反应条件对乳液法自组装合成氧化锰纳米粒子结构、形貌和尺寸的影响

通过改变反应物物质的量比、分散相与连续相体积比、反应体系温度和煅烧后处理温度,研究了反应条件对乳液法自组装合成氧化锰纳米粒子结构、形貌和尺寸的影响.结果表明:当高锰酸钾与油酸物质的量比在1∶5～1∶1之间时,形成蜂窝状纳米粒子和空心纳米粒子;当分散相(油酸)与连续相(水)的体积比为4∶200时,形成良好的空心球纳米结构;反应体系温度升高不利于产物的洗涤;不同温度的煅烧后处理不仅影响氧化锰纳米粒子的形貌,而且影响其结晶度和晶型.     

不同TiO2原料制备一维钛酸盐纳米材料

以不同二氧化钛为原料, 用水热法制备一维钛酸盐纳米材料. 原料一次粒径和晶体结构对一维纳米钛酸盐的形貌和结构的影响很大. 原料的一次粒径越小, 反应过程中产物的形貌和晶相转变越快; 纯锐钛矿相有利于钛酸盐纳米管的形成, 而少量金红石相则有利于纳米管向纳米线的进一步转变和晶相转变.     

晶相调控对金属纳米粒子催化性能的影响（英文）

尺寸在1–10 nm的金属纳米催化剂广泛地应用于石油化工,精细化学品合成,能源与环境保护等领域.大量研究表明,金属纳米粒子的催化性能与其微观结构,即尺寸、形貌和晶相等密切相关.近年来,对金属纳米粒子的尺寸和形貌效应已经有了较为系统深入的研究,但对晶相效应的研究则较少涉及.这主要是由于介稳晶相的金属纳米粒子在合成过程中或反应条件下极易转化为热力学稳定的晶相结构.根据金属原子密堆积形式,金属纳米粒子的晶相结构主要有立方面心(fcc)、立方体心(bcc)和六方密堆积(hcp)三种晶相;而金属合金由于d带电子存在着多种杂化方式,因而其晶相结构呈现出多样性且与单一金属有很大的不同.金属和合金纳米粒子晶相结构的调控,不仅会改变金属原子的配位环境,调控了其电子分布状态,还可影响反应物和产物的吸附、活化和脱附,进而调变催化性能.首先,我们简要总结了液相合成和固相转变调控金属纳米粒子晶相的原理和方法.纳米粒子的液相合成一般包括前驱体还原成核和晶核生长两个阶段,通过对液相合成条件的优化,尤其是表面活性剂的选择,可有效调控合成过程中的热力学和动力学因素,从而实现金属晶相的可控合成.固相转变则主要是对具有一定晶相结构的纳米粒子于一定气氛和温度条件下进行加热处理,利用金属粒子与活性气体之间(H2,CO等)的化学作用来实现晶相转变.利用上述方法,可以合成出fcc-Co、fcc-Ru、L10-Au Cu等热力学介稳的金属或合金纳米粒子.在此基础之上,我们分别以Co纳米粒子(fcc和hcp晶相)催化FT合成,Fe模型催化剂(fcc和bcc晶相)活化N2和CO,Ru纳米粒子(fcc和hcp晶相)催化CO氧化和氨硼烷水解制氢,Pd纳米粒子(Pd Hx物种)催化加氢等为例分析了晶相对金属纳米粒子催化性能的影响;在合金催化剂方面,以Pt3Co(无序的fcc和有序的L12),Au Pd Co(P3–m、Fm3–m和R3–m混合晶相)和Fe Pt纳米粒子(fcc和fct相)催化O2电化学还原、Pt Rh Sn(碲铂矿晶相和fcc晶相)和Zr Pt3纳米粒子(hcp和fcc晶相)催化乙醇电氧化、Ag3In合金(无序的Fm3–m相和有序的Pm3–m晶相)催化对硝基苯酚加氢、Pd Ru纳米粒子(fcc和hcp混合晶相)催化CO氧化等为例分析了合金催化剂的晶相对催化性能的影响.上述研究进展表明,金属纳米粒子的晶相也是影响制备剂高效金属催化剂的主要因素.最后,我们结合纳米催化的发展现状,提出了金属纳米粒子的晶相调控在纳米催化和纳米材料领域可能的发展态势.第一,通过对金属纳米粒子溶液相合成机理的深入研究,有助于发展出尺寸、形貌和晶相同时可控的新合成方法.第二,金属纳米粒子在晶相转化过程中往往伴随着烧结及组分的偏析等难题.利用氧化物包覆的核壳型或蛋壳型纳米结构以及碳纳米管的空间限域效应,或许有助于解决上述难题.第三,具有亚稳晶相结构的金属纳米粒子在反应条件下极易转变为热力学稳定的结构,因此,利用原位、动态、实时的表征技术对催化剂在真实工作状态下的微观结构进行细致的分析是阐明晶相效应的前提.     

