Miscibility,Crystallization, and Morphology of the Double-Crystalline Blends of Insulating Polyethylene and Semiconducting Poly(3-Butylthiophene)

A series of crystalline semiconducting poly(3-butylthiophene) (P3BT)/crystalline insulating polyethylene (PE) blends were prepared and the miscibility, crystallization, and structure/morphology were investigated. Even though phase separation was observed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS), several pieces of evidence indicated that limited miscibility should be present in PE/P3BT blends: small changes in both T_m and crystallinity of PE phase and a small portion of PE being dissolved in P3BT. The study of PE isothermal crystallization kinetics revealed that the introduction of P3BT significantly influenced the nucleation mechanism and growth geometry, i.e., PE was transformed from three-dimensional (3D) spherulitic to two-dimensional (2D) disc crystals. A striking reduction of nucleation density and an obvious ringed morphology of PE spherulites (2D) in PE/P3BT blends were also observed by polarized optical microscopy; it is proposed that the limited miscibility between PE and crystalline P3BT favors the formation of ringed PE spherulite in the blends. Additionally, preferred orientation of PE lamellae, with their b-axis largely constrained to the thin film plane, was observed by X-ray diffraction in PE/P3BT blend films. It is evidenced that the PE orientation was due to the b-axis being the crystal growth direction, which can only be in film plane.     

Investigation of the interface between polyethylene and functionalized graphene: A computer simulation study

The effect of surface chemical functionalization of a single graphene layer on its thermodynamic work of adhesion (WA) with polyethylene (PE) chains has been investigated using molecular dynamics (MD) simulation. For this purpose, amine (NH₂), carboxyl (COOH), hydroxyl (OH), and methyl (CH₃) functional groups were distributed randomly throughout the graphene surface using a Monte Carlo (MC) algorithm to achieve graphene functionalized structures with minimized potential energies. The MD simulation results showed that the thermodynamic WA between the PE and the functionalized graphene was larger than that between the PE and the pristine graphene. In fact, the electronegativity of functional groups and Van der Waals forces play influential roles in the thermodynamic WA between the PE and the functionalized graphene. In addition, the amount of thermodynamic WA was increased with increasing the functional group surface density, except for the graphene functionalized with the methyl groups. The segmental density of the PE chains near the single sheet surface was determined based on the density profile calculation. The polymer segments exhibited strong ordering and sharp density variations near the PE/graphene interface. The dynamic of chains was quantitatively characterized by calculating mean square displacement (MSD). Furthermore, the influence of functionality on the glass transition temperature (T_g) of the PE at the PE/graphene interface region was investigated. The results showed that the T_g at the PE/graphene interface was much higher than that of the bulk polymer. In fact, the functionalization of the graphene surface seems to considerably enhance the T_g of the polymer due to lowering the chains mobility.     

The effect of polymer type on electric breakdown strength on a nanosecond time scale

Based on the concepts of fast polarization, effective electric field and electron impact ionization criterion, the effect of polymer type on electric breakdown strength (E_BD) on a nanosecond time scale is investigated, and a formula that qualitatively characterizes the relation between the electric breakdown strength and the polymer type is derived. According to this formula, it is found that the electric breakdown strength decreases with an increase in the effective relative dielectric constants of the polymers. By calculating the effective relative dielectric constants for different types of polymers, the theoretical relation for the electric breakdown strengths of common polymers is predicted. To verify the prediction, the polymers of PE (polyethylene), PTFE (polytetrafluoroethelene), PMMA (organic glass) and Nylon are tested with a nanosecond-pulse generator. The experimental result shows E_BD (PTFE) > E_BD (PMMA) > E_BD (Nylon) > E_BD (PE). This result is consistent with the theoretical prediction.     

