Strong Nonbonded Interactions Research Articles

The Determination of Free Energy of Hydration of Water Ions from First Principles.

We model the autoionization of water by determining the free energy of hydration of the major intermediate species of water ions. We represent the smallest ions─the hydroxide ion OH-, the hydronium ion H3O+, and the Zundel ion H5O2+─by bonded models and the more extended ionic structures by strong nonbonded interactions (e.g., the Eigen H9O4+ = H3O+ + 3(H2O) and the Stoyanov H13O6+ = H5O2+ + 4(H2O)). Our models are faithful to the precise QM energies and their components to within 1% or less. Using the calculated free energies and atomization energies, we compute the pKa of pure water from first principles as a consistency check and arrive at a value within 1.3 log units of the experimental one. From these calculations, we conclude that the hydronium ion, and its hydrated state, the Eigen cation, are the dominant species in the water autoionization process.

Mechanistic Study of Novel Dipeptidyl Peptidase IV Inhibitory Peptides from Goat's Milk Based on Peptidomics and In Silico Analysis.

Two novel dipeptidyl peptidase IV (DPP-IV) inhibitory peptides (YPF and LLLP) were discovered from goat milk protein by peptidomics, in silico analysis, and in vitro assessment. A total of 698 peptides (<23 AA) were successfully identified by LC-MS/MS from goat milk hydrolysates (hydrolyzed by papaian plus proteinase K). Then, 105 potential DPP-IV inhibitory peptides were screened using PeptideRanker, the ToxinPred tool, Libdock, iDPPIV-SCM, and sequence characteristics. After ADME, physicochemical property evaluation, and a literature search, 12 candidates were efficiently selected and synthesized in vitro for functional validation. Two peptides (YPF and LLLP) were found to exert relatively high in vitro chemical system (IC50 = 368.54 ± 12.97 μM and 213.99 ± 0.64 μM) and in situ (IC50 = 159.46 ± 17.40 μM and 154.96 ± 8.41 μM) DPP-IV inhibitory capacities, and their inhibitory mechanisms were further explored by molecular docking. Our study showed that the formation of strong non-bonding interactions with the core residues from the pocket of DPP-IV (such as ARG358, PHE357, GLU205, TYR662, TYR547, and TYR666) might primarily account for the DPP-IV inhibitory activity of two identified peptides. Overall, the two novel DPP-IV inhibitory peptides rapidly identified in this study can be used as functional food ingredients for the control of diabetes.

Compelling DNA intercalation through 'anion-anion' anti-coulombic interactions: boron cluster self-vehicles as promising anticancer agents.

Anticancer drugs inhibit DNA replication by intercalating between DNA base pairs, forming covalent bonds with nucleotide bases, or binding to the DNA groove. To develop safer drugs, novel molecular structures with alternative binding mechanisms are essential. Stable boron hydrides offer a promising alternative for cancer therapy, opening up additional options like boron neutron capture therapy based on 10B and thermal neutron beams or proton boron fusion therapy using 11B and proton beams. These therapies are more efficient when the boron compound is ideally located inside cancer cells, particularly in the nucleus. Current cancer treatments often utilize small, polycyclic, aromatic, planar molecules that intercalate between ds-DNA base pairs, requiring only a spacing of approximately 0.34 nm. In this paper, we demonstrate another type of intercalation. Notably, [3,3'-Fe(1,2-C2B9H11)2]-, ([o-FESAN]-), a compact 3D molecule measuring 1.1 nm × 0.6 nm, can as well intercalate by strong non-bonding interactions preferentially with guanine. Unlike known intercalators, which are positive or neutral, [o-FESAN]- is a negative species and when an [o-FESAN]- molecule approaches the negatively charged DNA phosphate chain an anion-anion interaction consistently anti-electrostatic via Ccluster-H⋯O-P bonds occurs. Then, when more molecules approach, an elongated outstandingly self-assembled structure of [o-FESAN]--[o-FESAN]- forms moving anions towards the interthread region to interact with base pairs and form aggregates of four [o-FESAN]- anions per base pair. These aggregates, in this environment, are generated by Ccluster-H⋯O-C, N-H⋯H-B and Ccluster-H⋯H-B interactions. The ferrabis(dicarbollide) boron-rich small molecules not only effectively penetrate the nucleus but also intercalate with ds-DNA, making them promising for cancer treatment. This amphiphilic anionic molecule, used as a carrier-free drug, can enhance radiotherapy in a multimodal perspective, providing healthcare professionals with improved tools for cancer treatment. This work demonstrates these findings with a plethora of techniques.