ZrO_2晶相对MoO_x/ZrO_2催化剂结构及其甲醇选择氧化性能的影响

以纯的单斜氧化锆(m-ZrO2)和四方氧化锆(t-ZrO2)为载体,采用浸渍法分别合成了具有不同MoOx表面密度的MoOx/m-ZrO2和MoOx/t-ZrO2催化剂,并结合粉末X射线衍射,Raman光谱和H2程序升温还原等技术表征了不同ZrO2晶相对MoOx分散状态、结构以及甲醇氧化反应性能的影响.在低于锆表面MoOx的单层分散阈值(～5nm-2)时,m-ZrO2比t-ZrO2能够更有效地分散MoOx,形成高分散的孤立或二维结构的MoOx物种,避免了晶相MoO3的出现.当Mo表面密度超过单层分散阈值后,经过600℃焙烧,MoOx与ZrO2载体发生固相反应生成晶相ZrMo2O8,m-ZrO2比t-ZrO2更有利于ZrMo2O8的生成.提高Mo表面密度,催化剂表面的酸性随之增强,说明晶相ZrMo2O8比分散的MoOx物种具有更强的酸性.t-ZrO2与MoOx作用形成的强酸中心更有利于催化甲醇脱水生成二甲醚,但m-ZrO2使得MoOx具有更高的氧化还原能力和催化甲醇选择氧化反应的活性.ZrO2晶相对MoOx/ZrO2催化剂影响的研究结果将有助于研究VOx等其它金属氧化物催化剂以及发展酸性和氧化还原性双功能催化剂体系.     

侧基含纳米构筑单元的甲壳型液晶高分子的多层次自组装

甲壳型液晶高分子可以呈现超分子柱或片层的链构象,因此可以作为超分子液晶基元形成多种液晶相态,如六方柱状相、柱状向列相、六方柱状向列相、近晶相等.将纳米构筑单元,如一维的二联苯、二维的苯并菲、三维的多面体低聚倍半硅氧烷(POSS)等,引入到甲壳型液晶高分子中,所得聚合物可以自组装形成在亚十纳米和近纳米尺度的多级有序结构.这些结构具有尺寸可控及单分散的优点,可望在有机光电、纳米多孔膜以及纳米光刻等领域有着广阔的应用前景.本文主要介绍了将二联苯、偶氮苯、棒状多苯结构、苯并菲和POSS基元引入到甲壳型液晶高分子中制备多级组装结构的相关工作.     

