Molecular simulation of structure formation and rheological properties of mixtures of telechelic and monofunctional associating polymer

We study the phase behavior, the characteristic times, and the rheological properties under the steady shear flow of the mixtures consisting of telechelic and monofunctional associating polymers by a coarse‐grained molecular dynamics simulation. The mixtures form the transient networks, the closely packed spherical micelles, and the wormlike micelles. We confirm the molecular origins of the several characteristic times of the mixtures. The dependencies of the characteristic times on the composition ratio between telechelic and monofunctional associating polymers show good agreement with reported experimental results. Under the steady shear flow, the mixtures show the shear thinning induced by the change of the spatial configuration of the micelles. The telechelic associating polymers especially play an important role in connecting the micelles at the shear thinning regime and enhance the steady shear viscosity. Furthermore, at the wormlike micellar region, the mixtures show the second shear thinning initiated by the transformation of the association conformation of the telechelic associating polymers.     

Molecular dynamics simulation on the interaction between polymer inhibitors and anhydrite surface

Investigation on the microscopic interaction between polymer inhibitors and calcium sulfate will be helpful for understanding its scale inhibition mechanism and can provide a theoretical guidance to developing new scale inhibitors. In this work, molecular dynamics simulations with COMPASS force field have been performed to simulate the interaction between hydrolyzed polymaleic anhydride (HPMA), polyaspartic acid (PASP), polyepoxysuccinic acid (PESA), polyacrylic acid (PAA) and the (001) and (020) surfaces of anhydrite (AD) crystal with and without water. The results show that the sequence of binding energies between four polymer inhibitors and AD (001) and (020) with water is PESA > PASP > HPMA > PAA. The binding energy of the same polymer inhibitor on AD (001) is smaller than that on AD (020). Water molecules weaken the deformations of HPMA and PAA but aggravate those of PASP and PESA. Natural bond orbital (NBO) charges of the repeat units of polymer inhibitors were calculated by B3LYP/6‐31G* method. The Coulomb interaction is formed between the O atoms of polymer inhibitors and the Ca atoms of AD crystal. The system of polymer–AD is mainly contributed from the non‐bonding interaction. Polymer inhibitors do not interact directly with AD crystal, but indirectly through the interactions between inhibitor–H₂O and H₂O–AD, i.e. water molecules participate in scale inhibition of polymer inhibitors to AD crystal. Water molecules cannot be ignored when the interaction models are constructed, i.e. solvent effect cannot be ignored. Copyright © 2015 John Wiley & Sons, Ltd.     

Molecular dynamics simulation study of glass transition in hydrated Nafion

The thermal transition of Nafion is studied using a molecular dynamics simulation through a chemically realistic model. Static and dynamic properties of polymer melts with different water contents are investigated over a wide range of temperatures to obtain viscometric and calorimetric glass transition temperatures. The effect of cooling rate of the simulation on the glass transition of the hydrated polymer is also examined within the well‐known Williams–Landel–Ferry (WLF) equation. Variation of relaxation times versus temperature shows a fragile‐to‐strong transition. The hydration level has a significant impact on the static and dynamic properties of the polymer chains and water molecules confined in nanometric spaces between polymer chains. The results of this study are useful to predict the behavior of Nafion for various applications including fuel cells, sensors, actuators, and shape memory devices at different temperatures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 907–915     

Molecular dynamics simulations on the structures and properties of ε-CL-20-based PBXs——Primary theoretical studies on HEDM formulation design

