氮化硼诱导聚乙烯分子结晶的分子动力学模拟

采用分子动力学方法(MD)研究熔体条件下聚乙烯分子在氮化硼纳米管表面和氮化硼片层表面的结晶机理。通过对聚乙烯分子结晶过程中晶体构象的演变、空间内分子分布的变化以及分子扩散特性的研究,从微观角度比较了两种结构氮化硼纳米材料对聚乙烯结晶的影响。结果表明一维结构的氮化硼纳米管诱导聚乙烯结晶的能力远高于二维片层状的氮化硼,说明纳米材料的维度影响着高分子材料的结晶性能。     

Effects of Branch Content and Branch Length on Polyethylene Crystallization: Molecular Dynamics Simulation

Influences of branch content (BC) and branch length (BL) on isothermal crystallization of precisely branched polyethylene are studied by molecular dynamics simulation. Branch acts as a defect both in nucleation and crystal growth process. BC affects not only crystallization kinetics but also final morphologies. Crystallization rate and crystallinity decrease as BC increases. Morphology Regimes change from lamellae crystal to bundle crystal at critical BC (20/1000 C) because of different folding pattern. 50 CH₂ is the critical methyl sequence length to form lamellae crystal. Lamellae thicknesses of final morphologies decrease in gradient corresponding to Morphologies Regimes. BL has no influence on the crystallization kinetics, and only affects the final morphologies when more branches inclusion happens with BL increasing. Trans‐rich phenomenon in pre‐crystalline state is observed. Crystallization process begins at the end of induction stage when trans state population reaches a critical value, and this value is independent of BC and BL.     

Glass Transition Temperature and Chain Flexibility of Ethylene‐Norbornene Copolymers from Molecular Dynamics Simulations

Summary: Model chains of ethylene‐norbornene copolymers were built up using the results of ¹³C NMR spectral analysis of copolymer samples synthesized with metallocene‐based catalysts. Our models statistically reproduce the microstructure, composition, and tacticity of the copolymer chains of experimental samples. They were used to test if MD simulations are suitable to investigate the relationships between microstructure and macroscopic properties. In particular, MD simulations were applied to calculate the glass transition temperature and to study the chain flexibility by the analysis of ACF of specific virtual bonds. Plots of specific volume versus temperature computed for models of four copolymer samples having different microstructures and norbornene contents yield T_g values in good agreement with experiments. Moreover, comparison of the ACFs provides some qualitative indications about the relationship between chain stereochemistry and T_g.

ACF functions of the virtual bonds with microstructures NENE (bottom) and ENNE (top).     

Crystallization and melting of polyethylene copolymers: Differential scanning calorimetry and atomic force microscopy studies

The Hoffman–Lauritzen theory of secondary, surface nucleation and growth was primarily relied upon for about 40 years after its introduction in about 1960 to rationalize the crystallization of flexible chain polymers into lamellar crystals. However, in about 1998, Strobl and coworkers introduced a different model for crystallization, based on the stage‐wise formation of lamellae. Two major components of this model were as follows: (1) the concept of the formation of a mesomorphic melt as a precursor to crystallization and (2) the control of the melting temperature range of lamellar crystals of homogeneous polyolefin copolymers by an inner degree of order or perfection rather than on the crystal thickness. The first concept is in disagreement with the HL theory and the second with the Gibbs‐Thomson theory, which associates melting temperature with lamella thickness. In the present study, differential scanning calorimetry and atomic force microscopy were successfully employed to monitor the in situ quiescent crystallization of polyethylene homopolymer and copolymer. In the present study, evidence was not found to support the concept of lamellae with equal thickness melting over a broad temperature range. Some evidence was found that might be interpreted to support the concept of a mesomorphic melt as a precursor to crystallization. At present, the model promoted by Strobl and coworkers appears to be at an uncertain stage at which strong proof or disproof are not available. However, this alternative model has injected a new vitality into the study of crystallization of flexible chain polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2369–2388, 2006     

Molecular Dynamics Studies of Collapse Stages in the chain folding process of Polyethylene

Althoughtheequilibriumpropertiesofthepolymerhavebeenextensivelystudied,kineticphenomenasuchasthecondensingprocessandcollapsetransitionstillhavemanyunclearaspects,andmucheffortwastakentomakeathoroughinvestigationandstudy.Moleculardynamic(MD)simulationsofthefoldingandcollapseprocessforapolyethylenechainwasrecentlycarriedoutbymanyauthors1'2'3.Itwasreportedthatatwo-stagecollapseprocesswasseenwithouttorsionpotential',andthreestageswerefoundbysimplificationofthecomputationalmodel5.ButthecollaPsesta…     

Broadening the Polyethylene Molecular Weight Distribution by Periodic Variation of the Hydrogen Feed Rate

The ability to control the molecular weight distribution of the created polyethylene in gas‐phase fluidized‐bed reactors is discussed. This objective is achieved via nonlinear model predictive controller utilizing the hydrogen feed rate as the only manipulated variable. The idea for limited usage of manipulated variables is to avoid economically unfavorable operations such as excessive purge and/or reduced production rate. The simulation results indicated successful implementation of the control algorithm to attain the desired molecular weight distribution. The success depends on the improved hydrogen activities inside the reactor by employing a modified catalyst that is responsive to hydrogen variation and allowing a wider range for hydrogen feed rates.

