半晶态聚合物的分子动力学模拟

采用粗粒化聚乙烯醇模型,应用分子动力学方法模拟熔融态聚合物经过缓慢冷却、局部结晶形成半晶态聚合物的过程.静态结构因子的演变显示出在结晶初期小角散射强度的增大先于布拉格峰的出现,这与小角/大角X射线散射实验现象相一致.模拟得到的半晶态聚合物呈现为折叠链构成的晶区与非晶区交杂在一起的结构形态,与缨状微束结构模型相一致.研究发现在不同的冷却阶段具有不同的有序结构形成机制.从结晶温度到玻璃化温度的凝固过程中,存在分子链的伸展和伸直分子链之间平行排列两种形式的结构转变;而在玻璃化温度之后,材料的活性只允许调整伸直分子链之间的相对排列位置.     

梳型高分子聚合物向列相液晶的相变

本文根据Freiser关于向列相液晶分子相互作用模型,利用平均场理论,建立了梳型高分子聚合物向列相液晶分子的哈密顿量,导出序参数自洽方程,得到各向同性相至单轴向列相以及单轴至双轴向列相的相变,结果表明,当分子主、侧链耦合项中排斥作用及主、侧链中较强向列相场的场强增大时,单轴至双轴向列相相变温度升高,并从一级相变逐渐变为二级相变。 关键词：     

LJ模型的热力学特性、相平衡及用于真实分子的研究

结合描述硬球固体Helmholtz自由能的自由体积方法与描述硬球固体径向分布函数的拟合的分析表达式与一阶热力学摄动理论,用于描述Lennard Jones(LJ)固体的Helmholtz自由能.按照一个修正的WCA方法将LJ势分为短程排斥部分与长程吸引部分,将文献中一个用于求取液相的等价的硬球直径的简单的迭代法扩展到固相,用于求取固相的等价的硬球直径.在固体Helmholtz自由能的计算中,使用200壳层,以便获得精确的结果.体相LJ液体的热力学特性由一个最近提出的状态方程求取.该方法很好地描述了LJ固体的过量Helmholtz自由能与状态方程,满意地描述了Lennard Jones模型的相平衡;通过选取合适的LJ势参数,能很好地描述了真实分子的融化曲线.     

R134a PVT性质的分子动力学模拟

使用二中心LJ分子嵌入沿轴偶极矩(2CLJD)的分子模型,对新型制冷剂R134a的PVT性质进行了分子动力学(MD)模拟,并与实验数据进行了对比,结果表明通过合理地选择势能参数,利用2CLJD模型预测氟利昂类制冷剂的热物理性质是可行的。     

环状聚合物及其对应的线性链熔体在启动剪切场下流变特性的分子动力学模拟研究

采用非平衡分子动力学方法研究了环状聚合物及其对应的线性链熔体在启动剪切场下的结构与流变特性.模拟结果显示:低剪切速率下(■1×10~(-4)τ~(-1))环状链分子体系相比于同分子量的线性链体系并没有出现明显的过冲现象.该结果表明,在启动剪切场下环状分子与其对应的线性链相比较表现出了更弱的分子形变,同时模拟结果也与最近实验观察的结果一致.为了进一步探究这种现象背后的分子机理,在分子层面统计了不同流场强度下,链段的长度和取向角分布随着应变的演化,统计结果证明了环状聚合物分子链段弱的形变是导致其弱的剪切变稀和峰值应变的主要因素.本文还给出了过冲点和稳态在不同剪切速率下环状分子与其对应的线性链的流变特性(过冲应变、最大应力、最大黏性和稳态黏性)、结构和取向参数与维森伯格数(Wi_R)所满足的标度关系.     

梳型聚合物液晶的介电反应

计算了梳型聚合物双轴丝状相液晶侧链的扭曲缺陷能,对处于外电场的侧链,将其哈密顿量与一维Ising模型相比较,得到侧链在外电场中的极化率.结果表明,聚合物的电极化率远大于普通液晶.在低温下,两者之间的比值趋于聚合度N.对弛豫时间的计算表明,聚合物液晶的极化达到饱和的时间很长,在低温下是普通液晶的N倍,而介电损耗出现的频率则为普通液晶的1/N倍. 关键词：     

骨架柔性对短链烷烃分子在金属-有机骨架材料中扩散的影响

采用分子动力学方法,对短链烷烃在柔性和刚性具有相似拓扑结构的金属有机骨架材料(isoreticular metal-organic frameworks, IRMOFs)中的分子扩散进行了研究. 结果表明,分子在柔性骨架中的自扩散系数大于在刚性骨架中的自扩散系数,在柔性骨架中的活化能小于在刚性骨架中的活化能. 骨架的柔性对自扩散系数的影响随着温度的升高而增加,随着扩散分子数目以及扩散分子链长的增长而降低. 因此,在利用分子模拟方法研究烷烃在金属-有机骨架材料中的扩散行为时,尤其对于较高温度以及较短的烷烃分 关键词： 分子模拟 金属-有机骨架材料 柔性骨架 扩散     