Five polymer bonded explosives (PBXs) with the base explosiveε-CL-20 (hexanitrohexaazaisowurtzitane), the most important high energy density compound (HEDC), and five polymer binders (Estane 5703, GAP, HTPB, PEG, and F2314) were constructed. Molecular dynamics (MD) method was employed to investigate their binding energies (Ebind), compatibility, safety, mechanical properties, and energetic properties. The information and rules were reported for choosing better binders and guiding formulation design of high energy density material (HEDM). According to the calculated binding energies, the ordering of compatibility and stability of the five PBXs was predicted as ε-CL-20/PEG ＞ ε-CL-20/ Estane5703 ≈ε-CL-20/GAP ＞ ε-CL-20/HTPB ＞ ε-CL-20/F2314. By pair correlation function g(r) analyses, hydrogen bonds and vdw are found to be the main interactions between the two components. The elasticity and isotropy of PBXs based ε-CL-20 can be obviously improved more than pure ε-CL-20 crystal. It is not by changing the molecular structures of ε-CL-20 for each binder to affect the sensitivity. The safety and energetic properties of these PBXs are mainly influenced by the thermal capability (C°p) and density (ρ) of binders, respectively.     

Molecular Dynamics Studies on Thermal Behavior of a FAU-type Zeolite

IntroductionZeolitesaregenerallycrystallinealuminosilicates.TheirframeworksarecomposedofinterconnectedTO.(T=Si.Al)tetrahetralnetworkswhichhavenumerousmoleculardimensionalmicropores.Itisknownthatzeoliteframeworkscanvibratethermallyorbedistortedbychangeoftemperatureorthepresenceofsorbedmolecules.Thecatalyticpropertiesofthezeolitesandthestabilityoftheframeworkaregreatlyaffectedbythetemperatures.Itisthusmeaningfultomodeltheflexibleframeworksinasimulationinordertounderstandthepropertiesofthezeol…     

Molecular dynamics simulation of the interaction between methyl benzotriazole and Cu2O crystal

In circulating water system , methyl benzotriazole (TTA) is one of the common corrosion inhibitors for copper. But the inhibition mechanisms have not been clearly understood so far. In different number of water molecules, the interaction between TTA and Cu₂O (copper surface) was investigated with molecular dynamics (MD) method. The results showed that the MD simulation result with water was more consistent with the experiment results. In different number of water molecules, the sequence of the interaction energies between TTA and Cu₂O (001) was ? E ₁ (150H₂O) > ? E ₁(200H₂O) > ? E ₁(100H₂O) > ? E ₁(50H₂O) > ? E ₁(0H₂O). The number of water molecules had an important influence on the interaction between corrosion inhibitors and Cu₂O crystal. From non‐bond energy and pair correlation functions, the interaction energies of the model system were mainly contributed by the non‐bond interaction. Strong adsorption could be raised by the Coulomb interaction between the negatively charged functional groups in TTA and the positive copper ions in the Cu₂O (001) face, and further interaction between aggressive media and copper could be restricted. So, copper corrosion could be avoided. Chemical bonds and non‐bond interactions were formed between TTA and Cu₂O (001) in different number of water molecules. Water molecules could not be ignored during the MD simulation, too. The results obtained here may provide theoretical supports for developing new corrosion inhibitors.     

Molecular modeling of polymer-clay nanocomposite precursors: Lysine in montmorillonite interlayers

The layered structure of clays with interlayer cations leads to unique chemical and mechanical properties, which have been capitalized on in the field of polymer/layered silicate nanocomposites. Hydrophilic silica surfaces can become organophilic with the inclusion of alkylammonium cations, which improve the wetting characteristics of the polymer matrix. In fact, the molecular level interactions of amino acids, either natural or non-natural, with clay surfaces are at the heart of fields of study as diverse as nanocomposites fabrication, drug delivery, bio-remediation of soils and catalysis of biological polymers, to name a few. The ubiquity of these systems and the potential uses to which they could be put suggests the necessity of a deeper understanding of the interplay of bonds, conformations, and configurations between the molecules and the hosts. The interactions of the amino acid lysine with sodium montmorillonite were studied using theoretical molecular modeling methods. The interlayer spacing of montmorillonite was increased by incorporating water molecules and allowing the system to evolve with molecular mechanics. Care was taken to retain the sodium cations in the interlayer. The initial amino acid conformation was obtained surrounding the molecule with numerous discrete water molecules and minimizing the system at the semi empirical level. The optimized amino acid was then placed in the interlayer space in a series of initial positions. Molecular mechanics calculations were performed and the final positions were analyzed. The results tended to indicate the preponderance of configurations which included surface-sodium-amino acid complexes with a variety of spatial arrangements. These results were compared with molecular dynamics calculations of similar systems from the literature.     

