Songqing Hu

Effect of Interfacial Bonding on Interphase Properties in SiO2/Epoxy Nanocomposite: A Molecular Dynamics Simulation Study

Atomistic molecular dynamics simulations have been performed to explore the effect of interfacial bonding on the interphase properties of a nanocomposite system that consists of a silica nanoparticle and the highly cross-linked epoxy matrix. For the structural properties, results show that interfacial covalent bonding can broaden the interphase region by increasing the radial effect range of fluctuated mass density and oriented chains, as well as strengthen the interphase region by improving the thermal stability of interfacial van der Waals excluded volume and reducing the proportion of cis conformers of epoxy segments. The improved thermal stability of the interphase region in the covalently bonded model results in an increase of ∼21 K in the glass transition temperature (Tg) compared to that of the pure epoxy. It is also found that interfacial covalent bonding mainly restricts the volume thermal expansion of the model at temperatures near or larger than Tg. Furthermore, investigations from mean-square displacement and fraction of immobile atoms point out that interfacial covalent and noncovalent bonding induces lower and higher mobility of interphase atoms than that of the pure epoxy, respectively. The obtained critical interfacial bonding ratio when the interphase and matrix atoms have the same mobility is 5.8%. These results demonstrate that the glass transitions of the interphase and matrix will be asynchronous when the interfacial bonding ratio is not 5.8%. Specifically, the interphase region will trigger the glass transition of the matrix when the ratio is larger than 5.8%, whereas it restrains the glass transition of the matrix when the ratio is smaller than 5.8%.

Glass transition investigations on highly crosslinked epoxy resins by molecular dynamics simulations

Molecular dynamics simulations at the atomistic level were performed to investigate the glass transition of a highly crosslinked thermoset epoxy resin system composed of diglycidyl ether bisphenol A and isophorone diamine. The crosslinked model was first constructed using a cyclic dynamic method, and extended by investigating the effect of conversion degree on the static properties of local structure, internal energy and volume shrinkage. Based on this model, a systematic investigation on volume, energy and dynamic properties against temperature was made, which determined the glass transition temperature (Tg). The Tgs obtained from various volumetric and energy properties agree well with the differential scanning calorimetry experimental data available, yet a dynamic Tg obtained from the diffusion coefficient is relatively higher. Moreover, the investigation on epoxy segmental dynamics confirmed that the glass transition of the highly crosslinked epoxy resin has a strong dependence on the backbone bond torsional kinetics.

Atmospheric Corrosion Behavior of Extruded Aluminum Alloy 7075-T6 After Long-Term Field Testing in China

Abstract The long-term atmospheric corrosion behavior of extruded aluminum alloy 7075-T6 (UNS A97075) was investigated by weight loss, corrosion morphology, loss in mechanical properties, and corro...

Tunable Permeability of Cross-Linked Microcapsules from pH-Responsive Amphiphilic Diblock Copolymers: A Dissipative Particle Dynamics Study

Using dissipative particle dynamics simulation, we probe the tunable permeability of cross-linked microcapsules made from pH-sensitive diblock copolymers poly(ethylene oxide)-b-poly(N,N-diethylamino-2-ethyl methacrylate) (PEO-b-PDEAEMA). We first examine the self-assembly of non-cross-linked microcapsules and their pH-responsive collapse and then explore the effects of cross-linking and block interaction on the swelling or deswelling of cross-linked microcapsules. Our results reveal a preferential loading of hydrophobic dicyclopentadiene (DCPD) molecules in PEO-b-PDEAEMA copolymers. Upon reduction of pH, non-cross-linked microcapsules fully decompose into small wormlike clusters as a result of large self-repulsions of protonated copolymers. With increasing degree of cross-linking, the morphology of the microcapsule becomes more stable to pH change. The highly cross-linked microcapsule shell undergoes significant local polymer rearrangement in acidic solution, which eliminates the amphiphilicility and therefore enlarges the permeability of the shell. The responsive cross-linked shell experiences a disperse-to-buckle configurational transition upon reduction of pH, which is effective for the steady or pulsatile regulation of shell permeability. The swelling rate of the cross-linked shell is dependent on both electrostatic and nonelectrostatic interactions between the pH-sensitive groups as well as the other groups. Our study highlights the combination of cross-linking structure and block interactions in stabilizing microcapsules and tuning their selective permeability.

Exfoliation Corrosion of Extruded AA2024-T4 in the Industrial and Coastal-Industrial Environments

Exfoliation of extruded AA2024-T4 (UNS A92024) in the industrial and coastal-industrial environments was investigated by the field test and laboratory-accelerated test. Results show that the exfoliation corrosion was influenced by the alloy structure and the type of the atmospheric environment, especially the concentration of SO42− in the environment. In the field test, AA2024-T4 suffered slighter exfoliation in both environments than that in the coastal environments. The side wall with a thinner coarse-grain structure of extruded AA2024-T4 suffered more severe exfoliation than the bottom with a thicker coarse-grain structure after a 20-year exposure. Energy-dispersive spectroscopy (EDS) results show that the corrosion products of extruded AA2024-T4 exposed in both environments contained a small amount of S in addition to a large amount of Al and O. In the laboratory-accelerated test, both the critical relative humidity (RH) and induction period for exfoliation in the simulated coastal-industrial environm...

