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

Morphology-Driven Nanofiller Size Measurement Integrated with Micromechanical Finite Element Analysis for Quantifying Interphase in Polymer Nanocomposites

This study focused on an innovative practical method using computer vision for particle size measurement, which serves as a key precursor for predicting the elastic modulus of polymer nanocomposites. This approach involved the morphological segmentation of the nanodispersed phase. It aimed, for the first time, to address the impractical conditions resulting from the assumption of idealized single-particle sizes in a monodispersed system during modeling. Subsequently, a micromechanical finite element framework was employed to determine the interphase thickness and modulus in ultrahigh molecular weight polyethylene/nanozeolite composites, following the quantification of nanoparticle sizes. The size measurement approach relied on morphological images extracted from scanning electron microscopy micrographs of impact-fractured surfaces. To compute the interphase thickness, experimental data was fitted to an interphase-inclusive upper-bound Hashin-Shtrikman model, with the measured average particle size per composition serving as a crucial input. Subsequently, the interphase elastic modulus was computed based on its thickness, employing a hybrid modified-Hashin-Hansen and Maxwell model. These estimated interfacial variables were then utilized as inputs for the finite element model to determine the tensile modulus. A comparison between the model results and measured data revealed a maximum discrepancy of 3.29%, indicating the effectiveness of the methodology employed in quantifying interfacial properties.

A dissipative particle dynamics simulation of controlled loading and responsive release of theranostic agents from reversible crosslinked triblock copolymer vesicles

To control the transport stability and release efficiency of loaded theranostic drugs in triblock copolymer carriers, the reversible crosslinking ability is of great significance. A molecular level exploration of such a function is needed to extend existing stabilizing and responsive dissociation mechanisms of carriers. Here, dissipative particle dynamics simulations were used to first demonstrate the formation of triblock copolymer vesicular carriers. Chemical crosslinking was used to strengthen the structural stability of the vesicle shell to avoid drug leakage. Reversible decrosslinking along with dissociation of the vesicle and release of loaded drugs were then explored. The structural, energetic and dynamical properties of the system were discussed at the molecular level. The regulation mechanism of drug release patterns was revealed by systematically exploring the effect of intra and intermolecular repulsive interactions. The results indicate that the chemical crosslinking of copolymers enhanced the compactness of the vesicle shell with a strengthened microstructure, increased binding energy, and limited chain migration, thus achieving more stable delivery of drugs. In terms of drug release, we clarified how the pairwise interactions of beads in the solution system affect the responsive dissociation of the vesicle and associated release patterns (speed and amount) of drugs. More efficient delivery and smart release of theranostic drugs are achieved using such reversible crosslinked triblock copolymer vesicles.

Molecular Insights into Antioxidant Efficiency of Melanin: A Sustainable Antioxidant for Natural Rubber Formulations

N-(1,3-Dimethyl butyl)-N'-phenyl-p-phenylenediamine (6-PPD) is a worldwide antioxidant commonly added to delay the thermo-oxidative degradation of tire rubbers. Unfortunately, 6PPD and its transformation product 6PPD-quinone are toxic to aquatic organisms (e.g., coho salmon). Herein, we explore the free radical scavenging activity and protective mechanism of melanin (MLN) on natural rubber's (NR's) oxidative resistance using molecular dynamics (MD) and quantum mechanical (QM) calculations. The relationship between the molecular structure and the chemical nature of the antioxidant molecules via transition state calculations is explored to unravel the reaction mechanisms of antioxidants interacting with peroxy radicals (ROO·) of NR with the estimation of reaction barriers. Following this, the radical scavenging activity of antioxidants was quantified via a hydrogen atom transfer mechanism and bond dissociation energy calculations. Parallel MD simulations were considered to study the interfacial interactions of antioxidant molecules with polymer chains and fillers with a quantifiable structure-property correlation. Given these results, the nanocomposite (NR-MLN-SiO2) with natural antioxidant melanin manifested outstanding antioxidant properties by preferentially bagging the ROO· radicals, thus improving NR's thermal-oxidative aging relative to 6-PPD. The MD results revealed that the intermolecular interactions at the NR/antioxidant interface benefited the antioxidant MLN to bind tightly to the NR in NR-MLN-SiO2 composite, thus exhibiting improved dispersion, O2 barrier properties, and thermo-oxidative stability, which could extend the service life of NR products (e.g., tires). In addition, as a sustainable antioxidant, MLN could replace toxic antioxidants like 6-PPD. More importantly, the QM/MD simulations provided a fundamental understanding of the mechanistic pathways of antioxidant molecules in NR composites, which are conducive to designing high-performance and sustainable green elastomers.

