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

Prediction of molar absorptivities of color reagents and their color reactions with yttrium by artificial neural networks

The new topological indices A(x1)-A(x3) suggested in our laboratories were applied to the study of structure-property relationships between color reagents and their color reactions with yttrium. The topological indices of twenty asymmetrical phosphone bisazo derivatives of chromotropic acid were calculated. The work shows that QSPR can be used as a novel aid to predict the molar absorptivities of color reactions and in the long term to be helpful tool in-color reagent design. Multiple regression analysis and neural network were employed simultaneously in this study. The results demonstrated the feasibility and the effectiveness of the method.

Calculation of nonlinear optical properties of KNdP4O12

The second-order nonlinear optical (NLO) tensor coefficients of KNdP4O12 (KNP) are theoretically predicted from its crystal structural data, by using the chemical bond theory of complex crystals and the modified bond charge model. Linear and nonlinear optical contributions of each type of bond to the total linearity (chi) and nonlinearity (d(ij)) of KNP are quantitatively determined. The structure-property relationship of KNP is systematically investigated, from the chemical bond viewpoint. Based on the discussion of its structural modifications, we point out that NLO properties of I(NP can be improved effectively using the doping method. Theoretical predictions show KNP to be a promising: self-frequency-doubling laser material.

Calculation of nonlinearities of K2Ce(NO3)(5)center dot 2H(2)O and K2La(NO3)(5)center dot 2H(2)O

A quantitative investigation of structure-property relationships has been carried out in the nonlinear optical crystals K2Ce(NO3)(5) . 2H(2)O and K2La(NO3)(5) . 2H(2)O, from the chemical bond viewpoint. Chemical bond parameters and linear and nonlinear optical properties of each type of constituent chemical bond of both crystals are calculated. Theoretical results agree reasonably with experimental data, and explain quantitatively their nonlinear origins in this type of crystal. This theoretical method allows us to calculate accurately the nonlinearities of complex crystals.

High resolution solid-state NMR and DSC study of poly(ethylene glycol) silicate hybrid materials via sol-gel process

Hybrid materials incorporating poly(ethylene glycol) (PEG) with tetraethoxysilane (TEOS) via a sol-gel process were studied for a wide range of compositions of PEG by DSC and high resolution solid-state C-13- and Si-29-NMR spectroscopy. The results indicate that the microstructure of the hybrid materials and the crystallization behavior of PEG in hybrids strongly depend on the relative content of PEG. With an increasing content of PEG, the microstructure of hybrid materials changes a lot, from intimate mixing to macrophase separation. It is found that the glass transition temperatures (T-g) (around 373 K) of PEG homogeneously embedded in a silica network are much higher than that (about 223 K) of pure PEG and also much higher in melting temperatures T-m (around 323 K) than PEG crystallites in heterogeneous hybrids. Meanwhile, the lower the PEG content, the more perfect the silica network, and the higher the T-g of PEG embedded in hybrids. An extended-chain structure of PEG was supposed to be responsible for the unusually high T-g of PEG. Homogeneous PEG-TEOS hybrids on a molecular level can be obtained provided that the PEG. content in the hybrids is less than 30% by weight. (C) 1998 John Wiley & Sons, Inc.

Chemical bond analysis of nonlinearity of urea crystal

A novel and quantitative study on structure-property relationships has been carried out in urea crystal, based on the dielectric theory of complex crystals and the modified Levine bond charge model, mainly from the chemical bond viewpoint. For the first time, it was treated like this, and the bond parameters and linear and nonlinear characteristics of constituent chemical bonds were presented quantitatively. The theoretical result agrees satisfactorily with the experimental datum and can reasonably explain the nonlinear origin of urea crystal, that is, the C-N bond in the conjugated system of bonds O double left arrow C<--N-H. At the same time, the novel method should be a useful tool toward the future development of the search for new nonlinear optical (NLO) materials in the organic crystal field.