Structural crossover from nonmodulated to long-period modulated tetragonal phase and anomalous change in ferroelectric properties in the lead-free piezoelectric Na1/2Bi1/2TiO3-BaTiO3

The highly complex structure-property interrelationship in the lead-free piezoelectric (x) Na1/2Bi1/2TiO3 - (1 - x) BaTiO3 is a subject of considerable contemporary debate. Using comprehensive x-ray, neutron diffraction, dielectric, and ferroelectric studies, we have shown the existence of a new criticality in this system at x = 0.80, i.e., well within the conventional tetragonal phase field. This criticality manifests as a nonmonotonic variation of the tetragonality and coercivity and is shown to be associated with a crossover from a nonmodulated tetragonal phase (for x < 0.8) to a long-period modulated tetragonal phase (for x > 0.80). It is shown that the stabilization of long-period modulation introduces a characteristic depolarization temperature in the system. While differing qualitatively from the two-phase model often suggested for the critical compositions of this system, our results support the view with regard to the tendency in perovskites to stabilize long-period modulated structures as a result of complex interplay of antiferrodistortive modes Bellaiche and Iniguez, Phys. Rev. B 88, 014104 ( 2013); Prosandeev, Wang, Ren, Iniguez, ands Bellaiche, Adv. Funct. Mater. 23, 234 (2013)].

Phase-field elasticity model based on mechanical jump conditions

Computational models based on the phase-field method typically operate on a mesoscopic length scale and resolve structural changes of the material and furthermore provide valuable information about microstructure and mechanical property relations. An accurate calculation of the stresses and mechanical energy at the transition region is therefore indispensable. We derive a quantitative phase-field elasticity model based on force balance and Hadamard jump conditions at the interface. Comparing the simulated stress profiles calculated with Voigt/Taylor (Annalen der Physik 274(12):573, 1889), Reuss/Sachs (Z Angew Math Mech 9:49, 1929) and the proposed model with the theoretically predicted stress fields in a plate with a round inclusion under hydrostatic tension, we show the quantitative characteristics of the model. In order to validate the elastic contribution to the driving force for phase transition, we demonstrate the absence of excess energy, calculated by Durga et al. (Model Simul Mater Sci Eng 21(5):055018, 2013), in a one-dimensional equilibrium condition of serial and parallel material chains. To validate the driving force for systems with curved transition regions, we relate simulations to the Gibbs-Thompson equilibrium condition

Crystal chemistry and photomechanical behavior of 3,4-dimethoxycinnamic acid: correlation between maximum yield in the solid-state topochemical reaction and cooperative molecular motion

A new monoclinic polymorph, form II (P2(1)/c, Z = 4), has been isolated for 3,4-dimethoxycinnamic acid (DMCA). Its solid-state 2 + 2 photoreaction to the corresponding alpha-truxillic acid is different from that of the first polymorph, the triclinic form I (P (1) over bar, Z = 4) that was reported in 1984. The crystal structures of the two forms are rather different. The two polymorphs also exhibit different photomechanical properties. Form I exhibits photosalient behavior but this effect is absent in form II. These properties can be explained on the basis of the crystal packing in the two forms. The nanoindentation technique is used to shed further insights into these structure-property relationships. A faster photoreaction in form I and a higher yield in form II are rationalized on the basis of the mechanical properties of the individual crystal forms. It is suggested that both Schmidt-type and Kaupp-type topochemistry are applicable for the solid-state trans-cinnamic acid photodimerization reaction. Form I of DMCA is more plastic and seems to react under Kaupp-type conditions with maximum molecular movements. Form II is more brittle, and its interlocked structure seems to favor Schmidt-type topochemistry with minimum molecular movement.

A Deep-Blue Electroluminescent Device Based on a Coumarin Derivative

The design and synthesis is reported of 7-(9H-carbazol-9-yl)-4-methylcoumarin (Cz-Cm), comprising a carbazole donor moiety and a 4-methylcoumarin acceptor unit, for use in a blue organic light-emitting diode. A detailed solid state, theoretical and spectroscopic study was performed to understand the structure-property relationships. The material exhibits deep-blue emission and high photoluminescence quantum yield both in solution and in a doped matrix. A deep-blue electroluminescence emission at 430nm, a maximum brightness of 292cdm(-2) and an external quantum efficiency of 0.4% was achieved with a device configured as follows: ITO/NPD (30nm)/TCTA (20nm)/CzSi(10nm)/10wt% Cz-Cm:DPEPO (10nm)/TPBI (30nm)/LiF (1nm)/Al ITO=indium tin oxide, NPD=N,N-di(1-naphthyl)-N,N-diphenyl-(1,1-biphenyl)-4,4-diamine, TCTA=tris(4-carbazoyl-9-ylphenyl)amine, CzSi=9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole, DPEPO=bis2-(diphenylphosphino)phenyl]ether oxide, TPBI=1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene].