Synthesis, spectroscopic investigations, topological non-covalent interactions, chemical reactivity, molecular docking and molecular dynamic simulation on piperazine succinate - A potential antimicrobial compound

Piperazine Succinate (PZSN) has been synthesized and extensively analyzed using various spectroscopic techniques, including FT-IR, FT-Raman and NMR spectra. In optimizing geometry, a comparison between the bond lengths of C2-H25 in PZSN (1.0999 Å) and SN (1.0938 Å) reveals an increase of 0.0061 Å in the C21-H25 bond length for PZSN. This elongation can be attributed to the significant C21-H25‧‧‧N2 intermolecular hydrogen bonding interaction. Natural bond orbital analysis of the molecule indicates a significant charge transfer between LP(1) N2 → σ* (C21-H25) leads to a substantial stabilization energy of 6.73 kcal/mol, providing strong evidence for the presence of intermolecular hydrogen bonding interactions. The strong band observed in the IR spectrum at 3510 cm−1, assigned to N2-H16 stretching mode, which clearly supports the formation of a hydrogen bond between succinate C=O and piperazine NH group. The complete vibrational assignments and the force constants has been reported for PZSN by using B3LYP/6–311++G** method. Molecular parameters, such as Frontier Molecular Orbital (FMO) energies, Fukui functions and Molecular Electrostatic Potential (MEP), have been calculated to gain insights into the stability and reactivity of the molecule. According to the QTAIM analysis, the presence of a strong non-bonded interaction is supported by the largest interaction energy observed at the C21-H25‧‧‧N2 bond critical point (BCP), which is 3.42 kcal/mol. Furthermore, Reduced Density Gradient (RDG) and Independent Gradient Model (IGM) approaches have been performed to disclose the attractive contacts like hydrogen bonds, repulsive and van der Waals interconnections. The molecule's drug-likeness has been assessed using Lipinski's rule and ADMET contour analysis indicated its potential for pharmaceutical applications. Molecular docking modeling have been conducted on antimicrobial proteins to explore the structure-activity relationship and antimicrobial testing confirmed the molecule's efficacy against microbial infections. Molecular dynamic simulations have been utilized to examine the stability of protein-ligand complex.

Methanol-Assisted ADMET Polymerization of Semiaromatic Amides

A method for the acyclic diene metathesis polymerization of semiaromatic amides is described. The procedure uses second-generation Grubbs' catalyst and N-cyclohexyl-2-pyrrolidone (CHP), a high boiling, polar solvent capable of solubilizing both monomer and polymer. The addition of methanol to the reaction was found to significantly increase polymer molar mass although the role of the alcohol is currently not understood. Hydrogenation with hydrogen gas and Wilkinson's catalyst resulted in near-quantitative saturation. All polymers synthesized here exhibit a hierarchical semicrystalline morphology driven by ordering of aromatic amide groups via strong nonbonded interactions. Furthermore, the melting points can be tuned over a >100 °C range by precise substitution at just one of the backbone positions on each mer (<5% of the total).

Antifouling polymers for nanomedicine and surfaces: recent advances.

Antifouling polymers are materials that can resist nonspecific interactions with cells, proteins, and other biomolecules. Typically, they are hydrophilic polymers with polar or charged moieties that are capable of strong nonbonding interactions with water molecules. This propensity to bind water generates a surface hydration layer that reduces nonspecific interactions with other molecules and is paramount to the antifouling behavior. This property is especially useful for nanoscale applications such as nanomedicine and surface modifications at the molecular level. In nanomedicine, antifouling polymers such as poly(ethylene glycol) and its alternatives play a key role in shielding drug molecules and therapeutic proteins/genes from the immune system within nanoassemblies, thereby enabling effective delivery to target tissues. For coatings, antifouling polymers help to prevent adhesion of cells and molecules to surfaces and are thus valued in marine and biomedical device applications. In this Review, we survey recent advances in antifouling polymers in the context of nanomedicine and coatings, while shining the spotlight on the major polymer classes such as PEG, polyzwitterions, poly(oxazoline)s, and other nonionic hydrophilic polymers.

