Conformational properties and Rouse dynamics of different dendrimer molecules

A coarse-grained model previously proposed to perform Monte Carlo simulations for several dendrimer molecules with different topologies and chemical compositions in solution is employed now to obtain structural properties, such as the bead density profile, the asphericity and the molecular scattering factor, or form factor. It is also used to study the Rouse dynamics, including Rouse spring forces consistent with the equilibrium averages of distances between connected frictional beads and hydrodynamic interactions (Rouse-Zimm scheme). With this approach, the Rouse relaxation times and the frequency-dependent viscoelastic modulus are calculated. Since hydrodynamic interactions are included in their preaveraged form, the effect of the preaveraging approximation is explicitly discussed. The influence of the different structural and topological dependence on the dendrimer static and dynamic properties is analysed and discussed.     

Zigzag on a zigzag: Trap of hydrogen-bonded water molecules in a luminescent metal–organic network

A four-connected 2-D metal–organic coordination polymer {[Cd(mpdc)(phen)] · H₂O}(mpdc = 2,6-dimethylpyridine-3,5-dicarboxylate, phen = 1,10-phenanthroline) has been assemblied from cadmium ions and two kinds of organic ligands under hydrothermal conditions, characterized by single-crystal X-ray diffraction analysis. The 2-D metal–organic network was constructed from binuclear [Cd(O₅N₂)]₂ clusters bridged by mpdc²⁻ ligands. The most interesting is that water molecules trapped by the 2-D network look as if its motif is projected from the zigzag arrangement of cadmium atoms in the 2-D network via hydrogen bonding interactions. In addition, new compound exhibits strong luminescence at λ_max = 525 nm upon excitation at 367 nm.     

Ternary hydrogen-bonded polymer solutions

Ternary-phase diagrams have been experimentally determined at 100°C for systems containing a series of poly(n-alkyl methacrylates), poly(ethylene oxide) (PEO), and a solvent [4-ethyl phenol (EPh)]. A totally miscible phase diagram is experimentally determined for the poly(methyl methacrylate)/PEO/EPh system, while a closed-loop diagram is observed for the analogous system containing poly(ethyl methacrylate). The corresponding phase diagrams of analogous mixtures containing poly(n-propyl methacrylate) or poly(n-butyl methacrylate) exhibit large heterogeneous areas. Theoretically predicted phase diagrams calculated using an association model developed in our laboratory are in general accord with these observations for ternary hydrogen-bonded polymer/polymer solutions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1265–1271, 1998     

Tuning the transport dynamics of small penetrant molecules in glassy polymers

The effects of network structure of crosslinked poly(methyl methacrylate) (PMMA) discs on dynamic penetrant transport behavior were investigated through gravimetric integral sorption studies. Samples of P(MMA-co-(EG)_xDMA) were synthesized via an iniferter-mediated, thermally-initiated free radical polymerization procedure to produce polymers with controlled variations in network structure by altering the comonomer feed ratios of dimethacrylate crosslinkers and changing the crosslinker interchain bridge length. The front velocity in the Case II transport regime was shown to scale directly with the square root of the crosslinking density over varied thermal and structural conditions, and increasing the crosslinker interchain bridge length moved the observed penetrant transport behavior towards the Fickian regime. The demonstrated ability to tune the dynamic transport behavior through modification of the polymer network structure holds promise as a means to mitigate solvent-induced material failure due to cracking and crazing encountered in many high-tech applications.     

无定形PET中小分子扩散系数的分子动力学模拟

采用分子动力学模拟研究了相对分子质量在32～339范围内小分子在无定形PET中的扩散过程。基于Einstein关系式计算了扩散系数,讨论了模拟时间、密度对扩散系数的影响。结果表明：在较高的温度下,较短的模拟时间内就能观察到均方位移曲线的线性区;而在较低的温度下,需更长的模拟时间。扩散系数随密度增加而降低,聚合物密度越大,所需的模拟时间越长。通过比较扩散系数的计算值与实验值,发现两者的比值在一个数量级范围内,表明建立的聚合物模型可接受且能正确描述小分子在无定形PET中的扩散过程, 为获得迁移模型中关键参数——扩散系数提供了一种近似的计算方法。     

水分子在白云母表面吸附的分子动力学模拟

作为一种重要的化工分离技术,浮选过程中的固/液界面现象极为重要.利用PCFF_phyllosilicates力场对不同数量的水分子在白云母表面的吸附情况进行了分子动力学研究,考察了原子密度分布、氢键相对浓度分布、径向分布函数、均方位移和表面K⁺密度场等性质.结果表明:水分子达到单层覆盖后,靠近白云母(001)表面的前3层水分子的数目不随水分子的进一步增多发生改变;界面处的水分子比远离表面的水分子排列更为有序;原子密度分布、氢键相对浓度分布结果详细反映了白云母基质对水分子微观影响的"固体效应".研究还表明,表面K⁺在水化条件下移动性差,尤其是z方向流动性最差,K⁺密度场图也证明[Si₄Al₂]^-K⁺结构非常稳定.     

