计算机模拟技术在表面活性剂研究中的应用

根据表面活性剂溶液行为的模拟所需的时间和空间尺度,介绍了三种主要的计算机模拟方法:原子模拟、粗粒模拟和介观模拟.综述了这些模拟方法在表面活性剂单体、缔合体系及与聚合物相互作用等研究中的应用.指出了用计算机模拟方法研究表面活性剂体系的发展前景.     

Molecular dynamics simulation of the melting of a twist Σ=5 grain boundary

The existence of a possible grain boundary disordering transition of the melting type in a =5 (001) twist boundary of aluminium bicrystal below the melting temperature was investigated using a constant pressure molecular dynamics simulation. The calculated melting temperature T _cm of the bulk Al is about 960 K. The total internal energy, the structure factor, and the pair distribution function were calculated at different layers across the grain boundary. The mean atomic volume, the grain boundary energy, and the thermal expansion coefficients were also calculated using the same simulation method. This simulation also allows us to image the grain boundary structure at different temperatures. The equilibrium grain boundary structure at 300 K retains the periodicity of the coincident site lattice, so that the lowest energy structure corresponds to the coincident site arrangement of the two ideal crystals. With increasing temperature, the total internal energy of the atoms for both the perfect crystal and the grain boundary increases, as do the number of layers in the grain boundary. The grain boundary core exists and the perfect crystal structure still exists outside the grain boundary at 0.9375 T _cm. However, two atomic layers of the equilibrium grain boundary structure at 0.9375 T _cm lose the coincident site lattice periodicity and attain a structure with liquid-like disorder. Therefore, partial melting of the grain boundary has occurred at the temperature above 0.9375 T _cm which is in agreement with the experimental results.     

Thermotropic Liquid Crystals of Double-Chain Zwit-terionic Surfactants (C16)2NCnS

A series of double-chain sulfobetain zwitterionic surfactants with varying carbon number of the intercharge group (spacer), (C₁₆H₃₃)₂NCH₃(CH₂)_nSO₃, where n=2, 3, 4, and 6, referred to as (C₁₆)₂NC_nS, were synthesized. The influence of the spacer length on the thermal behaviors of (C₁₆)₂NC_nS in their pure state was studied using thermogravimetry (TG), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). Thermogravimetric analysis (TGA) showed that the thermal stability of (C₁₆)₂NC_nS lowered with increasing the spacer length. All compounds showed a complex polymorphism. Only for (C₁₆)₂NC₄S and (C₁₆)₂NC₆S, thermotropic liquid crystals were observed using POM. The former exhibited a smectic A (SmA) phase, whereas the latter formed a hexagonal columnar phase. These liquid crystals obtained both in the cooling and the second heating scans provided compelling evidence for the thermal stability of these compounds.     

基于因子分析的粮食收购政策执行效果评价

依据现有研究,通过归纳分析影响粮食收购政策执行效果的因素,构建评价其执行效果的模型.确定衡量粮食收购政策执行效果的指标因子,运用因子分析技术,对政策的执行效果进行分析,进而得到因子综合得分并用于对粮食收购政策执行效果进行评价.运用本文的因子综合得分法对部分示例省份粮食收购政策的执行效果进行评价和分析,在此基础上进一步提出完善建议.     

Effect of Terminal Modification on the Molecular Assembly and Mechanical Properties of Protein‐Based Block Copolymers

Accurate prediction and validation of the assembly of bioinspired peptide sequences into fibers with defined mechanical characteristics would aid significantly in designing and creating materials with desired properties. This process may also be utilized to provide insight into how the molecular architecture of many natural protein fibers is assembled. In this work, computational modeling and experimentation are used in tandem to determine how peptide terminal modification affects a fiber‐forming core domain. Modeling shows that increased terminal molecular weight and hydrophilicity improve peptide chain alignment under shearing conditions and promote consolidation of semicrystalline domains. Mechanical analysis shows acute improvements to strength and elasticity, but significantly reduced extensibility and overall toughness. These results highlight an important entropic function that terminal domains of fiber‐forming peptides exhibit as chain alignment promoters, which ultimately has notable consequences on the mechanical behavior of the final fiber products.     

An Unconventional Approach to Induce Liquid‐Crystalline Phases of Triazine‐Based Dendrons by Breaking Their Self‐Assembly into Dimers

Three triazine‐based dendrons ( 1 a – c ) were successfully prepared in 70–83 % yields. These newly prepared dendrons are found to be liquid crystalline (LC). Computational investigations on molecular conformations and dipoles of triazine‐based dendrons reveal that the substituent on the central triazine unit interrupts strong dipole or H‐bond interactions to avoid dimeric formation. The obtained dendrons, not favouring self‐assembly into dimers but showing LC behaviours, provides evidence for an approach contrary to the conventional method of inducing LC behaviours of dendrons by dimer or trimer formation, mostly through H‐bond interactions.     

Scalability of Coarse‐Grained Potentials Generated from Iterative Boltzmann Inversion for Polymers: Case Study on Polycarbonates

Using systematic coarse‐grained (CG) techniques such as iterative Boltzmann inversion (IBI) is an efficient means to simulate high molecular weight polymer melts within reasonable computational time. One drawback of such an approach is however the need to carry out extensive atomistic simulations in order to extrapolate the necessary distributions to derive the inter and intrabead force field parameters. Here it is shown that it is possible to use atomistic simulations of relative short oligomers to develop the CG model for high molecular weight polymers. In particular for the specific case of polycarbonates, it is found that the structural properties (end‐to‐end distance, radius of gyration and their distributions) are similar irrespective of whether the CG potentials are derived from 5‐mer or 10‐mer melt systems. Dynamical properties of the CG systems are smoother and faster than the atomistic ones. Scaling factor, derived by overlapping the CG mean square displacement curves (obtained from different CG IBI potentials) over the atomistic ones, also scales the autocorrelation functions. A prediction of the dynamical scaling factor in the case of the unavailability of atomistic simulations is also discussed. The dynamical properties of the CG melts are modeled reasonably well by all the CG potentials derived from atomistic simulations of short oligomers.

     

等效粒径与非均匀沙起动条件研究

阐述了泥沙起动机理，通过对等效粒径概念及表达式的引入，推得了泥沙起动公式，该公式的系数并非一个常量，而是位于某个区间，且随泥沙粒径、相对暴露度及起动标准而变，同时得到了均匀沙的起动公式，该公式系数同样位于一个区间，通过与部分均匀沙公式的系数比较可知，所给出的系数的取值区间基本涵盖了其他公式系数，说明采用等效粒径能反映泥沙的起动特性。     

Shape‐Persistent,Sterically Crowded Star Mesogens: From Exceptional Columnar Dimer Stacks to Supermesogens

Hexasubstituted C₃‐symmetric benzenes with three oligophenylenevinylene (OPV) arms and three pyridyl or phenyl substituents are shape‐persistent star mesogens that are sterically crowded in the center. Such molecular structures possess large void spaces between their arms, which have to be filled in condensed phases. For the neat materials, this is accomplished by an exceptional formation of dimers and short‐range helical packing in columnar mesophases. The mesophase is thermodynamically stable for the pyridyl compound. Only this derivative forms filled star‐shaped supermesogens in the presence of various carboxylic acids. The latter do not arrange as dimers, but as monomers along the columnar stacks. In this liquid crystal (LC) phase, the guests are completely enclosed by the hosts. Therefore, the host can be regarded as a new LC endoreceptor, which allows the design of columnar functional structures in the future.