Multiscale molecular modeling in nanostructured material design and process system engineering

Atomistic-based simulations such as molecular mechanics, molecular dynamics, and Monte Carlo-based methods have come into wide use for material design. Using these atomistic simulation tools, we can analyze molecular structure on the scale of 0.1–10 nm. However, difficulty arises concerning limitations of the time and length scale involved in the simulation. Although a possible molecular structure can be simulated by the atom-based simulations, it is less realistic to predict the mesoscopic structure defined on the scale of 100–1000 nm, for example the morphology of polymer blends and composites, which often dominates actual material properties. For the morphology on these scales, mesoscopic simulations such as the dynamic mean field density functional theory and dissipative particle dynamics are available as alternatives to atomistic simulations. It is therefore inevitable to adopt a mesoscopic simulation technique and bridge the gap between atomistic and mesoscopic simulations for an effective material design. Furthermore, it is possible to transfer the simulated mesoscopic structure to finite elements modeling tools for calculating macroscopic properties for the systems of interest.In this contribution, a hierarchical procedure for bridging the gap between atomistic and macroscopic modeling passing through mesoscopic simulations will be presented and discussed. The concept of multiscale (or many scale) modeling will be outlined, and examples of applications of single scale and multiscale procedures for nanostructured systems of industrial interest will be presented. In particular the following industrial applications will be considered: (i) polymer-organoclay nanocomposites of a montmorillonite–polymer–surface modifier system; (ii) mesoscale simulation for diblock copolymers with dispersion of nanoparticels; (iii) polymer–carbon nanotubes system and (iv) applications of multiscale modeling for process systems engineering.     

Creating new materials with melamine resins

Acid catalyzed condensation of hexa-methoxy methyl melamine (HMMM) in aqueous phase leads to new functional particles and up to now unknown lamellar mesoscopic gels. The investigation with transmission electron microscopy (TEM) shows that the polymer formation starts with nonspherical nanoparticles. Dynamic light scattering experiments reveal a particle size of about 60–100 nm. Atomic force microscopy (AFM) measurements disclose nonuniform flat particles with an aspect ratio of about 0.3. These nanoparticle dispersions form thermoreversible gels. Molecular modeling investigations indicate energy minimized layer-by-layer condensation of the melamine resin molecules. The next step in growth is the nucleation of the nanoparticles via the narrow sides. This forms nonperfect lamellar layers. This time, we get a thermoreversible gel which is fluid at 80 °C and gets fixed at 20 °C. Out of these platelet structures as precursors, a mesoporous, nonthermoreversible gel with essentially lamellar sides and pore sizes about 10 μm is formed. Scanning electron microscopy (SEM) studies show very uniform wall and plate sizes with a directed three-dimensional structure.     

Using modified Pechini method to synthesize α-Al2O3 nanoparticles of high surface area

A modified Pechini method for the preparation of a high surface area α-alumina is proposed. The synthesis of these nanoparticles was carried out using a polymer as a chelating agent. The polymer was prepared from citric acid and acrylic acid by the melt blending method. The resulting α-alumina (98.16%) after calcination at 900 °C consisted of cylindrical nanoparticles of 100–200 nm in length and <25 nm in diameter with a relatively high surface area (18 m² g^?1).     

Multiscale analysis of simultaneous uptake of two reactive gases in the human lungs and its application to methemoglobin anemia

We present a novel multiscale modeling and simulation methodology for quantifying the simultaneous uptake of two reactive gases in the human lungs, and apply it to predict pulmonary hypoxemia in patients suffering from methemoglobin anemia (resulting from excess inhaled nitric oxide (NO)). We start with the convection–diffusion–reaction equations at each scale of the lung and apply a spatial averaging technique (based on Liapunov–Schmidt method of the classical bifurcation theory) to obtain low-dimensional multiscale models. Our simulations for methemoglobin anemia show that while breathing in room air, the O₂ saturation in the patient's hemoglobin falls to below 94% at 50 ppm NO, and above 203 ppm, NO causes severe hypoxemia by reducing the O₂ saturation to below its critical value of 88%. We predict that patients respond to O₂ therapy up to inhaled NO levels of 271 ppm, above which they are candidates for methylene blue therapy.     