晶相调控对金属纳米粒子催化性能的影响

尺寸在1–10 nm的金属纳米催化剂广泛地应用于石油化工,精细化学品合成,能源与环境保护等领域。大量研究表明,金属纳米粒子的催化性能与其微观结构,即尺寸、形貌和晶相等密切相关。近年来,对金属纳米粒子的尺寸和形貌效应已经有了较为系统深入的研究,但对晶相效应的研究则较少涉及。这主要是由于介稳晶相的金属纳米粒子在合成过程中或反应条件下极易转化为热力学稳定的晶相结构。根据金属原子密堆积形式,金属纳米粒子的晶相结构主要有立方面心(fcc)、立方体心(bcc)和六方密堆积(hcp)三种晶相;而金属合金由于d带电子存在着多种杂化方式,因而其晶相结构呈现出多样性且与单一金属有很大的不同。金属和合金纳米粒子晶相结构的调控,不仅会改变金属原子的配位环境,调控了其电子分布状态,还可影响反应物和产物的吸附、活化和脱附,进而调变催化性能。首先,我们简要总结了液相合成和固相转变调控金属纳米粒子晶相的原理和方法。纳米粒子的液相合成一般包括前驱体还原成核和晶核生长两个阶段,通过对液相合成条件的优化,尤其是表面活性剂的选择,可有效调控合成过程中的热力学和动力学因素,从而实现金属晶相的可控合成。固相转变则主要是对具有一定晶相结构的纳米粒子于一定气氛和温度条件下进行加热处理,利用金属粒子与活性气体之间(H2, CO等)的化学作用来实现晶相转变。利用上述方法,可以合成出fcc-Co、fcc-Ru、L10-AuCu等热力学介稳的金属或合金纳米粒子。在此基础之上,我们分别以Co纳米粒子(fcc和hcp晶相)催化FT合成, Fe模型催化剂(fcc和bcc晶相)活化N2和CO, Ru纳米粒子(fcc和hcp晶相)催化CO氧化和氨硼烷水解制氢, Pd纳米粒子(PdHx物种)催化加氢等为例分析了晶相对金属纳米粒子催化性能的影响;在合金催化剂方面,以Pt3Co(无序的fcc和有序的L12), AuPdCo(P3–m、Fm3–m和R3–m混合晶相)和FePt纳米粒子(fcc和fct相)催化O2电化学还原、PtRhSn (碲铂矿晶相和fcc晶相)和ZrPt3纳米粒子(hcp和fcc晶相)催化乙醇电氧化、Ag3In合金(无序的Fm3–m相和有序的Pm3–m晶相)催化对硝基苯酚加氢、PdRu纳米粒子(fcc和hcp混合晶相)催化CO氧化等为例分析了合金催化剂的晶相对催化性能的影响。上述研究进展表明,金属纳米粒子的晶相也是影响制备剂高效金属催化剂的主要因素。最后,我们结合纳米催化的发展现状,提出了金属纳米粒子的晶相调控在纳米催化和纳米材料领域可能的发展态势。第一,通过对金属纳米粒子溶液相合成机理的深入研究,有助于发展出尺寸、形貌和晶相同时可控的新合成方法。第二,金属纳米粒子在晶相转化过程中往往伴随着烧结及组分的偏析等难题。利用氧化物包覆的核壳型或蛋壳型纳米结构以及碳纳米管的空间限域效应,或许有助于解决上述难题。第三,具有亚稳晶相结构的金属纳米粒子在反应条件下极易转变为热力学稳定的结构,因此,利用原位、动态、实时的表征技术对催化剂在真实工作状态下的微观结构进行细致的分析是阐明晶相效应的前提。     

纳米TiO_2催化剂微晶结构对光催化反应的影响

采用正交试验方法,调控Ti（SO4）2原料浓度、沉淀剂NH4HCO3浓度、沉淀pH值、焙烧温度和焙烧时间等制备条件制备得到了25个锐钛矿相TiO2光催化剂.对TiO2光催化降解十二烷基苯磺酸钠（SDBS）的催化活性与TiO2锐钛矿相10个晶面的法向粒子尺寸、晶格畸变应力、X射线衍射强度之间的关系进行了分析.发现TiO2光催化SDBS的降解遵循一级反应动力学;其主要影响因素为（101）晶面结晶情况,而与其余晶面的相关性不大;光催化反应需要晶格畸变较少的结晶较完整的（101）晶面;晶粒尺寸减小,比表面积增大有利于提高反应速率;光催化反应过程主要在结晶的锐钛矿相（101）晶面表面上发生,而无定形TiO2催化活性较低.     

