Hisao Takeuchi

Structure formation during the crystallization induction period of a short chain-molecule system: A molecular dynamics study

A molecular dynamics simulation has been performed for a short chain-molecule system to study structure formation during the crystallization induction period. A model simulating n-alkane having 20 methylene units was used for the short-chain molecule. A model system containing either 250 or 2000 chains was quenched from a high temperature, and the structure formation was examined at constant temperature and pressure. The model system containing 250 chains eventually forms an ordered phase after the induction period, during which macroscopic quantities, such as volume, remain almost constant. In spite of these small changes in the macroscopic quantities, development of local parallel order is significant during the induction period. We also found that the development of local parallel order causes density fluctuations which appear as a weak small-angle peak in the structure factor. A characteristic length scale corresponding to the density fluctuations becomes larger as time elapses in the induction period...

Dynamics of Small Molecules in a Dense Polymer Matrix: Molecular Dynamics Studies

Abstract The anomalous diffusion regime appearing in the self-diffusion of small molecules in bulk amorphous polymers has been extensively studied by molecular dynamics simulations. A rather long simulation of duration ∼ 10 −8 s is performed on a polyethylene-like simple polymer model containing either oxygen molecules or helium atoms as a diffusant. Dynamic properties evaluated for these diffusants are the mean-square displacement, the van Hove self correlation function, and the self part of the density autocorrelation. It is first confirmed that the anomalous diffusion regime appears in a few hundred picoseconds for oxygen molecule, while the Einstein relation adopted beyond this regime results in the self-diffusion coefficient of the order of ∼ 10−5 cm2/s. This anomaly is still observed for helium that diffuses much faster than oxygen. In the anomalous diffusion regime, it is found that the correlation functions for the two diffusants show characteristic features and become essentially the same as time...

Monte Carlo simulations of the exchange kinetics of polymers adsorbed on a solid surface

The exchange kinetics of polymers adsorbing on a solid surface is extensively studied by dynamic Monte Carlo simulations. A model employed simulates a semidilute polymer solution placed in contact with a solid surface that attracts polymer segments by the adsorption interaction (X s ). The exchange process of polymer chains, between the solution and the adsorbed polymer layer, is examined under various conditions. The exchange kinetics shows two characteristic regimes with increasing chain length. One is the diffusion-controlled regime found with a small X s , and the other the detachment-controlled regime with a large X s . These two regimes are well described by a kinetic theory. Various dynamic quantities show that the diffusion-controlled regime is not due to sluggish dynamics near the surface, but rather to bulk diffusion of chains. The diffusion-controlled regime found in this study is considered to appear at the high temperature limit.

Structural inhomogeneity of non-regioregular poly(3-hexylthiophene) in a solution and solid films structures and electronic properties

We study structures and electronic properties of non-regioregular poly(3-hexylthiophene) (P3HT) in a chloroform solution and in solid films prepared under various conditions. The extent of conjugation seems to be inhomogeneous for a slowly dried cast film. However, a rather uniform extent of conjugation is suggested for a spin-coated film. In photoluminescence excitation (PLE) spectra probed at 710 nm, large differences are found between these spin-coated and cast films. Based on structural features, exciton behavior is discussed.

Dynamics of adsorbed polymer chains as studied by Monte Carlo simulation

The exchange kinetics of polymers adsorbed on a solid surface was studied by a Monte Carlo simulation. We found two regimes of the exchange kinetics: one being controlled by the bulk diffusion, and the other by the detachment of adsorbed chains. These features were discussed in view of a kinetic theory and experimental observations.