Modeling the Role of Bulk and Surface Characteristics of Carbon Fiber on Thermal Conductance across the Carbon-Fiber/Matrix Interface

The rapid heating of carbon-fiber-reinforced polymer matrix composites leads to complex thermophysical interactions which not only are dependent on the thermal properties of the constituents and microstructure but are also dependent on the thermal transport between the fiber and resin interfaces. Using atomistic molecular dynamics simulations, the thermal conductance across the interface between a carbon-fiber near-surface region and bismaleimide monomer matrix is calculated as a function of the interface and bulk features of the carbon fiber. The surface of the carbon fiber is modeled as sheets of graphitic carbon with (a) varying degrees of surface functionality, (b) varying defect concentrations in the surface-carbon model (pure graphitic vs partially graphitic), (c) varying orientation of graphitic carbon at the interface, (d) varying interface saturation (dangling vs saturated bonds), (e) varying degrees of surface roughness, and (f) incorporating high conductive fillers (carbon nanotubes) at the interface. After combining separately equilibrated matrix system and different surface-carbon models, thermal energy exchange is investigated in terms of interface thermal conductance across the carbon fiber and the matrix. It is observed that modifications in the studied parameters (a–f) often lead to significant modulation of thermal conductance across the interface and, thus, showcases the role of interface tailoring and surface-carbon morphology toward thermal energy exchange. More importantly, the results provide key bounds and a realistic degree of variation to the interface thermal conductance values at fiber/matrix interfaces as a function of different surface-carbon features

Molecular Modeling of Interfaces between Hole Transport and Active Layers in Flexible Organic Electronic Devices

Molecular modeling methods are used to understand the interfacial properties between the hole-transport and active layers in organic photovoltaic (OPV) devices. The hole-transport layer (HTL) consists of a blend of poly­(styrene-sulfonate) and poly­(3,4-ethylenedioxythiophene) (PEDOT:PSS), whereas the active layer (AL) consists of a blend of poly­(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Simulation results on the HTL confirm the interpenetrating lamellar structure with alternating PSS and PEDOT domains as observed in experiments. In addition, interfacial results show high PCBM interactions with the HTL, which result in PCBM migration to the HTL surface. The observed PCBM concentration profile is discussed from the perspective of attractive interactions, and it is shown that these interactions are governed by the side chain of PCBM. Calculations also suggest that OPV device performance could be improved by, for example, increasing the number of benzene rings and backbone −CH₂– groups in the PCBM side chain, which would be expected to reduce PCBM concentration at the HTL surface. The results yield important insights into molecular interactions associated with the HTL and AL interfaces that contribute to final device morphology and thus provide guidelines toward materials design approaches for optimized device performance

Molecular Modeling of Cross-Linked Polymers with Complex Cure Pathways: A Case Study of Bismaleimide Resins

To date, molecular modeling of cross-linking polymers has focused on those involving single-reaction cure mechanisms, such as epoxies and the epoxide–amine reaction. In this work, we have developed a novel cross-linking framework that is capable of undertaking complex cure mechanisms involving several simultaneous reaction pathways with minimal user input. As a case study, a bismaleimide (BMI) resin is considered herein which possesses multiple cure reactions and reaction pathways. Using an adaptable molecular dynamics simulation method, we highlight our framework by implementing five distinct cure reactions of Matrimid-5292 (a BMI resin) and predicting the corresponding thermomechanical properties. The method is used to establish the influence of different cure reactions and extent of curing on mass density, glass transition temperature, coefficient of thermal expansion, elastic moduli, and thermal conductivity. The developed method is further validated by comparison of these properties to experimentally observed trends

Importance of Interfaces in Governing Thermal Transport in Composite Materials: Modeling and Experimental Perspectives