Intramolecular conflict : conformation and self-assembly of architecturally complex macromolecules in solution

The solution behavior of linear polymer chains is well understood, having been the subject of intense study throughout the previous century. As plastics have become ubiquitous in everyday life, polymer science has grown into a major field of study. The conformation of a polymer in solution depends on the molecular architecture and its interactions with the surroundings. Developments in synthetic techniques have led to the creation of precision-tailored polymeric materials with varied topologies and functionalities. In order to design materials with the desired properties, it is imperative to understand the relationships between polymer architecture and their conformation and behavior. To meet that need, this thesis investigates the conformation and self-assembly of three architecturally complex macromolecular systems with rich and varied behaviors driven by the resolution of intramolecular conflicts. First we describe the development of a robust and facile synthetic approach to reproducible bottlebrush polymers (Chapter 2). The method was used to produce homologous series of bottlebrush polymers with polynorbornene backbones, which revealed the effect of side-chain and backbone length on the overall conformation in both good and theta solvent conditions (Chapter 3). The side-chain conformation was obtained from a series of SANS experiments and determined to be indistinguishable from the behavior of free linear polymer chains. Using deuterium-labeled bottlebrushes, we were able for the first time to directly observe the backbone conformation of a bottlebrush polymer which showed self-avoiding walk behavior. Secondly, a series of SANS experiments was conducted on a homologous series of Side Group Liquid Crystalline Polymers (SGLCPs) in a perdeuterated small molecule liquid crystal (5CB). Monodomain, aligned, dilute samples of SGLCP-b-PS block copolymers were seen to self-assemble into complex micellar structures with mutually orthogonally oriented anisotropies at different length scales (Chapter 4). Finally, we present the results from the first scattering experiments on a set of fuel-soluble, associating telechelic polymers. We observed the formation of supramolecular aggregates in dilute (≤0.5wt%) solutions of telechelic polymers and determined that the choice of solvent has a significant effect on the strength of association and the size of the supramolecules (Chapter 5). A method was developed for the direct estimation of supramolecular aggregation number from SANS data. The insight into structure-property relationships obtained from this work will enable the more targeted development of these molecular architectures for their respective applications.

Rate and Microstructure Effects on the Dynamics of Carbon Nanotube Foams

Soft hierarchical materials often present unique functional properties that are sensitive to the geometry and organization of their micro- and nano-structural features across different lengthscales. Carbon Nanotube (CNT) foams are hierarchical materials with fibrous morphology that are known for their remarkable physical, chemical and electrical properties. Their complex microstructure has led them to exhibit intriguing mechanical responses at different length-scales and in different loading regimes. Even though these materials have been studied for mechanical behavior over the past few years, their response at high-rate finite deformations and the influence of their microstructure on bulk mechanical behavior and energy dissipative characteristics remain elusive.

In this dissertation, we study the response of aligned CNT foams at the high strain-rate regime of 10² - 10⁴ s^-1. We investigate their bulk dynamic response and the fundamental deformation mechanisms at different lengthscales, and correlate them to the microstructural characteristics of the foams. We develop an experimental platform, with which to study the mechanics of CNT foams in high-rate deformations, that includes direct measurements of the strain and transmitted forces, and allows for a full field visualization of the sample’s deformation through high-speed microscopy.

We synthesize various CNT foams (e.g., vertically aligned CNT (VACNT) foams, helical CNT foams, micro-architectured VACNT foams and VACNT foams with microscale heterogeneities) and show that the bulk functional properties of these materials are highly tunable either by tailoring their microstructure during synthesis or by designing micro-architectures that exploit the principles of structural mechanics. We also develop numerical models to describe the bulk dynamic response using multiscale mass-spring models and identify the mechanical properties at length scales that are smaller than the sample height.

