Sasan Nouranian

Review of Hierarchical Multiscale Modeling to Describe the Mechanical Behavior of Amorphous Polymers

Modern computational methods have proved invaluable for the design and analysis of structural components using lightweight materials. The challenge of optimizing lightweight materials in the design of industrial components relates to incorporating structure-property relationships within the computational strategy to incur robust designs. One effective methodology of incorporating structure-property relationships within a simulation-based design framework is to employ a hierarchical multiscale modeling strategy. This paper reviews techniques of multiscale modeling to predict the mechanical behavior of amorphous polymers. Hierarchical multiscale methods bridge nanoscale mechanisms to the macroscale/continuum by introducing a set of structure-property relationships. This review discusses the current state of the art and challenges for three distinct scales: quantum, atomistic/coarse graining, and continuum mechanics. For each scale, we review the modeling techniques and tools, as well as discuss important recent contributions. To help focus the review, we have mainly considered research devoted to amorphous polymers.

Dynamic mechanical analysis and optimization of vapor-grown carbon nanofiber/vinyl ester nanocomposites using design of experiments

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 <0.50 parts of nanofiber per hundred parts resin produced a 20% increase in the storage modulus vs. that of the neat cured resin. Statistical response surface models predicted nanocomposite storage and loss moduli as a function of the four factors and their interactions. Nanofiber type and weight fraction were the key interacting factors influencing the mean storage modulus. Nanofiber weight fraction, mixing method, and dispersing agent had coupled effects on the mean loss modulus. Employing this methodology, optimized nanocomposite properties can be predicted as a function of nanocomposite formulation and processing.

Molecular simulation of pH-dependent diffusion, loading, and release of doxorubicin in graphene and graphene oxide drug delivery systems

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.

An interatomic potential for saturated hydrocarbons based on the modified embedded-atom method

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.

Data mining and knowledge discovery in materials science and engineering

In this study, data mining and knowledge discovery techniques were employed to validate their efficacy in acquiring information about the viscoelastic properties of vapor-grown carbon nanofiber (VGCNF)/vinyl ester (VE) nanocomposites solely from data derived from a designed experimental study. Formulation and processing factors (VGCNF type, use of a dispersing agent, mixing method, and VGCNF weight fraction) and testing temperature were utilized as inputs and the storage modulus, loss modulus, and tan delta were selected as outputs. The data mining and knowledge discovery algorithms and techniques included self-organizing maps (SOMs) and clustering techniques. SOMs demonstrated that temperature had the most significant effect on the output responses followed by VGCNF weight fraction. SOMs also showed how to prepare different VGCNF/VE nanocomposites with the same storage and loss modulus responses. A clustering technique, i.e., fuzzy C-means algorithm, was also applied to discover certain patterns in nanocomposite behavior after using principal component analysis as a dimensionality reduction technique. Particularly, these techniques were able to separate the nanocomposite specimens into different clusters based on temperature and tan delta features as well as to place the neat VE specimens (i.e., specimens containing no VGCNFs) in separate clusters. Most importantly, the results from data mining are consistent with previous response surface characterizations of this nanocomposite system. This work highlights the significance and utility of data mining and knowledge discovery techniques in the context of materials informatics.

Reactive Molecular Simulation of the Damage Mitigation Efficacy of POSS-, Graphene-, and Carbon Nanotube-Loaded Polyimide Coatings Exposed to Atomic Oxygen Bombardment

Reactive molecular dynamics simulation was employed to compare the damage mitigation efficacy of pristine and polyimide (PI)-grafted polyoctahedral silsesquioxane (POSS), graphene (Gr), and carbon nanotubes (CNTs) in a PI matrix exposed to atomic oxygen (AO) bombardment. The concentration of POSS and the orientation of Gr and CNT nanoparticles were further investigated. Overall, the mass loss, erosion yield, surface damage, AO penetration depth, and temperature evolution are lower for the PI systems with randomly oriented CNTs and Gr or PI-grafted POSS compared to those of the pristine POSS or aligned CNT and Gr systems at the same nanoparticle concentration. On the basis of experimental early degradation data (before the onset of nanoparticle damage), the amount of exposed PI, which has the highest erosion yield of all material components, on the material surface is the most important parameter affecting the erosion yield of the hybrid material. Our data indicate that the PI systems with randomly oriented Gr and CNT nanoparticles have the lowest amount of exposed PI on the material surface; therefore, a lower erosion yield is obtained for these systems compared to that of the PI systems with aligned Gr and CNT nanoparticles. However, the PI/grafted-POSS system has a significantly lower erosion yield than that of the PI systems with aligned Gr and CNT nanoparticles, again due to a lower amount of exposed PI on the surface. When comparing the PI systems loaded with PI-grafted POSS versus pristine POSS at low and high nanoparticle concentrations, our data indicate that grafting the POSS and increasing the POSS concentration lower the erosion yield by a factor of about 4 and 1.5, respectively. The former is attributed to a better dispersion of PI-grafted POSS versus that of the pristine POSS in the PI matrix, as determined by the radial distribution function.

Taguchi-based optimization of adhesion of polyurethane to plasticized poly(vinyl chloride) in synthetic leather

In this investigation, the effects of formulation and processing factors on the adhesion between polyurethane (PU) and plasticized poly(vinyl chloride) (pPVC) layers was studied using the Taguchi method for experimental design. Among the many factors, nine parameters were screened and tested at two or three levels, taking advantage of the Taguchi L 27 orthogonal array. The factors studied were PVC type, PVC K-value, plasticizer type and content, filler type and content, fusion temperature and fusion time of PVC, and PU type. Using the results of T-peel adhesion test at 60°C as a response, the data were analyzed by appropriate software based on the ANOVA technique. The effect of the various factors on the adhesion was found to be in the following descending order: PU type, PVC fusion temperature, PVC type, plasticizer content, PVC K-value, PVC fusion time, interaction between plasticizer type and PVC fusion temperature, plasticizer type, filler type, filler content, interaction between filler type and PVC fusion temperature, and interaction between PVC K-value and filler type.

Interatomic Potential for Hydrocarbons on the Basis of the Modified Embedded-Atom Method with Bond Order (MEAM-BO)

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.

High Strain Rate Behavior of Carbon Nanofiber Reinforced Vinyl Ester

The dynamic uniaxial compressive response of neat vinyl ester resin with and without the addition of a dispersing agent and vinyl ester reinforced with both 0.50 and 1.00 parts (by weight) per hundred (parts of) resin of vapor grown carbon nanofibers was analyzed using a classical split Hopkinson pressure bar at strain rates ranging from 750 s -1 to 2350 s -1 . Dynamic force equilibrium was verified at all strain rates in order to validate the small strain data and the observed dynamic elastic modulus. Both the effects of the dispersing agent and the nanofiber loading level conducive for optimal yield stress and stiffness were shown to be highly strain rate dependent. Due to the viscoelastic nature of the vinyl ester resin/matrix, the differences in the observed dynamic stress-strain response (notably yield stress and stiffness) of all four material configurations likely was attributed to the coupled effect of the strain rate and the associated internal temperature rise experienced during split Hopkinson pressure bar tests.

Effects of Moulding Condition and Curing Atmosphere on the Flexural Properties of Vinyl Ester

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.