Molecular dynamics simulations on the direct sputtering of Al2O3 insulating film in a magnetic tunneling junction

The molecular dynamics (MD) simulation is used to study the direct deposition of aluminum-oxide (Al₂O₃) to grow an insulating thin film on cobalt substrate in a magnetic tunneling junction (MTJ). The direct deposition of Al₂O₃ may produce a very thin film with very smooth surface and form an averagely oxidized insulator. A high magnetoresistance ratio is desired in an MTJ device, so that smooth surface and good structural integrity for the insulating thin film is anticipated. The MD simulation is an effective way in determining optimal processing parameters that can be used to fabricate high-quality MTJ devices. The Buckingham and many-body tight-binding potentials are applied in the MD simulations for different interactions between particles. The effects of the cluster size and the incident energy on the thin film’s surface morphology and Al₂O₃ purity are investigated. Some optimal parameters that could be used as the reference for practical processing purposes are derived.     

Theoretical investigations of rotational phenomena and dielectric properties in a nematic liquid crystal

The molecular dynamics (MD) simulation, based on a realistic atom-atom interaction potential, was performed on 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic phase. The rotational viscosity coefficients (RVCs) γ _i, (i = 1, 2) and the ratio of the RVCs λ = - γ ₂/γ ₁ were investigated. Furthermore, static and frequency-dependent dielectric constants and ε were calculated using parameters obtained from the MD simulation. Time correlation functions were computed and used to determine the rotational diffusion coefficient, D _⊥. The RVCs and λ were evaluated using the existing statistical-mechanical approach (SMA), based on a rotational diffusion model. The SMA rests on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMA, γ _i are dependent on the orientational order and rotational diffusion coefficients. The former was characterized using: i) orientational distribution function (ODF), and ii) a set of order parameters, both derived from analyses of the MD trajectory. A reasonable agreement between the calculated and experimental values of γ _i and λ was obtained. Received 22 March 2000 and Received in final form 8 October 2000     

Atomic-level investigation of Al and Ni thin film growth on Ni(1 1 1) surface: Molecular dynamics simulation

Using molecular dynamics (MD) simulation, the structural characteristics of Al and Ni thin film growth on Ni(1 1 1) substrate according to the incident energy of adatoms were investigated. In case of Al on Ni(1 1 1), Al adatoms were grown basically through the layer-by-layer growth mode. On the other hand, Ni thin films on Ni(1 1 1) surface at low incident energy were shown to favor island growth. The steering effect due to atomic attraction, which results in rougher surface, was significantly observed at low incident energy. The growth mode of Ni film was, however, changed to follow layer-by-layer growth mode for the incident energy of 6 eV. The different aspects of surface morphology between Al and Ni deposition on Ni(1 1 1) surface could be successfully explained by the surface diffusion and impact cascade diffusion.     

The effect of polymer type on electric breakdown strength on a nanosecond time scale

Based on the concepts of fast polarization,effective electric field and electron impact ionization criterion,the effect of polymer type on electric breakdown strength(E BD) on a nanosecond time scale is investigated,and a formula that qualitatively characterizes the relation between the electric breakdown strength and the polymer type is derived.According to this formula,it is found that the electric breakdown strength decreases with an increase in the effective relative dielectric constants of the polymers.By calculating the effective relative dielectric constants for different types of polymers,the theoretical relation for the electric breakdown strengths of common polymers is predicted.To verify the prediction,the polymers of PE(polyethylene),PTFE(polytetrafluoroethelene),PMMA(organic glass) and Nylon are tested with a nanosecond-pulse generator.The experimental result shows E BD(PTFE) > E BD(PMMA) > E BD(Nylon) > E BD(PE).This result is consistent with the theoretical prediction.     

Prediction of ion channel transport from Grote—Hynes and Kramers theories

Generalized Langevin equation based Grote—Hynes (GH) theory and Langevin equation based Kramers theory are used to calculate the transmission coefficient for K⁺ diffusion through a model of the biological potassium ion channel IRK1, which contains a high potential barrier in the selectivity filter. The ion friction kernel is determined from a molecular dynamics (MD) simulation of the force on a stationary ion at the barrier top. The GH and Kramers estimates of the transmission coefficient are compared with those obtained from MD simulations of ion diffusion at the barrier top of the IRK1 channel. It is found that the GH estimate agrees with the value determined by rigorous MD, but the Kramers estimate is about 40% too small. The success or failure of GH and Kramers theories for various other systems is discussed and compared with these results.     