无有机模板剂体系水热合成Beta沸石中晶种的导向机理(英文)

在无有机模板剂体系中研究了不同硅铝比和晶粒度Beta沸石晶种的结构导向行为,采用X射线衍射、X射线荧光光谱、扫描电镜、透射电镜、紫外-拉曼光谱、红外光谱和N2物理吸附等方法对不同晶化时间固相产物和Beta沸石产物进行了表征.结果表明,不同Beta沸石晶种,包括全硅晶种,均能够导向合成Beta沸石,而且晶种在晶化诱导期都发生溶解.但是,晶种的硅铝比、晶粒度、预处理(焙烧)以及晶种加入的时间对晶种的溶解行为、Beta沸石晶化过程和产物都有重要影响.形貌研究还发现,含铝晶种不仅溶解后的残体通过提供晶核聚集的"固载化"表面导向了新生Beta沸石小晶体的密集生长,而且溶解下来的结构片段也提供了分散的晶核导向形成相对分散的Beta沸石小晶体;全硅晶种则仅通过溶解下来的结构片段提供分散的晶核.在无模板体系中,使用适当高硅铝比、小晶粒和经过焙烧处理的Beta沸石作为晶种有利于合成得到高结晶度的Beta沸石纯相.     

无有机模板剂体系水热合成Beta沸石中晶种的导向机理(英文)

在无有机模板剂体系中研究了不同硅铝比和晶粒度Beta沸石晶种的结构导向行为,采用X射线衍射、X射线荧光光谱、扫描电镜、透射电镜、紫外-拉曼光谱、红外光谱和N2物理吸附等方法对不同晶化时间固相产物和Beta沸石产物进行了表征.结果表明,不同Beta沸石晶种,包括全硅晶种,均能够导向合成Beta沸石,而且晶种在晶化诱导期都发生溶解.但是,晶种的硅铝比、晶粒度、预处理(焙烧)以及晶种加入的时间对晶种的溶解行为、Beta沸石晶化过程和产物都有重要影响.形貌研究还发现,含铝晶种不仅溶解后的残体通过提供晶核聚集的"固载化"表面导向了新生Beta沸石小晶体的密集生长,而且溶解下来的结构片段也提供了分散的晶核导向形成相对分散的Beta沸石小晶体;全硅晶种则仅通过溶解下来的结构片段提供分散的晶核.在无模板体系中,使用适当高硅铝比、小晶粒和经过焙烧处理的Beta沸石作为晶种有利于合成得到高结晶度的Beta沸石纯相.     

微波辐射法合成SAPO-5分子筛中硅铝比对产物结晶度和甲苯吸附性能的影响

采用硅溶胶作硅源, 用微波辐射法在酸性条件下(pH=4.5-5.0)合成了SAPO-5分子筛, 利用X射线衍射(XRD), 傅立叶变换红外光谱(FT-IR), 扫描电镜(SEM)和Brunauer-Emmett-Teller (BET)比表面积分析对样品的结晶度、形貌和比表面等进行表征, 考察了晶化原料硅铝比对分子筛结晶度和晶体颗粒长径比(c/a)的影响, 测定了分子筛对甲苯的吸附性能. 结果显示, 采用微波辐射法合成的SAPO-5分子筛, 在硅铝摩尔比为0.50附近生成产物的结晶度最好, 晶体颗粒的长径比最小(约为1.0). 尽管不同硅铝比条件下得到的样品比表面测定结果变化不大, 但对甲苯的吸附实验显示, 在硅铝比为0.50时样品的吸附速率和饱和吸附量均达到了最大值. 这与表征所得的结晶度和长径比的变化一致, 即结晶度好,长径比小的SAPO-5分子筛具有更好的甲苯吸附性能.     

