A molecular dynamics study of N–A–S–H gel with various Si/Al ratios

In this paper, the atomic structures of sodium aluminosilicate hydrate (N–A–S–H) gels with different Si/Al ratios are studied by molecular dynamics simulation. An N–A–S–H gel model was obtained from the polymerization of Si(OH)₄ and Al(OH)₃ monomers with the use of a reactive force field (ReaxFF). The simulated atomic structural features, such as the bond length, bond angle, and simulated X-ray diffraction pattern of the gel structure are in good accordance with the experimental results in the literature. Si–O–Al is found to be preferred over Si–O–Si in the N–A–S–H gel structure according to the amount of T–O–T bond angles and distribution of Si⁴(mAl). Pentacoordinate Al is identified in all simulated N–A–S–H models. It provides strong support to current knowledge that pentacoordinate Al in geopolymer does not only come from raw material. Furthermore, the structural analysis results also show that N–A–S–H gel with lower Si/Al ratios has a more cross-linked and compacted structure.     

环氧树脂密度与二面角扭转能的仿真计算

基于COMPASS分子力场,利用分子动力学模拟方法和Materials Studio软件建立了低固化度交联耦合的双酚A型环氧树脂交联结构模型,并利用环氧树脂交联体系模型模拟计算了不同温度下交联环氧树脂的密度和二面角扭转能,以此预测了环氧树脂的玻璃化转变温度(Tg)。结果表明,计算得到的Tg与实验值具有良好的一致性,分子动力学模拟方法可以应用于复杂聚合物体系结构与性质的研究中。     

Effect of ultraviolet irradiation on the cross-linking process and ceramic yield of liquid hyperbranched polycarbosilane

Liquid hyperbranched polycarbosilanes (LHBPCSs) with different reactant molar ratios of chloropropene/chloromethyltrichlorosilane were synthesised by Grignard coupling, followed by reduction. Different from traditional thermal cross-linking for LHBPCS, ultraviolet (UV) irradiation cross-linking was used. The molecular weight and hardness of LHBPCS increased with prolonging UV irradiation time. According to Fourier transform infrared result, the –Si–H, –Si–H₂ and allyl groups were consumed. Possible cross-linking reactions (hydrosilylation, allyl group polymerisation and dehydrocoupling) were discussed. The ceramic yield of LHBPCS showed an increase trend with increasing the UV irradiation time. After 1?h of UV irradiation for LHBPCS-1 (the molar ratio of chloropropene to chloromethyltrichlorosilane was 1:9), 78.0?wt-% ceramic yield was obtained. In addition, LHBPCS with higher allyl group content had fewer pores in it. By heating LHBPCS-1 cross-linked by UV irradiation to 1400°C, a non-porous SiC ceramic was obtained.     

Dynamic response of transiently trapped entanglements in polymer networks

The structure and viscoelastic response of polymer networks are highly sensitive to the presence of pendant chains. These imperfections, that are unavoidable produced during a cross-linking reaction, reduce the cross-linking density and affect the damping response of elastomers. In this work the dynamics of pendant chains present in a cross-linked network is investigated using end-linked poly(dimethyl-siloxane) networks with well defined structure. For this purpose, model networks containing 10 and 20 wt% of two different monodisperse pendant chains with molecular weights well above the critical entanglement molecular weight and some of their blends were prepared. It was found that, within this range of concentration of pendant chains, the long-time dynamic response of the networks was nearly insensitive to the content of pendant material but deeply influenced by the average molar mass of these defects. While the equilibrium behavior of the networks can be well described by a mean field theory for rubber elasticity, the long time relaxational dynamics can be rationalized in terms of the Pearson-Helfand picture for the arm retraction process. Within this theoretical picture, the dynamics can be explained in terms of the molecular architecture of the network, the Rouse time and the weight average molar mass of the pendant material.     