Thermal management in polymeric composite materials has become increasingly critical in the air-vehicle industry because of the increasing thermal load in small-scale composite devices extensively used in electronics and aerospace systems. The thermal transport phenomenon in these small-scale heterogeneous systems is essentially controlled by the interface thermal resistance because of the large surface-to-volume ratio. In this review article, several modeling strategies are discussed for different length scales, complemented by our experimental efforts to tailor the thermal transport properties of polymeric composite materials. Progress in the molecular modeling of thermal transport in thermosets is reviewed along with a discussion on the interface thermal resistance between functionalized carbon nanotube and epoxy resin systems. For the thermal transport in fiber-reinforced composites, various micromechanics-based analytical and numerical modeling schemes are reviewed in predicting the transverse thermal conductivity. Numerical schemes used to realize and scale the interface thermal resistance and the finite mean free path of the energy carrier in the mesoscale are discussed in the frame of the lattice Boltzmann–Peierls–Callaway equation. Finally, guided by modeling, complementary experimental efforts are discussed for exfoliated graphite and vertically aligned nanotubes based composites toward improving their effective thermal conductivity by tailoring interface thermal resistance

Effects of Titanium-Containing Additives on the Dehydrogenation Properties of LiAlH₄: A Computational and Experimental Study

Metal hydrides are attractive materials for use in thermal storage systems to manage excessive transient heat loads and for hydrogen storage applications. This paper presents a combined computational and experimental investigation of the influence of Ti, TiO₂, and TiCl₃ additives on the dehydrogenation properties of milled LiAlH₄. Density functional theory (DFT) is used to predict the effect of Ti-containing additives on the electronic structure of the region surrounding the additive after its adsorption on the LiAlH₄ (010) surface. The electron distribution and charge transfer within the LiAlH₄/additive system is evaluated. Electronic structure calculations predict covalent-like bonding between the Ti atom of the adsorbate and surrounding H atoms. The hydrogen (H) binding energy associated with the removal of the first H from the modified LiAlH₄ surface is calculated and compared with experimental dehydration activation energies. It is seen that the weaker H binding corresponds to the larger amount of charge transferred from the Ti atom to adjacent H atoms. A reduction in charge transfer between the Al atom and surrounding H atoms is also observed when compared to charge transfer in the unmodified LiAlH₄ surface. This reduction in charge transfer between Al–H weakens the covalent bond within the [AlH₄]⁻ tetrahedron, which in turn reduces the dehydrogenation temperature exhibited by LiAlH₄ when Ti-containing additives are used

Hydrogenation of Penta-Graphene Leads to Unexpected Large Improvement in Thermal Conductivity

Penta-graphene (PG) has been identified as a novel two-dimensional (2D) material with an intrinsic bandgap, which makes it especially promising for electronics applications. In this work, we use first-principles lattice dynamics and iterative solution of the phonon Boltzmann transport equation (BTE) to determine the thermal conductivity of PG and its more stable derivative, hydrogenated penta-graphene (HPG). As a comparison, we also studied the effect of hydrogenation on graphene thermal conductivity. In contrast to hydrogenation of graphene, which leads to a dramatic decrease in thermal conductivity, HPG shows a notable increase in thermal conductivity, which is much higher than that of PG. Considering the necessity of using the same thickness when comparing thermal conductivity values of different 2D materials, hydrogenation leads to a 63% reduction in thermal conductivity for graphene, while it results in a 76% increase for PG. The high thermal conductivity of HPG makes it more thermally conductive than most other semiconducting 2D materials, such as the transition metal chalcogenides. Our detailed analyses show that the primary reason for the counterintuitive hydrogenation-induced thermal conductivity enhancement is the weaker bond anharmonicity in HPG than PG. This leads to weaker phonon scattering after hydrogenation, despite the increase in the phonon scattering phase space. The high thermal conductivity of HPG may inspire intensive research around HPG and other derivatives of PG as potential materials for future nanoelectronic devices. The fundamental physics understood from this study may open up a new strategy to engineer thermal transport properties of other 2D materials by controlling bond anharmonicity via functionalization