Unstructured Self-Assembled Molecular Lamella Induces Ultrafast Thermal Transfer through a Cathode/Separator Interphase in Lithium-Ion Batteries

Thermal issues associated with lithium-ion batteries (LIBs) can dramatically affect their life cycle and overall performance. However, the effective heat transfer is deeply restrained by the high thermal resistance across the cathode (lithium cobalt oxide, LCO)-separator (polyethylene, PE) interface. This work presents a new approach to tailoring the interfacial thermal resistance, namely, unstructured self-assembled lamella (USAL). Compared to the popular self-assembled monolayers, although the USAL gives a redundant interface and amorphous molecule patterns, it can also provide many benefits, including easy assembly, more thermal bridges, and ready pressurization. Three small organic molecules (SOMs) were assembled into an LCO-PE interface, providing unique functional groups, -NH2, -SH, and -CH3, to illustrate its energy conversion efficiency. Through molecular dynamics simulations, our results show that the USAL can facilitate interfacial heat transfer remarkably. A 3-aminopropyl trimethoxysilane (APTMS)-coated LCO-PE system with 11.4 Å thickness demonstrates the maximum enhancement of thermal conductance, about 320% of the pristine system. Such enhancement is attributed to the developed double heat passages by strong non-bonded interactions across LCO-SOM and PE-SOM interfaces, a tuned temperature field, and high compatibility between SOMs and PE. Importantly, due to SOMs' amorphous morphology, the pressure can be imposed and further enhance the interfacial heat transfer. Results show the improved thermal conductance rises the most for the APTMS-coated LCO-PE system with 11.4 Å thickness at 10 GPa, almost 685% higher than that of the pristine system. The high efficiency of heat transfer comes as a result of the enhanced binding strength across the LCO-SOM and SOM-PE interface, the reduced phonon scattering in PE and SOMs, and the high LCO stiffness. These investigations are expected to provide a new perspective for modulating the heat transfer across the interphase of LIBs and achieve more effective thermal management for the multi-material system.

Interfacial modification to anomalously facilitate thermal transport through cathode-separator composite in lithium-ion batteries

Effective thermal management is the key to ensuring lithium-ion batteries (LIBs)’ lifetime and performance. However, the high thermal resistance across the cathode-separator interphase considerably limits the fast heat transfer. Adopting the self-assembled monolayers (SAMs) approach, various organic molecules were selected as the heat controllers to modulate the interfacial thermal conductance (ITC) between the cathode, lithium cobalt oxide (LCO), and the separator, polyethylene (PE). Silane-based SAMs molecules with different groups, including –NH2, –SH, and –CH3, were assembled into the LCO-PE composite’s interphase. Through molecular dynamics (MD) simulations, our results demonstrate SAMs molecules-decorated LCO-PE nanocomposites give a notably improved interfacial heat transfer but are of different magnitude. Such difference mainly results from the different non-bonded interactions and compatibility between SAMs molecules and PE. Of the three SAMs molecules, the assembled 3-aminopropyl trimethoxysilane (APTMS) featuring –NH2 groups improves the ITC the most, about 303.29% in comparison with the pristine interface. Furthermore, these findings help elucidate the underlying mechanisms of how SAMs molecules improve heat transfer across the LCO-PE interphase. Specifically, such enhancement is greatly attributed to the unique SAMs molecules, which build the new heat transfer pathways between LCO and PE, straighten SAMs molecules’ morphology, remove the discontinuities in the temperature field, develop the strong non-bonded interactions between SAMs molecules and PE, and strengthen the coupling vibration of two materials. These investigations provide a new perspective for designing composite’s interphase to mediate the heat transfer and achieve more effective thermal management across the interphase.

Boron Nitride- and Graphene-Supported Trimetallic Yolk–Shell and Hollow Nanoparticles