Electron beam cross-linking of EVA/TPU blends

Abstract

Ethylene vinyl acetate (EVA) copolymer and thermoplastic polyurethane (TPU) with different blending ratios were melt mixed in an internal mixer. The blends were then exposed to electron beam (EB) irradiation with different doses of 50, 100, 150, 200 and 250 kGy. FTIR spectroscopy and dynamic mechanical thermal analysis (DMTA) were used to investigate the effect of cross-linking and blending ratio on chemical structure as well as solid state viscoelastic properties of the blends. FTIR spectroscopy showed interchain cross-linking during melt blending and also radiation cross-linking during solid state irradiation. Observation of one damping peak for blends at almost all blending ratios was an indication of miscibility of these blends. The results indicated formation of interchain cross-linking stabilised with exposure to EB irradiation. Mechanical properties of the blends were investigated via stress–strain curves. Modulus showed a monotonic increasing trend with the radiation dose, but tensile strength and elongation at break were initially increased and then decreased with increasing radiation dose. This was attributed to two competing parallel factors of strain induced crystallisation and degree of cross-linking.     

疏水缔合聚合物交联弱凝胶流变性能研究

疏水缔合聚合物交联弱凝胶体系含有化学交联和物理缔合双重性能,采用Bohlin CVO流变仪测试了该体系的流变性,并采用Carreau模型和幂律模型对测试数据进行拟合。结果表明,疏水缔合聚合物交联弱凝胶体系完全满足这些模型,其零剪切粘度和稠度系数均随着交联剂和聚合物浓度的增加而增加。     

Modeling of branching and gelation in living copolymerization of monomer and divinyl cross-linker using dynamic lattice liquid model (DLL) and Flory-Stockmayer model

The random living copolymerization of vinyl and divinyl monomers with fast initiation and slow propagation was simulated using two Monte Carlo methods: one, purely statistical, corresponding to the mean-field Flory-Stockmayer (FS) theory and the dynamic lattice liquid model (DLL). The results were compared with experiments in which atom transfer radical polymerization (ATRP) method was used. Molecular weights, polydispersities and the cross-linking/cyclization of macromolecules were modeled as a function of conversion for various cross-linker concentrations. The results obtained by the DLL and FS methods and experiments are presented and the sources of discrepancies are discussed. The DLL method yields gel points, molecular weight distributions and critical exponents closer to experimental values and gives an insight into cross-linking processes near the gel point (leading to gelation). More realistic gel structure is obtained due to an appropriate modeling of intra-chain cyclization and diffusion effects, especially close to and above the gel point.     

Supramolecular domains in mixed peptide self-assembled monolayers on gold nanoparticles

Self-organization in mixed self-assembled monolayers of small molecules provides a route towards nanoparticles with complex molecular structures. Inspired by structural biology, a strategy based on chemical cross-linking is introduced to probe proximity between functional peptides embedded in a mixed self-assembled monolayer at the surface of a nanoparticle. The physical basis of the proximity measurement is a transition from intramolecular to intermolecular cross-linking as the functional peptides get closer. Experimental investigations of a binary peptide self-assembled monolayer show that this transition happens at an extremely low molar ratio of the functional versus matrix peptide. Molecular dynamics simulations of the peptide self-assembled monolayer are used to calculate the volume explored by the reactive groups. Comparison of the experimental results with a probabilistic model demonstrates that the peptides are not randomly distributed at the surface of the nanoparticle, but rather self-organize into supramolecular domains.     

Atomistically informed stochastic multiscale model to predict the behavior of carbon nanotube-enhanced nanocomposites

A comprehensive, point-information-to-continuum-level analysis framework is presented in this paper to accurately characterize the behavior of carbon nanotube (CNT)-enhanced composite materials. Molecular dynamics (MD) simulations are performed to study sub-nanoscale interactions of the CNT with the polymeric phase of the nanocomposite. The effect of cross-linking between the epoxy resin and the hardener on the mechanical properties of the polymer is investigated; furthermore, the effect of CNT weight fraction on the probability distribution of polymer cross-linking degree is also studied through stochastic models. The stochastic distributions obtained from MD simulations provide a basis to simulate local variations in the matrix properties in the continuum model at the microscale. The inclusion of an atomistically informed elastic–plastic model at the microscale reveals a significant deviation of the mechanical properties from those obtained based on classical homogenization approaches. Microstructural variability arising from heterogeneous cross-linking degree in the polymer phase and variations in fiber geometry and spacing is also found to cause deviations in the mechanical response when compared to the assumption of a perfectly ordered fiber–matrix microstructure.     

