Simulation study of the order formation dynamics in the melt crystallization of flexible chain molecules induced by rigid molecular nuclei

Effects of surface affinity on the ordering dynamics of self-assembled monolayers of chain molecules: Transition from a parallel to a perpendicular structure

The effects of surface interactions on the ordering dynamics of self-assembled monolayers (SAM) of chain molecules were studied using molecular dynamics simulations. When the strength of surface-chain interactions was equal to or less than that of chain-chain interactions, domains of chain molecules adsorbed perpendicular to the surface ("upright" chains) formed on the surface. Although chain molecules adsorbed parallel to the surface ("lying" chains) were initially observed on the surface, they did not develop into two-dimensionally aligned structures. In contrast, when the strength of surface-chain interactions was at least twice that of chain-chain interactions, the proportion of upright chain molecules was initially small, and the reorientation of lying chains was observed shortly afterwards. In this case, the reorientation from lying to upright configuration developed slowly from the domain boundaries of two-dimensionally aligned structures late in the calculation period. Although the orientation processes of chain molecules on surfaces were strongly influenced by the strength of surface-chain interactions, the total adsorption rate on the surface was not. We also analyzed the maximum area of domains formed by lying chains. The development of two-dimensionally aligned domains required strong surface-chain interactions to prevent the spontaneous formation of nuclei of upright domains.

Molecular dynamics study of the effects of chain properties on the order formation dynamics of self-assembled monolayers of long-chain molecules

The order formation dynamics of self-assembled monolayers (SAM) of long-chain molecules were studied using coarse-grained molecular dynamics simulations. The primary kinetic processes of surface order formation from solution are adsorption to the surface and surface diffusion. For long-chain molecules, the degrees of freedom of the chain structure and motion add various complexities to the order formation dynamics. Specifically, the strength of the chain interaction, the chain flexibility and the chain length play a significant role, and this work focused on the effects of these chain properties on the order formation dynamics. The adsorption dynamics of SAM molecules can be explained by the same theoretical framework as the polymer brush. On the other hand, the evolution of highly ordered structure is specific to SAM systems. Simulation results revealed that the development of oriented domains can be grouped into three types, isolated island growth, packing growth, and growth suppression, which depend on temperature and chain flexibility. In packing growth, oriented domains are formed gradually due to the decrease in free volume as the surface density becomes high, while the tilt of the adsorbed chain molecules does not become upright gradually as a whole. Rather, inside the oriented domains, the adsorbed chains adopt "standing" states with tilt angles almost equal to the final values, which contributes to the gradual increase in the total tilt order. The effect of chain length was also studied. In the case of semirigid chain molecules, longer-chain systems showed slightly slower growth in adsorption but faster growth in oriented domains. These simulation results reveal how chain properties influence the dynamics of oriented structure formation on surfaces.

Molecular dynamics study of crystallization of polymer systems confined in small nanodomains

We study the ordering dynamics of polymer crystallization using molecular dynamics simulations, in which the polymers are confined to small nanodomains surrounded by a noncrystalline medium. Although the adsorption process and surface diffusion of polymer chains play a major role in the case of the crystalline interface, the domain interface which consists of noncrystalline medium does not have such a crystal substrate to induce the growth of a crystal layer, which may lead to different ordering dynamics. We found that existence of a noncrystalline domain interface has two opposite effects. In the case of semiflexible polymer systems, the domain interface accelerates crystallization in the initial period, whereas it suppresses crystallization in the intermediate or late period. When the rigidity of polymer chains increases, the acceleration effect of the initial crystallization induced by the domain interface is sometimes hidden by spontaneous homogeneous nucleation. These ordering behaviors can be explained by the restriction on the local chain direction near the domain surface in the crystal nucleation stage and the restriction of large orientation relaxation and coalescence in the crystal growth stage. Simulation results reveal that the confinement by the noncrystalline medium does not have a simple effect on the polymer crystallization dynamics, but has various conflicting effects combined with the time stage, the rigidity of polymer chains, and the strength of the surrounding domain interface.

Molecular dynamics in n-alkanes: Premelting phenomena and rotator phases

Molecular dynamics simulations of the n-alkanes C18H38, C19H40, and C20H42 are reported for temperatures just below the melting point. Besides thermodynamic and average structural data for the ordered phase, we discuss the molecular motions initiating the rotator phases observed in spontaneous phase transitions in isothermal, isostress simulations. The RI phase of C19H40 is initiated by particular cork-screw-like jumps combining a quarter turn about the long molecular axis and a half-chain-period translation along the axis. This motion occurs between the minimum-energy conformation of the ordered crystal and a secondary minimum. Transient analogs of the RI and RII phases of the odd alkanes are found on melting C18H38 and C20H42. Collective motions within lamellae of molecules are prominent in the dynamics.