In this study, the stability of yolk–shell and hollow Ni@PdAu and PdAu@Ni nanoparticles in free state and supported on boron nitride and graphene substrates was investigated at 300 K using molecular dynamics simulations. To this end, analyses of several parameters such as relative stability, surface energy, coordination number, number of surface atoms, bond length, displacement vector, and atomic strain were employed. Results show that the yolk–shell and hollow Ni@PdAu nanoparticles are more stable than yolk–shell and hollow PdAu@Ni nanoparticles in free and supported states due to the less surface energy. Furthermore, results show that the hollow nanoparticles are more stable than yolk–shell nanoparticles in all states. In addition, results show that the void and hollow core in yolk–shell and hollow nanoparticles are unstable and collapse due to the contraction and expansion of the core and shell, which lead to the formation of the core–shell and quasi–Janus structures in hollow and yolk–shell nanoparticles, respectively. Moreover, nanoparticles create a quasi–ellipsoid structure with low density and high coordination number on the boron nitride. The yolk–shell and hollow nanoparticles are the most stable on the boron nitride due to the smaller displacement and variation of the atomic bond length and lower atomic strain, which are caused by the strong nonbonded interaction, good atomic match, and placement of the Ni, Au, and Pd atoms on the middle position of the boron nitride hexagonal structure. Generally, this study demonstrated that the stability of yolk–shell and hollow nanoparticles can be tuned by the substrate–nanoparticle interaction.

Seawater transport through quartz pores of coastal aquifers: A molecular dynamics study

Classical molecular dynamics (MD) simulations were carried out to analyze the diffusion of saltwater in aquifers at a salinity of 3.5 wt%, and temperatures between 5 and 50 °C. Porous systems were built from the quartz stable alpha phase to represent typical compositions and structures reported for aquifer surfaces. The SPC/E model was used to describe the interactions between water molecules in the system, while the quartz intramolecular bonds were characterized through a Buckingham–type potential. It was demonstrated from MD trajectories that the diffusivity in small pores of both, water molecules and NaCl ions, depends strongly on the crystallographic orientation of quartz surfaces exposed to the quartz/saltwater interface. The three most stable quartz surfaces, favor the diffusion of Na+ over Cl– in confinement, principally due to strong nonbonded interactions between chloride and silicon atoms with partial positive charges that remain exposed at surface terminations. The exposed quartz surfaces also determined the structure of confined water and the distribution of ions along the normal direction of the pore surfaces. The results reported in this study provide a molecular-scale description of the diffusive transport of seawater through quartz pores, which represents a potential scenario for seawater intrusion in coastal aquifers.

Thermodynamic, structural, and mechanical properties of fluoropolymers from molecular dynamics simulation: Comparison of force fields

Copolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) make up the largest volume of fluoroelastomers sales. Within this family, Viton A (VDF/HFP 60/40 wt%, 66% fluorine) is most important commercially. Determination of thermomechanical properties of such fluorinated polymers is still limited to experimentation or empirical models, while computational prediction of their physical properties is still at its infancy. Considerable efforts have been made to develop molecular force fields for the prediction of properties of various crystalline as well as amorphous fluorinated substances. These force fields were parametrized based on small molecules or oligomers, hence their transferability to longer and more complex (i.e. copolymer) chain architectures must be tested. In this work, thermodynamic, structural, and mechanical properties of the Viton A fluoroelastomer are evaluated using atomistic molecular dynamics (MD) simulations for several force fields and compared with available experimental data. Differences in molecular structure of the chains modeled by different force fields are shown to substantially influence thermodynamic and mechanical behavior. OPLS-derived force fields give rise to extended chains that are more mobile and result in softer material that lack in molecular structure. Other force fields rely on strong nonbonded interactions that interfere with chain dynamics and mechanics. PCFF is shown to most satisfactorily predict structural properties and thermal volumetric behavior around Tg, demonstrating relatively successful transferability to long linear polymer chains.

Halogenated derivatives of methotrexate as human dihydrofolate reductase inhibitors in cancer chemotherapy

Methotrexate is a widely used anti-metabolite in cancer chemotherapy. A series of halogenated drugs is designed from Methotrexate to assess their interactions with human dihydrofolate reductase. The aim of this study is to evaluate the performance of the modified drugs compared to the parent Methotrexate. Density Functional Theory is employed to optimize these drugs. Molecular docking calculation of these optimized drugs against dihydrofolate reductase is performed to find out binding affinity. In addition, molecular dynamics simulation is considered for the complexes of best two modified drugs with their receptors. Modifications by the halogens show significant changes in the charge distribution, dipole moment, thermodynamic stability, enthalpy and free energy. The highest binding affinity value (−36.401 KJ/mol) was obtained for M14. Hybrid quantum mechanics/molecular mechanics calculation shows a binding energy of −255.140 KJ/mol. Modified drugs have significant hydrogen and non-covalent bonding interactions with amino acids of the receptor. Molecular dynamics simulation disclosed that the root-mean-square-deviation of the alpha carbon associated with M6-1KMV and M14-1KMV complexes is 2.367 Å and 2.622 Å, respectively. Moreover, the interactions between modified drugs and receptor are mostly persevered in 25 nanosecond molecular dynamics simulation. Ensemble-based docking also confirmed that modified drugs show strong non-bonding interactions with different crystallographic and molecular dynamics based conformers. The best scored drugs show considerable pharmacokinetic properties. Modified derivatives M5, M6, M8, M10, M13 and M14 show the better binding affinity and a good number of hydrogen and other non-bonding interactions with the target protein which are similar to other anticancer drugs.Communicated by Ramaswamy H. Sarma