Interface damage and fracture mechanisms of a ceramic/polymer interface based on atomic-scale simulations

The performance of ceramic/polymer composite materials is significantly affected by their internal interfaces. To reveal the intrinsic interface fracturing mechanism of ceramic/polymer interfaces, an interfacial model composed of SiO₂ and polypropylene (PP) is investigated using the molecular dynamics method. The interface damage is quantified by the increase in the interface free volume and deformation of a single PP chain. As stretching speeds increase, the free volume and outflowing atoms of PP chains decrease with the same interfacial displacement, which results in the increase of the interface strength and fracture energy. At low stretching speeds, the interface damage mechanism is determined by a competition between attractions of the PP single chains from SiO₂ and PP. In contrast, at higher stretching speeds, the interface fracture is more brittle and the interface strength and fracture energy are both higher owing to the smaller cavity ratio. The results of this study contribute to an in depth understanding of the fracture mechanism of ceramic/polymer interfaces in many systems.

CNT-Reinforced Self-Healable Epoxy Dynamic Networks Based on Disulfide Bond Exchange

The design and utilization of polymers with healing capability have drawn increasing attention owing to their enhanced chain mobility and opportunity to heal minor cracks in composites. Rehealable thermoset polymers promise reduction in the maintenance cost and thus prolonged lifetime, reshaping, and recyclability. Introducing reversible covalent bonds is the mainstay strategy to achieve such plasticity in crosslinked polymers. Herein, we report a dynamic epoxy, which includes associative covalent adaptive networks (CANs) based on disulfide exchange bonds. Epoxy resin is chosen to study rehealing, as it is one of the most critical thermosetting polymers for various industries from aerospace to soft robotics. This study enlightens us about not only the consequences of CANs in the epoxy but also various factors such as soft segments and carbon nanotubes (CNTs). Epoxy dynamic networks are investigated in an attempt to explore the synergistic effect of the soft-segmented resins and CNTs on the healing and reshaping characteristics of epoxy networks along with varying stiffness. This research discusses epoxy dynamic networks in three main aspects: crosslink density, CAN density, and CNTs. Introducing soft segments into the epoxy network enhances the healing efficiency due to the increased chain mobility. A higher CAN density accelerates network rearrangement, improving the healing efficiency. It should also be noted that even with a low weight fraction of nanotubes, CNT-reinforced samples restored their initial strength more than neat samples after healing. The tensile strength of dynamic networks is at least 50 MPa, which is significant for their utility in primary or secondary structural components.

Molecular Simulations and Network Analyses of Surface/Interface Effects in Epoxy Resins: How Bonding Adapts to Boundary Conditions

In this study, we unravel the atomic structure of a covalent resin near boundaries such as surfaces and composite constituents. For this, a molecular simulation analysis of epoxy resin hardening under various boundary conditions was performed. On the atomic level of detail, molecular dynamics simulations were employed to study crosslinking reactions and self-organization of the polymer network within nm scale slab models. The resulting structures were then coarsened into a graph theoretical description for connectivity analysis of the nodes and combined with characterization of the node-to-node vector orientation. On this basis, we show that the local bonding of epoxy resins near interfaces tends to avoid under-coordinated linker sites. For both epoxy–vacuum surface models and epoxy–silica/epoxy cellulose interfaces, we find almost fully cured polymer networks. These feature a local increase in network linking lateral to the surface/interface, rather than the dangling of unreacted epoxy groups. Consequently, interface tension is low (as compared to the work of separating bulk epoxy), and the reactivity of the resin surface appears negligible.

Unraveling Nanoscale Elastic and Adhesive Properties at the Nanoparticle/Epoxy Interface Using Bimodal Atomic Force Microscopy

The addition of a small fraction of solid nanoparticles to thermosetting polymers can substantially improve their fracture toughness, while maintaining various intrinsic thermomechanical properties. The underlying mechanism is largely related to the debonding process and subsequent formation of nanovoids at a nanoscale nanoparticle/epoxy interface, which is thought to be associated with a change in the structural and mechanical properties of the formed epoxy network at the interface compared with the matrix region. However, a direct characterization of the local physical properties at this nanoscale interface remains significantly challenging. Here, we employ a recently developed bimodal atomic force microscopy technique for the direct mapping of nanoscale elastic and adhesive responses of an amine-cured epoxy resin filled with ∼50 nm diameter silica nanoparticles. The obtained elastic modulus and dissipated energy maps with high spatial resolution evidence the existence of a ∼20-nm-thick interfacial epoxy layer surrounding the nanoparticles, which exhibits a reduced modulus and weaker adhesive response in comparison with the matrix properties. While the presence of such a soft and weak-adhesive interfacial layer is found not to affect the architecture of structural heterogeneities in the epoxy matrix, it conceivably supports the toughening mechanism related to the debonding and plastic nanovoid growth at the silica/epoxy interface. The incorporation of this soft interfacial layer into the Halpin-Tsai model also provides a good explanation for the effect of the silica fraction on the tensile modulus of cured epoxy nanocomposites.