Influence of polymer matrix composition and architecture on polymer nanocomposite formation: Coarse-grained molecular dynamics simulation

Coarse-grained molecular dynamics simulations of stacks of two-dimensional platelets immersed in a polymer melt were performed to investigate aspects of the polymer matrix that promote the formation of intercalated or exfoliated nanocomposite structures. Such factors include temperature, copolymer architecture, and blend composition. Increasing the polymer-sheet attractive interaction led to binding of the sheets, where individual beads simultaneously attract two neighboring sheets, thus kinetically blocking intercalation by occupying the perimeter of the affected gallery. Polymers with a small polymer-sheet attraction, but having a strongly attractive chain end (end-functionalized polymers) minimized the bonding of adjacent sheets. These systems exhibited some sheet sliding because a majority of the confined polymer beads only interacted weakly with adjacent sheets; however, the number density of intercalated polymer was low. Mixtures of end-functionalized and nonfunctionalized polymers, however, yielded better intercalation efficiency. For the mixed system, the reduced number of highly attractive beads provided sufficient interaction for intercalation to occur, enabling greater intercalation rates, less sheet-bridging, and incorporation of the nonfunctionalized polymers into the galleries. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3272–3284, 2003     

Molecular dynamics simulation of mechanical properties of TATB/fluorine-polymer PBXs along different surfaces

TATB (1,3,5-triamino-2,4,6-trinitrobenzene) is the well-known high insensitive explosive. With TATB as the main body (90% and above) the polymer bonded explosives ( PBXs) contain a small amount of poly-mers (5%―10%). The composite materials with good saf…     

Multiscale modeling of viscoelastic properties of polymer nanocomposites

A methodology for simple multiscale modeling of mechanical properties of polymer nanocomposites has been developed. This methodology consists of three steps: (1) obtaining from molecular dynamics simulations the viscoelastic properties of the bulklike polymer and approximating the position-dependent shear modulus of the interfacial polymer on the basis of the polymer-bead mean-square displacements as a function of the distance from the nanoparticle surface, (2) using bulk- and interfacial-polymer properties obtained from molecular dynamics simulations and performing stress–relaxation simulations of the nanocomposites with material-point-method simulations to extract the nanocomposite viscoelastic properties, and (3) performing direct validation of the average composite viscoelastic properties obtained from material-point-method simulations with those obtained from the molecular dynamics simulations of the nanocomposites. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1005-1013, 2005     

Molecular dynamics simulation of surface morphology and thermodynamic properties of polyaniline nanostructured film

To develop predictive models in nanostructured films, there is an ongoing research to validate molecular dynamics (MD) simulation results with experimental data. The morphology and surface topography of polyaniline (PANI) nanostructured film coated on a TiO₂ nanocrystalline surface were investigated by scanning electron microscopy and atomic force microscopy, respectively. The atomistic model of the simulated PANI was generated using energy minimization with a condensed‐phase optimized molecular potential for atomistic studies force field function to reach a thermodynamic equilibrium state. Various parameters of PANI such as density, energy, cavity size, and free volume distributions are calculated. MD simulation has also been used to obtain specific volume (V) as a function of temperature (T). It is demonstrated that this V–T curve can be used to determinate glass transition temperature T_g, reliably. Although experimental data available for the PANI film are very limited, simulation results such as density and T_g are in good agreement with the experimental values reported in the literature. Comparison of the surface topography of PANI demonstrates a reasonable trend between atomic force microscopy image analysis and the MD simulation results at various temperatures. Copyright © 2014 John Wiley & Sons, Ltd.     

