Taku Ozawa

Fragment Molecular Orbital Based Parametrization Procedure for Mesoscopic Structure Prediction of Polymeric Materials

In the analyses of miscibility behaviors of macromolecules and polymers, dissipative particle dynamics (DPD) simulations are generally performed. In these simulations, the so-called χ parameters describing the effective interactions among particles are crucial. It has been known that such parameters can be obtained within the classical or empirical force field frameworks. However, there is a potential problem that charge transfer and polarization occasionally occur. Additionally, satisfactory reference parameters are not available for some cases. Therefore, we developed a new procedure to evaluate the set of parameters by using the ab initio fragment molecular orbital (FMO) method which can provide the set of interaction energies among segments as polymer units. Moreover, we evaluated the anisotropy of molecules by using the FMO-based effective interaction parameters for three standard binary mixture systems (hexane-nitrobenzene, polyisobutylene-diisobutyl ketone, and polyisoprene-polystyrene). The calculated values showed good agreement with the experimental values with about 10% errors.

Theoretical analyses on water cluster structures in polymer electrolyte membrane by using dissipative particle dynamics simulations with fragment molecular orbital based effective parameters

The mesoscopic structures of polymer electrolyte membrane (PEM) affect the performances of fuel cells. Nafion® with the Teflon® backbone has been the most widely used of all PEMs, but sulfonated poly-ether ether-ketone (SPEEK) having an aromatic backbone has drawn interest as an alternative to Nafion. In the present study, a series of dissipative particle dynamics (DPD) simulations were performed to compare Nafion and SPEEK. These PEM polymers were modeled by connected particles corresponding to the hydrophobic backbone and the hydrophilic moiety of sulfonic acid group. The water particle interacting with Nafion particles was prepared as well. The crucial interaction parameters among DPD particles were evaluated by a series of calculations based on the fragment molecular orbital (FMO) method in a non-empirical way (Okuwaki et al., J. Phys. Chem. B, 2018, 122, 338–347). Through the DPD simulations, the water and hydrophilic particles aggregated, forming cluster networks surrounded by the hydrophobic phase. The structural features of formed water clusters were investigated in detail. Furthermore, the differences in percolation behaviors between Nafion and SPEEK revealed much better connectivity among water clusters by Nafion. The present FMO-DPD simulation results were in good agreement with available experimental data.

Polymer Blends: Interfacial Strength

The interfacial structure affects properties, such as the elastic modulus and impact resistance, of a polymeric material with a phase-separated structure. By changing the solubility between components or by adding a compatibilizer, the interfacial thickness and structure are drastically changed. In this study, a multiscale simulation is performed for the qualitative evaluation of interfacial phenomena. First, one-dimensional structures of the interface are calculated using self-consistent field theory included in SUSHI. The initial structures of coarse-grained molecular dynamics are then created, reflecting the results of self-consistent field theory using the density-biased Monte Carlo method included in COGNAC. This method allows the creation of a fully relaxed initial structure, which is difficult to obtain using only coarse-grained molecular dynamics. Finally, an elongation simulation is conducted using coarse-grained molecular dynamics included in COGNAC. Through these simulations, the effects of the interfacial structure on the fracture at the interface and the stress–strain behavior are evaluated.

Composites: Interfacial Strength

Nanocomposites have higher functionality than the host polymer itself. It is important to have knowledge of the interface between organic and inorganic materials in order to understand properties of nanocomposites. In addition, a microscopic picture is needed for an understanding of the interfacial phenomenon. In this chapter, we analyze the properties of an interface between graphene and polyethylene employing molecular dynamics simulation. The COGNAC engine in OCTA is used for this simulation. We use the united atom model in modeling polyethylene chains, and the structure of interface between graphene and the bulk polymer is constructed. We then simulate the normal separation of polyethylene chains from graphene. The forces applied to the graphene during the separation process are evaluated. Techniques described in this chapter will provide a qualitative understanding of the phenomena. Sample files for the simulations and scripts for modeling systems are provided, and they can be used in more realistic simulations.

MUFFIN: Multiphase Simulator

MUFFIN (MUltiFarious FIeld simulator for a Non-equilibrium system) consists of simulators based on the continuum model that calculate the dynamics of multiphase structures of soft matter. As the numerical methods, the finite difference method and finite element method are used. Using these simulators, it is possible to simulate polymer phase separation under a flow, the dynamics of an electrolyte solution considering an electric field, the reaction and diffusion of a fluid in a microchannel flow, large deformation of gels, deformation of elastic material with a phase-separated structure, and the light transmission of polymeric materials containing spherulites. This chapter presents the background and fundamental theory of representative simulators, together with some simple tutorials. We select two application examples. The first is the phase separation of polymer melts under the effect of hydrodynamics, and the second is the deformation of elastic material with a phase-separated structure.

Analysis of Relaxation Mechanism of Thread-Like Micelle Solution

Dissipative particle dynamics is applied to study the crossing dynamics at an entanglement point of surfactant thread-like micelles in an aqueous solution. This chapter investigates the possibility of a phantom crossing, which is the relaxation mechanism for the pronounced viscoelastic behavior of a surfactant thread-like micellar solution. When two thread-like micelles are encountered at an entanglement point under a condition close to thermal equilibrium, they fuse to form a four-armed branch point. Then, a phantom crossing reaction occurs occasionally, or one micelle is cut down at the branch point. When increasing the repulsive forces between hydrophilic parts of the surfactants, fusion occurs less and the thread-like micelle is frequently broken down at an entanglement point. In these three schemes (i.e., a phantom crossing, a cut at the branch point, and a break at the entanglement point), the breakage occurs somewhere along the thread-like micelle. The breakage is considered as an essential process in the relaxation mechanism, and a phantom crossing can be seen as a special case of these processes. To explain the experimental evidence that a terminal of thread-like micelles is scarcely observed, a mechanism is also proposed where the generated terminal merges into the connected micelle part between two entanglement points owing to thermal motion.