Molecular Dynamics Study of the Influence of Nano SiO2 on the Thermodynamic Properties of PMIA Composites

The poly-m-phenyleneisophthalamide (PMIA) is widely used in the electrical field due to its numerous favorable characteristics, but its poor thermal conductivity limits its application. In this study, PMIA was modified with nano-silica (SiO2) to improve its thermal and mechanical properties. Using iso-phthalic acid and m-phenylenediamine as monomers, the changes in the thermodynamic properties and microstructure parameters of SiO2-modified PMIA were analyzed using molecular dynamics before and after modification in the temperature range of 250~450 K. It was found that adding SiO2 improves the Young’s modulus and Shear modulus of PMIA, and the mechanical properties of PMIA, and SiO2/PMIA composites deteriorate with increasing temperature, but the mechanical properties of SiO2/PMIA composites are always better than those of pure PMIA in the temperature range of electrical equipment. Meanwhile, after doping SiO2 with the radius of 8 Å, the glass transition temperature of PMIA increases by 27.11 K, and its thermal conductivity increases from 0.249 W m−1 K−1 to 0.396 W m−1 K−1. When SiO2 is added to PMIA, the thermal expansion coefficient of PMIA will decrease in both glass and rubber states, and its thermal stability will improve. In terms of microstructure parameters, the free volume distribution of the SiO2/PMIA model is less easily dispersed than that of the PMIA model, indicating that the addition of SiO2 can improve the related properties of PMIA by hindering the movement of molecular chains.

Molecular insights into the ‘defects’ network in the thermosets and the influence on the mechanical performance

The introduction of ‘defects’ to the thermoset crosslinking network is one of the most applicable strategies for improving the modulus and toughness simultaneously. However, the reinforcement effect disappears when the ‘defects’ proportion exceeds the threshold. The speculated mechanism was that the aggregation and entanglement of the ‘defects’ chains changed the matrix topology, making the stacking structure more compact. However, the ‘defects’ are hardly directly observed in the experiment. As the result, the relationship between the ‘defects’ proportion and the package state of the matrix, and the effect on the material's mechanical performance was not explored. Herein, the network of bisphenol-A diglycidyl (DGEBA) with diethyltoluenediamine (DETDA) as the hardener was constructed using MD simulation, and n-butylamine was decorated on the matrix by replacing a proportion of DETDA acting as the ‘defects’. The results indicated that the aliphatic chains aggregated and entangled at a low concentration, occupying the voids in the rigid aromatic crosslinking structure, thus lowering the free volume. The strong non-bonding interactions drew the matrix segments close together, thus reinforcing the resin. However, the microphases formed by the aliphatic chains no longer filled the voids but created a new free volume and loosened the network when the content increased, which reduced the mechanical performance of the material. The experimental results were consistent with the findings in the simulations. The moduli of the resin increased with the increase in the n-butylamine content first and then declined. The maximum moduli of the thermosets was 3.4 GPa in S₃₀, which was about 25% higher compared with the control; the corresponding elongation at break was 8.9%, which was about 46% improved compared with the control.

The introduction of ‘defects’ to the thermoset crosslinking network is one of the most applicable strategies for improving the modulus and toughness simultaneously.

Atomistic-scale analysis of the deformation and failure of polypropylene composites reinforced by functionalized silica nanoparticles

Interfacial adhesion between polymer matrix and reinforcing silica nanoparticles plays an important role in strengthening polypropylene (PP) composite. To improve the adhesion strength, the surface of silica nanoparticles can be modified by grafted functional molecules. Using atomistic simulations, we examined the effect of functionalization of silica nanoparticles by hexamethyldisilazane (HMDS) and octyltriethoxysilane (OTES) molecules on the deformation and failure of silica-reinforced PP composite. We found that the ultimate tensile strength (UTS) of PP composite functionalized by OTES (28 MPa) is higher than that of HMDS (25 MPa), which is in turn higher than that passivated only by hydrogen (22 MPa). To understand the underlying mechanistic origin, we calculated the adhesive energy and interfacial strength of the interphase region, and found that both the adhesive energy and interfacial strength are the highest for the silica nanoparticles functionalized by OTES molecules, while both are the lowest by hydrogen. The ultimate failure of the polymer composite is initiated by the cavitation in the interphase region with the lowest mass density, and this cavitation failure mode is common for all the examined PP composites, but the cavitation position is dependent on the tail length of the functional molecules. The present work provides interesting insights into the deformation and cavitation failure mechanisms of the silica-reinforced PP composites, and the findings can be used as useful guidelines in selecting chemical agents for surface treatment of silica nanoparticles.