The ability to control the geometry of microstructural features, and their local interactions, allows the creation of novel hierarchical materials with desired functional properties. The fundamental understanding provided by this work on the key structure-function relations that govern the bulk response of CNT foams can be extended to other fibrous, soft and hierarchical materials. The findings can be used to design materials with tailored properties for different engineering applications, like vibration damping, impact mitigation and packaging.

Cinética de cristalização não-isotérmica de resíduos de polietileno de alta densidade (PEAD)

Nos últimos anos, a presença dos polímeros nos resíduos sólidos urbanos tem aumentado significativamente. Dentre todos os tipos de polímeros encontrados no lixo urbano, o polietileno de alta densidade (PEAD) ganha destaque, pois está presente em grande quantidade. A reciclagem de plásticos, neste âmbito, se configura como uma importante forma de reduzir a quantidade deste material nos lixões e aterros. Entretanto, sabe-se que os artefatos produzidos com material reciclado possuem propriedades inferiores. Outro ponto importante é o conhecimento da relação estrutura-propriedade, este conhecimento é fundamental na aplicação de qualquer material. Sendo assim, foram caracterizadas amostras de PEAD (embalagens pós-consumo) retiradas do lixo urbano através da calorimetria exploratória diferencial (DSC). Utilizando-se os modelos cinéticos de Avrami modificado e Liu, foi possível verificar que dependendo do tipo de carga ou colorante encontrado, há diferenças importantes na formação e morfologia dos cristais.

毛乌素沙地沙生灌木的生理生态学结构与功能

毛乌素沙地处于鄂尔多斯生态过渡带，在生态学研究上具有重要的理论与实践意义。在这样一个典型的半干旱区，水分是影响生态系统功能的最主要的限制因子，在沙基质土壤条件下，灌木得到充分发育，而乔木及草本植物则处于相对次要的地位。由于自然植被的初级生产力主要由灌木形成，故对于灌木在生态系统中的功能的系统研究是十分重要的，但目前又未得到应有重视。在荒漠化防治当中，这一点显得尤其急迫。此外，为了较好地预测该地区生态系统对全球气候变化的响应模式，急需要建立对主要乡土灌木功能型的划分方案。本研究运用植物生理生态学的方法，从植物的结构与功能的统一性出发，探讨了上述若干问题。主要结果与结论如下： 1．讨论了气候及土壤基质作为基本的环境背景，对沙生灌木生态功能的塑造作用，尤其探讨了若干土壤物理特性与植物水分利用的关系问题。 2．光学显微解剖及电子显微镜的观察表明，在自东南至西北横贯毛乌素沙地的水分梯度上生长的灌木当中，存在鲜明的叶片结构与功能特征与水分胁迫程度之间的对应性，在自东南向西北随干旱胁迫强度的增加，灌木的旱生结构也明显增加，甚至于在鄂尔多斯西部可以见到不少超旱生植物。同时，对灌木水分关系的研究支持了以解剖学观察为基础的认识，即：不同的景观条件下，也存在灌木的抗旱性与其生长的土壤的水分可利用性之间的对应性。这显示了水分因素在灌木的生态功能发挥中所起的主导作用。 3．通过分析沙丘的水分平衡，得到如下认识：(1)沙地条件的蒸发散主要来自植物的蒸腾作用，而土壤蒸发只占10%稍多：(2)沙地凝结水对灌木的水分需求而言微乎其微：(3)对多数沙生灌木而言，适宜的种植密度应为30-40%，即相当于半固定沙丘的覆盖度。 4．在上述三点的基础上，探讨了在不同类型的干旱（气候干旱、基质干旱及生理干旱）影响下，灌木在种类组成及干旱适应对策方面的多样性，将毛乌素沙地的灌木概括为两个功能类型：(1)干旱忍耐者;(2)竞争消费者。上述将极其多样的灌木物种在水分生态功能的相似性这个层面上概括成较少的类别的偿试，可望促使该地区的灌木生态学基础研究走向深入。 5．本研究对个别灌木的生态特性的新认识： ● 关于沙地柏(Sabina vulgaris)：(1)现已知道其蒸腾速率极低，本研究进一步认识到，低的蒸腾速率主要来自于气孔在叶表面上局限性分布：(2)该灌木虽十分抗旱，但过大的密度常引起叶片水分亏缺，在其根系不能利用地下潜水时．叶片水分亏缺更严重。 ● 关于油蒿(Artermisia．ordosica)及籽蒿（Asphaerocephala)：现已广泛地观察到，这两种灌木在沙丘演替不同阶段呈不同的优势分布：籽蒿主要生长于流动性沙丘（这里水分条件往往很好），但随沙丘逐渐被固定，就会被油蒿取代。本研究从叶片结构与功能两方面解释了上述现象，主要依据是(l)籽蒿比油蒿往往有更大的蒸腾速率：(2)籽蒿叶片近轴面1/3以下处有极为密集的气孔分布，而在油蒿却未发现;(3)籽蒿具有极其高的零膨压时的渗透势值(ψлp)。 ● 关于蒙古岩黄芪(Hedysarum mongolicum):该灌木具有独特的 干旱适应方式（如，幼茎 可行光合作用，及叶片富含粘液细胞），但本研究提示，该灌木的抗旱性是较弱的（相比与其它同生境下的灌木而言）。主要依据是：(1)很高的零膨压时的渗透势值;(2)海绵与栅栏组织均有较充分的发育。 ● 关于沙柳(Salix psammophila):在沙基质中有广泛的适应性， 但其适应主要体现在形态上（根茎比）的可塑性，而在生理（尤其是蒸腾作用）方面的调整则不十分显著。生长于丘顶的沙柳具有极其发达的根系，这可减缓土壤水分不足对植物的不利影响;生长于丘间低地的沙柳具有好的水分供给，但在特定的微生境下，其叶片温度比生长лллл于丘顶的沙柳叶片高，尤其是在强的光照条件下，其叶片温度会大幅度升高。这有可能对植物形成不利的影响。