Structure and chemical reactivity of lithium-doped graphene on hydrogen-saturated silicon carbide

Herein, we studied the structure and hydrogenation of graphene supported on hydrogen-terminated SiC, with intercalated Li atoms. Strong nonbonded interactions occur between graphene and the Si–H groups. The latter were found to be significantly augmented by the intercalation of Li atoms, which enhanced the SiH::pi interactions. Although the electronic structure of graphene did not experience significant changes when supported on hydrogen-terminated SiC, a small gap of 0.04 eV was computed at the HSEH1PBE/6-31G* level. The clustering of Li atoms over graphene was prevented by the SiC support. A significant enhancement of reactivity could be corroborated due to the presence of intercalated Li atoms, given that the C–H binding energy was increased by almost 1 eV with respect to pristine graphene. We expect that hydrogen-terminated SiC with intercalated Li atoms can be used to enhance the chemical reactivity of graphene, given that it strongly interacts with graphene but does not compete for the electrons donated by Li.

A Two-Dimensional Supramolecular Carpet with Widely Uniform Interfacial Structure

Thin films composed of organic molecules are receiving much attention for their practical uses in various organic devices such as organic light-emitting diodes (OLEDs), organic thin film transistors (OTFTs), and organic photovoltaics (OPVs). Particularly, it has been recognized that interfaces between organic thin films and metal electrodes play crucial roles in operation characteristics of organic devices in recent years [1-3]. The interfaces between organic thin films and metal electrodes are where charge-carriers are injected into organic active layers, and various interactions at the interfaces (i.e., surface-molecule and intermolecular interactions) are of great importance in the formation of intermolecular networks and organic epitaxy which have a strong correlation with their electronic structures [1,3]. Both geometric and electronic structures of the interfaces between organic thin films and metal electrodes have profound effects on the performance of organic devices, and it is necessary to understand and control such interfacial structures for further development and enhancement of organic devices. Despite extensive studies, however, the formation of organic thin films with widely uniform interfacial structures on metal surfaces has been suffered from the limitation of molecular ordering imposed by irregular surface structures, due to the relatively weak intermolecular interactions between organic molecules compared to metallic, covalent, and coordinate bonds. Thus, a new approach is required to achieve the formation of widely uniform organic thin films without breaking structural integrity and to enhance the film uniformity. Here we demonstrate that well-balanced interfacial interactions, based on a careful design of the system, enable the formation of a two-dimensional supramolecular carpet with widely uniform interfacial structure and high adaptability on a metal surface, which can be extended over multiple surface steps and terraces without breaking structural integrity [4]. Strong intermolecular interactions in a planar fashion, symmetry of compounds, and localized surface-molecule interactions were considered in our strategies and principles of the system design. For this work, bis([1,2,5]thiadiazolo)-tetracyanoquinodimethane (BTDA-TCNQ) was used as a key element to form the two-dimensional supramolecular carpet [5], and its geometric and electronic structures were investigated using scanning tunneling microscopy and scanning tunneling spectroscopy under ultrahigh-vacuum conditions. Tetracyanoquinodimethane (TCNQ) is one of the strongest organic electron acceptors which is a widely used class in organic electronics, and it forms a strongly bonded donor-acceptor complex with tetrathiafulvalene (TTF). TCNQ and TTF are self-organized into ordered domains of the strongly bonded donor-acceptor complexes if TCNQ and TTF satisfy a 1:1 stoichiometry. Based on these molecular properties, we designed a model system of BTDA-TCNQ, which integrates both structural properties of TCNQ and TTF with a single molecule. The strong non-bonding intermolecular interactions induced by two electrostatically opposite symmetry axes of the molecule, balanced with surface-molecule interactions and site-specific rearrangements of the BTDA-TCNQ molecules near surface step edges, enables “seamless” growth mode for the formation of supramolecular carpet. It results in widely uniform interfacial structures covering over multiple surface steps and terraces even on the pre-annealed amorphous gold (Au) surface prepared on a glass substrate. In addition, the electronic structures projected onto the supramolecular carpet on the Au surface show different types and dimensionalities corresponding to the energy region and the local position of the system. These result suggest that the supramolecular carpet has great potential for applications in organic electronics, and also provide important guidelines to develop novel materials for seamless growth mode on various surfaces. [1] S. R. Forrest, Chem. Rev. 97, 1793 (1997). [2] H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater. 11, 605 (1999). [3] D. E. Hooks, T. Fritz, and M. D. Ward, Adv. Mater. 13, 227 (2001). [4] J.-H. Kim, J.-C. Ribierre, Y. S. Yang, C. Adachi, M. Kawai, J. Jung, T. Fukushima, and Y. Kim, Nat. Commun. 7, 10653 (2016) [5] Y. Yamashita, T. Suzuku, T. Mukai, and G. Saito, J. Chem. Soc. Chem. Commun. 1985, 1044 (1985).

