Guoqiang Xu

Monte Carlo simulation on the glass transition of free-standing atactic polypropylene thin films on a high coordination lattice

A recently developed high coordination lattice based Monte Carlo method is applied to investigate the glass transition of free-standing atactic polypropylene thin films. In this method the short-range interactions are described by a rotational isomeric state model and the long-range interactions are obtained from a discretized form of a Lennard-Jones function. Simulations of five thin films of C150H302 with thickness of 20–100 A at Tg are performed by a stepwise cooling from the melts to the glass states. The temperature dependence of the thickness of the thin films is obtained from the fitting of the density profile by a hyperbolic function. The glass transition temperatures of the thin films are evaluated from the change in thermal expansion coefficients at Tg. The Tg decreases as the films become thinner. For all but the thinnest film, 1/Tg is inversely proportional to the thickness of the film. The proportionality constant is within the range cited in experimental studies of other systems. The depress...

Study on structure formation of short polyethylene chains via dynamic Monte Carlo simulation

Abstract Monte Carlo (MC) simulations of structure formation for short polyethylene chains at low temperature are performed based on a recent developed method that uses coarse-grained chains on a high coordination lattice. Local short-range interactions based on rotational isomeric state (RIS) model and long-range interactions obtained from Lennard–Jones (LJ) potential are introduced during the simulation. Properties evaluated from the simulations are the mean square dimensions, anisotropy of the radius of gyration tensor, local conformation determined by the occupancy of trans state and orientation correlation functions, energy of the system, and chain packing reflected by the pair correlation functions and structure factors. All of these parameters reveal an ordering process that produces an approximation to a hexagonal crystal phase. The hexagonal structure is imposed by the presence of a diamond lattice underlying the high coordination lattice on which the simulation is performed. Folding of the chains in the crystal is mandatory, because they have fully extended lengths in excess of the dimension of the simulated periodic box. Nevertheless, the simulations demonstrate that a high degree of crystallinity can be achieved in reasonable computer time. The simulation technique should be applicable to other choices of periodic boundary conditions that do not affect the results as strongly as in the present case.

Configuration selection in the simulations of the crystallization of short polyethylene chains in a free-standing thin film

The configuration of thin films of short polyethylene chains during the crystallization has been investigated using a recently developed Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Thin films select their own crystal configurations, in which the chains have different orientations with respect to the surface normal. Two kinds of crystal structures, with and without grain boundary in the center of the thin films, in which all the chains are parallel to the free surface, have been found after a deep quench from an equilibrated thin film above the melting temperature in the previous simulations [J. Chem. Phys. 116, 2277 (2002)]. However, another crystal configuration shows up after a quench from the thin film formed by increasing the periodic boundary conditions in one direction from a homogeneous melt. In this configuration all the chains are parallel with each other and to the z axis. This configuration was found both in the experi...

Monte Carlo simulation of the crystallization and annealing of a freestanding thin film of n-tetracontane

A freestanding thin film of n-tetracontane chains is simulated by a Monte Carlo (MC) method on a high coordination lattice. The coarse-grained chains, represented by 20 beads each, can be reverse-mapped into the fully atomistic description, C40H82. The Hamiltonian includes a short-range interaction based on a rotational isomeric state model and a long-range interaction obtained from a Lennard-Jones potential energy function. When the melt is instantaneously quenched from 473 to 298 K, crystallization initiates in the surface region and propagates into the interior of the film, as was found in a prior molecular dynamics simulation of a united atom model of polyethylene [M. Ito, M. Matsumoto, and M. Doi, Fluid Phase Equilibria, 144, 395 (1998)]. Several repetitions of the MC simulation, starting from different configurations of the melt at 473 K, reveal that two distinctly different structures can be obtained. Usually the independently initiated crystals at the two surfaces of the thin film produce a disord...

Monte Carlo simulation on separation and co-crystallization of a mixture of short polyethylene chains in a thin film

Thin films of a mixture of two kinds of short polyethylene chains, n-tricontane and n-tetracontane, are simulated both at high and low temperatures by a dynamic Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Films are obtained from an equilibrated model for the melt by increasing one of the three periodic boundaries to a very large value. The melting temperatures (Tm) of the two pure components in the simulation are obtained from a sharp transition of various parameters, such as the probability of trans conformation of C–C bonds, orientation order parameters, energies, etc. At high temperature, 420 K (above Tm of the two pure components), the chains have a trend to separate with each other. The shorter chains are enriched on the free surface due to their larger fraction of chain ends. At low temperature (below Tm of the two pure components), two situations exist, which depend on the prior history of the mixture. If the quench takes ...

A Method for Handling Branch Points in Monte Carlo Simulations

A general procedure is described for introducing branch point relaxation into a Metropolis Monte Carlo simulation of molecules expressed on the discrete space of a lattice. The lattice may be of any type. When applied to regular tri- and tetrafunctional stars represented by self-avoilding walks on a high-coordination lattice, the method produces the expected behavior for the mean square dimensions, displacement of the center of the mass, and the times for internal relaxation of the branches. The method is easily generalized to branch points of higher functionality and molecules that contain multiple branch points.

Similarities and differences in the rapid crystallization induced in n-tetracontane by an instantaneous deep quench of the free-standing nanofiber and free-standing thin film

The crystallization of a confined short polyethylene chain, n-tetracontane, quenched from the melt has been investigated by a dynamic Monte Carlo method on a hight coordination lattice. The periodic boundary conditions force the formation of a free-standing nanofiber exposed to a vacuum. After the deep quench, the nanofiber adopts a configuration dominated by extended chains aligned parallel to the fiber axis. The vicinity of the fiber axis is less dense, and less well ordered, than portions of the fiber located further from the fiber axis. Annealing finds that this low-density region inside the fiber is not as easily removed as is the grain boundary that usually develops inside a free-standing thin film upon rapid crystallization.