Adsorption of Epoxy Oligomers on Iron Oxide Surfaces: The Importance of Surface Treatment and the Role of Entropy

Epoxy-based coatings are widely used in a range of industries as protective coatings. The performance of the final solid-polymer system is dependent on the physicochemical properties of the interface and the interaction between the polymer and the solid substrate. In this study, we perform atomistic molecular dynamics simulations to investigate the binding of a common component in epoxy resins, diglycidyl ether of bisphenol A (DGEBA), on two iron oxide surfaces, hematite (0001) and magnetite (100), and investigate the effect of surface hydroxylation on the binding energy. We show that adsorption of DGEBA on hematite is more favorable than on magnetite and that the adsorbed molecules are highly localized on the pristine hematite surface but mobile on highly hydroxylated hematite surfaces and magnetite surfaces irregardless of surface hydroxylation fraction. A high degree of hydroxylation significantly reduces the binding energy of DGEBA on hematite but not on magnetite. The free-energy calculations confirm the trends observed upon hydroxylation, but the magnitude of the potential of mean force is lower than the binding energy due to the entropic contributions. Therefore, it can be suggested that DGEBA will adsorb more strongly on a surface containing a higher content of hematite than magnetite and that the presence of hydroxyl groups will weaken this adsorption. The presence of hydroxyl groups increases mobility of the chains, which can affect the coating rigidity.

Nanoscale mechanical and electrical characterization of the interphase in polyimide/silicon nitride nanocomposites

Polymer nanocomposites (pNC) have attracted wide interests in electrical insulation applications. Compared to neat matrices or microcomposites, pNC provide significant improvements in combined electrical, mechanical and thermal properties. In the understanding of the reasons behind these improvements, a major role was attributed to the interphase, the interaction zone between the nanoparticles (NP) and the matrix. Because of their nanoscale dimensions, the interphase properties are mostly theoretically described but rarely experimentally characterized. The aim of this study is to propose a nanoscale measurement protocol in order to probe mechanical (Young modulus) and electrical (dielectric permittivity) interphase features using, respectively, the peak force quantitative nanomechanical (PF-QNM) and the electrostatic force microscopy (EFM) modes of the atomic force microscopy. Measurements are performed on polyimide/silicon nitride (Si₃N₄) nanocomposite and the effect of a silane coupling agent treatment of Si₃N₄NP is considered. In order to accurately probe mechanical properties in PF-QNM mode, the impacting parameters such as the applied force, the deformation and the topography are taken into account. The interphase region has shown a higher elastic modulus compared to the matrix and a higher width (W_I) value for treated NP. From EFM measurements combined to a finite element model feeded with theW_Ivalues obtained from PF-QNM, the interphase permittivity is determined. The corresponding values are lower than the matrix one and similar for untreated and treated NP. This is in total agreement with its higher elastic modulus and implies that the interphase is a region around the NP where the polymer chains present a better organization and thus, a restricted mobility.

Damage-tolerant 3D-printed ceramics via conformal coating

Ceramic materials, despite their high strength and modulus, are limited in many structural applications due to inherent brittleness and low toughness. Nevertheless, ceramic-based structures, in nature, overcome this limitation using bottom-up complex hierarchical assembly of hard ceramic and soft polymer, where ceramics are packaged with tiny fraction of polymers in an internalized fashion. Here, we propose a far simpler approach of entirely externalizing the soft phase via conformal polymer coating over architected ceramic structures, leading to damage tolerance. Architected structures are printed using silica-filled preceramic polymer, pyrolyzed to stabilize the ceramic scaffolds, and then dip-coated conformally with a thin, flexible epoxy polymer. The polymer-coated architected structures show multifold improvement in compressive strength and toughness while resisting catastrophic failure through a considerable delay of the damage propagation. This surface modification approach allows a simple strategy to build complex ceramic parts that are far more damage-tolerant than their traditional counterparts.

Molecular simulation of nano polyhedral oligomeric silsesquioxane doping effect on the properties of two-component crosslinked epoxy resin

Epoxy resin (EP) has been extensively used in the field of insulation for its excellent electrical strength, mechanical property, chemical stability, and low cost. In this paper, computer molecular simulation is used to analyze the influence of nano-POSS (Nano-Polyhedral Oligomericsils Esquioxane) doping on the properties of epoxy composite from the micro point of view, which can provide a scientific basis for the optimization of the epoxy system. Two kinds of nano-POSS fillers with different mass fractions were doped into the base material of diglycidyl ether of bisphenol A (DGEBA) and 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecar (OSC). By molecular dynamics (MD) simulation the influence of nanofillers on the microstructure, thermal and mechanical properties of the composite were analyzed. Simulation results show that the doping of nano-POSS can improve the thermal and mechanical properties of the epoxy composite. Different nano-POSS has little effect on the glass transition temperature (T_g), coefficient of thermal expansion (CTE), and mechanical properties of the epoxy system, while the filling amount has an obvious improvement effect. Compared with EP/methyl-POSS system, the thermal and mechanical properties of the EP/phenyl-POSS system are better. At the same time, the doping of nano-POSS changed the microstructure parameters of epoxy composite. With the increase of nano-POSS filler content, fractional free volume (FFV) and mean square displacement (MSD) of both EP/POSS systems increased after the first drop. Besides, when the content of nano-POSS exceeded a certain range, the aggregation of filler itself hindered the accumulation of epoxy molecular chain segments.