Density functional theory of freezing of a system of conjugated oligomers parameterised via Gay–Berne potential

Density functional theory (DFT) of freezing is used to study the isotropic–nematic, isotropic–smectic A and nematic–smectic A phase transitions in a system of large, semi-flexible conjugated oligomers parameterised within Gay–Berne (GB) potential. The pair correlation functions of the isotropic fluid, used as structural inputs in the DFT, are calculated by solving the Percus–Yevick integral equation theory. Large number of spherical harmonic coefficients of each orientation-dependent functions has been considered to ensure the numerical accuracy at different densities and temperatures for the system of these model GB ellipsoids having large aspect ratio (length-to-breadth ratio). We found that the system of GB ellipsoids parameterised for conjugated oligomers shows stable isotropic, nematic and smectic A phases. At low temperatures, on increasing the density, isotropic fluid makes a direct transition to smectic A phase. Nematic phase get stabilised in between the isotropic and smectic A phases on increasing the temperature. Using the transition parameter obtained through the DFT, we have plotted the temperature–density and pressure–temperature phase diagrams which are found to be qualitatively similar to the one obtained in simulations for the systems with low aspect ratio GB particles.     

Influence of the smectic A-nematic transitional order on the formation of the smectic phases of 4-n-hexyloxybenzylidene-4′-n-butyIaniline and 4-n-butyloxybenzylidene-4′-n-octyIaniline from an electrically deformed nematic phase

Abstract

The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,_th 5U _th and 10U _th, where U _th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Apolar 2‐alkoxyalkoxy‐substituted nematic liquid crystals

The synthesis is reported of a series of apolar nematic liquid crystals incorporating an alkoxyalkoxy chain attached in a terminal position to the molecular core. The dependence of the mesomorphism and phase transition temperatures on the nature of the terminal chain with one, two or three oxygen atoms was studied. Some compounds with a completely alicyclic molecular core and a terminal 2‐methoxyethoxy chain exhibit a nematic phase with a low melting point, birefringence and viscosity and a relatively high clearing point in the absence of smectic phases. They do not to absorb light in the near UV and may be useful as components of UV‐stable nematic mixtures for use in photoluminescent liquid crystal displays. The presence of the methoxyethoxy chain leads to a relatively large dipole moment perpendicular to the molecular long axis.     

Nonuniform liquid-crystalline phases of parallel hard rod-shaped particles: From ellipsoids to cylinders

In this article we consider systems of parallel hard superellipsoids, which can be viewed as a possible interpolation between ellipsoids of revolution and cylinders. Superellipsoids are characterized by an aspect ratio and an exponent alpha (shape parameter) which takes care of the geometry, with alpha=1 corresponding to ellipsoids of revolution, while alpha=infinity is the limit of cylinders. It is well known that, while hard parallel cylinders exhibit nematic, smectic, and solid phases, hard parallel ellipsoids do not stabilize the smectic phase, the nematic phase transforming directly into a solid as density is increased. We use computer simulation to find evidence that for alpha>or=alpha(c), where alpha(c) is a critical value which the simulations estimate to be approximately 1.2-1.3, the smectic phase is stabilized. This is surprisingly close to the ellipsoidal case. In addition, we use a density-functional approach, based on the Parsons-Lee approximation, to describe smectic and columnar orderings. In combination with a free-volume theory for the crystalline phase, a theoretical phase diagram is predicted. While some qualitative features, such as the enhancement of smectic stability for increasing alpha and the probable absence of a stable columnar phase, are correct, the precise location of coexistence densities is quantitatively incorrect.     

Novel bent-shaped liquid crystalline compounds II. Synthesis and properties of the 1,3-bis4-[4-(4-alkyloxybenzoyloxy)benzylidene]aminophenoxypropan-2-ols

A series of new symmetric dimer compounds was synthesized, constaining 2-hydroxy-1,3-dioxypropylene as the central linkage and terminal alkyl chains with different lengths. The chemical structures of the liquid crystal dimers (2ES-n) were examined by FTIR and ¹H NMR spectroscopy. Their mesomorphism, thermodynamic properties and optical textures were investigated by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction. For homologues with terminal propyloxy and butyloxy chains, no liquid crystalline phase was observed. Homologues with pentyloxy and hexyloxy terminal chains showed nematic phases, while those with heptyloxy, octyloxy, nonyloxy and decyloxy terminal chains displayed nematic phases and smectic phases. The results confirmed that the liquid crystalline phase changes from nematic to smectic as the terminal chain length increases.     

