The structural formation of a single polymer chain with 500 CH 2 groups is studied by the molecular dynamics simulations. Our simulations show that the bond-orientationally ordered structure at low temperatures is formed from a random-coil structure at high temperatures by a gradual stepwise cooling. From the radii of gyration and the bond-orientational order parameters, it is found that the anisotropy of a polymer chain also grows during the growth of the bond-orientational order. In the bond-orientationally ordered structure at low temperatures, 16 stems form a structure with deformed hexagonal symmetry and the stems in the outer layer have a tilted configuration. Furthermore, the gauche states are localized in the fold surface and the conformational states in the fold surface change more readily than those in the orientationally-ordered region.

Computer simulation of the structural formation of polymer chains has recently become the focus of attention in physics, chemistry and material science. We aim at understanding the mechanisms of the structural for-.mation of polymer chains at the molecular level. To this end, we carry out the molecular dynamics simulations of 100 short polymer chains, each of which consists of 20 CH 2 groups, and analyze the formation process of the orientationally ordered structure.The united CH 2 groups interact via the bonded potentials (bond-stretching, bond-bending and torsional potentials) and the non-bonded potential (12-6 LennardJanes potential). The atomic force field used here is the DREIDING potential 1 ). We use the velocity version of the Verlet algorithm and apply the Nose-Hoover method in order to keep the temperature of the system constant. The integration time step and the relaxation constant for the heat bath variable are 0.001 ps and 0.1 ps, respectively. The cutoff distance for the Lennard-Janes potential is 10.5 A. The polymer chains are exposed to vacuum. The total momentum and the total angular momentum are taken to be zero in order to cancel overall translation and rotation of chains. At first, we prepare random configuration of short polymer chains at high temperature (T = 700 K) and then it is quenched to various low temperatures (T = 300, 320, ... , 460 K) 2 ).We show, in Fig. 1 At last they coalesce into a large cluster and a highly ordered monolayer structure is formed [ Fig. 1 (d)].In order to investigate the growth process of the global bond-orientational order, we calculate the global bond-orientational order parameter S, which is defined bywhere N and n are respectively the number of polymer chains and the number of CH 2 groups per polymer chain (N = 100, n = 20) and 1/Ji is the angle between the subbond vector of the m-th chain bf\ which is formed by connecting centers of two adjacent bonds i and i-1 of the m-th chain, and the director (c-axis) of the layer. The parameter S would take a value of 1.0, 0.0 or -0.5, respectively, for polymer chains whose sub-bonds are perfectly parallel, random or perpendicular to the director.We show the time dependence of S at T = 440 K in

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.

We report a new methodology for studying diffusion of individual polymer chains in a melt state, with special emphasis on the effect of chain topology. A perylene diimide fluorophore was incorporated into the linear and cyclic poly(THF)s, and real-time diffusion behavior of individual chains in a melt of linear poly(THF) was measured by means of a single-molecule fluorescence imaging technique. The combination of mean squared displacement (MSD) and cumulative distribution function (CDF) analysis demonstrated the broad distribution of diffusion coefficient of both the linear and cyclic polymer chains in the melt state. This indicates the presence of spatiotemporal heterogeneity of the polymer diffusion which occurs at much larger time and length scales than those expected from the current polymer physics theory. We further demonstrated that the cyclic chains showed marginally slower diffusion in comparison with the linear counterparts, to suggest the effective suppression of the translocation through the threading-entanglement with the linear matrix chains. This coincides with the higher activation energy for the diffusion of the cyclic chains than of the linear chains. These results suggest that the single-molecule imaging technique provides a powerful tool to analyze complicated polymer dynamics and contributes to the molecular level understanding of the chain interaction.

Molecular dynamics simulations are carried out to study the structure formation of 100 short chain molecules, each of which consists of 20 CH 2 groups. Our simulations show that the orientationally ordered structure is formed from a random configuration by quenching. The global orientational order starts to increase suddenly after a certain duration and grow in a stepwise fashion afterwards. This behavior is also found in the growth process of the local orientationally-ordered domains. It is found from the microscopic analysis of the structure formation process, that parallel ordering of chain molecules starts to occur after the chain molecules stretch to some extent. From the analysis of the obtained orientationally ordered structure and the molecular mobility, we also find the following characteristic features: ͑i͒ The chain molecules are packed hexagonally at 400 K and the transition from the hexagonal phase toward the orthorhombic phase takes place as the temperature decreases. ͑ii͒ The gauche bonds in the same chain molecule tend to form gauche pairs. The gauche pairs with the same sign form the double gauche defects and those with the opposite sign form the kink defects. ͑iii͒ In the hexagonal phase, the chain molecules become longitudinally mobile. This result, which is obtained by the microscopic analysis of the chain motion, is the microscopic evidence to confirm the existence of the chain sliding diffusion in the hexagonal phase which underlies the sliding diffusion theory of polymer crystallization proposed by Hikosaka ͓Polymer 28, 1257 ͑1987͒; 31, 458 ͑1990͔͒.