     

Effect of Vacancy Defects on the Young's Modulus and Fracture Strength of Graphene: A Molecular Dynamics Study

The Young's modulus of graphene with various rectangular and circular vacancy defects is investigated by molecular dynamics simulation. By comparing with the results calculated from an effective spring model, it is demonstrated that the Young's modulus of graphene is largely correlated to the size of vacancy defects perpendicular to the stretching direction. And a linear reduction of Young's modulus with the increasing concentration of mono‐atomic‐vacancy defects (i.e., the slope of ?0.03) is also observed. The fracture behavior of graphene, including the fracture strength, crack initiation and propagation are then studied by the molecular dynamics simulation, the effective spring model, and the quantized fracture mechanics. The blunting effect of vacancy edges is demonstrated, and the characterized crack tip radius of 4.44 Å is observed.     

真空条件下β-环糊精和对-甲基苯酚包结物动态结构的分子动力学模拟

本文报道了真空状态下主体β-环糊精和客体对-甲基苯酚形成的包结物在一定时间间隔内动态微观结构的分子动力学模拟结果。研究表明，对-甲基苯酚在环糊精腔内的运动具较大的自由度，但由于存在范德华作用，其运动受到一定的限制，同时通过计算观察到包结前后主客体的某些微观结构和性质发生了较为明显的变化。本文还给出了计算得到的主客体包结物的时间平均构象。     

The Dynamics of the Neuropeptide Y Receptor Type 1 Investigated by Solid-State NMR and Molecular Dynamics Simulation

We report data on the structural dynamics of the neuropeptide Y (NPY) G-protein-coupled receptor (GPCR) type 1 (Y1R), a typical representative of class A peptide ligand GPCRs, using a combination of solid-state NMR and molecular dynamics (MD) simulation. First, the equilibrium dynamics of Y1R were studied using ¹⁵N-NMR and quantitative determination of ¹H-¹³C order parameters through the measurement of dipolar couplings in separated-local-field NMR experiments. Order parameters reporting the amplitudes of the molecular motions of the C-H bond vectors of Y1R in DMPC membranes are 0.57 for the Cα sites and lower in the side chains (0.37 for the CH₂ and 0.18 for the CH₃ groups). Different NMR excitation schemes identify relatively rigid and also dynamic segments of the molecule. In monounsaturated membranes composed of longer lipid chains, Y1R is more rigid, attributed to a higher hydrophobic thickness of the lipid membrane. The presence of an antagonist or NPY has little influence on the amplitude of motions, whereas the addition of agonist and arrestin led to a pronounced rigidization. To investigate Y1R dynamics with site resolution, we conducted extensive all-atom MD simulations of the apo and antagonist-bound state. In each state, three replicas with a length of 20 μs (with one exception, where the trajectory length was 10 μs) were conducted. In these simulations, order parameters of each residue were determined and showed high values in the transmembrane helices, whereas the loops and termini exhibit much lower order. The extracellular helix segments undergo larger amplitude motions than their intracellular counterparts, whereas the opposite is observed for the loops, Helix 8, and termini. Only minor differences in order were observed between the apo and antagonist-bound state, whereas the time scale of the motions is shorter for the apo state. Although these relatively fast motions occurring with correlation times of ns up to a few µs have no direct relevance for receptor activation, it is believed that they represent the prerequisite for larger conformational transitions in proteins.     

Conformational Dynamics of Minimal Elastin‐Like Polypeptides: The Role of Proline Revealed by Molecular Dynamics and Nuclear Magnetic Resonance

Previous molecular dynamics studies of the elastin‐like peptide (ELP) GVG(VPGVG) predict that this ELP undergoes a conformational transition from an open to a more compact closed state upon an increase in temperature. These structural changes occurring in this minimal elastin model at the so‐called inverse temperature transition (ITT), which takes place when elastin is heated to temperatures of about 20–40 ^oC, are investigated further in this work by means of a combined theoretical and experimental approach. To do this, additional extensive classical molecular dynamics (MD) simulations of the capped octapeptide are carried out, analyzed, and compared to data obtained from homonuclear magnetic resonance (NMR) spectroscopy of the same octapeptide. Moreover, in the previous simulations, the proline residue in the ELP is found to act as a hinge, thereby allowing for the large‐amplitude opening and closing conformational motion of the ITT. To explore the role of proline in such elastin repeating units, a point mutant (P5I), which replaces the proline residue with an isoleucine residue, is also investigated using the aforementioned theoretical and experimental techniques. The results show that the site‐directed mutation completely alters the properties of this ELP, thus confirming the importance of the highly conserved proline residue in the ITT. Furthermore, a correlation between the two different methods employed is seen. Both methods predict the mutant ELP to be present in an unstructured form and the wild type ELP to have a β‐turn‐like structure. Finally, the role of the peptidyl cis to trans isomerization of the proline hinge is assessed in detail.     