Random function priors for exchangeable arrays with applications to graphs and relational data

A fundamental problem in the analysis of structured relational data like graphs, networks, databases, and matrices is to extract a summary of the common structure underlying relations between individual entities. Relational data are typically encoded in the form of arrays; invariance to the ordering of rows and columns corresponds to exchangeable arrays. Results in probability theory due to Aldous, Hoover and Kallenberg show that exchangeable arrays can be represented in terms of a random measurable function which constitutes the natural model parameter in a Bayesian model. We obtain a flexible yet simple Bayesian nonparametric model by placing a Gaussian process prior on the parameter function. Efficient inference utilises elliptical slice sampling combined with a random sparse approximation to the Gaussian process. We demonstrate applications of the model to network data and clarify its relation to models in the literature, several of which emerge as special cases.

三维定量结构-性质/活性相关性研究

该研究中,建议了一些新的方法,并且应用到QSAR/QSPR研究中,较好的预测了化合物的活性.研究工作主要集中在如下几个方面:[1]用相似度矩阵构造数学模型,预测化合物的活性.相似度矩阵是通过提取分子三维结构特征—广义二面角,原子三角和原子对空间距离来得到.[2]在三维空间对分子进行投影,计算出每一个平面上的投影面积来描述分子的形状.并进一步对分子投影进行了研究.用每一个平面的投影边缘代替面积预测化合物的活性.得到了更好的结果.[3]传统的拓朴指数很难区分属于3-D范畴的手性异构体.研究中扩展了由该实验室提出的Am指数,并利用该指数预测了手性药物的活性.[4]比较不同物种的β-球蛋白基因的第一个外显子.该方法不是直接进行DNA序列的比较,而是将DNA编码序列转化为图,然后模拟表征分子图的方法对之进一步处理.[5]利用VC++编写了一个包含三十多个分子拓扑指数的软件.

Facile synthesis of 9,10-diarylphenanthrenes and poly(9,10-diarylphenanthrene)s

One-pot reduction of 9,10-diaryl-9,10-dihydrophenanthrene-9,10-diols to 9,10-diarylphenanthrenes was achieved with Zn/H+ in acetic acid. Accordingly, various novel phenanthrenes and polyphenanthrenes with efficient blue emission were easily synthesized.