Design, synthesis and biological activity evaluation of novel pefloxacin derivatives as potential antibacterial agents

A facile and efficient method for the synthesis of novel derivatives of pefloxacin hydrazone via condensation of various aldehydes and pefloxacin acid hydrazide using conventional as well as solvent-free microwave irradiation methods was reported. The biological activity of these compounds was screened using array of techniques and found to exhibit promising broad spectrum of antibacterial activity against Gram-positive and Gram-negative bacteria, moderate antimycobacterial activity against Mycobacterium smegmatis and moderate antifungal activity opposed to Aspergillus clavatus and Candida albicans. Cytotoxic assay of the title compounds was evaluated against human Pc-3 cancer cell lines, and interestingly, very good anticancer properties were shown by the compounds. To explore the binding mode of the compounds and understand the key active site residues with Staphylococcusaureus DNA gyrase, a molecular docking study has been performed. The results of the docking studies of the reported derivatives were quite promising and manifested strong non-bonded interactions with DNA Gyrase.

Ferrocene-based guanidine derivatives: In vitro antimicrobial, DNA binding and docking supported urease inhibition studies

Some novel ferrocenyl guanidines 1–8 were synthesized and characterized by different spectroscopic methods, elemental analysis and single crystal X-rays diffraction techniques. The crystallographic studies revealed that the existence of the strong non-bonding interactions facilitate these molecules to interact with biological macro-molecules like DNA that described to inherit good biological activities. The DNA interaction studies carried out by cyclic voltammetry (CV) and UV–visible spectroscopy are in close agreement with the binding constants (K) (0.79–5.4) × 105 (CV) and (0.72–5.1) × 105 (UV–vis). The shift in peak potential, current and absorption maxima of the studied ferrocenyl guanidines in the presence of DNA revealed that CV coupled with UV–vis spectroscopy could provide an opportune to characterize metal-based compounds–DNA interaction mechanism, a prerequisite for the design of new anticancer agents and understanding the molecular basis of their action. The compounds 1–8 have been screened for their antibacterial, antifungal and urease inhibition potency. A concurrent in silico study has also been applied on ferrocene moiety impregnated guanidines 1–8 to identify most active compounds having for inhibiting the activity of urease (pdb id 3LA4). Most of the compounds were found as potent inhibitors of urease and the compound 1 was found to be the most active with an IC50 of 16.83 ± 0.03 μM. The docking scores are in close agreement with the in vitro obtained IC50 values of inhibitors 1–8.