Elucidating the Role of Interfacial Hydrogen Bonds on Glass Transition Temperature Change in a Poly(Vinyl Alcohol)/SiO2 Polymer-Nanocomposite by Noncovalent Interaction Characterization and Atomistic Molecular Dynamics Simulations

A thorough experimental investigation of polymer-glass transition temperature (T_g ) is performed on poly(vinyl alcohol) (PVA) and fumed silica nanoparticle (SiNP) composite. This is done together with atomistic molecular dynamics simulations of PVA systems in contact with bare and fully hydroxylated silica. Experimentally, PVA-SiNP composites are prepared by simple solution casting from aqueous solutions followed by its characterization using Fourier-transform infrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), and dynamic scanning calorimetry (DSC). Both theoretical and experimentally deduced T_g are correlated with the presence of hydrogen bonding interactions involving OH functionality present on the surface of SiNP and along PVA polymer backbone. Further deconvolution of FTIR data show that inter-molecular hydrogen bonding present between PVA and SiNP surface is directly responsible for the increase in T_g . SiNP filler and PVA matrix ratio is also optimized for a desired T_g increase. An optimal loading of SiNP exists, in order to yield the maximum T_g increase arising from the competition between hydrogen bonding and crowding effect of SiNP.

Role of Interface Structure and Chain Dynamics on the Diverging Glass Transition Behavior of SSBR-SiO2-PIL Elastomers

Intermolecular interactions between the constituents of a polymer nanocomposite at the polymer-particle interface strongly affect the segmental mobility of polymer chains, correlated with their glass transition behavior, and are responsible for the improved dynamical viscoelastic properties. In this work, we emphasized on the evolution of characteristic interfaces and their dynamics in silica (SiO2 NP)-reinforced, solution-polymerized, styrene butadiene rubber (SSBR) composites, whose relative prevalence varied with the phosphonium ionic liquid (PIL) volume fraction, used as an interfacial modifier. The molecular origins of such interfaces were examined through systematic dielectric spectroscopy, molecular dynamics (MD) simulations, and dynamic-mechanical analyses. The PIL facilitated H-bonding, cation-π, surface-phenyl, and van der Waals interfacial interactions between SSBR and SiO2 NP, thereby regulating the polymer chain dynamics, orientation, and mean-square displacement. Specifically, the mass density profiles from MD simulations revealed the dynamic gradient of polymer chains in the interfacial region as a function of radial distance from the center of mass of the SiO2 NP surface. The results showed a structuring effect to result in well-resolved density peaks at specific radial distances with the tangential orientation of styrene monomers in the vicinity of the SiO2 NP surface. These domino effects highlighted strong interfacial interactions to have an indispensable effect on the viscoelastic performance and thermal motion of SSBR molecular chains, leading to a higher glass transition temperature (T g) by ∼15 K, validating the experimental data. More importantly, our results gave new insights into the fundamental understanding of the fact that the strength of intermolecular interactions induced by PIL at the polymer-particle interface is the key to control the α-relaxation dynamics and T g optimization, desired for specific applications.

Orbital Overlapping through Induction Bonding Overcomes the Intrinsic Delamination of 3D-Printed Cementitious Binders

3D printing of cementitious materials holds a great promise for construction due to its rapid, consistent, modular, and geometry-controlled ability. However, its major drawback is low cohesion in the interlayer region. Herein, we report a combined experimental and computational approach to understand and control fabrication of 3D-printed cementitious materials with significantly enhanced interlayer strength using multimaterial 3D printing, in which the composition, function, and structure of the materials are programmed. Our results show that the intrinsic low interlayer cohesion is caused by excess moisture and time lag that block the majority of valuable interactions in the interlayer zone between the adjacent cement matrices. As a remedy, a thin epoxy layer is introduced as an intermediator between the adjacent extruded layers, both to improve the interlayer cohesion and to extend the possible time delay between printed adjacent layers. Our ab initio calculations demonstrate that an orbital overlap between the calcium ions, as the main electrophilic part of the cement structure, and the hydroxyl groups, as the nucleophilic part of the epoxy, create strong interfacial absorption sites. These electronic absorptions lead to several iono-covalent bonds between the cement matrix and epoxy, leading to significant improvements in tensile, shear, and compressive strengths as well as ductility of the 3D-printed composites. This is verified by our experimental data, which showed an average of 84% improvement in interlayer bonding. The upward augmentation of interlayer bonding helps 3D printing cementitious material to overcome their intrinsic limitation of weak interlayer cohesion, thereby mitigating/eliminating the key bottleneck of additive manufacturing in constructing materials.

