Masato Hashimoto

Molecular dynamics simulation of amphiphilic molecules in solution : Micelle formation and dynamic coexistence

The micelle formation and the dynamic coexistence in amphiphilic solution are investigated by molecular dynamics simulation of coarse-grained rigid amphiphilic molecules with explicit solvent molecules. Our simulations show that three kinds of isolated micelles (disk, cylindrical, and spherical micelles) are observed at a lower temperature by quenching from a random configuration of amphiphilic molecules in solution at a higher temperature. The micellar shape changes from a disk into a cylinder, and then into a sphere as the hydrophilic interaction increases whereas it is not so sensitive to the variation of the hydrophobic interaction. This fact indicates that the hydrophilic interaction plays an important role in determining the micellar shape in the range of the interaction parameters used. It is also found that in a certain interaction parameter range, two kinds of micellar shapes coexist dynamically. From the detailed analyses of the dynamic coexistence, it is ascertained that the dynamic coexistence of a cylindrical micelle and a spherical micelle accompanies the coalescence and fragmentation of micelles while that of a disk micelle and a cylindrical micelle does not, but exhibits the continuous change between them.

Molecular dynamics simulation of micelle formation in amphiphilic solution

The micelle formation in amphiphilic solution is investigated by molecular dynamics (MD) simulation of coarse-grained rigid amphiphiles with explicit solvent molecules. In our simulation model, the intensity of the hydrophilic interaction and the hydrophobic interaction can be varied independently. Our simulations show that various kinds of micellar structures are formed at a lower temperature by quenching from a random configuration of amphiphilic molecules in solution at a higher temperature. The micellar shape changes from a disc (bilayer) into a cylinder and then into a sphere as the intensity of the hydrophilic interaction increases. It is also found that the micelle formation proceeds in a stepwise fashion through the coalescence of smaller micelles. From the analysis of the orientational order for the amphiphilic molecules, it is concluded that the orientational order parameters can be used to distinguish the micellar shapes clearly.

Ferroelectric Hysteresis Behavior and Structural Property of Even-Numbered Nylon Film

Nylon 6 film quenched from the molten state into liquid nitrogen exhibits an electric displacement vs electric field hysteresis loop at room temperature with the remanent polarization of 26 mC/m2, as well as an exothermic anomaly around 340 K in the specific heat due to the heating process, which is attributed to the independent dipole rotation in the nematic-type unstable structure with loose molecular packing and imperfect hydrogen bonding.

Molecular dynamics simulation of self-organization in amphiphilic solution

The micelle formation in amphiphilie solution is investigated by means of a a molecular dynamics simulation of coarse-grained amphipliilic molecules with explicit solvent molecules. A random configuration of amphiphilic molecules in solution at high temperature is quenched to a lower temperature. Our simulations show that the micella shapes change from a cylindrical micelle to a planar bilayer as the number density increases. At higher densities, we also find the following characteristic features. (1) The potential energy relaxes in a stepwise manner (2) The radius of gyration R g of the largest micelles increases with time in a stepwise fashion, (3) The sharp bumps in R g occur during coalescence of micelles.

Molecular Dynamics Simulation for Structure Formation of Single Polymer Chain in Solution

The structure formation of a single polymer chain in solution with explicit solvent molecules is investigated by molecular dynamics simulation. The orientationally ordered structure is formed at a low temperature by quenching from a random conformation at a high temperature. The growth of the global and local orientational order proceeds in a stepwise manner at T ≥350 K, whereas it proceeds in a gradual manner at T =300 K. From the detailed analyses of the parallel ordering process, it is found that a conformational change of the polymer chain occurs at first, and then parallel ordering starts to take place. In comparison with the simulation results of an isolated polymer chain in vacuum, it is ascertained that the stem length of the orientationally ordered structure formed in solution becomes 2–3 times longer than that formed in vacuum.

Atomic Force Microscopy of Isotactic Polystyrene Crystals

The growth of isotactic polystyrene crystals from a thin melt film is examined by atomic force microscopy. The crystals protrude from the film up to about 100 nm to form a hexagonal spiral with a hollow screw dislocation at the center; the distance between neighboring steps of the growth spiral is discussed on the basis of the critical nucleus. The entire crystal has collapsed to the substrate. An amorphous layer a few nanometers thick covers the surface of the crystal. At the crystal-liquid interface, a concave region about 2 nm deep extends ca. 400 nm.

Ferroelectricity in odd- and even-numbered nylons

Abstract The D–E hysteresis and switching time were measured as well as X-ray, FTIR and thermal analyses for as-quenched nylon 6 and cold-drawn nylon 11 after quenching. The maximum remanent polarization of the former was 58 mC/m2 and that of the latter was 52 mC/m2. Such ferroelectricity in nylon 6 was partly related with an exothermic anomaly in C P at 336 K and attributed to the independent dipole rotation in the nematic-type unstable structure with loose molecular packing and imperfect hydrogen bonding, while the intermolecular steric hindrance mainly affected the D–E hysteresis disappearance in the cold-drawn nylon 11. Such results indicated that the number of CH2 groups between adjacent amide groups in a nylon chain (even or odd: polar or non-polar property in a chain with all-trans conformation) was not the determining factor in the cause of the ferroelectric property of nylons.

The Role of Intermolecular Electrostatic Interaction on Appearance of the Periodic Band Structure in Type I Collagen Fibril

Intermolecular electrostatic interaction between staggered tropocollagens is calculated, focussing on the condition such that D=(G+L)/K in the collagen fibril with a periodic band structure, where D, G and L are lengths of band period, gap region and tropocollagen molecule respectively, and K=[L/D]+1 where [L/D] is the highest integer below L/D. By changing D, G and K, only two cases are found where the interaction energy is minimum at every position with multiples of D; i.e., D=234, G=124, K=5 and D=235, G=129, K=5. Such results propose the origin of a periodic band structure or the Hodge-Petruska scheme and a pseudohexagonal crystal in type I collagen fibrils.

Ferroelectricity and Amorphous Structure in Quenched Nylon 6 Film

The electric displacement versus electric field (D–E) hysteresis appeared for the as-quenched nylon 6 film approximately 30 µm thick. Maximum remanent polarization was 60 mC/m2. Temperature dependence of the d-spacing of the amorphous phase was measured by a high-temperature X-ray camera to show continuous decrease in the d-spacing from 0.439 to 0.434 nm with increasing temperature from 330 to 340 K without appearance of the crystalline phase. It was found that the D–E hysteresis disappeared when the d-spacing became smaller than approximately 0.42 nm with annealing the specimen, where no crystalline phase was observed. Such results suggest that ferroelectricity is caused in the amorphous phase with d-spacing larger than 0.42 nm.

Structural Analysis of Collagen Fibrils in Rat Skin Based on Small-Angle X-Ray-Diffraction Pattern

A superhelix structure is proposed to explain the short band period (65 nm) observed in rat skin collagen fibrils in comparison with that of tendon collagen fibrils (67 nm). In the former case, each molecule forms an angle of 12–13° with the fibril axis and almost ten tropocollagen molecules aggregate to form a tilting plane in a bundle or a subfibril. The adjacent bundles may be staggered with respect to each other by a length of ten amino acid residues along the chain axis to form the band structure observed to be normal to the fibril axis. This model is found to be quantitatively consistent with small-angle X-ray diffraction data obtained from uniaxially oriented collagen fibrils in native rat skin as well as a transmission electron microscopic image of freeze-etched fibrils in glycerinated rat skin.