Yellow light-emitting polymer bearing fluorescein dye units: Photophysical property and application as luminescence converter of a hybrid LED

Yellow-light emitting polymer particles (spherical shape ∼500–700 nm) were prepared from a solution of polyester bearing fluorescein dye units in tetrahydrofuran, via simple evaporation of the solvent. The morphology of the polymer particles was dependent on the concentration of a base such as triethylamine (TEA), probably due to a change in the balance between interactions among the polymer chains and interactions of the polymer chain with TEA molecules. Thus when a polymer solution containing an appropriate amount of TEA (pH 7.76) was evaporated, the resulting polymer particles became more distinct in the shape and larger in size (∼1–2 μm). A hybrid light-emitting diode (LED) was fabricated, employing the polymer and a blue LED (460 nm) as a luminescence converter and a primary light source, respectively. When the polymer content was 10 wt.% in epoxy, a white emission was observed (1.81 lm at 20 mA).     

An investigation of the parameters effecting the agglomerate size of a PZT ceramic powder prepared with a sol–gel technique

This paper describes the production of Pb_1.0Zr_0.9Ti_0.1O₃ ceramic powder, by using metal organic precursors as starting materials and a polyvinylpyrrolidone (PVP) as an agglomeration control agent. In this study, the effects of water content, aging time and polyvinylpyrrolidone molecular weight on the agglomeration behaviour of the powders were investigated. Least agglomerated powder, with size population ranges 60–200 nm and 200–800 nm, was produced from gel which contained the lower molecular weight of polymer and was unhydrolysed and unaged.     

Short channeled Zr-Ce-SBA-15 supported palladium catalysts for toluene catalytic oxidation

Short channeled Zr-Ce-SBA-15 (ZCS) mesoporous materials were synthesized through hydrothermal routes without addition of mineral acids. 0.5 wt.% palladium was loaded on ZCS and SBA-15 via an ethanol reduction approach. Pd/ZCS possesses unique hexagonal platelet morphologies with short channels running parallel to the thickness in the range of 400–500 nm, while Pd/SBA-15 has a fibrous macrostructure with channels at the micrometer scale. Palladium species present in supports as well dispersed PdO nanocrystals with diameter of ca. 5–6 nm. Comparing with Pd/SBA-15, Pd/ZCS shows enhanced catalytic activity for toluene oxidation, which is ascribed to short channeled supports facilitating the molecular diffusion.     

The influence of a hierarchical porous carbon network on the coherent dynamics of a nanoconfined room temperature ionic liquid: A neutron spin echo and atomistic simulation investigation

The molecular-scale dynamic properties of the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, or [C₄mim⁺][Tf₂N⁻], confined in hierarchical microporous–mesoporous carbon, were investigated using neutron spin echo (NSE) and molecular dynamics (MD) simulations. Both NSE and MD reveal pronounced slowing of the overall collective dynamics, including the presence of an immobilized fraction of RTIL at the pore wall, on the time scales of these approaches. A fraction of the dynamics, corresponding to RTIL inside 0.75 nm micropores located along the mesopore surfaces, are faster than those of RTIL in direct contact with the walls of 5.8 nm and 7.8 nm cylindrical mesopores. This behavior is ascribed to the near-surface confined-ion density fluctuations resulting from the ion–ion and ion–wall interactions between the micropores and mesopores as well as their confinement geometries. Strong micropore–RTIL interactions result in less-coordinated RTIL within the micropores than in the bulk fluid. Increasing temperature from 296 K to 353 K reduces the immobilized RTIL fraction and results in nearly an order of magnitude increase in the RTIL dynamics. The observed interfacial phenomena underscore the importance of tailoring the surface properties of porous carbons to achieve desirable electrolyte dynamic behavior, since this impacts the performance in applications such as electrical energy storage devices.     