Effects of Different Grafting Density of Amino Silane Coupling Agents on Thermomechanical Properties of Cross-Linked Epoxy Resin

In order to study the influences of amino silane coupling agents with different grafting densities on the surface of nano silica on the thermomechanical properties of cross-linked epoxy resin, the molecular dynamics method was used to establish an amorphous model and calculate the mechanical properties, glass transition temperature, mean square displacement, hydrogen bond, binding energy, and radial distribution function of the composite models in this paper. The results are as follows: with the increase of the grafting density of an amino silane coupling agent on the surface of nano silica particles, the mechanical properties and glass transition temperature of epoxy resin showed a trend of increasing first and then decreasing. When the grafting ratio was 9%, the mechanical properties and glass transition temperature of the epoxy resin were the largest, and the glass transition temperature was increased by 41 K. At the same time, it was found that the higher the grafting ratio, the lower the chain movement ability, but the higher the binding energy. Besides, the binding energy between the nanoparticles of the grafted silane coupling agent and epoxy resin was negatively correlated with the temperature. By analyzing the hydrogen bond and radial distribution function, the results showed that the improvement of the grafted silane coupling agent on the surface of the nanoparticle to the thermomechanical properties of the epoxy resin was related to the OH···O and NH···O hydrogen bonds. The analysis results indicated that the proper grafting density should be selected based on the established model size, selected nanoparticle diameter, and epoxy resin materials in order to better improve the thermomechanical properties of the epoxy resin.

Insights into Nano-Scale Physical and Mechanical Properties of Epoxy/Boehmite Nanocomposite Using Different AFM Modes

Understanding the interaction between nanoparticles and the matrix and the properties of interphase is crucial to predict the macroscopic properties of a nanocomposite system. Here, we investigate the interaction between boehmite nanoparticles (BNPs) and epoxy using different atomic force microscopy (AFM) approaches. We demonstrate benefits of using multifrequency intermodulation AFM (ImAFM) to obtain information about conservative, dissipative and van der Waals tip-surface forces and probing local properties of nanoparticles, matrix and the interphase. We utilize scanning kelvin probe microscopy (SKPM) to probe surface potential as a tool to visualize material contrast with a physical parameter, which is independent from the mechanics of the surface. Combining the information from ImAFM stiffness and SKPM surface potential results in a precise characterization of interfacial region, demonstrating that the interphase is softer than epoxy and boehmite nanoparticles. Further, we investigated the effect of boehmite nanoparticles on the bulk properties of epoxy matrix. ImAFM stiffness maps revealed the significant stiffening effect of boehmite nanoparticles on anhydride-cured epoxy matrix. The energy dissipation of epoxy matrix locally measured by ImAFM shows a considerable increase compared to that of neat epoxy. These measurements suggest a substantial alteration of epoxy structure induced by the presence of boehmite.

Enhanced stability of crosslinked and charged unimolecular micelles from multigeometry triblock copolymers with short hydrophilic segments: dissipative particle dynamics simulation

High micellar stability and well-performed drug loading and release are two conflicting factors for unimolecular micelles as an ideal drug delivery system. Achieving the formation of unimolecular micelles with short hydrophilic blocks is a challenging and promising approach to solve this bottleneck and limitation of current unimolecular micelle systems. In this work, dissipative particle dynamics (DPD) simulation is used to study the synergetic effect of crosslinking and electrostatic repulsion on stability of unimolecular micelles and to analyze the micro-mechanism and factors influencing this synergetic stabilization strategy. The strategy can generate unimolecular micelles with extremely high stability for various supramolecular polymers with short hydrophilic chains. Protonation of DEAEMA blocks leads to a large improvement in micellar hydrophilicity. The protonated middle layer further shrinks through crosslinking to produce the largest charge density, enlarging the electrostatic repulsion between colloidal particles. Additionally, the crosslinking and protonation treatment maximizes the extension degree of hydrophilic EO segments due to the increasing steric hindrance and poor compatibility between DEAHEMA and EO blocks. In this study, the relation between shrinkage degree of hydrophobic cores and stability of unimolecular micelles is first reported. The above-mentioned transition of micellar structures and properties results in the maximum degree of core shrinkage (R_g of MMA blocks) corresponding to the high stability of unimolecular micelles. Further study shows that the increasing cyclization degree, the mode of end cyclization, and the crosslinking and electrostatic repulsion of the middle layer all exert favorable effects on the stability of unimolecular micelles due to controlled shrinkage of hydrophobic cores.