不同组分比的HMX/DMI共晶炸药结构与性质的理论研究

利用分子动力学,研究了分子摩尔比对HMX/DMI共晶炸药几个重要晶面成键能的影响,对于不同分子的摩尔比的力学性质也进行了估算,借助M06-2x/6-311+G(2df,2p)方法对HMX/DMI复合物的溶剂效应也进行了研究。计算结果表明,(020)和(100)取代基模型具有最高的成键能和稳定性,1∶1和2∶1的化合物最稳定且具有最高的力学性能。分子间相互作用能和N–NO_2键离解能的变化对HMX/DMI共晶炸药的稳定性有较大影响。制备稳定的HMX/DMI共晶炸药应选用较低介电常数作溶剂。     

The atomic scale structure of nanographene platelets studied by X-ray diffraction,high-resolution transmission electron microscopy and molecular dynamics

The atomic structure of commercially available nanographene platelets has been studied by high energy X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and molecular dynamics simulations using the reactive empirical bond order potential. Atomic models of the structure have been constructed and then relaxed using the molecular dynamics method and the model based simulations are compared with the experimental data both in reciprocal and real space. All model relaxations and the X-ray diffraction experiments have been carried out at 300 K. The proposed models consisting of about 2500 carbon atoms arranged within four graphitic layers with a diameter of 46 Å, reproduced correctly all features of the experimental data. The atomic arrangement within an individual layer can be described in terms of the paracrystalline ordering, in which lattice distortions propagate proportionally to the square root of interatomic distances. The paracrystalline structure was simulated by introducing the topological point defects such as the Stone–Thrower–Wales defects, single- and double-vacancies, randomly distributed in the network. Such defects lead to curvature of individual layers and this effect was also analyzed. The generated models are related to the observations by high-resolution transmission electron microscopy.     

Effects of cross-linked networks on dielectric properties of epoxy resins based on molecular dynamics

In this paper, three epoxy resin systems commonly used in power equipment are prepared to obtain cross-linked networks with structural differences. The relationship between microscopic structures and dielectric properties of epoxy resins is investigated and discussed. Experimental results show that the polarization and conductance losses are inhibited in the anhydride-cured systems with methyl groups. The molecular dynamics (MD) simulation shows that rigid anhydride structures (such as methyl groups) play a major role in reducing the local segment mobility and increasing the free volume at cross-linking points. The decrease in local segment mobility has been confirmed by the decrease of mean square displacement (MSD) at the cross-linking points, which is consistent with the change of measured polarization and conductance loss. In the glassy state, the dielectric properties of different anhydride-cured systems can be reflected by local MSD at the cross-linking points. At high temperatures near the glass transition temperature, both the free volume and network mobility increase significantly, which reflects the increased dielectric relaxation strength and conductance loss. The understanding of the structure–property relationships could provide a theoretical foundation for epoxy modification in a controlled manner for power equipment applications.     

Structure of Sodium Aluminosilicate Glass Surfaces

Sodium aluminosilicate (NAS) glass surfaces with compositions containing approximately 63% SiO₂ and Al/Na ratios, R , of 0.25 R 2.0 were simulated using the molecular dynamics technique with a multibody interaction potential. There were changes to the surface structure and composition in comparison to bulk NAS glasses. The changes included an increased concentration of sodium and oxygen and the formation of nonbridging oxygen at the outermost surfaces, although the increases were smaller with increased Al concentration. In addition, the formation of small-membered rings and three-coordinated aluminum occurred in the subsurface regions. These changes were accompanied by a change in the ratio of Al/Na in the region extending to 4 Å below the surface.     