Broad-band dielectric response of PbMg1/3Nb2/3O3 relaxor ferroelectrics: Single crystals,ceramics and thin films

Data of the extensive study of dielectric response of relaxor PbMg_1/3Nb_2/3O₃ (PMN) single crystals, ceramics (standard and textured) and thin films (thickness 500 nm, sapphire substrate) in the broad frequency range (3 × 10⁻³ to 10¹⁴ Hz) were combined, summarized and analyzed. Influence of the mesoscopic structure, possible strain and defects in ceramics and thin film on both relaxational and phonon dynamics is discussed. The phonon response of PMN single crystal and thin film appears to be very similar, including the soft mode behaviour. Similar to PMN crystals, the dielectric response of PMN ceramics and films is mainly determined by relaxational dynamics of polar nanoclusters. Flipping and breathing of the clusters are assumed to be the dominant mechanisms, which can be resolved in the frequency spectra of the complex permittivity. The mesoscopic structure and defects in the ceramics do not result in any significant contribution to additional mechanisms, but influence the dynamics of nanoclusters and lead to pinning of the flipping contribution. In thin films the dielectric response due to cluster dynamics is much more reduced.     

How small nanodiamonds can be? MD study of the stability against graphitization

How small nanodiamonds can be is a crucial question for biomedical applications. To answer this question, we present here molecular dynamic simulations of the annealing of very small diamond clusters (diameter between 0.3 and 1.3 nm) of various shape in vacuum and in the presence of oxygen. Isothermal cycles of 30 ps were carried out at 500, 1000, 1500, and 2000 K with 10 ps ramps between them. Predominantly {100} faceted diamond clusters as small as 1 nm (~ 250 atoms) survive these short anneals up to 1500 K. Longer anneals at 1500 K, as well “accelerated” MD at very high temperatures, indicate that the diamond core is still preserved when thermal equilibration is reached. The primary effect of oxygen seems to be the saturation of threefold-coordinated surface carbon atoms and the etching of lower coordinated ones. Oxygen accelerates the graphitization somewhat but does not affect the critical size. Our result means that nanodiamonds with a core of only 0.8 nm can be kinetically stable up to 1500 K. This is significantly less than the lower limit of the thermodynamic stability (~ 1.9 nm).     

Thermal properties of poly(lactic acid) fumed silica nanocomposites: Experiments and molecular dynamics simulations

Poly(lactic acid) (PLA) fumed silica nanocomposites were prepared by twin-screw extruder. Thermal properties were investigated by experiments and molecular dynamics simulations. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used and 1.34 °C increase of the glass transition temperature (T_g) and 12 °C improvement of thermal stability were observed for PLA–silica nanocomposites as compared to neat PLA. Molecular dynamics simulations (NPT ensemble) were carried out using modified OPLS-AA force field, and T_g and root-mean-square radii of gyration (R_g) were calculated. A good agreement between the simulation results and experiments was obtained.     

Structure and hydrogen adsorption properties of low density nanoporous carbons from simulations

We systematically model the hydrogen adsorption in nanoporous carbons over a wide range of carbon bulk densities (0.6–2.4 g/cm³) by using tight binding molecular dynamics simulations for the carbon structures and thermodynamics calculations of the hydrogen adsorption. The resulting structures are in good agreement with the experimental data of ultra-microporous carbon (UMC), a wood-based activated carbon, as indicated by comparisons of the microstructure at atomic level, pair distribution function, and pore size distribution. The hydrogen adsorption calculations in carbon structures demonstrate both a promising hydrogen storage capacity (excess uptake of 1.33 wt.% at 298 K and 5 MPa, for carbon structures at the lower range of densities) and a reasonable heat of adsorption (12–22 kJ/mol). This work demonstrates that increasing the heat of adsorption does not necessarily increase the hydrogen uptake. In fact, the available adsorption volume is as important as the isosteric heat of adsorption for hydrogen storage in nanoporous carbons.     

