Search

As a glycerol-based polyester, poly(glycerol azelaic acid) has shown great potential for biomedical applications, such as tissue engineering.

As a glycerol-based polyester, poly(glycerol azelaic acid) (PGAz) has shown great potential for biomedical applications, such as tissue engineering. However, it tends to show low mechanical strength and a relatively fast biodegradation rate, limiting its capability of mimicking and supporting a broad range of hard tissues such as bone. Moreover, the typical thermal curing process of poly(glycerol

A combined experimental and molecular dynamics (MD) simulation approach was used to investigate the effects of the nanoconfinement of a highly CO2/CH4-selective ionic liquid (IL), 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]), in porous poly(vinylidene fluoride) (PVDF) matrices on the gas separation performance of the resulting membranes. The observed experimental CO2/CH4 permselectivity increased by about 46% when the nominal pore diameter in PVDF, which is a measure of nanoconfinement, decreased from 450 to 100 nm, thus demonstrating nanoconfinement improvements of gas separation. MD simulations corroborated these experimental observations and indicated a suppression in the sorption of CH4 by [EMIM][SCN] when the IL nanoconfinement length decreased within the nonpolar PVDF surfaces. This is consistent with the experimental observation that the CH4 permeance through the IL confined in nonpolar PVDF is significantly less than the CH4 permeance through the IL confined in a water-wetting polar formulation of PVDF. The potential of mean force calculations further indicated that CO2 has more affinity to the nonpolar PVDF surface than CH4. Also, a charge/density distribution analysis of the IL in the PVDF-confined region revealed a layering of the IL into [EMIM]- and [SCN]-rich regions, where CH4 was preferentially distributed in the former and CO2 in the latter. These molecular insights into the nanoconfinement-driven mechanisms in polymer/IL membranes provide a framework for a better molecular design of such membranes for critical gas separation and CO2 capture applications.

Drug stability and retention on nanocarriers is essential for maximizing the drug targeting and therapeutic efficiency. PEGylation of graphene oxide (GO) as a drug nanocarrier is widely known to prolong its circulation time in the body, thereby increasing the probability of drug delivery system interactions with the proteins in the blood stream. Herein, molecular dynamics (MD) simulations were performed to investigate the interactions between doxorubicin (DOX)-loaded GO and PEGylated GO (PEGGO) nanocarriers with human serum albumin (HSA), a prevalent human blood protein and among the first to be adsorbed on the DOX-loaded nanocarriers. The results indicate that drug stability and retention on PEGGO nanocarriers are far more superior to the GO nanocarriers (control) when in contact with HSA. It is also demonstrated in this work that the PEGGO nanocarriers retain the DOX molecules irrespective of the HSA Sudlow site I and II orientations, thereby revealing their robustness in DOX loading.

A combined experimental and molecular dynamics (MD) simulation approach was used to investigate the effects of the nanoconfinement of a highly CO

Hyperbranched polyethyleneimine functionalized silica (PEI-SiO

Three-dimensional graphene (3DGr) is explored as an unconventional support material for supported ionic liquid membranes (SILMs) in gas separations. Herein, molecular dynamics/grand canonical Monte...

All-atom Molecular Dynamics (MD) simulations were employed to investigate the structural and free volume evolution (correlated with damage) during creep of model amorphous polyethylene (PE) at various applied stress states (tension, shear, compression), stress levels (10–200 MPa), and temperatures (175–325 K). The Modified Embedded-Atom Method for saturated hydrocarbons is applied to show that the phenomenological macroscale creep response of PE can be captured through MD simulations. The model adequately predicts the three classical stages of creep (primary, secondary, and tertiary) and provides detailed insight into the underlying molecular mechanisms. The calculated glass transition temperature (Tg) was found to be very close to the experimental Tg. Simulations were performed at temperatures below Tg (175 K) to above Tg (325 K) and demonstrate that the transition from glassy to rubbery state is reflected in the chain dynamics and damage evolution. Under all the stress states and temperatures simulated, the evolution of void volume, nucleation, growth, and coalescence are shown to directly correlate with specific stages of the creep response and the underlying chain dynamics within each stage. A correlation between the steady-state creep rate and steady-state void nucleation rate is found, suggesting that secondary creep is heavily driven by void nucleation, while tertiary creep is driven by void growth and coalescence.

