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    ABSTRACT: 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 5) pressure-volume-temperature (PVT) data of a dense high-pressure methane system with the density of 0.5534 g/cc. 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 MEAM. 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.
    Physical Chemistry Chemical Physics 01/2014; · 3.83 Impact Factor
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    ABSTRACT: A design of experiments and response surface modeling were performed to investigate the effects of formulation and processing 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 article 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 mechanical properties of nanocomposites over a range of anticipated operating environments.
    Journal of Applied Polymer Science 10/2013; 130(3):2087-2099. · 1.40 Impact Factor
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    ABSTRACT: Modern computational methods have proved invaluable for the design and analysis of structural components using lightweight materials. The challenge of optimizing light-weight materials in the design of industrial components relates to incorporating structure-property relationships within the computational strategy to incur robust de-signs. 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 re-lationships. 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.
    Journal of Engineering Materials and Technology 07/2013; 131(4):041206. · 0.72 Impact Factor
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    ABSTRACT: 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.
    Journal of Applied Polymer Science 04/2013; 128(2):1070-1080. · 1.40 Impact Factor
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    ABSTRACT: 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.
    Polymers and Polymer Composites 03/2013; 21(2):61-64. · 0.31 Impact Factor
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    ABSTRACT: 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.
    Advanced Engineering Informatics 01/2013; 27(4):615–624. · 1.59 Impact Factor
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    ABSTRACT: A full factorial design of experiments and response surface methodology were used to investigate the effects of formulation, 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.
    Journal of Applied Polymer Science 01/2013; 130(1):234-247. · 1.40 Impact Factor
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    ABSTRACT: 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.
    Engineering Failure Analysis 12/2012; 26:230–239. · 1.13 Impact Factor
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    ABSTRACT: 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 10Å 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.
    Carbon. 03/2012; 50(3):748-760.
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    Sasan Nouranian
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    ABSTRACT: The use of nanoreinforcements in automotive structural composites has provided promising improvements in their mechanical properties. For the first time, a robust statistical design of experiments approach was undertaken to demonstrate how key formulation and processing factors (nanofiber type, use of dispersing agent, mixing method, nanofiber weight fraction, and temperature) affected the dynamic mechanical properties of vapor-grown carbon nanofiber (VGCNF)/vinyl ester (VE) nanocomposites. Statistical response surface models were developed to predict nanocomposite storage and loss moduli as functions of significant factors. Only ∼0.50 parts of nanofiber per hundred parts resin produced a roughly 20% increase in the storage modulus versus that of the neat VE at room temperature. Optimized nanocomposite properties were predicted as a function of design factors employing this methodology. For example, the use of high-shear mixing (one of the mixing methods in the design) with the oxidized VGCNFs in the absence of dispersing agent or arbitrarily with pristine VGCNFs in the presence of dispersing agent was found to maximize the predicted storage modulus over the entire temperature range (30-120 °C). To study the key concept of interphase in thermoset nanocomposites, molecular dynamics simulations were performed to investigate liquid VE resin monomer interactions with the surface of a pristine VGCNF. A liquid resin having a mole ratio of styrene to bisphenol A-diglycidyl dimethacrylate monomers consistent with a 33 wt% styrene VE resin was placed in contact with both sides of pristine graphene sheets, overlapped like shingles, to represent the outer surface of a pristine VGCNF. The relative monomer concentrations were calculated in a direction progressively away from the surface of 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 styrene concentration 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.
    04/2011; ProQuest, UMI Dissertation Publishing., ISBN: 1249037719
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    ABSTRACT: 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
    Journal of Composite Materials 01/2011; 45(16):1647-1657. · 0.94 Impact Factor
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    ABSTRACT: 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 Å 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.
    Carbon. 01/2011; 49(10):3219-3232.
  • 01/2011: pages 128-140; , ISBN: 1615038434
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    S. Nouranian, H. Garmabi, N. Mohammadi
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    ABSTRACT: 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 L27 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.
    Journal of Adhesion Science and Technology 01/2007; 21(8):705-724. · 0.90 Impact Factor

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