Stability of nematic and smectic phases in rod-like mesogens with orientation-dependent attractive interactions

The stability of isotropic (I), nematic (N), smectic A (Sm A), and hexatic (Hex) liquid crystalline phases is studied for a fluid of molecules with a rod-like shape and dispersive interactions dependent on orientation. The fluid is modeled with the spherocylindrical Gay-Berne-Kihara interaction potential proposed in a recent work, with parameters favoring parallel pair orientations. The liquid crystal phase diagram is characterized for different molecular aspect ratios by means of Monte Carlo simulations in the isobaric-isothermal ensemble. Three types of triple points are observed, namely, I-Sm A-Hex, I-N-Sm A, and N-Sm A-Hex, leading to island-shape domains for the smectic A phase. The resulting phase diagrams are compared with those derived previously for prolate fluids of ellipsoidal and spherocylindrical symmetry. It is concluded that the stability of the layered phases with respect to the nematic phase is enhanced in the spherocylindrical fluids due to geometrical constraints. Furthermore, the anisotropy of the dispersive interactions induces a stronger dependence of the overall phase diagram on temperature and aids in the energetic stabilization of the hexatic crystalline phase with respect to the fluid smectic A phase.     

Generation of frustrated liquid crystal phases by mixing an achiral nematic-smectic-C mesogen with an antiferroelectric chiral smectic liquid crystal

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmC(a)(*) phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB(*)) phase, two or three blue phases, the untilted SmA(*) phase, as well as all three chiral smectic-C-type "subphases," SmC(alpha)(*), SmC(beta)(*), and SmC(gamma)(*). The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB(*) at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC(*) exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmC(a)(*) transition or a temperature range of the SmC(beta)(*) subphase of about 50 K.     

Determination of elastic constants of a binary system (7CPB+9.CN) showing nematic,induced smectic Ad and re-entrant nematic phases

The temperature variation of the splay and bend elastic constants of a binary system exhibiting nematic-induced smectic A_d and re-entrant nematic phases measured by electric field-induced Freedericksz transition method has been reported. As bend deformation is not permitted in the smectic A phase, bend elastic constant (K₃₃) could only be determined in the nematic and re-entrant nematic phases. In both the nematic phases, the splay elastic constant has the same order of magnitude and does not show any pretransitional effect. However, in the induced smectic A_d phase, the value of K₁₁ is about one to two orders higher than that in the nematic phases. The bend elastic constant shows a strong pretransitional effect near the nematic–smectic and smectic–re-entrant nematic phase transitions. The influence of the presence of the induced smectic phase is observed even in those mixtures which have no induced smectic phases. As the smectic phase is approached, the ratio K₃₃/K₁₁ increases rapidly and diverges to infinity.     

Direct observation of the main-chain conformation of a combined liquid-crystal polymer

The conformation of the main-chain (backbone) of a combined main-chain/side-chain liquid crystalline polymer has been qualitatively determined by small angle neutron scattering in the oriented nematic, the smectic A and the smectic C phases. The polymer backbone presents only a weak anisotropy, of prolate shape, in the nematic and the smectic C phases. A stronger reorientation of the backbones in the direction of the applied magnetic field is measured for the S_A phase. However, this anisotropy remains small compared to the stretching of a main-chain liquid crystal polymer and the smectic structure results apparently from side-chain ordering. On the other hand, hydrodynamic measurements show that the combined polymer, in solvent, is as flexible as a polystyrene chain. This result is compatible with an explanation for the weak observed anisotropy.