Desorption of water vapor in hydrogen-bonded polymer blends

Successive desorption experiments of water vapor in poly(methyl methacrylate) (PMMA) were performed at temperatures from 31.0 to 45.0°C. The solubility of water in PMMA was found to be independent of temperature in agreement with literature findings. But the results for diffusion showed stronger dependence on water concentration than those in literature. The diffusion coefficients of water in PMMA became almost independent of temperature at high water concentrations. However, at lower water concentrations, the temperature effect on diffusion was more pronounced. The observed weak temperature dependence of diffusivity at high concentrations is likely due to a high degree of clustering of water molecules found in the PMMA we prepared. Two modified polystyrenes containing 5 and 15 mol %, respectively, 4-hydroxystyrene as comonomer units were blended with PMMA to form hydrogen-bonded polymer blends. Successive desorption experiments of water vapor in the hydrogen-bonded polymer blends were carried out at 31.0°C. The solubility of water in both blends was found to increase with increasing composition of PMMA. The diffusion coefficients for PMMA and its blends increased with increasing concentration of water first, reached a maximum, then decreased with water concentration. When the desorption results were plotted with the previous study of absorption, hysteresis phenomenon of sorption existed in all blend compositions for our experimental time span. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 39–45, 1998     

光响应性氢键层层自组装薄膜的研究

为了研究偶氮苯结构对氢键复合物薄膜组装的影响,本文将分散红1(DR1)和对氨基偶氮苯(Aazo)引入到聚丙烯酸(PAA)侧链上,在氢键的作用下,通过层层自组装(LbL)技术与聚环氧乙烷(PEO)形成复合物薄膜.利用核磁(mH NMR)光谱、傅里叶红外光谱仪(FT-IR)、紫外可见光谱(UV-Vis)和石英晶体微天平(Q...     

Preparation and characterization of hydrogen-bonded P4VP-bisazobenzene complexes

Photoresponsive supramolecular complexes contain 4-hydroxyethyloxy-4??-(4-nitrophenylazo)azobenzene(BisAzo) molecule as a hydrogen bonding donor and poly(4-vinylpyridine) (P4VP) as a hydrogen bonding acceptor, and strong phenol pyridine hydrogen bonding is formed between them. FT-IR spectrum verifies the hydrogen bonding has formed between the phenol hydroxyl of BisAzo and pyridine ring of P4VP. The glass- transition temperature (Tg) is determined with differential-scanning calorimetry(DSC) and a decrease of complexes compared with pure P4VP is observed due to the attachment of BisAzo onto the backbone of P4VP. Polarized optical microscopy (POM) is performed to study the structure of supramolecular complexes,with a different texture observed which is different from both constituents alone. X-ray diffraction patterns are investigated with the same films already used for POM observation, indicating a lamellar structure with a periodic thickness of 4.1?nm. The supramolecular complexes P4VP/(BisAzo)_x are prepared with the molar ratio of BisAzo to the pyridine group varied at x?=?0.25, 0.5, 0.75 and 1.0, no mass aggregation is observed even at a high concentration of chromophore.     

玻碳电极的物理预处理方法

用循环伏安技术研究了玻碳电极在 K3 Fe( CN) 6和肾上腺素体系中的电化学性能 ,证实了用 Si C打磨玻碳电极的物理预处理方法的可行性。实验结果表明 ,在预处理玻碳电极上 ,K3 Fe( CN) 6在 0 .5mol/L KCl中的电还原反应为可逆反应 ,肾上腺素在 0 .5mol/L H2 SO4 介质中的电氧化反应为准可逆反应 ,其异相电子转移速率常数为 1 .0 7× 1 0 -2 cm/s     

玻碳电极上碘仿电解合成的机理

本文采用循环伏安法研究了在Na2CO3溶液介质中KI在金电极和玻碳电极上的电化学行为.在0.2V～1.2V电位范围内玻碳电极上仅发生I-被氧化为I2的反应,而在金电极上除了I-被氧化为I2外,还发生生成IO3-的氧化反应.同时通过不同电位扫描速度下的循环伏安行为分析发现电极过程为传质控制步骤.     

玻璃中的玻璃态夹杂物——线道和节瘤

线道和节瘤是玻璃中的玻璃态夹杂物。详细分析了它们产生的各种原因,为生产中解决条纹节瘤缺陷提供了依据。