Effect of Aminosilane Coupling Agents with Different Chain Lengths on Thermo-Mechanical Properties of Cross-Linked Epoxy Resin

In this paper, a molecular dynamics simulation method was used to study the thermo-mechanical properties of cross-linked epoxy resins doped with nano silica particles that were grafted with 3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and 3-[2-(2-aminoethylamino)ethylamino]-propyl-trimethoxysilane with different chain lengths. Firstly, a set of pure epoxy resin models, and four sets of SiO₂/EP composite models were established. Then, a reasonable structure was obtained through a series of optimizations using molecular dynamics calculations. Next, the mechanical properties, hydrogen bond statistics, glass transition temperature, free volume fraction, and chain spacing of the five models were studied comparatively. The results show that doped nano silica particles of surfaces grafted with 3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, and 3-[2-(2-aminoethylamino)ethylamino]-propyl-trimethoxysilane with different chain lengths enhanced mechanical properties such as elastic modulus, shear modulus, and volume modulus obviously. The glass transition temperature increased by 15⁻16 K, 40⁻41 K, and 24⁻27 K, respectively. Finally, the data show that the cross-linked epoxy resin modified by nanoparticles grafted with N-(2-aminoethyl)-3-aminopropyl trimethoxysilane had better effects for improving thermo-mechanical properties by the comparatively studying the five groups of parameter models under the same conditions.

Anticorrosive Effects of Some Thiophene Derivatives Against the Corrosion of Iron: A Computational Study

It is known that iron is one of the most widely used metals in industrial production. In this work, the inhibition performances of three thiophene derivatives on the corrosion of iron were investigated in the light of several theoretical approaches. In the section including DFT calculations, several global reactivity descriptors such as E_HOMO, E_LUMO, ionization energy (I), electron affinity (A), HOMO-LUMO energy gap (ΔE), chemical hardness (η), softness (σ), as well as local reactivity descriptors like Fukui indices, local softness, and local electrophilicity were considered and discussed. The adsorption behaviors of considered thiophene derivatives on Fe(110) surface were investigated using molecular dynamics simulation approach. To determine the most active corrosion inhibitor among studied thiophene derivatives, we used the principle component analysis (PCA) and agglomerative hierarchical cluster analysis (AHCA). Accordingly, all data obtained using various theoretical calculation techniques are consistent with experiments.

Effects of chemically heterogeneous nanoparticles on polymer dynamics: insights from molecular dynamics simulations

The dispersion of solid nanoparticles within polymeric materials is widely used to enhance their performance. Many scientific and technological aspects of the resulting polymer nanocomposites have been studied, but the role of the structural and chemical heterogeneity of the nanoparticles has just started to be appreciated. For example, simulations of polymer films on planar heterogeneous surfaces revealed unexpected, non-monotonic activation energy to diffusion on varying the surface composition. Motivated by these intriguing results, here we simulate via molecular dynamics a different, fully three-dimensional system, in which the heterogeneous nanoparticles are incorporated in a polymer melt. The nanoparticles are roughly spherical assemblies of strongly and weakly attractive sites, in fractions of f and 1 - f, respectively. We show that the polymer diffusion is still characterized by a non-monotonic dependence of the activation energy on f. The comparison with the case of homogeneous nanoparticles clarifies that the effect of the heterogeneity increases on approaching the polymer glass transition.

Computational Thermomechanical Properties of Silica⁻Epoxy Nanocomposites by Molecular Dynamic Simulation

Silica⁻epoxy nanocomposite models were established to investigate the influence of silane coupling agent on the structure and thermomechanical properties of the nanocomposites through molecular dynamics simulation. Results revealed that incorporating silica nanoparticles into a polymer matrix could improve thermomechanical properties of the composites and increase their glass transition temperature and thermal conductivity. Their thermomechanical properties were further enhanced through silane coupling agent modification on the surface of fillers. Compared with that of pure epoxy, the glass transition temperatures of the silica⁻epoxy composites with grafting ratios of 5% and 10% increased by 17 and 28 K, respectively. The thermal conductivities of the two models at room temperature respectively increased by 60.0% and 67.1%. At higher temperature 450 K, thermal conductivity of the nanocomposite model with a high grafting ratio of 10% demonstrated a considerable increase of approximately 50% over the pure epoxy resin (EP) model. The elastic and shear modulus of the nanocomposite models decreased at temperatures below their glass transition temperatures. These observations were further addressed in the interpretation from three aspects: segmental mobility capability, radial distribution function, and free volume fraction. Our computational results are largely consistent with existing experimental data, and our simulation model got fully validated.

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.