Design and synthesis of a novel d10–d10 mixed metal-based polymer with superior luminescent properties to select Ca2 + and Zn2 +

A novel luminescent probe based on Ag–Cu mixed metal coordination polymer has been designed and synthesized. Luminescent experiment indicates two peaks with maxima near 425 (Ag) and 660 nm (Cu), which shows high selectivity to Ca^2 + and Zn^2 +.     

A novel,conjugated polymer containing fluorene,pyridine and unsymmetric carbazole moieties: Synthesis,protonation and electrochemical properties

An optically active, conjugated polymer bearing unsymmetric pendant carbazole chromophores was prepared via the Suzuki coupling of 9,9-dioctylfluorene-2,7-diboronic acid and a novel pyridine-containing compound. The polymer had a T_g of 192 °C and T_d10 at 437 °C under a nitrogen atmosphere and exhibited absorption bands at 320–400 nm and displayed an additional absorption bands at 380–480 nm after protonation with aq. HCl solution. The photoluminescence of the polymer shifted from 360–460 nm to 460–560 nm after protonation and the photoluminescence quantum yield of the polymer in THF solution was 0.88. The emission color of the polymer film changed from blue (439 nm) to yellow (551 nm) under an applied bias voltage of 2.5 V.     

Nonlinear optical,poly(amide-imide)–clay nanocomposites comprising an azobenzene moiety synthesised via sequential self-repetitive reaction

Poly(N-acylurea)–clay nanocomposites consisting of a modified montmorillonite and poly(N-acylurea) were prepared from which poly(amide-imide)–clay nanocomposites were subsequently obtained via the sequential self-repetitive reaction of poly(N-acylurea). The moderate T_g of poly(N-acylurea) allows the nonlinear optically active polymer to exhibit high poling efficiency; in situ poling and curing increased the T_gs of poly(amide-imide)–clay nanocomposites. Electro-optical coefficients, r₃₃ of ～17–20 pm/V (830 nm), were achieved; high temporal stability (120 °C) and waveguide optical losses of 3.4–3.9 dB/cm at 1310 nm were also obtained for poly(amide-imide)–clay nanocomposites.     

Confinement effects in irradiation of nanocrystalline diamond

Swift heavy ion irradiation does not generate amorphous tracks in diamond, contrary to what happens in graphite or in diamond-like carbon. Since nanocrystalline diamond is of interest for several technological applications we investigate the reason for this difference, by means of large scale atomistic simulations of ion tracks in nanocrystalline diamond, using a thermal spike model, with up to 2.5 million atoms, and grain sizes in the range 5–10 nm. We conclude that tracking can be achieved under these conditions, when it is absent in single crystal diamond: for 5 nm samples the tracking threshold is below 15 keV/nm. Point defects are observed below this threshold. As the energy loss increases the track region becomes amorphous, and graphitic-like, with predominant sp² hybridization. This higher sensitivity to irradiation can be related to a very large decrease in thermal conductivity of nanocrystalline diamond, due to grain boundary confinement of the heat spike which enhances localized heating of the lattice.     

Dynamic,conformational and topological properties of ring–linear poly(ethylene oxide) blends from molecular dynamics simulations

We present results for the equilibrium conformational and dynamic properties of ring–linear poly(ethylene oxide) (PEO) blends from detailed molecular dynamics (MD) simulations with a thoroughly validated and very accurate forcefield. The simulations have been performed in the isothermal–isobaric (NPT) statistical ensemble with blends where the two types of chains (ring and linear) have the same size. Simulations with two different chain lengths, corresponding to molecular weights equal to 1800 and 5000 g/mol, allowed us to study the dependence of these properties on molecular length. Overall, the presence of linear chains seems to considerably slow down the orientational relaxation of ring molecules and to lower their diffusivity, to a degree that depends strongly on chain length and level of contamination of the melt by linear chains. The longer the size of the molecules the more pronounced the decrease in ring diffusivity is, at a given mass fraction of linear chains. To explain the reduction in the relaxation and mobility of ring molecules when they mix with linear chains to form a blend, selected configurations from the MD simulations were subjected to a detailed topological analysis which revealed significant threading of the cyclic molecules by the linear ones. Our simulation data indicate that, due to threading, ring dynamics in ring–linear PEO melts is strongly heterogeneous. An analysis of the statistics of the lifetimes of ring–linear topological constraints (TCs) reveals a long tail on the long time scale, demonstrating that many of these TCs are long-lived. By inspecting individual ring–linear PEO pairs we found that, in many cases, the lifetimes of these TCs are up to one order of magnitude larger than the typical time characterizing ring relaxation in the pure ring melt. This phenomenon was more pronounced in the blend with the longer molecules (molecular weight = 5000 g/mol).     

