Determination of interaction parameters in a bottom-up approach employed in reactive dissipative particle dynamics simulations for thermosetting polymers.

The limitations in previous dissipative particle dynamics (DPD) studies confined simulations to a narrow resin range. This study refines DPD parameter calculation methodology, extending its application to diverse polymer materials. Using a bottom-up approach with molecular dynamics (MD) simulations, we evaluated solubility parameters and bead number density governing nonbonded interactions via the Flory-Huggins parameter and covalent-bonded interactions. Two solubility parameter methods, Hildebrand and Krevelen-Hoftyzer, were compared for DPD simulations. The Hildebrand method, utilizing MD simulations, demonstrates higher consistency and broader applicability in determining solubility parameters for all DPD particles. The DPD/MD curing reaction process was examined in three epoxy systems: DGEBA/4,4'-DDS, DGEBA/MPDA and DGEBA/DETA. Calculations for the curing profile, gelation point, radial distribution function and branch ratio were performed. Compared to MD data for DGEBA/4,4'-DDS, the maximum deviation in secondary reactions between epoxy and amine groups according to DPD simulations with Krevelen-Hoftyzer was 14.8%, while with the Hildebrand method, it was 1.7%. The accuracy of the DPD curing reaction in reproducing the structural properties verifies its expanded application to general polymeric material simulations. The proposed curing DPD simulations, with a short run time and minimal computational resources, contributes to high-throughput screening for optimal resins and investigates mesoscopic inhomogeneous structures in large resin systems.

Effects of the chain length of nonaromatic epoxy resins on thermomechanical and optical properties: experiments, and ab initio and molecular dynamics simulations.

Epoxy resin has been extensively used in the field of advanced electronic materials as an adhesive and encapsulant owing to its excellent material properties. However, recently, there has been a demand for further improvement in heat resistance, high transparency, environmental resistance, and enhanced handling properties for high-brightness light-emitting diodes. Conventional aromatic epoxy resins lack light resistance; therefore, a colorless and transparent epoxy resin without aromatic rings is desirable. In this study, tris(2,3-epoxypropyl) isocyanurate (TEPIC) was used as a nonaromatic epoxy resin, and three types of TEPIC with different side-chain lengths were prepared. The ultraviolet (UV)-visible absorption properties of TEPIC were evaluated using time-dependent density functional theory, and the practicality of the numerical prediction of light resistance was verified. TEPIC yields a UV absorbance spectrum with a lower intensity than those of conventional aromatic epoxy resins, suggesting that TEPIC is expected to have high light resistance. In addition, their thermomechanical properties and the influence of molecular structure were evaluated using both molecular dynamics (MD) simulations and experiments. The MD simulation and experimental results were in good agreement, indicating that the long side chains of TEPIC suppress triaxial deformation-induced failure and improve ductility instead of decreasing strength and stiffness. In addition, the longer side chains form a dense molecular structure with less free volume. These results indicate that numerical approaches can be used to predict various properties of epoxy resins and interpret them from the molecular structure. Accordingly, these approaches can be used to aid the material development process.

Reaction-induced morphological transitions in a blend of diblock copolymers and reactive monomers: dissipative particle dynamics simulation.

The dissipative particle dynamics (DPD) method is applied to the morphological transitions of microphase-separated domains in a mixture of symmetric AB-diblock copolymers and reactive C-monomers, where polymerization and cross-linking reactions take place among C-monomers. The initial structure for the DPD simulation is an equilibrated cylindrical domain structure prepared by the density-biased Monte Carlo method with density profiles obtained from the self-consistent field theory. By introducing a cross-linking reaction among reactive C-monomers, we confirmed that the DPD simulation reproduces the morphological transitions observed in experiments, where the domain morphology changes due to segregation between A-blocks of diblock copolymers and cross-linking networks of C-monomers. When the cross-linking reaction of C-monomers is sufficiently fast compared to the deformation of the domains, the initial cylindrical domains are preserved, while the distance between the domains increases. On the other hand, when the formation of the cross-linking network is slow, the domains can deform and reconnect with each other in the developing cross-linking network. In this case, we observe morphological transitions from the initial domain morphology with a large-curvature interface to another domain morphology with a smaller-curvature interface, such as the transition from the cylindrical phase to the lamellar phase. We calculated the spatial correlations in the microphase-separated domains and found that such correlations are affected by the speed of the formation of the cross-linking network depending on whether the bridging between microphase-separated domains occurs in a nucleation and growth process or in a spinodal decomposition process.