Data mining and knowledge discovery techniques were employed herein to acquire new information on the viscoelastic, flexural, compressive, and tensile properties of vapor-grown carbon nanofiber (VGCNF)/vinyl ester (VE) nanocomposites. Formulation and processing factors (curing environment, presence or absence of dispersing agent, mixing method, VGCNF weight fraction, VGCNF type, high-shear mixing time, and sonication time) and testing temperature were utilized as inputs and the true ultimate strength, true yield strength, engineering elastic modulus, engineering ultimate strength, flexural modulus, flexural strength, storage modulus, loss modulus, and tan delta were selected as outputs. The data mining and knowledge discovery algorithms used in this study include self-organizing maps (SOMs) and clustering techniques. SOMs demonstrated that temperature and tan delta had the most significant effects on the output responses followed by the VGCNF high-shear mixing time, and sonication time. SOMs were also used to produce optimal responses using certain combination(s) of inputs. Fuzzy C-means algorithm (FCM) was also applied to discover patterns in the nanocomposite behavior subsequent to a principal component analysis (PCA), which is a dimensionality reduction technique. Utilizing these techniques, the nanocomposite specimens were separated into different clusters based on the testing temperature (30 °C and 120 °C being the most dominant responses), tan delta, high-shear mixing time, and sonication time. Furthermore, the VGCNF/VE specimens were separated into a cluster based on their viscoelastic responses (storage and loss moduli) at the same temperature. The FCM results indicate that, while all nanocomposite properties in the new framework are essential, the viscoelastic responses of the VGCNF/VE specimens are the most significant. This work highlights the utility of data mining and knowledge discovery techniques in the context of materials informatics for the discovery of patterns and trends in the material behavior that are not immediately known.

Molecular dynamics (MD) simulations were performed to investigate the loading and dynamics of doxorubicin (DOX) anticancer drug on graphene oxide (GO) and poly(ethylene glycol) (PEG) decorated GO (PEGGO) nanocarriers in an aqueous environment at human body temperature (310 K) and physiological pH level of 7.4. Mechanisms of DOX adsorption on PEGGO as a function of PEG chain length were revealed. While the total DOX-nanocarrier interaction energy was the same for the DOX/GO (control), DOX/Sh-PEGGO (short PEG chains consisting of 15 repeat units), and DOX/L-PEGGO (long PEG chains consisting of 30 repeat units) within the margin of error, the PEG-DOX interactions increased with an increase in the PEG chain length. At the same time, the PEG-DOX solvent-accessible contact area almost doubled going from the short to long PEG chains. PEGylation of the GO effectively causes an increase in the average water density around the nanocarrier, which can act as a barrier, leading to the DOX migration to the solvated PEG-free part of the GO surface. This effect is more pronounced for shorter PEG chains. The DOX-DOX solvent-accessible contact area is smaller in the DOX/GO system, which means the drug molecules are less aggregated in this system. However, the level of DOX aggregation is slightly higher for the PEGGO systems. The computational results in this work shed light on the fact that increasing the PEG chain length benefits DOX loading on the nanocarrier, revealing an observation that is difficult to acertain through experiments. Moreover, a detailed picture is provided for the DOX adsorption and retention in PEGGO drug delivery systems, which would enable the researchers to improve the drug's circulation time, as well as its delivery and targeting efficiency.

We introduce highly antifouling Polymer-Nanoparticle-Nanoparticle/Polymer (PNNP) hybrid membranes as multi-functional materials for versatile purification of wastewater. Nitrogen-rich polyethylenimine (PEI)-functionalized halloysite nanotube (HNT-SiO

Nonreactive molecular dynamics simulations were performed to determine the thermal conductivities and through-thickness temperature profiles of unprotected cross-linked epoxy, as well as protected epoxy with graphene (Gr) and montmorillonite (MMT)/Gr surface coatings against lightning strike damage. Three representative hot surface temperatures of 500, 1000, and 10000 K were used for thermal analysis. The MMT/Gr double-layer coating provided the most efficient thermal shielding of the epoxy sublayer, and the epoxy/MMT/Gr system exhibited a 55% lower thermal conductivity than the neat epoxy. The results imply that the MMT/Gr double-layer coating may be used for lightning strike protection.