Sinter forging of rapidly quenched eutectic Al2O3–ZrO2(Y2O3)-glass powders

Spray dried agglomerates of Al₂O₃–ZrO₂ (1% Y₂O₃) with 4 wt.% borosilicate glass were arc plasma sprayed and rapidly quenched into water. Because of the rapid quenching the particles <25 μm were mostly amorphous. After annealing 1 h at 1200 °C the scale of the microstructure of the particles was on the order 30 nm. Hot forging of this powder yielded dense specimens with the width of the ZrO₂ phase still less than 100 nm. Since the particle size ranged from 5 to 25 μm and the scale of the particle microstructure was <100 nm, densification was controlled by creep of the particles rather than by the typical hot pressing mechanism of diffusion along the neck between particles to fill the pores. Thus, the scale of the microstructure controls densification rather than the particle size. These powders offer an alternate source for manufacturing nanostructured parts and should be more suitable for hot pressing or forging than nanoparticulate powders.     

Diazobenzene chromophore-doped silica films with large two-photon absorption cross-section

The diazobenzene chromophore, C.I Direct Red 28 (Congo Red) was modified using isocyanatopropyl silane and subsequently incorporated into a silica matrix, which significantly increased the thermal stability of the dye. The third-order nonlinearity of the ensuing hybrid dye–silica film was investigated using Third Harmonic Generation and Z-scan measurements at 1064 nm and 400 nm. The observed high, nonlinear refractive index (up to 10^?8 esu) of the film, as well as the large two-photon absorption cross-section at 1064 nm, was attributed to the enlarged and strengthened conjugated electron system as well as the symmetrical molecular structure of the hybrid material; optical Kerr studies showed that the hybrid films had an ultrafast response time of 0.36 ps at 400 nm.     

Synthesis,characterization, and photophysics of electroluminescent fluorene/dibenzothiophene- and fluorene/dibenzothiophene-S,S-dioxide-based main-chain copolymers bearing benzimidazole-based iridium complexes as backbones or dopants

A series of electroluminescent copolymers containing fluorene-2,8-disubstituted dibenzothiophene (PFD), fluorene-2,8-disubstituted dibenzothiophene-S,S-dioxide (PFDo) and phosphorescent benzimidazole-based iridium (Ir) complexes in the backbones were synthesized by the Suzuki coupling reaction. The thermal stabilities, HOMO/LUMO levels and triplet energy gap (E_T) values were enhanced with increasing contents of dibenzothiophene (D) or dibenzothiophene-S,S-dioxide (Do) segments in the copolymers. The relative intensities of phosphorescence and fluorescence were affected by the energy transfer and back transfer efficiencies between the polymer backbones and iridium units as evidenced by solid state PL and EL spectra. PLED devices with a configuration of ITO/PEDOT:PSS (50 nm)/metal-free copolymers (P1–P5), Ir-copolymers (P7–P13) and Ir-doped copolymers (P3 doped with Ir-complexes 6 and 8) (60–80 nm)/TPBI (40 nm)/LiF (1 nm)/Al (120 nm) were fabricated, and the electroluminescence (EL) efficiencies depended on the chemical constituents and triplet energies of the copolymers. The space-charge-limited current (SCLC) flow technique was used to measure the charge carrier mobilities of these copolymers, where both hole and electron mobilities were in the following order: the metal-free copolymers (P2, P3 and P5) > Ir-doped copolymers (P3 + 3 or 10 mol% Ir-complex 6) > Ir-copolymers (P7, P8, P12 and P13).