Thermoset resin curing simulation using quantum-chemical reaction path calculation and dissipative particle dynamics.

Thermoset resin, which is commonly used as a matrix in carbon-fiber-reinforced plastic, requires curing procedures. We propose a curing simulation technique involving a dissipative particle dynamics (DPD) simulation, which can simulate a larger system and longer time period than those of conventional all-atom molecular dynamics (AA-MD) simulations. The proposed curing DPD simulation can represent the thermoset resin exothermic reaction process precisely by considering each reactivity according to the reaction types calculated via quantum-chemical reaction path calculations. The cure reaction process given by the curing DPD simulation agrees well with that given by a conventional curing AA-MD simulation, but with run-time and computational-resource reductions of 1/480 and 1/10 times, respectively. We also conduct reverse mapping, through which the AA-MD system can be reconstructed from the DPD system, to evaluate the structural and thermomechanical properties. The X-ray diffraction pattern and thermomechanical properties of the reconstructed system agree well with those of the systems derived from the curing AA-MD simulation and experimental setup. Therefore, a cured-resin AA-MD system can be obtained from a curing DPD simulation at an extremely low computational cost, and the thermomechanical properties can be evaluated precisely using this system. The proposed curing simulation technique can be applied in high-throughput screening for better materials properties and in large system calculations.

Dissipative Particle Dynamics Simulation for Reaction-Induced Phase Separation of Thermoset/Thermoplastic Blends.

Reaction-induced phase separation occurs during the curing reaction when a thermoplastic resin is dissolved in a thermoset resin, which enables toughening of the thermoset resin. As resin properties vary significantly depending on the morphology of the phase-separated structure, controlling the morphology formation is of critical importance. Reaction-induced phase separation is a phenomenon that ranges from the chemical reaction scale to the mesoscale dynamics of polymer molecules. In this study, we performed curing simulations using dissipative particle dynamics (DPD) coupled with a reaction model to reproduce reaction-induced phase separation. The curing reaction properties of the thermoset resin were determined by ab initio quantum chemical calculations, and the DPD parameters were determined by all-atom molecular dynamics simulations. This enabled mesoscopic simulations, including reactions that reflect the intrinsic material properties. The effects of the thermoplastic resin concentration, molecular weight, and curing conditions on the phase-separation morphology were evaluated, and the cure shrinkage and stiffness of each cured resin were confirmed to be consistent with the experimental trends. Furthermore, the local strain field under tensile deformation was visualized, and the inhomogeneous strain field caused by the phase-separated structures of two resins with different stiffnesses was revealed. These results can aid in understanding the toughening properties of thermoplastic additives at the molecular level.

In-situ shearing process observation system for soft materials via transmission electron microscopy.

We developed an in-situ shear test system suitable for transmission electron microscopy (TEM) observations, which enabled us to examine the shear deformation behaviours inside soft materials at nanoscale resolutions. This study was conducted on a nanoparticle-filled rubber to investigate its nanoscale deformation behaviour under a large shear strain. First, the shear deformation process of a large area in the specimen was accurately examined and proven to exhibit an almost perfect simple shear. At the nanoscale, voids grew along the maximum principal strain during shear deformation. In addition, the nanoscale regions with rubber and silica aggregates exhibited deformation behaviours similar to the global shear deformation of the specimen. Although the silica aggregates exhibited displacement along the shearing directions, rotational motions were also observed owing to the torque generated by the local shear stress. This in-situ shear deformation system for TEM enabled us to understand the nanoscale origins of the mechanical properties of soft materials, particularly polymer composites. Graphical Abstract.