Efficient Electrophosphorescence from a Platinum Metallopolyyne Featuring a 2,7-Carbazole Chromophore

The synthesis, thermal and emission properties of an electrophosphorescent platinum(II) metallopolyyne polymer consisting of 9-butylcarbazole-2,7-diyl spacer P1 are described. The optical and electronic properties of P1 are compared to their molecular diplatinum(II) and digold(I) model complexes. The photophysical properties of P1 are somehow analogous to its 2,7-fluorene-linked congener but differs significantly from that for the 3,6-carbazole derivative. Its optical band gap is notably reduced as compared to that for the 3,6-carbazole analog. Multi-layer polymer light-emitting diodes (PLEDs) were fabricated with P1 as the emitting layer which gave a strong green-yellow electrophosphorescence. The best PLED can reach the maximum current efficiency of 4.7 cd . A(-1) at 5 wt.-% doping level, corresponding to an external quantum efficiency of 1.5%. This represents the first literature example of efficient PLEDs exhibiting pure triplet emission under electrical excitation for metallopolyynes without the concomitant singlet emission.

Synthesis, properties, and gas permeation performance of cardo poly(arylene ether sulfone)s containing phthalimide side groups

Novel bisphenol monomers (1a-d) containing phthalimide groups were synthesized by the reaction of phenolphthalein with ammonia, methylamine, aniline, and 4-tert-butylanilne, respectively. A series of cardo poly (arylene ether sulfone)s was synthesized via aromatic nucleophilic substitution of 1a-d with dichlorodiphenylsulfone, and characterized in terms of thermal, mechanical and gas transport properties to H-2, O-2, N-2, and CO2. The polymers showed high glass transition temperature in the range 230-296 degrees C, good solubility in polar solvents as well as excellent thermal stability with 5% weight loss above 410 degrees C. The most permeable membrane studied showed permeability coefficients of 1.78 barrers to O-2 and 13.80 barrers to CO2, with ideal selectivity. factors of 4.24 for O-2/N-2 pair and 28.75 for CO2/CH4 pair. Furthermore, the structure-property relationship among these cardo poly(arylene ether sulfone)s had been discussed on solubility, thermal stability, mechanical, and gas permeation properties. The results indicated that introducing 4-tert-butylphenyl group improved the gas permeability of polymers evidently.

Isomeric polyimides

This review deals with polyimides based on isomeric dianhydrides and diamines, and with chiral polyimides. First, however, a summary is presented of recent work on the synthesis of isomeric dianhydrides, the reaction of mellophanic dianhydride with diamines, and the tendency toward cyclization in reactions of some dianhydrides and diamines. Then turning to polymers, the discussion covers solubility, thermal and dielectric properties, permeability and permselectivity for gas separation, and rheology of isomeric polyimides. Several useful general rules have been found: i.e. the glass transition temperature of polyimides based on isomeric dianhydrides with a given diamine decreases in the order 3,3'- > 3,4'- > 4,4-dianhydride if the polymers are of comparable molecular weight, whereas the thermal stability and the T-beta/T-g ratio (in absolute temperatures) increase in the order of 3,3'- < 3,4'- < 4,4'-dianhydride. Polyimides from 3,3'- or 3,4'-dianhydride have higher solubility than those from 4,4'-dianhydride. Polyimides from 3,4'-dianhydrides exhibit much lower melt viscosity than those from the other isomeric anhydrides. The dielectric constants of polyimides derived from m,m'-diamines are lower than those from p,p'-diamines. Polyimides based on 3,3'- or 3,4'-dianhydrides have higher permeability and slightly lower permselectivity than polyimides based on 4,4'-dianhydrides.

Synthesis and characterization of blue-light-emitting poly(aryl ether)s containing oligofluorenes in the main chain

A series of blue light-emitting poly(aryl ether)s (PAEs) containing ter- or pentafluorenes in the main chain have been synthesized via nucleophilic substitution polycondensation reaction. The energy levels of the polymers were tuned by introducing hole-transporting triaryamine groups in the side chains and/or incorporating electron-transporting oxadiazole segments in the main chain. The optical properties of the polymers are dominantly determined by the well-defined oligofluorene segments, and therefore all polymers show high photoluminescence quantum yield. Differential scanning calorimeter (DSC) characterizations indicate that they are vitrified polymers with high glass transition temperature (up to 156 degrees C). The polymers comprising pentafluorenes exhibit electroluminescent properties equal to or better than fully conjugated fluorene homopolymers. With the device structure of ITO/PEDOT:PSS/polymer/Ca/Al, an external quantum efficiency of 1.4% along with Commission Internationale de L'Eclairage (CIE) coordinates of (0.17, 0.09) has