Ionic Liquid-Silica Precursors via Solvent-Free Sol-Gel Process and Their Application in Epoxy-Amine Network: A Theoretical/Experimental Study

This work describes the solvent-free sol-gel synthesis of epoxy-functionalized silica-based precursors in the presence of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) containing different anions: chloride (Cl^-) and methanesulfonate (MeSO₃^-). The IL-driven sol-gel mechanisms were investigated in detail using experimental characterizations (²⁹Si NMR and ATR FTIR spectroscopy) and a theoretical computational method based on density functional theory (DFT). We observed complex IL influence on both hydrolysis and condensation steps, involving especially H-bonding and Coulomb coupling stabilization of the process intermediates. The obtained IL-silica precursors and their further xerogels were widely characterized (rheology measurements, MALDI TOF, ²⁹Si NMR, ATR FTIR, and DFT simulation), which allowed observation of their precise silica structures and established their most energetically favorable conformations. The detected silica structures were dependent on the IL type and varied from highly condensed 3D cage-like to branched ladder-like and cyclic ones. The application of prepared IL-silica precursors as reinforcing additives into the epoxy-amine network led to an improvement in the organic/inorganic interphase interactions through chemical and physical bonding. Uniform and well-dispersed silica aggregates, in the size of ∼30 nm, were formed when ≤6.8 wt % of each IL-silica precursor was applied into the epoxy-amine network. The use of imidazolium-based ILs contributed to a significant improvement in thermomechanical properties of hybrids and reduced their UV absorption ability compared to that of the reference matrix. All hybrids exhibited an increase in energy to break (up to ∼53%), elongation at break (up to ∼43%), shear storage modulus in the rubbery region (up to 4 times), and thermo-oxidative stability.

Design Rules for Enhanced Interfacial Shear Response in Functionalized Carbon Fiber Epoxy Composites

Carbon-fiber reinforced composites are ideal light-weighting candidates to replace traditional engineering materials. The mechanical performance of these composites results from a complex interplay of influences operating over several length and time scales. The mechanical performance may therefore be limited by many factors, one of which being the modest interfacial adhesion between the carbon fiber and the polymer. Chemical modification of the fiber, via surface grafting of molecules, is one possible strategy to enhance interactions across the fiber-polymer interface. To achieve systematic improvements in these modified materials, the ability to manipulate and monitor the molecular structure of the polymer interphase and the surface grafted molecules in the composite is essential, but challenging to accomplish from a purely experimental perspective. Alternatively, molecular simulations can bridge this knowledge gap by providing molecular-scale insights into the optimal design of these surface-grafted molecules to deliver superior mechanical properties. Here we use molecular dynamics simulations to predict the interfacial shear response of a typical epoxy/carbon-fiber composite for both pristine fiber and a range of surface graftings. We allow for the dynamic curing of the epoxy in the presence of the functionalized surface, including cross-link formation between the grafted molecules and the polymer matrix. Our predictions agree with recently reported experimental data for these systems and reveal the molecular-scale origins of the enhanced interfacial shear response arising from functionalization. In addition to the presence of interfacial covalent bonds, we find that the interfacial structural complexity, resulting from the presence of the grafted molecules, and a concomitant spatial homogeneity of the interphase polymer density are beneficial factors in conferring high interfacial shear stress. Our approach paves the way for computational screening processes to design, test, and rapidly identify viable surface modifications in silico, which would enable rapid systematic progress in optimizing the match between the carbon fiber treatment and the desired thermoset polymer matrix.

Temperature-dependent surface nanomechanical properties of a thermoplastic nanocomposite

In polymer nanocomposites, particle-polymer interactions influence the properties of the matrix polymer next to the particle surface, providing different physicochemical properties than in the bulk matrix. This region is often referred to as the interphase, but detailed characterization of its properties remains a challenge. Here we employ two atomic force microscopy (AFM) force methods, differing by a factor of about 15 in probing rate, to directly measure the surface nanomechanical properties of the transition region between filler particle and matrix over a controlled temperature range. The nanocomposite consists of poly(ethyl methacrylate) (PEMA) and poly(isobutyl methacrylate) (PiBMA) with a high concentration of hydrophobized silica nanoparticles. Both AFM methods demonstrate that the interphase region around a 40-nm-sized particle located on the surface of the nanocomposite could extend to 55-70nm, and the interphase exhibits a gradient distribution in surface nanomechanical properties. However, the slower probing rate provides somewhat lower numerical values for the surface stiffness. The analysis of the local glass transition temperature (T_g) of the interphase and the polymer matrix provides evidence for reduced stiffness of the polymer matrix at high particle concentration, a feature that we attribute to selective adsorption. These findings provide new insight into understanding the microstructure and mechanical properties of nanocomposites, which is of importance for designing nanomaterials.

Effect of epoxy monomer structure on the curing process and thermo-mechanical characteristics of tri-functional epoxy/amine systems: a methodology combining atomistic molecular simulation with experimental analyses

A methodology, which combined molecular simulation with experimental research, was established to expound the performance of a tri-functional epoxy/amine system.

Toward understanding the adsorption mechanism of large size organic corrosion inhibitors on an Fe(110) surface using the DFTB method

Compared with the conventional DFT approach, SCC-DFTB method is more effective to deal with the adsorption issues of large size organic corrosion inhibitors on metal surface.