Several groups of polyimide (PI)-based nanomaterials reinforced with polyhedral oligomeric silsequioxane (POSS) nanoparticles were subjected to atomic oxygen (AO) exposure to investigate the effects of POSS and glassification plasma pre-treatment. Various characterizations revealed the clear effects of AO degradation, such as decreased transmissions of all tested films. POSS significantly enhanced the stability of PI in terms of mass loss under an AO environment. One polyimide film sample containing 10 wt% POSS also exhibited excellent stability in mechanical properties measured by dynamic mechanical analyzer (DMA). Surface topography and roughness of all films were qualitatively and quantitatively analyzed using the atomic force microscope (AFM). After AO irradiation the POSS-filled films showed a much smoother surface than that of neat PI film. The results of mass loss, mechanical property and AFM topography collectively indicate the exceptional self-healing capability of the POSS nanoparticles upon AO erosion that enables it to protect polyimide due to the formation of a thin glass layer. A further molecular dynamics simulation of the neat PI and PI-POSS system revealed reduced mass loss, damage propagation depth, and erosion yield of the nanocomposites with increasing POSS concentration, which are consistent with the experimental findings.

Molecular dynamics simulations were performed to gain fundamental molecular insights on the concentration-dependent adsorption and gas transport properties of the components in a CH4/CO2 gaseous mixture in single- and double-layered nanoporous graphene (NPG) and graphene oxide (NPGO) separation platforms. While these platforms are promising for a variety of separation applications, much about the relevant gas separation mechanisms in these systems is still unexplored. Based on the gas adsorption results in this work, at least two layers of CO2 are formed on the gas side of both NPG and NPGO, while no adsorption is observed for pure CH4 on the single-layered NPG. In contrast, increasing the CH4 concentration in the CH4/CO2 mixture leads to an enhancement of the CH4 adsorption on both separation platforms. The through-the-pore diffusion coefficients of both CO2 and CH4 increase with an increase in the CH4 concentration for all NPG and NPGO systems. The permeance of CO2 is smaller than that of CH4, suggestin...

Sintering of multiple single-crystal titanium (Ti) nanoparticles (NPs) during additive manufacturing by using ultrafast laser was simulated using molecular dynamics (MD). The aim was to better understand how factors such as sintering temperature and heating rate would influence the mechanical properties of the ultrafine-sized sintered products, i.e., Ti “NP-chains.” For this purpose, the effects of heating and strain rate on the tensile behavior of the final sintered products were studied in detail. Ti NP-chain precursors with weak neck connections were first created through solid-state sintering process at room temperature. They were later heated very rapidly to 800 K, 1200 K, or 1500 K with two different heating rates of 0.04 K/ps and 0.2 K/ps, and maintained at these high-temperature levels for 1 ns to mimic the fast temperature rise and short equilibration due to femtosecond/picosecond laser irradiation. The formed Ti NP-chains with different neck connection strengths were then cooled to 298 K. Those final NP-chains were subjected to uniaxial tension at three different strain rates of 0.001%/ps, 0.01%/ps, and 0.1%/ps. Our simulation results indicate a strong correlation between the tensile strength of the final NP-chain product and the heating rate during the previous short sintering process (including the ultrafast temperature rise and up to 1-ns high-temperature equilibration). A slower heating rate to a higher temperature level yields larger neck connection diameters in the final NP-chain product, resulting a higher tensile strength. Furthermore, our results demonstrate that high strain rates applied to the NP-chains with stronger neck connections result in an improvement in the tensile strength and ductility of the final products. In contrast, sintered products resulting from a lower temperature level show an elastic-brittle-damage behavior. Due to the weak neck connections and limited crystal sliding, the heating rate effect during sintering does not have a significant effect on the tensile strength.

Element recovery from the Martian regolith using ionic liquids (ILs) is an active area of research within the field of in-situ resource utilization. In this work, we performed a classical molecular dynamics (MD) simulation study to better understand the solvation thermodynamics and structures of potential cationic and anionic species originating from the Martian regolith in two select ILs, i.e., 1-ethyl-3-methylimidazolium acetate ([emim][Ac]) and 1-ethyl-3-methylimidazolium hydrogen sulfate ([emim][HSO4]), at two temperatures of 298.15 and 473.15 K. The studied cationic and anionic species represent the stable ions, i.e., a series of tetra-, tri-, di-, and monovalent cations, as well as several silicate, phosphate, chromate, titanate, and select halide anions, based on the mineral composition of the Martian regolith. We calculated the solvation free energies (SFEs) of these ionic species in the ILs using the free energy perturbation method. Moreover, we investigated the solvation environment of these ionic solutes by generating the relevant radial distribution functions and calculating the running coordination numbers of ILs' anions and cations surrounding the solutes. Overall, the average absolute values of the SFEs for cationic solutes increase with increasing ion valency (charge) and size of the solute at both temperatures. For anionic solutes, a more complex effect of anion molecular size and charge is responsible for the trends observed in the absolute values of the SFEs. For example, we found orthosilicate to be the most soluble anionic species in both ILs. On the other hand, the dichromate anion was found to be essentially insoluble in both ILs. Comparing between the solvation efficiencies of the ILs, [emim][Ac] shows larger negative SFE values than [emim][HSO4] for all cationic solutes at both temperatures. While the temperature effect on the solvation of cationic solutes is mixed, higher temperatures generally favor the dissolution of the anionic solutes in both ILs. Our results provide molecular insights into the solvation thermodynamics of various potential ionic species that may be extracted from the Martian regolith using suitable ILs.