Molecular dynamics studies of the interaction between water and oxide surfaces

The water-surface interaction is a research target of great importance for a broad spectrum of technological applications and fundamental scientific disciplines. In the present study, a comparative analysis is performed to clarify the structural and diffusion properties of water on a number of oxide surfaces. Based on the molecular dynamics (MD) simulation method, the water-surface interaction mechanism was investigated for the oxide materials TiO₂ (anatase), Al₂O₃ (corundum), and Fe₂O₃ (hematite). A comparison of the water-TiO₂ interaction with the water-Al₂O₃ and water-Fe₂O₃ systems demonstrates the specificity of the adsorption and layer formation on the atomic/molecular level scale. The obtained MD analysis data point to a considerable enhancement of water-TiO₂ surface adsorption and a relatively high density distribution profile near the surface. The novel data on water structure and diffusion on oxide surfaces are discussed from the point of view of possible material innovation and design.     

Vibrational spectrum at a water surface: a hybrid quantum mechanics/molecular mechanics molecular dynamics approach

A hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulation is applied to the calculation of surface orientational structure and vibrational spectrum (second-order nonlinear susceptibility) at the vapor/water interface for the first time. The surface orientational structure of the QM water molecules is consistent with the previous MD studies, and the calculated susceptibility reproduces the experimentally reported one, supporting the previous results using the classical force field MD simulation. The present QM/MM MD simulation also demonstrates that the positive sign of the imaginary part of the second-order nonlinear susceptibility at the lower hydrogen bonding OH frequency region originates not from individual molecular orientational structure, but from cooperative electronic structure through the hydrogen bonding network.     

Computational analysis of effect of modification on the interfacial characteristics of a carbon nanotube-polyethylene composite system

In this study, the non-covalent association of single-walled nanotube (SWNT) with polyethylene (PE) molecule and the influence of sidewall modification on the interfacial bonding between the SWNTs and polymer were investigated using molecular mechanics (MM) and molecular dynamics (MD) simulations. The model of interaction between the initially separated PE and SWNT fragments, which can be either wrapping or filling, was computed. The possible extension of polymers wrapping or filling SWNTs can be used to structurally bridge the SWNTs and polymers to significantly improve the load transfer between them when SWNTs are used to produce nanocomposites. The interfacial bonding characteristics between the single-walled nanotubes, on which -COOH, -CONH₂, -C₆H₁₁, or -C₆H₅ groups have been chemically attached, and the polymer matrix were also investigated by performing pullout simulations. The results show that appropriate functionalization of nanotubes at low densities of functionalized carbon atoms drastically increase their interfacial bonding and shear stress between the nanotubes and the polymer matrix, where chemisorption with -C₆H₅ groups to as little as 5.0% of the nanotube carbon atoms increases the shear stress by about 1700%. Furthermore, this suggests the possibility to use functionalized nanotubes to effectively reinforce other kinds of polymer-based materials as well.     

Molecular dynamics study of self-diffusion in Zr

We employed a recently developed semi-empirical Zr potential to determine the diffusivities in hcp and bcc Zr via molecular dynamics simulation. The point defect concentration was determined directly from molecular dynamics (MD) simulation rather than from theoretical methods using T = 0 calculations. Our MD simulation indicates that the diffusion proceeds via the interstitial mechanism in hcp Zr, and both vacancy and interstitial mechanisms contribute to diffusivity in bcc Zr. The agreement with the experimental data is excellent for hcp Zr and rather good for bcc Zr at high temperatures, but there is considerable disagreement at low temperatures.     

Displacive and order-disorder behavior of KDP-type ferroelectrics on the local scale

The modified strong dipole-proton coupling (MSDPC) model, which predicted several static and dynamic dielectric properties of KH₂PO₄ or KDP-type ferroelectrics, was used to investigate the properties of these crystals on the local scale. Results calculated by molecular dynamics (MD) simulation show that both order-disorder and displacive characteristics of one PO₄ dipole are present in KDP and KD₂PO₄ (DKDP). These results correlate with experimental data from NMR and neutron scattering studies of local properties.     