Engineering Physical Properties of Asphalt Binders through Nanoclay–Asphalt Interactions

Increasing traffic volumes, heavier loads, higher tire pressures, and performance problems with asphalt concrete mixtures under adverse conditions are leading state highway departments to using modified binders to enhance the physical properties of asphalt mix. This paper reports the advantages of modified nanoclays (NC) as asphalt modifiers. Dynamic mechanical analysis, flexural creep stiffness, and flexural tests were conducted on the NC-asphalt nanocomposite. In addition, Fourier transform infrared (FTIR) testing was conducted to evaluate the nature of the interactions between NC and asphalt. Increasing NC concentration in asphalt enhances temperature susceptibility of asphalt, as well as increasing the complex modulus in addition to decreasing phase angle. FTIR experiments indicate a significant change in Si─O vibration from NC, indicating strong nonbonded interactions of Si─O tetrahedra with asphalt. These changes in Si─O vibrations suggest both distortion in Si─O tetrahedra as well as stronger interactions between asphalt and NC. Further, X-ray diffraction (XRD) testing results show intercalation of asphalt in clay galleries indicated by an enlarged d-spacing of up to 43.17 Å. The d-spacing decreases with NC content. These experiments suggest that the addition of engineered nanoclays to asphalt has tremendous potential in tailoring the properties of asphalt based on type of application.

Molecular Dynamics Simulation of Interactions on Graphene/Polypyrrole Nanocomposites Interface

Graphene receives world-wide attention due to its unique two-dimensional structure and exceptional electrical and mechanical properties. The graphene/polypyrrole (Gr/PPy) nanocomposites, have been synthesized via dispersing the graphene into polymers, which can be widely used in microwave absorption, supercapacitors and electronic devices. Molecular dynamics simulations are carried out to investigate the microscopic mechanism of interactions on Gr/PPy nanocomposites interface. The interaction configurations, intermolecular interaction energy and pair correlation functions between graphene and PPy are computed. The curves of temperature, energy evolution and interaction energy analysis indicate that Gr/PPy system reaches equilibrium in a relatively short time, and it is a thermodynamic equilibrium system. The interaction configurations show that there is an attractive interaction between the graphene and PPy. Pair correlation functions reveal that there are van der Waals interactions and hydrogen bond between the graphene and PPy, which are strong short-range non-bonded interactions. The strong interactions mainly arise from the sp2 hybridized π-conjugated structure of graphene and PPy, which engenders π-π interaction.

Mechanical behavior of highly cross-linked polymer networks and its links to microscopic structure

Highly cross-linked polymer (HCP) networks are becoming increasingly important as high-performance adhesives and multifunctional composite materials. Because of their cross-linked molecular architectures, HCPs can be strong but brittle. One key goal in improving the performance of an HCP is to increase toughness without sacrificing strength. Using large scale molecular-dynamics simulation, we compare and characterize the mechanical behavior of two model HCPs under tensile deformation. In the first case, bond angles among any three connected monomers are unconstrained and in the second case we impose harmonic tetrahedral bond angle constraints. We perform a detailed microstructural analysis that establishes a unique correlation between macroscopic mechanical behavior and the microscopic structure of an HCP. While, in the unconstrained system, strain-hardening behavior is observed that is attributed to the formation of microvoids, the void growth is completely arrested in the constrained system and no strain hardening is observed. Moreover, after the initial strain-hardening phase, the unconstrained system displays the same stress-strain behavior as that of a constrained network. Strain hardening makes the unconstrained system ductile while it retains the same tensile strength as the constrained system. We suggest that bond angle flexibility of cross-linkers might be a possible means to control ductility in an HCP network at a constant cross-linker density. We have also studied the effect of temperature, strain rate, and intermonomer nonbonded interaction strength on the stress-strain behavior. Interestingly at a strong intermonomer nonbonded interaction strength, no strain hardening is observed even in the unconstrained system and fracture sets in at around 1% strain, similar to what is observed in an experimental system such as epoxy and vinyl-ester based thermosets. This indicates that strong nonbonded interactions play a key role in making an HCP strong but brittle.

A molecular dynamics study on the strength and ductility of high Tg polymers

High temperature performance of polymers depend mainly on their glass transition temperature Tg. Polymers with high Tg, however, often tend to have low toughness. In this work, a model polyethylene like polymer is analysed to understand the reason for this phenomenon. To this end, molecular dynamics simulations are performed to investigate the effect of various factors on glass transition and yielding. Two generic modifications to the model polymer are suggested for enhancing its Tg and the effect of these modifications on the stress–strain behaviour are studied. Results indicate that when non-bonded interactions govern the energetics of the deformation process, a polymer with a high energetic barrier to torsional changes will lead to a higher Tg without significantly compromising the toughness. On the other hand, strong non-bonded interactions also promote high Tg but lead to a brittle material.