In this study, the adsorption of doxorubicin (DOX), an anticancer drug, on pristine graphene (PG) and graphene oxide (GO) nanocarriers with different surface oxygen densities and in an aqueous environment with varying pH levels was investigated using molecular dynamics (MD) simulation. The drug loading and release on the GO nanocarrier was also simulated using pH as the controller mechanism. Overall, the DOX/nanocarrier interactions become stronger as the graphene surface oxygen density increases. Although pH has a negligible effect on the single-molecule drug adsorption on the GO surfaces under acidic and neutral conditions, significantly stronger DOX/nanocarrier interactions occur for the GO nanosheet with a lower surface oxygen density (GO-16, with an O/C ratio of 1 : 6) at basic pH levels. Moreover, the DOX/nanocarrier interactions are greatly weakened in the GO nanosheet with higher surface oxygen density (GO-13, with an O/C ratio of 1 : 3) under basic conditions. These observations are partly attributed to a more favorable geometry of the DOX molecule on the GO-16 surface as opposed to a loosely attached DOX molecule on the edges of the GO-13 nanosheet. When comparing the adsorption kinetics and transport properties of the DOX molecule in different GO systems, the drug diffusion coefficient increases with decreasing pH value (going from basic to neutral to acidic) due to the reduced total water-nanocarrier interactions. The latter observation is an indication of the more facilitated transport of the DOX molecule in an aqueous medium towards the nanocarrier surface at lower pH levels. Finally, we have confirmed the loading and release of the DOX molecules on the GO nanocarrier under neutral (pH = 7) and acidic (pH = 5) conditions, respectively. The former signifies the blood pH level, whereas the latter is reminiscent of the pH of a tumorous cell. The computational results presented in this work reveal the underlying mechanisms of DOX loading and release on PG and GO surfaces, which may be used to design better graphene-based nanocarriers for the DOX delivery and targeting applications.

Molecular simulations were performed to study the energetics and geometries of bond rupture in single alkane molecules using three reactive hydrocarbon potentials: (1) modified embedded-atom method (MEAM) for saturated hydrocarbons, (2) ReaxFF, and (3) second-generation REBO. The total energy/force versus strain, strain at fracture, and strain energy release were compared for a homologous series of normal alkanes (ethane to undecane) with generalization to polyethylene. The C C bond distances and C C C bond angles were quantified, and a fragment analysis was performed. Overall, the MEAM and ReaxFF potentials are in reasonable agreement with first-principles data with MEAM matching DFT-calculated lowest energy fragments.

The two-phase solid–liquid coexisting structures of Ni, Cu, and Al are studied by molecular dynamics (MD) simulations using the second nearest-neighbor (2NN) modified-embedded atom method (MEAM) potential. For this purpose, the existing 2NN-MEAM parameters for Ni and Cu were modified to make them suitable for the MD simulations of the problems related to the two-phase solid–liquid coexistence of these elements. Using these potentials, we compare calculated low-temperature properties of Ni, Cu, and Al, such as elastic constants, structural energy differences, vacancy formation energy, stacking fault energies, surface energies, specific heat and thermal expansion coefficient with experimental data. The solid–liquid coexistence approach is utilized to accurately calculate the melting points of Ni, Cu, and Al. The MD calculations of the expansion in melting, latent heat and the liquid structure factor are also compared with experimental data. In addition, the solid–liquid interface free energy and surface anisotropy of the elements are determined from the interface fluctuations, and the predictions are compared to the experimental and computational data in the literature.

In the context of data mining and knowledge discovery, a large dataset of vapor-grown carbon nanofiber (VGCNF)/vinyl ester (VE) nanocomposites was thoroughly analyzed and classified using support vector machines (SVMs) into ten classes of desired mechanical properties. These classes are high true ultimate strength, high true yield strength, high engineering elastic modulus, high engineering ultimate strength, high flexural modulus, high flexural strength, high impact strength, high storage modulus, high loss modulus, and high tan delta. Resubstitution and 3-folds cross validation techniques were applied and different sets of confusion matrices were used to compare and analyze the classifier’s resulting classification performance. The designed SVMs model is resourceful for materials scientists and engineers, because it can be used to qualitatively assess different nanocomposite mechanical responses associated with different combinations of the formulation, processing, and environmental conditions. In addition, the lead time required to develop VGCNF/VE nanocomposites for particular engineering application will be significantly reduced using the designed SVMs classifier. This work specifically present a framework for a fast and reliable classification of a large material dataset with respect to desired mechanical properties, and can be used for all materials within the context of materials science and engineering.

