Computer simulation of polymer surfaces.

摘要

One of the main objectives of computer simulation is to isolate the effect of a specific variable in a physical or chemical system of interest, but with ambiguity in experimental interpretation. The area of polymer surface or interface contains such an ambiguity due to absence of a major thermodynamic driving force and difficulty of the complete control of experimental design.; Considering the length and the time scales that define a phenomenon observed in polymeric systems, the appropriate choice of a method among the currently available methodologies in computational chemistry that have been developed mostly for small molecules is very demanding because of the Imitation of computational resources.; In this study, a computationally efficient Monte Carlo simulation on a high coordination lattice employing the RIS scheme for short range interactions and a Lennard-Jones potential for long-range interaction has been applied to various boundary situations which define the material status and distinguish the properties of the material at an interface or surface from those in the bulk state. The polymer surfaces of interest in this study include a free polymer surface, a surface near an attractive solid substrate, a polymer surface generated by compression between two repulsive hard walls, and a polymer-polymer interface.; Several focuses are on the change of the static properties and dynamic properties at the interfaces, which includes density profiles, distribution of a specific constituent of a polymer chain at the interfaces, chain orientation, local conformational state, and chain diffusivity. Each property at an interface is greatly affected by the characteristic of the imposed heterogeneity. One common feature is that the chains are confined at an interface along the direction normal to a surface regardless of the detailed nature of the surface. In addition, the effect of a surface gradually diminishes toward a bulk region and each property has its own effective range in which the properties are different from those in the bulk state. The length scale over which the effect of a free surface can extend in terms of a macroscopic property like density is related to the chain dimension as around 2 R_g where R_g is the radius of gyration of the chain. This length varies with the introduction of a strong boundary such as a solid substrate. With a strongly attractive interaction between polymer chains and a solid substrate, the effect can extend much farther than 2 R_g. The length scale can be easily changed by a perturbation such as a small change of chemical constituents of chain ends.; The simulation method is also used to elucidate a microscopic cause for the decrease of chain mobility of polybutadiene with increase of its 1,2-units, in conjunction with an experimental observation and a theoretical interpretation. After ruling out other possible causes, the simulation reproduces the experimental trend with the lower rate of the transitional jump between rotational states of sp3-sp3 bonds in 1,2-units.