Multiscale investigation on molecular structure and mechanical properties of thermal-treated rigid polyurethane foam under high temperature

In order to study the relationship between the molecular structure and mechanical properties of rigid polyurethane foam (RPUF) during the mechanical and chemical failure process, the variation of the molecular structure and mechanical properties of RPUF treated in temperature range of 323–473 K were characterized by both theoretical and experimental methods. The molecular structure stability of RPUF varied with thermal treatment temperature was characterized by density functional theory method. The mechanical properties of base material of RPUF were simulated by means of molecular dynamics (MD) simulation. Then the related parameters obtained from the MD simulation were assigned into a representative volume element model of RPUF for the finite element analysis. The results indicated that the vibrational frequencies of isocyanate groups and amino acid ester groups in RPUF molecule increased while the molecular orbital energy gap of RPUF decreased with the increase of treatment temperature. It indicated that the RPUF molecule had high chemical reactivity at high temperature. The results of the multiscale simulation of mechanical properties showed that the defects and voids in RPUF generated under high temperature would grow with the increase of thermal treatment temperature, which intensified the stress concentration in RPUF and decreased the tensile properties of RPUF.     

计算机模拟溶剂对对苯二酚晶习的影响

针对苯二酚的单晶结构,采用BFDH和AE模型对对苯二酚的理论晶习进行了模拟,然后利用分子动力学模拟计算了对苯二酚各晶面在水溶液中与水分子的相互作用能,并利用修正的AE模型对水溶液可能出现的晶习进行了模拟计算,计算结果与实际晶习一致。     

Molecular simulation study of aluminum–noble gas interfacial thermal accommodation coefficients

Thermal accommodation coefficients (TAC) between aluminum and noble gases were studied with molecular dynamics (MD) simulations. Gases interacting with aluminum substrates were modeled by MD with gas velocities sampled from the Maxwell–Boltzmann distribution to give accumulated TAC results. Different implementations of the equation to calculate the TAC, which differ in the amount of information gleaned from MD and the corresponding simulation results, were carefully discussed. The best formula for MD modeling in finite simulation time was determined. Additionally, the influence of the combining rules applied on aluminum–noble gas interatomic potential was characterized with the well‐known Lennard–Jones 12–6 potential combined with Lorentz–Berthelot and Fender–Halsey mixing rules. The results were compared with experimental values and previous analytical model. TACs simulated with Fender–Halsey rules present excellent agreement with the experimental values. Detailed TAC distributions and accumulated TAC convergence are also included. © 2017 American Institute of Chemical Engineers AIChE J, 63: 338–345, 2018     

Formation of 3D networks in polylactic acid by adjusting the cross-linking agent content with respect to processing variables: a simple approach

High-performance biodegradable polymers have attracted considerable attention over the years because of their eco-friendly nature. The effects of processing variables on the efficiency of crosslinking, and the rheological and thermal properties of cross-linked polylactic acid (XPLA) have not been comprehensively addressed yet. In this work, XPLA was prepared through solution casting followed by curing in an oven. Enhancements in properties could be quantified in terms of structural changes in 3D structure of XPLA by varying the amount of dicumyl peroxide (DCP) as a cross--linking agent and curing temperature and time. The XPLAs were characterized by differential scanning calorimetry, thermo-gravimetric analysis, swelling, and rheological techniques. The swelling data revealed an increase in gel fraction by 1.32% per 1 °C temperature rise in the range of 125–195 °C. The results were also indicative of an increase in gel faction by 0.32% per minute in the time range of 5–100 min. Maximum variation in gel fraction occurred at 195 °C with high peroxide content. At this temperature, the variation rate of gel content was about 14.99%. With gel formation evolution, especially at 85% completion stage, the melting point was vanished. Rheological measurements showed that the Newtonian plateau disappeared for the cross-linked samples, simultaneously with the onset of shear thinning and zero-shear viscosity, through which the molecular weight obtained by the Mark–Houwink equation shifted to lower frequencies. A mathematical model based on the Charlesby–Pinner equation was developed for predicting the gel content of the XPLA as a function of curing time and peroxide concentration. The Flory–Huggins parameter also changed during the cross-linking process as a function of cross-linking density. This study is focused on adjusting cross-linking density and processing factors, like temperature and time, to achieve an XPLA with desirable properties.     