The research objective herein is to understand the relationships between the interatomic potential parameters and properties used in the training and validation of potentials, specifically using a recently developed modified embedded-atom method (MEAM) potential for saturated hydrocarbons (C–H system). This potential was parameterized to a training set that included bond distances, bond angles, and atomization energies at 0 K of a series of alkane structures from methane to n-octane. In this work, the parameters of the MEAM potential were explored through a fractional factorial design and a Latin hypercube design to better understand how individual MEAM parameters affected several properties of molecules (energy, bond distances, bond angles, and dihedral angles) and also to quantify the relationship/correlation between various molecules in terms of these properties. The generalized methodology presented shows quantitative approaches that can be used in selecting the appropriate parameters for the interatomic potential, selecting the bounds for these parameters (for constrained optimization), selecting the responses for the training set, selecting the weights for various responses in the objective function, and setting up the single/multi-objective optimization process itself. The significance of the approach applied in this study is not only the application to the C–H system but that the broader framework can also be easily applied to any number of systems to understand the significance of parameters, their relationships to properties, and the subsequent steps for designing interatomic potentials under uncertainty.

Standard casting methods, using polymer/RTIL solutions, can produce membranes with improved stability. We evaluated the performance of RTIL-membranes cast using either solvent casting (homogeneous polymer/RTIL film) or phase-inversion (biphasic polymer/RTIL films). The study used a model casting polymer, cellulose acetate (CA), and the RTIL 1-ethyl-3-methylimidazolium thiocyanate, ([emim][SCN]). The gas separation performances of the resulting homogeneous and biphasic CA/[emim][SCN] membranes were compared with a conventional supported ionic liquid membrane (SILM) of [emim][SCN]. The evaluations of CO 2 removal from CH 4 supported our hypotheses that the biphasic phase-inversion cast membranes have higher selectivities and membrane stability compared to the homogeneous film membranes. While the homogeneous membranes had the highest gas permeances, the biphasic membranes had three-fold higher selectivities. The highest CO 2 /CH 4 mixed-gas selectivity reported was 89 ± 12 for the biphasic membrane. It appears that selectivity in cast membranes is a function of free RTIL content. The order of membrane stability vs. cross-membrane pressure was biphasic ≈ SILM > homogeneous, with the biphasic breakthrough pressure being two-fold higher than the homogeneous membrane's. Therefore, the phase-inversion casting method (biphasic membranes) is a viable alternative to SILM membranes for fabricating RTIL-membranes. Infrared spectra and atomic force micrographs characterized the homogeneous and biphasic membrane morphologies.

In this work, we developed an interatomic potential for saturated hydrocarbons using the modified embedded-atom method (MEAM), a reactive semi-empirical many-body potential based on density functional theory and pair potentials. We parameterized the potential by fitting to a large experimental and first-principles (FP) database consisting of (1) bond distances, bond angles, and atomization energies at 0 K of a homologous series of alkanes and their select isomers from methane to n-octane, (2) the potential energy curves of H2, CH, and C2 diatomics, (3) the potential energy curves of hydrogen, methane, ethane, and propane dimers, i.e., (H2)2, (CH4)2, (C2H6)2, and (C3H8)2, respectively, and (4) pressure-volume-temperature (PVT) data of a dense high-pressure methane system with the density of 0.5534 g cc(-1). We compared the atomization energies and geometries of a range of linear alkanes, cycloalkanes, and free radicals calculated from the MEAM potential to those calculated by other commonly used reactive potentials for hydrocarbons, i.e., second-generation reactive empirical bond order (REBO) and reactive force field (ReaxFF). MEAM reproduced the experimental and/or FP data with accuracy comparable to or better than REBO or ReaxFF. The experimental PVT data for a relatively large series of methane, ethane, propane, and butane systems with different densities were predicted reasonably well by the MEAM potential. Although the MEAM formalism has been applied to atomic systems with predominantly metallic bonding in the past, the current work demonstrates the promising extension of the MEAM potential to covalently bonded molecular systems, specifically saturated hydrocarbons and saturated hydrocarbon-based polymers. The MEAM potential has already been parameterized for a large number of metallic unary, binary, ternary, carbide, nitride, and hydride systems, and extending it to saturated hydrocarbons provides a reliable and transferable potential for atomistic/molecular studies of complex material phenomena involving hydrocarbon-metal or polymer-metal interfaces, polymer-metal nanocomposites, fracture and failure in hydrocarbon-based polymers, etc. The latter is especially true since MEAM is a reactive potential that allows for dynamic bond formation and bond breaking during simulation. Our results show that MEAM predicts the energetics of two major chemical reactions for saturated hydrocarbons, i.e., breaking a C-C and a C-H bond, reasonably well. However, the current parameterization does not accurately reproduce the energetics and structures of unsaturated hydrocarbons and, therefore, should not be applied to such systems.

