the nucleation and growth of nematic droplets,and then slowly settle and coalesce to form a birefringent nematic layer in the bottom of the colloidal suspension.Based on the fractionation experiment,we found that the particle diameter increased from top to bottom,and the partial polydispersity is also decreased significantly.Section 2:Liquid crystal phase transition of polydisperse Mg₂A1 LDH particles in microgravity.In this section,we firstly conducted a microgravity experiment of liquid crystal phase transition,and observed the formation of MCLCs without gravity,providing direct experimental evidence to support the Onsager theory without gravity.For the three samples with particle concentration of 22,23,25 wt%,no MCLCs formed in microgravity after 18 days,while MCLCs formed in gravity within only 3 days.For the three samples with particle concentration of 27,28,30 wt%,new birefringence textures occurred,indicating the formation of MCLCs.The MCLCs grew around the concentrated regions and finally were able to distribute throughout the cuvette.However,the MCLCs formed in gravity growing solely in the vertical direction and being limited to the bottom of the suspension.Furthermore,the birefringence after magnetic stirring in gravity also relaxed faster than that in microgravity.These results fill the gap of the liquid crystal phase transition in microgravity,and promote the understanding of the self-assembly of colloidal particles.更多还原