РП Спецсеминар Совр. проблемы биофизики, биологии и биотех-и 2015 с печатью

Silica nanostructures find applications in drug delivery, catalysis, and composites, however, understanding of the surface chemistry, aqueous interfaces, and biomolecule recognition remain difficult using current imaging techniques and spectroscopy. A silica force field is introduced that resolves numerous shortcomings of prior silica force fields over the last 30 years and reduces uncertainties in computed interfacial properties relative to experiment from several 100% to less than 5%. In addition, a silica surface model database is introduced for the full range of variable surface chemistry and pH (Q², Q³, Q⁴ environments with adjustable degree of ionization) that have shown to determine selective molecular recognition. The force field enables accurate computational predictions of aqueous interfacial properties of all types of silica, which is substantiated by extensive comparisons to experimental measurements. The parameters are integrated into multiple force fields for broad applicability to biomolecules, polymers, and inorganic materials (AMBER, CHARMM, COMPASS, CVFF, PCFF, INTERFACE force fields). We also explain mechanistic details of molecular adsorption of water vapor, as well as significant variations in the amount and dissociation depth of superficial cations at silica–water interfaces that correlate with ζ-potential measurements and create a wide range of aqueous environments for adsorption and self-assembly of complex molecules. The systematic analysis of binding conformations and adsorption free energies of distinct peptides to silica surfaces will be reported separately in a companion paper. The models aid to understand and design silica nanomaterials in 3D atomic resolution and are extendable to chemical reactions

Prediction of Specific Biomolecule Adsorption on Silica Surfaces as a Function of pH and Particle Size

Silica nanostructures are biologically available and find wide applications for drug delivery, catalysts, separation processes, and composites. However, specific adsorption of biomolecules on silica surfaces and control in biomimetic synthesis remain largely unpredictable. In this contribution, the variability and control of peptide adsorption on silica nanoparticle surfaces are explained as a function of pH, particle diameter, and peptide electrostatic charge using molecular dynamics simulations with the CHARMM-INTERFACE force field. Adsorption free energies and specific binding residues are analyzed in molecular detail, providing experimentally elusive, atomic-level information on the complex dynamics of aqueous electric double layers in contact with biological molecules. Tunable contributions to adsorption are described in the context of specific silica surface chemistry, including ion pairing, hydrogen bonds, hydrophobic interactions, and conformation effects. Remarkable agreement is found for computed peptide binding as a function of pH and particle size with respect to experimental adsorption isotherms and ζ-potentials. Representative surface models were built using characterization of the silica surfaces by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), thermalgravimetric analysis (TGA), ζ-potential, and surface titration measurements. The results show that the recently introduced interatomic potentials (Emami et al. <i>Chem. Mater.</i> <b>2014</b>, <i>26</i>, 2647) enable computational screening of a limitless number of silica interfaces to predict the binding of drugs, cell receptors, polymers, surfactants, and gases under realistic solution conditions at the scale of 1 to 100 nm. The highly specific binding outcomes underline the significance of the surface chemistry, pH, and topography

Oligosiloxane-Based Epoxy Vitrimers: Adaptable Thermosetting Networks with Dual Dynamic Bonds

Embedding dynamic covalent bonds into polymer compositions transforms static thermosets into active materials with the reprocessability of thermoplastics and the bulk properties of cross-linked networks. This class of next-generation materials, called covalent adaptable networks, shows significant promise in composites, soft optoelectronics, and robotics. Herein, we synthesized two oligosiloxane-based epoxy networks that provide fast dynamic bond exchange. Oligosiloxane diepoxides were cured with stoichiometric amounts of 1,2-phenylenediacetic acid to generate epoxy acid networks with two dynamic covalent bonding mechanisms. The resulting polymer networks provided access to fast stress-relaxation times (1–10 min) at temperatures of only 130 °C with excellent reprocessability