A design of experiments and response surface modeling were performed to investigate the effects of formulation and proc- essing factors on the flexural moduli and strengths of vapor-grown carbon nanofiber (VGCNF)/vinyl ester (VE) nanocomposites. VGCNF type (pristine, surface-oxidized), use of a dispersing agent (no, yes), mixing method (ultrasonication, high-shear mixing, and a combination of both), and VGCNF weight fraction (0.00, 0.25, 0.50, 0.75, and 1.00 parts per hundred parts resin (phr)) were selected as independent factors. Response surface models were developed to predict flexural moduli and strengths as a continuous function of VGCNF weight fraction. The use of surface-oxidized nanofibers, a dispersing agent, and high-shear mixing at 0.48 phr of VGCNF led to an average increase of 19% in the predicted flexural modulus over that of the neat VE. High-shear mixing with 0.60 phr of VGCNF resulted in a remarkable 49% increase in nanocomposite flexural strength relative to that of the neat VE. This ar- ticle underscores the advantages of statistical design of experiments and response surface modeling in characterizing and optimizing polymer nanocomposites for automotive structural applications. Moreover, response surface models may be used to tailor the me- chanical properties of nanocomposites over a range of anticipated operating environments. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2087-2099, 2013

A full factorial design of experiments and response surface methodology were used to investigate the effects of formula- tion, processing, and operating temperature on the viscoelastic properties of vapor-grown carbon nanofiber (VGCNF)/vinyl ester (VE) nanocomposites. Factors included VGCNF type (pristine, oxidized), use of a dispersing agent (DA) (no, yes), mixing method (ultrasonication, high-shear mixing, and a combination of both), VGCNF weight fraction (0.00, 0.25, 0.50, 0.75, and 1.00 parts per hundred parts resin (phr)), and temperature (30, 60, 90, and 120 � C). Response surface models (RSMs) for predicting storage and loss moduli were developed, which explicitly account for the effect of complex interactions between nanocomposite design factors and operating temperature on resultant composite properties; such influences would be impossible to assess using traditional single- factor experiments. Nanocomposite storage moduli were maximized over the entire temperature range (� 20% increase over neat VE) by using high-shear mixing and oxidized VGCNFs with DA or equivalently by employing pristine VGCNFs without DA at � 0.40 phr of VGCNFs. Ultrasonication yielded the highest loss modulus at � 0.25 phr of VGCNFs. The RSMs developed in this investigation may be used to design VGCNF-enhanced VE matrices with optimal storage and loss moduli for automotive structural applications. Moreover, a similar approach may be used to tailor the mechanical, thermal, and electrical properties of nanomaterials over a range of anticipated operating environments. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 234-247, 2013

The effects of moulding condition and curing atmosphere on the flexural properties of a neat 33 wt.%-styrene epoxy vinyl ester (VE) were investigated. Specimens were prepared using either open or closed moulds, and thermally cured under either air or nitrogen atmosphere. Four-point bending tests were performed with both the top (“air-side”) and the bottom (“mould-side”) surfaces of the cured specimens in tension. The mean flexural moduli for nitrogen-cured and closed-mould specimens were 3% and 9% higher than for air-cured specimens, respectively. However, the mean flexural strength for open-mould air-cured specimens with their air-sides loaded in tension were 65% lower than the mean flexural strengths of open-mould nitrogen-cured or closed-mould specimens. This likely resulted from partial VE resin curing inhibition due to oxygen diffusion into the free surface region of the open-mould air-cured specimens. This creates gradients in the local stiffness and strength in the near-surface region due to lower crosslink density. This effect may be particularly important for thin specimens. These results underscore the significance of exposure to air during open-mould curing on the cured VE flexural properties. Such assessments are crucial for composite part manufacturing utilizing VEs.