向列相液晶$lt;i$gt;n$lt;/i$gt;CB(4-$lt;i$gt;n$lt;/i$gt;-alkyl-4′-cyanobiphenyls, $lt;i$gt;n$lt;/i$gt;=5—8)的旋转黏度及其奇偶效应的分子动力学模拟

在全原子力场模型的基础上,对向列液晶nCB（4-n-alkyl-4′-cyanobiphenyls, n=5—8）进行了等压等温NPT系综下的分子动力学（MD）模拟.对MD的轨迹分析得到了二阶和四阶序参数以及描述液晶分子翻转运动的取向时间相关函数（TCF）,并通过一个近似的单指函数对TCF拟合得到了相关时间.在此基础上,计算了nCB（n=5—8）的旋转扩散系数（Rotational diffusion coefficient, RDC）.利用基于统计力学模型的Nemtsov-Zakharov方法和Fialkowski方法,分别计算了它们的旋转黏度系数（rotational viscosity coefficient, RVC）,进而分别讨论了RVC,RDC和相关时间的奇偶效应.与文献中的实验结果比较表明,该方法得到了合理的计算结果和一致的奇偶效应. 关键词： 旋转黏度 奇偶效应 分子动力学模拟 向列相液晶     

向列相液晶nCB(4-n-alkyl-4'-cyanobiphenyls,n=5-8)的旋转黏度及其奇偶效应的分子动力学模拟

在全原子力场模型的基础上,对向列液晶nCB(4-n-alkyl-4'-cyano biphenyls,n=5-8)进行了等压等温NPT系综下的分子动力学(MD)模拟.对MD的轨迹分析得到了二阶和四阶序参数以及描述液晶分子翻转运动的取向时间相关函数(TCF),并通过一个近似的单指函数对TCF拟合得到了相关时间.在此基础上,计算了nCB(n=5-8)的旋转扩散系数(Rotat ional diffusion coefficient,RDC).利用基于统计力学模型的Nemtsov-Zakharov方法和Fialkowski方法,分别计算了它们的旋转黏度系数(rotational viscosity coefficient,RVC),进而分别讨论了RVC,RDC和相关时间的奇偶效应.与文献中的实验结果比较表明,该方法得到了合理的计算结果和一致的奇偶效应.     

单个纳米粒子对聚合物结晶行为的影响

采用粗粒化模型,应用分子动力学方法研究了单个纳米粒子对聚合物结晶行为的影响.通过改变纳米粒子与聚合物单体之间作用方式(吸引作用或排斥作用)、纳米粒子与聚合物单体之间作用强度和聚合物分子链的长度,计算整个系统和局部区域的有序参数,研究了三个不同因素下纳米粒子对聚合物结晶行为的不同影响.研究表明,在聚合物基体中添加单个纳米粒子,纳米粒子对整个系统的结晶影响不明显,但是纳米粒子对其周围聚合物单体的结晶存在局部强化作用.当纳米粒子与聚合物单体之间为吸引作用且作用强度较大时,纳米粒子对聚合物结晶表现出明显的局部强化作用,聚合物分子链长度也有着一定的影响,在较大吸引作用强度下,长链样本比短链样本有着更为显著的局部强化作用.     

Preparation of Main-Chain Thermotropic Polyesters With Two Types of Mesogens Connected Alternatively by Aliphatic Spacers

A homologous series of main-chain thermotropic polyesters (PBTn, n = 2, 4, 6) containing biphenyl and triad aromatic ester mesogenic units interconnected by n-methylene spacers in the main chain were prepared from terephthaloyl bis(4-oxybenzoyl chloride) (TOBC) and 4,4′-bis(ω-hydroxyalkyloxy)biphenyls (n-BP, n = 2, 4, 6) by solution polycondensation. The chemical structures of the monomers and PBTn were characterized by spectroscopic techniques and elemental analysis. The solubility, supermolecular structures, thermal and liquid crystalline properties of PBTn were investigated by differential scanning calorimetry (DSC), polarizing microscopy (PLM), small-angle X-ray scattering (SAXS), thermogravimetric analyses (TGA), and wide-angle X-ray diffraction (WAXD), and they were found to be closely dependent on the length of the methylene spacers. The increasing methylene units led to improved solubility, broader liquid crystalline range, reduced mesophase-isotropic transition point, higher thermal stability, and decreasing crystallinity. The decomposition temperatures (5% mass loss) of the polymers were above 380°C in nitrogen atmosphere and only PBT4 and PBT6 exhibited smectic mesophases as well as high viscosity in the mesophase.     