Effect of graphene dispersion on the equilibrium structure and deformation of graphene/eicosane composites as surrogates for graphene/polyethylene composites: a molecular dynamics simulation

Molecular dynamics simulations are used to investigate the effect of graphene dispersion on the equilibrium structure and deformation of graphene/eicosane composites. Two graphene sheets with four different interlayer distances are incorporated, respectively, into a eicosane matrix to form graphene/eicosane composites representing different graphene dispersions. With greater graphene dispersion, the “adsorption solidification” of the eicosane increases, where eicosane molecular lamination, orientation, and extension become more uniform and stronger. In addition, eicosane molecular motion is inhibited more in the direction perpendicular to graphene surfaces. When these graphene/eicosane composites are deformed, the free volume initially increases slowly due to small, scattered voids. After reaching the yield strains, the free volume rises sharply as the structures of composites are damaged, and small voids merge into large voids. The damage always occurs in the region of the composite with the weakest “adsorption solidification.” Since this effect is stronger when the graphene sheets are more dispersed, more complete dispersion results in higher composite yield stresses. Lessons from these simulations may provide some insights into graphene/polyethylene composites, where suitable models would require very long equilibration times.

Controllable Multigeometry Nanoparticles via Cooperative Assembly of Amphiphilic Diblock Copolymer Blends with Asymmetric Architectures

Multigeometry nanoparticles with high complexity in composition and structure have attracted significant attention for enhanced functionality. We assess a simple but versatile strategy to construct hybrid nanoparticles with subdivided geometries through the cooperative assembly of diblock copolymer blends with asymmetric architectures. We report the formation of multicompartmental, vesicular, cylindrical, and spherical structures from pure AB systems. Then, we explore the assemblies of binary AB/AC blends, where the two incompatible, hydrophobic diblock copolymers subdivide into self-assembled local geometries, and the complexity of the obtained morphologies increases. We expand the strategy to ternary AB/AC/AD systems by tuning the effect of phase separation of different hydrophobic domains on the surface or internal region of the nanoparticle. The kinetic control of the coassembly in the initial stage is crucial for controlling the final morphology. The interactions of copolymers with different block lengths and chemistries enable the stabilization of interfaces, rims and ends of subdomains in the hybrid multigeometry nanoparticles. With further exploration of size and shape, the dependence of local geometry on the volume fraction is discussed. We show an efficient approach for controllable multigeometry nanoparticle construction that will be useful for multifunctional and hierarchical nanomaterials.

Dissipative particle dynamics simulations reveal the pH-driven micellar transition pathway of monorhamnolipids

Dissipative particle dynamics (DPD) simulation has been used to study the effect of pH on the morphology transition of micelles assembled by monorhamnolipids (monoRLs). Results show that micellar structures and transition modes with increasing mass concentrations are multiform due to the changeable hydrophilicity of pH-responsive beads at different pH levels. Various chaotic multilayer aggregations of monoRLs are observed at low pH (pH<4.0) whereas well-ordered single-layer structures are obtained at high pH (pH>7.4). At medium pH region (4.0<pH<7.4), morphologies with semi-chaotic structures will transform from multilayer to single layer with increasing pH due to the hydrophilicity increase of micelles. In addition, three micellar transition modes are built by varying the hydrophilicity of pH-responsive beads and validated by solvent accessible surface areas (SASAs) as a function of mass concentrations. This work is expected to trigger further studies on the stimuli-driven phenomena of glycolipids.

pH-Induced evolution of surface patterns in micelles assembled from dirhamnolipids: dissipative particle dynamics simulation

Dissipative particle dynamics (DPD) simulation is used to study the effect of pH on the morphological transition in micelles assembled from dirhamnolipids (diRLs), and analyze the pH-driven mechanism and influence factors of micellar surface patterns. At pH < 4.0, various multilayer structures with homogeneous surface patterns are observed, whereas diRLs can self-assemble into novel anisotropic morphologies with phase-separated surface patterns at pH > 7.4, such as patchy spherical micelles, rod-like micelles with helical surface patterns and a lamellar phase with anisotropic surface patterns. The change in a surface pattern results from the diverse molecular arrangement in the course of assembly due to the deprotonation of carboxyl groups. Further studies show that influence factors, such as molecular structure, solvent selectivity and intramolecular interaction, are closely associated with the changes in surface patterns and topological structures. In detail, decreasing the critical packing parameter of rhamnolipids, increasing the solution polarity and weakening the compatibility between rhamnose rings and alkyl chains are all beneficial to the formation of phase-separated surface patterns. Remarkably, a wider variety of surface patterns (randomly anisotropic surface patterns) can be further obtained with the different factors mentioned above. This work is expected to extend the applications of diRLs to advanced functional materials like drug delivery, optoelectronics and nanofiltration membranes.

Molecular dynamics simulations of the aggregation behaviour of overlapped graphene sheets in linear aliphatic hydrocarbons

Abstract Molecular dynamics simulations were used to investigate the aggregation of two partially overlapped graphene sheets in hexane, dodecane and eicosane. When partially overlapped graphene sheets are adjacent to one another, they will expel the adsorbed layers of the solvent molecules on the graphene surface, and the amount of overlap will increase. When the overlapped regions of the graphene sheets are separated by solvent molecules, they cannot expel the adsorption layers between them, and so the sheets remain separated. The driving force for aggregation is the van der Waals interaction between the two graphene sheets, while the van der Waals interaction between the graphene sheets and the solvent molecules inhibits graphene aggregation. The diffusion rate of the hydrocarbon molecules with shorter chain lengths is higher. Thus, they diffuse faster during graphene aggregation, which leads to a higher rate of graphene overlapping in the shorter hydrocarbons. This work provides useful insights into graphene aggregation in linear hydrocarbon solvents of varying lengths at the nanoscale.