Effect of inorganic material surface chemistry on structures and fracture behaviours of epoxy resin.

The mechanisms underlying the influence of the surface chemistry of inorganic materials on polymer structures and fracture behaviours near adhesive interfaces are not fully understood. This study demonstrates the first clear and direct evidence that molecular surface segregation and cross-linking of epoxy resin are driven by intermolecular forces at the inorganic surfaces alone, which can be linked directly to adhesive failure mechanisms. We prepare adhesive interfaces between epoxy resin and silicon substrates with varying surface chemistries (OH and H terminations) with a smoothness below 1 nm, which have different adhesive strengths by ~13 %. The epoxy resins within sub-nanometre distance from the surfaces with different chemistries exhibit distinct amine-to-epoxy ratios, cross-linked network structures, and adhesion energies. The OH- and H-terminated interfaces exhibit cohesive failure and interfacial delamination, respectively. The substrate surface chemistry impacts the cross-linked structures of the epoxy resins within several nanometres of the interfaces and the adsorption structures of molecules at the interfaces, which result in different fracture behaviours and adhesive strengths.

Uncovering the Mechanism of Size Effect on the Thermomechanical Properties of Highly Cross-Linked Epoxy Resins.

Epoxy resins are widely used as matrix resins, especially for carbon-fiber-reinforced plastic, due to their outstanding physical and mechanical properties. To date, most research into cross-linking processes using simulation has considered only a distance-based criterion to judge the probability of reaction. In this work, a new algorithm was developed for use with the large-scale atomic/molecular massively parallel simulator (LAMMPS) simulation package to study the cross-linking process; this new approach combines both a distance-based criterion and several kinetic criteria to identify whether the reaction has occurred. Using this simulation framework, we investigated the effect of model size on predicted thermomechanical properties of three different structural systems: diglycidyl ether of bisphenol A (DGEBA)/4,4'-diaminodiphenyl sulfone (4,4'-DDS), DGEBA/diethylenetriamine (DETA), and tetraglycidyl diaminodiphenylmethane (TGDDM)/4,4'-DDS. Derived values of gel point, volume shrinkage, and cross-linked resin density were found to be insensitive to model size in these three systems. Other thermomechanical properties, i.e., glass-transition temperature, Young's modulus, and yield stress, were found to reach stable values for systems larger than ∼40 000 atoms for both DGEBA/4,4'-DDS and DGEBA/DETA. However, these same properties modeled for TGDDM/4,4'-DDS did not stabilize until the system size reached 50 000 atoms. Our results provide general guidelines for simulation system size and procedures to more accurately predict the thermomechanical properties of epoxy resins.

Effects of vacancy defects on the interfacial shear strength of carbon nanotube reinforced polymer composite.

We investigate the effects of the vacancy defects (i.e., missing atoms) in carbon nanotubes (CNTs) on the interfacial shear strength (ISS) of the CNT-polyethylene composite with the molecular dynamics simulation. In the simulation, the crystalline polyethylene matrix is set up in a hexagonal array with the polymer chains parallel to the CNT axis. Vacancy defects in the CNT are introduced by removing the corresponding atoms from the pristine CNT (i.e., CNT without any defect). Three patterns of vacancy defects with three different sizes are considered. Two types of interfaces, with and without cross-links between the CNT and the matrix are also considered here. Polyethylene chains are used as cross-links between the CNT and the matrix. The Brenner potential is used for the carbon-carbon interaction in the CNT, while the polymer is modeled by a united-atom potential. The nonbonded van der Waals interaction between the CNT and the polymer matrix and within the polymer matrix itself is modeled with the Lennard-Jones potential. To determine the ISS, we conduct the CNT pull-out from the polymer matrix and the ISS has been estimated with the change of total potential energy of the CNT-polymer system. The simulation results reveal that the vacancy defects significantly influence the ISS. Moreover, the simulation clarifies that CNT breakage occurs during the pull-out process for large size vacancy defect which ultimately reduces the reinforcement.