Structural and stereochemical isomerism effects in thermotropic liquid crystalline polymers

Summary A critical survey is presented of the major contributions so far available on the influence of structural and stereochemical isomerism on the physical properties of thermotropic liquid crystalline polymers. The structure-property correlation in liquid crystalline polymeric materials is still far beyond to be satisfactorily rationalizable in terms of chemical and physical responses, as predictable at molecular level and modulable, from a practical point of view, at the stage of polymer formulation. In this connection it will be pointed out how isomerism effects in the polymer-repeating units can play sometimes a key role. For convenience, structural and stereochemical isomerism features shall be treated separately and the systems divided in ?side-chain? and ?main-chain? type, in accordance with the relative position of the mesogens within the repeating units. Work presented at the Second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Mechanical model study of relationship of molecular configuration and multiphase in liquid crystal materials

A mechanical model of liquid crystals (LCs) is applied to study the polymorphism of homologous series of terphenyl compounds. With a semi-experimental molecular orbit method, we calculate the moment of inertia which represents the rotation state to describe the phase transition temperature obtained from experimental data. We propose a novel explanation of the phase sequence or polymorphism of LC materials using the two key parameters, the moment of inertia and critical rotational velocity. The effect of molecular polarity on the appearance of liquid crystalline is also discussed.     

Molecular simulation of model liquid crystals in a strong aligning field

We report a computer simulation study of systems of perfectly aligned molecules interacting through the Gay–Berne (GB) potential model for two different values of the molecular anisotropy parameter κ, namely 3 and 4.4. The models are appropriate to gauge the effects of strong aligning fields on the thermodynamics and structural properties of thermotropic liquid crystals. According to our results, one of the main effects of the external field is to increase the range of stability of the smectic A phase, which indicates the existence of a strong coupling between orientational and translational order. For the κ?=?3?GB model the smectic phase, which is not stable in the absence of the field, is promoted when the molecules are constrained to be parallel. According to the simulation results, the smectic A-nematic transition is, in general, continuous; however, this transition appears to be first order at low pressure for the κ?=?4.4?GB fluid model.     

Influence of dispersive long-range interactions on properties of vapour–liquid equilibria and interfaces of binary Lennard-Jones mixtures

The influence of dispersive long-range interactions on properties of vapour–liquid equilibria and interfaces of six binary Lennard-Jones (LJ) mixtures was studied by molecular dynamics (MD) simulations and density gradient theory (DGT). The mixtures were investigated at a constant temperature T, at which the low-boiling component, which is the same in all mixtures, is subcritical. Two different high-boiling components were considered: one is subcritical, the other is supercritical at T. Furthermore, the unlike dispersive interaction was varied such that mixtures with three different types of phase behaviour were obtained: ideal, low-boiling azeotrope, and high-boiling azeotrope. In a first series of simulations, the full LJ potential was used to describe these mixtures. To assess the influence of the long-range interactions, these results were compared with simulations carried out with the LJ truncated and shifted (LJTS) potential applying the corresponding states principle. The dispersive long-range interactions have a significant influence on the surface tension and the interfacial thickness of the studied mixtures, whereas the relative adsorption and the enrichment are hardly affected. Furthermore, the influence of the long-range interactions on Henry's law constants and the phase envelopes of the vapour–liquid equilibrium was investigated. The long-range interactions have practically no influence on the composition dependency of the investigated mixture properties.     

Expressions for forces and torques in molecular simulations using rigid bodies

Expressions for intermolecular forces and torques, derived from pair potentials between rigid non-spherical units, are presented. The aim is to give compact and clear expressions, which are easily generalized, and which minimize the risk of error in writing molecular dynamics simulation programs. It is anticipated that these expressions will be useful in the simulation of liquid crystalline systems, and in coarse-grained modelling of macromolecules.     

Hydrazide‐based non‐symmetric liquid crystal dimers: synthesis and mesomorphic behavior

Hydrazide‐based non‐symmetric liquid crystal dimers were synthesized. The liquid crystalline properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and powder X‐ray diffraction (XRD). These non‐symmetric liquid crystal dimers are evidenced to display the monolayer smectic C phase. The effects of the lateral intermolecular hydrogen bonding as well as the length of the terminal alkyl chains and the spacers on the mesophase are discussed. Our studies reveal that intermolecular hydrogen bonding between the hydrazide groups and microsegregation effect is the driving force for the formation of the monolayer smectic C structure. Copyright © 2007 John Wiley & Sons, Ltd.