Prediction of Structure and Energy of Trans-1,4-Polybutadiene Glassy Surface by Atomistic Simulations of Free-Standing Ultrathin Films

Atomistic modeling of amorphous trans-1,4-polybutadiene (TPBD), using molecular mechanics and molecular dynamics (MD) simulations, is performed to generate three-dimensionally periodic bulk and two-dimensionally periodic thin film condensed phases. The condensed structures are constructed using multiple polymer chains. Structural and energetic relaxations and sampling of properties are performed using MD in the canonical ensemble (NVT) by a procedure that relieves local high-energy spots and brings the system to realistic thermodynamic states. The calculated surface energy for TPBD, 30.72 erg/cm², is in excellent agreement with the reported experimental value of 31 erg/cm². The structure of the surface layers is probed in terms of the atomic mass density variations, bond-bond orientation function profiles, and the distribution of the dihedral angles about the rotatable backbone bonds. The thickness of the surface layer over which the density varies smoothly but rapidly is found to be approximately 15 Å. The level of agreement of the calculated surface energy with the experimental value is superior in comparison to previous investigations in the literature using the atomistic approach for flexible polymers.     

MECHANISMS OF TOUGHNESS IMPROVEMENT OF SEMI-CRYSTALLINE POLYMERS

Many normally ductile semi-crystalline polymers, such as polyamide-6 (or 6,6) (Nylon), high density polyethylene (HDPE), and isotactic polypropylene (iPP) are known to be brittle under impact loading.

Some general principles for improvement of toughness of semi-crystalline polymers that rely on the reduction of plastic resistance of the cavitated matrix are presented. As typical cases, the mechanisms of the dramatic toughness jumps achievable in both Nylon and HDPE through the incorporation of either flexible rubbery particles or rigid CaCO₃ particles are discussed. In both cases these result in the establishment of a material component of reduced plastic resistance in the form of a layer of oriented crystallization of well-defined thickness around the particles. The toughness jumps occur when the average interparticle ligament thickness is reduced below a critical value, specific to the particular polymer, and the component of reduced plastic resistance percolates through the structure.     

F8BT薄膜表面形貌及与Al形成界面的电子结构和反应

基于共轭聚合物光电器件的性能与聚合物的表面形貌、分子取向、以及与金属电极形成的界面结构密切相关. 本文利用原子力显微镜(AFM)、同步辐射光电子能谱(SRPES)和近边X射线吸收精细结构谱(NEXAFS)等, 研究了聚(9, 9-二辛基芴并苯噻二唑)(F8BT)薄膜的表面形貌、分子取向及其与Al 电极形成界面过程的结构变化. 结果表明, 在略低于F8BT玻璃转变温度(T_g=130 ℃)条件下对F8BT薄膜进行退火, 可明显增加薄膜的表面粗糙度, 薄膜中F8BT 的分子取向角约为49°, 9, 9-二辛基芴单元(F8)与苯噻唑单元(BT)几乎在同一平面. 在Al/F8BT 界面形成过程中, Al与F8BT中的C, N和S均发生不同程度的化学反应, 并导致价带结构和未占据分子轨道(LUMO)态密度的变化. Al对F8BT进行n型掺杂引起F8BT能带弯曲的同时, 未占据能级被部分占据, 更多的电子将被注入到LUMO+1中. 通过考察价带电子结构、芯能级位移及二次截止边的变化, 绘制了清晰的Al/F8BT界面能级图.     

Molecular modeling study on inhibition performance of imidazolines for mild steel in CO2 corrosion

Corrosion inhibiting performance of 1-hydroxyethyl-2-heptadecylimidazoline (A) and 1-aminoethyl-2-heptadecylimidazoline (B) for mild steel was evaluated by combination of quantum chemistry calculation, molecular mechanics, and molecular dynamics simulation. The calculated results by quantum chemistry method demonstrated that frontier orbitals of A and B molecules are mainly located on imidazoline rings, and molecule B possesses higher reactivity than molecule A. The calculated results by molecular mechanics and molecular dynamics simulation presented that these two inhibitor molecules could form dense and high-coverage membranes to prevent diffusion of reactive corrosive species to metal surface. Furthermore, the adsorption energy, cohesive energy, and adsorption angle demonstrated that the binding affinity and stability of B membrane was remarkably greater than that of A, which indicated that B had better inhibition performance in CO₂ corrosion. The calculated results were well accorded with previous reported experimental results. These researches implied that molecular modeling might be an effective approach to assess inhibition performance, which has potential application in design of new inhibitors.     