In this paper, we develop a new modified embedded atom method (MEAM) potential that includes the bond order (MEAM-BO) to describe the energetics of unsaturated hydrocarbons (double and triple carbon bonds) and also develop improved parameters for saturated hydrocarbons from those of our previous work. Such quantities like bond lengths, bond angles, and atomization energies at 0 K, dimer molecule interactions, rotational barriers, and the pressure-volume-temperature relationships of dense systems of small molecules give a comparable or more accurate property relative to experimental and first-principles data than the classical reactive force fields REBO and ReaxFF. Our extension of the MEAM potential for unsaturated hydrocarbons (MEAM-BO) is a step toward developing more reliable and accurate polymer simulations with their associated structure-property relationships, such as reactive multicomponent (organic/metal) systems, polymer-metal interfaces, and nanocomposites. When the constants for the BO are zero, MEAM-BO reduces to the original MEAM potential. As such, this MEAM-BO potential describing the interaction of organic materials with metals within the same MEAM formalism is a significant advancement for computational materials science.

In this fundamental study, a series of molecular dynamics simulations were performed in vacuo to investigate the energetics and select geometries of 20 standard amino acids (AAs) on pristine graphene (PG) and graphene oxide (GO) surfaces as a function of graphene surface oxygen density. These interactions are of key interest to graphene/biomolecular systems. Our results indicate that aromatic AAs exhibit the strongest total interactions with the PG surfaces due to π-π stacking. Tryptophan (Trp) has the highest aromaticity due to its indole side chain and, hence, has the strongest interaction among all AAs (−16.66 kcal/mol). Aliphatic, polar, and charged AAs show various levels of affinity to the PG sheets depending on the strength of their side chain hydrophobic interactions. For example, arginine (Arg) with its guanidinium side chain exhibits the strongest interaction with the PG sheets (−13.81 kcal/mol) following aromatic AAs. Also, glycine (Gly; a polar AA) has the weakest interaction with the PG sheets (−7.29 kcal/mol). When oxygen-containing functional groups are added to the graphene sheets, the π-π stacking in aromatic AAs becomes disrupted and perfect parallelism of the aromatic rings is lost. Moreover, hydrogen bonding and/or electrostatic interactions become more pronounced. Charged AAs exhibit the strongest interactions with the GO surfaces. In general, the AA-GO interactions increase with increasing surface oxygen density, and the effect is more pronounced at higher O/C ratios. This study provides a quantitative measure of AA-graphene interactions for the design and tuning of biomolecular systems suitable for biosensing, drug delivery, and gene delivery applications.

This paper characterizes the failure of a polymeric clamp hanger component using finite element analysis coupled with experimental methods such as scanning electron microscopy, X-ray computed tomography, and mechanical testing. Using Fourier transform infrared spectroscopy, the material was identified as a polypropylene. Internal porosity that arose from the manufacturing procedure was determined using three dimensional X-ray computed tomography. From static mechanical experiments, the forces applied on the component were determined and used in a finite element simulation, which clearly showed the process of fracture arising from the pre-existing processing pores. The fracture surfaces were observed under a scanning electron microscope confirming the finite element simulation results illustrating that low-cycle fatigue fracture occurred in which the fatigue cracks nucleated from the manufacturing porosity.

The effects of vapor-grown carbon nanofiber (VGCNF) weight fraction, high-shear mixing time, and ultrasonication time on the Izod impact strengths of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design. A response surface model (RSM) for predicting impact strengths was developed using regression analysis. RSM predictions suggested that an 18% increase in impact strength was possible for nanocomposites containing only 0.170 parts per hundred parts resin (phr) of VGCNFs (∼0.1 v%) that were high-shear mixed for 100 min when compared to that of neat VE. In general, the predicted impact strengths increased for high-shear mixing times above 55 min and VGCNF weight fractions below 0.400 phr. The predicted strengths decreased as the VGCNF weight fraction was further increased. Scanning electron micrographs of the nanocomposite fracture surfaces showed that increased impact strength could be directly correlated to better nanofiber dispersion in the matrix. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Surface oxidation effects on the liquid vinyl ester (VE) monomer distributions near two oxidized vapor-grown carbon nanofiber (VGCNF) surfaces were studied using molecular dynamics simulations. Two overlapping graphene sheets containing oxygenated functional groups represented the oxidized VGCNF surfaces. Two liquid VE bisphenol-A dimethacrylates (designated VE1 and VE2, respectively) and styrene constituted the resin. Temporally and spatially averaged relative monomer concentrations, calculated in a direction away from the oxidized graphene surfaces, showed increased styrene and VE1 concentrations. Monomer molar ratios found within a 10 A thick region adjacent to the oxidized graphene sheets were substantially different from those in the bulk resin. Curing should result in the formation of a very thin interphase region of different composition. The crosslink structure of such an interphase will be distinct from that of an unoxidized VGCNF surface. The enhanced VE1 concentration near this oxidized surface should give a higher crosslink density, leading to a stiffer interphase than that adjacent to unoxidized VGCNF surfaces. VGCNF–matrix adhesion may also be modified by the different interphase monomer molar ratios. These studies may facilitate multiscale material design by providing insight into carbon nanofiber–matrix interactions leading to improved macroscale composite properties.