Molecular dynamics simulation of the free surface of liquid formamide

Molecular dynamics simulations of liquid formamide (HCONH₂) were carried out using the GROMOS software. The formamide molecule is represented by all of its atoms with all internal degrees of freedom. In contrast to other simulations dealing with bulk properties, this study focuses on the interface liquid–vacuum for the first time. We show that the molecular plane is tilted out of the surface, exposing the HCO group to the vacuum. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63 : 1123–1131, 1997     

Molecular dynamics of a calix[4]arene-containing polymer in dichloromethane solution: ability of the solvent molecules to fill the cavity of the macrocycle

The structure and dynamics of the dichloromethane solvent around the calix[4]arene units contained in the molecular actuator poly(calix[4]arene bis-bithiophene) have been examined using a 1-micros molecular dynamics simulation. Results indicate that a solvent molecule fills the cavity associated to the cone conformation of the macrocycle during a significant period of time, especially when the actuator is not contracted. The position of such solvent molecule presents a fourfold symmetry with a maximum orientation toward the center of each ring contained in the calix[4]arene. Frequently, the solvent molecule located inside the cavity is rapidly exchanged for another molecule of the bulk. Thus, the number of dichloromethane molecules that reside more than 0.5 ns inside the cavity is relatively small. Finally, we detected that a significant number of solvent molecules are able to migrate from the cavity of one calix[4]arene to the cavity/ies of other/s, suggesting that the dynamics of the bulk solvent is important for the formation of these inclusion complexes.     

A steered molecular dynamics simulation on the elastic behavior of adsorbed star polymer chains

<正>Elastic behavior of 4-branched star polymer chain with different chain length N adsorbed on attractive surface is investigated using steered molecular dynamics(SMD) simulation method based on the united-atom(UA) model for branched alkanes.The simulation is realized by pulling up the chain via a linear spring with a constant velocity v = 0.005 nm/ps.At the beginning,the chain lies extensionally on adsorbed surface and suffers continuous deformations during the tensile process.Statistical parameters as mean-square radii of gyration S~2_(xy),S~2_z,shape factor δ,describing the conformational changes,sectional density den which gives the states of the chain,and average surface attractive energy U_a,average total energy U,average force f probed by the spring,which characterize the thermodynamic properties, are calculated in the stimulant process.Remarkably,distinguishing from the case in linear chains that there only exists one long plateau in the curve of f,the force plateau in our study for star chains is multiple,denoting different steps of desorption,and this agrees well with the experimental results in essence.We find during the tensile process,there are three characteristic distances Z_c,Z_t and Z_0 from the attractive surface,and these values vary with N.When Z=Z_c,the chain is stripped from the surface,but due to the form of wall-monomer interaction,the surface retains weak influence on the chain till Z = Z_c.From Z=Z_t,parameters U_a,U and f respectively reach a stable value,while the shape and the size of the chain still need adjustments after Z_t till Z_0 to reach their equilibrium states.Specifically,for short chain of N= 41,Z_t and Z_0 are incorporated.These results may help us to deepen the knowledge about the elastic behavior of adsorbed star polymer chains.     

Adsorption of a single polymer chain on a surface: A molecular dynamics simulation study

We performed simulations of the physical adsorption of a single globular chain on a surface of hemispherical shape by means of molecular dynamics simulations. For the chain, we took advantage of a united atom model. Interactions within the chain were limited to stretching, bending, and torsional as well as nonbonded interactions between the nonadjacent atoms. The interaction between each chain element and the surface formation are reigned by a Lennard–Jones potential. In this article, we focused on differences in the behavior of the adsorbed globule to the free unadsorbed one particularly in two different zones of the immediate vicinity of the surface. There were strong indications for a localized acceleration of the dynamics as compared with the bulk that appears in an increase of trans–gauche switches. For explanation we came up with an adsorption scenario. Special attention was given to the shift of the percentage of trans and gauche conformations within the globule in dependence on the strength of the adsorption potential that might be related to crystallization or glass transition. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2333–2339, 2001     