The effect of surface modification by nitrogen plasma on photocatalytic degradation of polyvinyl chloride films

The solid-phase photocatalytic degradation of poly(vinyl chloride) (PVC) films was investigated under the ambient air in order to assess the feasibility of developing photodegradable polymers. Nitrogen plasma was used to modify PVC films to enhance the photocatalytic degradation of PVC with nano-sized anatase TiO₂. The plasma parameter varied in this study is discharge power from 30 to 120 W for a constant treatment time of 60 s and a constant gas pressure of 10 Pa. The photodegradation of the plasma-treated PVC-TiO₂ films was compared with that of pure PVC films and PVC-TiO₂ films performing weight loss monitoring, scanning electron microscopy (SEM) analysis, contact angle measurements, electron spin resonance (ESR), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The wettability of the plasma-treated PVC is improved significantly. ESR revealed that the signal of radicals on the surface of the plasma-treated PVC film was enhanced after the treatment. Furthermore, the weight loss indicated that TiO₂ speeds up the photocatalytic degradation of PVC chains. The SEM image of the plasma-treated PVC-TiO₂ film showed a lot of crack on the film surface after irradiation. XPS indicated that the C and Cl atomic concentration reached minimum values on the surface of plasma-treated PVC-TiO₂ under identical photocatalytic condition. The experimental results reveal that plasma treatment can obviously enhance the photocatalytic degradation of PVC.     

Numerical simulation on the diffusion behavior of penetrant molecules in poly-<Emphasis Type="Italic">p</Emphasis>-xylylene and its chlorine substituent polymers

Models of poly-p-xylylene and its chlorine substituent polymers were established and structural optimized with Molecular Dynamics (MD) in Material Studio (MS) software. The densities and glass transition temperatures (T _g) calculated from the optimized models were consistent very well with the experimental data. The free volumes and their distributions in these polymeric models were obtained using the Atom Volumes and Surfaces tool. The results showed that the amount of free volume for each model was very small, especially for those free volume cavities of diameter larger than 2.8 Å, which made it difficult for the molecules of diameter larger than 2.8 Å penetrating in the polymer. The diffusion, solubility and permeability coefficients of some gaseous molecules in these models were also calculated based on the combined methods of MD and Great Canonical Monte Carlo (GCMC), which presented the same or 1–2 higher order of magnitude with experimental values. The trajectories of penetrant molecules diffusing in polymeric models demonstrated that the molecules transferred or moved in amorphous regions between spherical crystals with a “hopping diffusion” behavior.     

Dynamical structure of α-Ag0.99Cu0.01I crystal by Cu NMR chemical shift, spin-lattice relaxation time and molecular dynamics simulation

Solid ⁶³Cu NMR and Molecular Dynamics (MD) methods have been used to investigate the dynamical structure of Ag_0.99Cu_0.01I crystal, through the viewpoint of the chemical shift and the spin-lattice relaxation. In the superionic phase (α-phase), we obtained the temperature variation of the chemical shift as −0.2 ppm/K, and the activation energy of the Cu⁺ ion diffusion as 11 kJ/mol. The temperature dependence of the chemical shift was explained by the calculated chemical shielding surface based on ab initio MO calculation, and by the probability density of Cu⁺ ion estimated by MD simulation. The spin-lattice relaxation was also analyzed by using the MD method in which we assumed two jumping models as the diffusion process of the mobile cations. It is concluded that the temperature dependence of the chemical shift is dominated by the shielding in the vicinity of the 24(h) site, and the observed activation energy is due to the diffusion process of the mobile cations jumping from the 6(b) intrasub-lattice to the nearest-neighbor 6(b) sub-lattice.