A molecular dynamics simulation study was performed to investigate the role of liquid vinyl ester (VE) resin monomer interactions with the surface of pristine vapor-grown carbon nanofibers (VGCNFs). These interactions may influence the formation of an interphase region during resin curing. A liquid resin having a mole ratio of styrene to bisphenol-A-diglycidyl dimethacrylate VE monomers consistent with a commercially available 33 wt.% styrene VE resin was placed in contact with both sides of two pristine graphene sheets overlapped like shingles to represent the outer surface of a pristine VGCNF. The relative monomer concentrations were calculated in a direction away from the graphene sheets. At equilibrium, the styrene/VE monomer ratio was higher in a 5 A thick region adjacent to the nanofiber surface than in the remaining liquid volume. The elevated concentration of styrene near the nanofiber surface suggests that a styrene-rich interphase region, with a lower crosslink density than the bulk matrix, could be formed upon curing. Furthermore, styrene accumulation in the immediate vicinity of the nanofiber surface might, after curing, improve the nanofiber–matrix interfacial adhesion compared to the case where the monomers were uniformly distributed throughout the matrix.

A design of experiments approach demonstrated how four formulation and processing factors (i.e., nanofiber type, use of dispersing agent, mixing method, and nanofiber weight fraction) affected the dynamic mechanical properties of carbon nanofiber/vinyl ester nanocomposites. Only

The effects of selected factors such as vapor-grown carbon nanofiber (VGCNF) weight fraction, applied stress, and tem- perature on the viscoelastic responses (creep strain and creep compliance) of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design (CCD). Nanocomposite test articles were fabricated by high-shear mixing, casting, curing, and post curing in an open-face mold under a nitrogen environment. Short-term creep/creep recovery experiments were conducted at pre- scribed combinations of temperature (23.8-69.2 � C), applied stress (30.2-49.8 MPa), and VGCNF weight fraction (0.00-1.00 parts of VGCNF per hundred parts of resin) determined from the CCD. Response surface models (RSMs) for predicting these viscoelastic responses were developed using the least squares method and an analysis of variance procedure. The response surface estimates indi- cate that increasing the VGCNF weight fraction marginally increases the creep resistance of the VGCNF/VE nanocomposite at low temperatures (i.e., 23.8-46.5 � C). However, increasing the VGCNF weight fraction decreased the creep resistance of these nanocompo- sites for temperatures greater than 50 � C. The latter response may be due to a decrease in the nanofiber-to-matrix adhesion as the temperature is increased. The RSMs for creep strain and creep compliance revealed the interactions between the VGCNF weight frac- tion, stress, and temperature on the creep behavior of thermoset polymer nanocomposites. The design of experiments approach is useful in revealing interactions between selected factors, and thus can facilitate the development of more physics-based models. V C 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42162.

(Received 10 July 2014; revised manuscript received 10 December 2014; published 12 January 2015) In this paper, molecular dynamics (MD) simulations based on the modified-embedded atom method (MEAM) and a phase-field crystal (PFC) model are utilized to quantitatively investigate the solid-liquid properties of Fe. A set of second nearest-neighbor MEAM parameters for high-temperature applications are developed for Fe, and the solid-liquid coexisting approach is utilized in MD simulations to accurately calculate the melting point, expansion in melting, latent heat, and solid-liquid interface free energy, and surface anisotropy. The required input properties to determine the PFC model parameters, such as liquid structure factor and fluctuations of atoms in the solid, are also calculated from MD simulations. The PFC parameters are calculated utilizing an iterative procedure from the inputs of MD simulations. The solid-liquid interface free energy and surface anisotropy are calculated using the PFC simulations. Very good agreement is observed between the results of our calculations from MEAM-MD and PFC simulations and the available modeling and experimental results in the literature. As an application of the developed model, the grain boundary free energy of Fe is calculated using the PFC model and the results are compared against experiments.