Influence of Cross‐Linking Density on the Glass Transition and Structure of Chemically Cross‐Linked PVA: A Molecular Dynamics Study

Mechanical properties and glass transitions of cross‐linked polymer networks depend strongly on both the network topology and cross‐linking density. A model is developed using a dynamic cross‐linking approach based on a cutoff distance criterion followed by a high‐temperature annealing procedure. The analysis focused on on the influence of cross‐linking degree on chain packing and hydrogen‐bond structure and on the roles played by various energy components in the glass transition process. T_g was calculated using two different methods; (i) from the intersection of lines drawn through points in a plot of specific volume versus temperature and (ii) from plots of different molecular energy components as a function of temperature.

     

预先负载原料法的制备过程对复合分子筛形成的影响

采用预先负载原料法合成了具有核/壳结构的复合分子筛, 并研究了铝源类型和制备过程对反应的影响. 结果表明, 以异丙醇铝为铝源时, 晶化时间较短, 产物中没有SAPO-5晶体, 延长晶化时间可以提高SAPO-5的结晶度. 将负载磷的ZSM-5分别与不同组成的凝胶混合, 经室温陈化后可以明显加快晶化速度. 制备过程中凝胶的水含量和搅拌方式也根据铝源类型的不同有所差别. 当以拟薄水铝石为原料时, 在相同的处理条件下, 产物的结晶度都较高.     

Molecular Bottle Brushes with Positioned Selenols: Extending the Toolbox of Oxidative Single Polymer Chain Folding with Conformation Analysis by Atomic Force Microscopy

A synthesis route to controlled and dynamic single polymer chain folding is reported. Sequence-controlled macromolecules containing precisely located selenol moieties within a polymer chain are synthesized. Oxidation of selenol functionalities lead to diselenide bridges and induces controlled intramolecular crosslinking to generate single chain collapse. The cyclization process is successfully characterized by SEC as well as by ¹H NMR and 2D HSQC NMR spectroscopies. In order to gain insight on the molecular level to reveal the degree of structural control, the folded polymers are transformed into folded molecular brushes that are known to be visualizable as single molecule structures by AFM. The “grafting onto” approach is performed by using triazolinedione−diene reaction to graft the side chain polymers. A series of folded molecular brushes as well as the corresponding linear controls are synthesized. AFM visualization is proving the cyclization of the folded backbone by showing globular objects, where non-folded brushes show typical worm-like structures. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 154–162     

Simulation of Molecular Weight Distributions Obtained by Pulsed Laser Polymerization (PLP): New Analytical Expressions Including Intramolecular Chain Transfer to the Polymer

A new approach for the simulation of PLP (pulsed laser polymerization) is presented. This approach allows one to obtain new analytical solutions for different polymerization schemes, including either chain transfer to the monomer or intramolecular chain transfer to the polymer. The first results of the simulation of PLP experiments on n‐butyl acrylate at 20 °C and ambient pressure are presented.

MWDs simulated for PLP of n‐butyl acrylate, in bulk at 20 °C and ambient pressure using three models: the model with intramolecular chain transfer to the polymer (solid line), the model with chain transfer to monomer (dashed line), and the classical model (dotted line).     

The Nature of [PdCl2(C2H4)(H2O)] as an Active Species in the Wacker Process: New Insights from Ab Initio Molecular Dynamics Simulations

First‐principles molecular dynamics coupled with metadynamics have been used to gain a deeper insight into the reaction mechanism of the Wacker process by determining the nature of the active species. An explicit and dynamic representation of the aqueous solvent, which was essential for modeling this reaction, was efficiently included into the simulations. Prompted by our earlier results, which showed that the configuration of the catalytically active species [PdCl₂(H₂O)(C₂H₄)] was crucial in the subsequent steps of the Wacker process, herein we focused on the preceding equilibria that led to the formation of both the cis and trans isomers. Starting from the initial catalyst, [PdCl₄]^2?, the free‐energy barriers for the forward and backward reactions were calculated. These results confirmed the relevance of the trans intermediate in the reaction mechanism, whilst conversely, they showed that the cis configuration played no role in it. This sole participation of the trans intermediate has some very important implications; besides the mechanistic interpretation of the initial steps in the Wacker reaction mechanism, the analysis of these equilibria provides additional information about the chemical nature of these ligand‐substitution processes.