Mechanical and rheological properties of poly(ethylene-co-ethyl acrylate) as a function of carbon black content

Carbon black (CB) filled poly(ethylene-co-ethyl acrylate) (EEA-CB) is used as conductive phase in conductive polymer composites (CPC). Electrical conductivity of CPC obtained from blends of immiscible polymers results from CB particles localization within the material, which depends on composition and processing conditions. It is particularly important to control viscosity of such systems as this parameter determines both the phase morphology and conductive pathway structure. The small scale, at which CB particle aggregation/dispersion phenomena occur within CPC, makes direct observations difficult. But the effect of carbon black particles/polymers interactions is clearly visible by means of rheological measurements. A strong reinforcing effect of CB on CPC in both solid and liquid states has been noticed. This phenomenon has been analyzed using classical models as a function of temperature and CB content. The results confirm the necessity of CPC rheology control especially during the process to achieve good reproducibility of electrical properties.     

Molecular dynamic study of dielectric polarization and ferroelectricity in a model polar polymer

Molecular dynamics simulations are used to explore the polarization response of a lamellar crystal consisting of folded chains of a highly simplified model polar polymer. The system is based on a united atom model of polyethylene with constrained bond lengths and bond angles, and it is endowed with artificial partial charges placed on the united atoms to give it a simple polar character. Simulations performed with various temperatures, electric field directions, and electric field application histories reveal a complicated sequence of reorientation processes, including pronounced ferroelectric behavior. The sequence includes a weak, temperature‐independent prompt response, and a slow‐rising delay regime with stretched exponential behavior and thermally‐activated reorientation parameters consistent with trans‐gauche (TG) barrier crossings in the amorphous phase. When the delay regime has progressed sufficiently, a primary large‐amplitude response due to organized rotation of large subsegments in the crystalline phase occurs in a rapid manner that requires relatively few TG barrier crossings. A final, extremely slow rise in residual polarization completes the sequence. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 740–759     

PETN基PBX结合能和力学性能的理论研究

PETN(季戊四醇四硝酸酯)是著名的硝酸酯类猛炸药,用量子力学(QM)、分子力学(MM)和分子动力学(MD)方法,计算模拟其与高聚物组成的PBX(高聚物粘结炸药)的结合能和力学性能.以AM1-MO法和MM方法取PETN与系列高聚物的尺寸匹配原子簇模型,经几何全优化计算,发现两种方法求得的结合能彼此线性相关.对PETN超晶胞及其与系列氟聚物组成的双组分PBX,实施COMPASS力场下的分子动力学(MD)周期性模拟,求得其弹性系数、拉伸模量、体模量、剪切模量和泊松比,发现添加少量高聚物确能有效改善炸药的力学性能.     

Molecular weight effects on gelation and rheological properties of konjac glucomannan–xanthan mixtures

Fractions of konjac glucomannan (KGM) with various viscosity‐average molecular weights (M_v) ranging from 4.00 × 10⁵ to 2.50 × 10⁶ were prepared by hydrolysis degradation in hydrochloride acid/ethanol. Effect of M_v of KGM on the critical gelation temperature (T_gel) determined by Winter–Chambon criterion and the elasticity of KGM/xanthan mixed gels, a kind of binary gel formed by synergistic gelation, were investigated by dynamic viscoelastic measurements. It was shown that the value of T_gel of the gel was shifted to a higher temperature and the gel strength was enhanced when M_v of KGM was increased. The critical M_v (1.91 × 10⁶) was observed, above which the T_gel and elasticity of the mixed gels showed no or slight increase. It was suggested that T_gel and elasticity of KGM/xanthan mixed gels mainly depend on the structure of junction zones driven by the strong interaction between KGM and xanthan, which was gradually improved with increasing M_v of KGM. It was found that the critical strain and yield stress of the mixed gels increased monotonically with the increasing M_v of KGM. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 313–321, 2010