Mathematical modelling of enzymatically cross-linked polymer–phenol conjugates using deterministic and stochastic methods

In this study, two different modelling approaches, namely, a deterministic and a stochastic one, are developed to model the enzymatic cross-linking of polymer–phenol conjugates. A comprehensive kinetic mechanism is postulated to describe the elementary reactions in the cross-linking of polymer–phenol chains in the presence of the horseradish peroxidase (HRP)–H₂O₂ initiation system. In the first approach, a moments-based model is derived to account for the conservation of all molecular species and leading moments of the number chain length distribution (NCLD) in the reactive system. In the second approach, a stochastic Monte Carlo kinetic model is formulated to follow the time evolution of a sample of cross-linkable polymer chains and calculate the weight chain length distribution (WCLD). From the numerical solutions of both models, the dynamic evolution of the concentrations of all the reactive species, the gelation onset time, the sol and gel mass fractions as well as the number and weight average molecular weights of the cross-linkable polymer chains are calculated. The two derived models are validated using experimental kinetic measurements on the enzymatic cross-linking of tyramine-modified hyaluronic acid and carboxymethyl-chitin. It is shown that both models can accurately predict the gelation onset time of the two cross-linkable systems over a wide range of variations in HRP and H₂O₂ concentrations. Finally, the MC model predictions on the weight average number of polymer chains in the cross-linked molecules are compared to Flory's analytical solution on the tetrafunctional cross-linking of polymer chains of uniform length.     

Simulation of Impedance Spectra for Core–Shell Grain Structures Using Finite Element Modeling

The volume fraction of core‐ and shell‐regions is an important parameter in the control of temperature‐dependent electrical properties of core–shell‐microstructured electroceramics such as BaTiO₃. Here, we highlight the potential unreliability of using capacitance ratios, obtained by simulating impedance spectra, to extract accurate volume fractions of the two regions. Two microstructures were simulated using a finite element approach: an approximation to a core–shell structure (the encased model) and a series‐layer model (SLM). The impedance response of the microstructures was simulated for a range of input volume fractions. The volume fractions obtained from the simulation agreed with the input values for the SLM microstructure but differed for the encased model. Current density and electric field plots revealed that this discrepancy was caused by differences between the physical and electrical microstructures of the encased model. A stream trace analysis of current density demonstrated that the current follows the path of least resistance through the core, leaving regions of shell with lower current density. These differences are important when attempting to extract volume fractions from encased microstructures with small cores. In the present case, core volume fractions less than 0.7 produce differences in excess of 25%.     

CFD study of droplet atomisation using a binary nozzle in fluidised bed coating

An Eulerian–Lagrangian computational fluid dynamics (CFD) model was built to describe two-fluid atomisation in a tapered fluidised bed coater using the air-blast/air-assisted atomiser model. Atomisation was modelled both with and without the inclusion of the solid phase (i.e. gas–liquid and gas–solid–liquid multiphase modelling). In addition, a multi-fluid flow model (Eulerian–Eulerian framework) combined with a population balance model was used as an alternative approach for modelling the spray produced by a two-fluid nozzle. In this approach, the CFD solver couples the population balance equation along with the Navier–Stokes equations for predicting the droplet diameter and mass fraction distribution. Comparison between simulated spray pattern (gas–liquid model) and that experimentally visualised by means of UV illumination was made and a good agreement was obtained. Parametric studies were done in order to investigate the effects of operating conditions on spray cone and liquid mass fraction inside the reactor. Furthermore, comparison of time-averaged fluidised bed behaviour with the inclusion of sprays obtained by both gas–solid–liquid multiphase modelling methods is presented.