Atomistic Simulation of Orientation of Methyl Groups and Methylene Bisectors, and Surface Segregation, in Freely Standing Thin Films of Atactic Poly(ethylene-co-propylene)

Monte Carlo simulation of molecular and structural properties of random copolymer thin films

Molecular and structural properties of random copolymer thin films were studied by Monte Carlo simulation of coarse-grained copolymer model on the high coordination lattice. Random copolymer thin films with 50% comonomer fraction with varied interaction strength between comonomer units were studied. Intramolecular interaction was represented by Flory's rotational isomeric state (RIS) model of polyethylene (PE). The non-bonded interactions were treated by Lennard-Jones potential with different parameter sets for comonomer units. When the interaction strength of comonomer is increased, the density of thin films is higher in the inner region and significantly dropped near the free surface along the normal direction of the film. Higher bulk densities and narrower interfacial thicknesses are observed for random copolymer with stronger bead interaction. End beads were segregated at the free surface region and tend to orient perpendicularly to the surface. Copolymer size and shape were significantly changed as a function of bead interaction. For orientation of the whole chain, the largest molecular axis tends to orient along the film surface, but changed toward random orientation for weaker bead interaction. The intra-chain energies were decreased at the surface region while the non-bonded energies were increased. The stronger the comonomer bead attraction, the more change in energetics across the thin film.

A comparative molecular simulation study of the glass former ortho-terphenyl in bulk and freestanding films

We performed molecular dynamics simulations of the low-molecular weight organic glass former ortho-terphenyl in bulk and freestanding films. The main motivation is to provide molecular insight into the confinement effect without explicit interfaces. Based on earlier models of ortho-terphenyl we developed an atomistic model for bulk simulations. The model reproduces literature data both from simulations and experiments starting from specific volume and diffusivity to mean square displacement and radial distribution functions. After characterizing the bulk model we form freestanding films by the elongation and expansion method. These films give us the opportunity to study the dynamical heterogeneity near the glass transition through in-plane mobility and reorientation dynamics. We finally compare the model in bulk and under confinement. We found qualitatively a lower glass transition temperature for the freestanding film compared to the bulk.

Interior segment regrowth configurational-bias algorithm for the efficient sampling and fast relaxation of coarse-grained polyethylene and polyoxyethylene melts on a high coordination lattice

We demonstrate the application of a modified form of the configurational-bias algorithm for the simulation of chain molecules on the second-nearest-neighbor-diamond lattice. Using polyethylene and poly(ethylene-oxide) as model systems we show that the present configurational-bias algorithm can increase the speed of the equilibration by at least a factor of 2-3 or more as compared to the previous method of using a combination of single-bead and pivot moves along with the Metropolis sampling scheme [N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth, A. H. Teller, and E. Teller, J. Chem. Phys. 21, 1087 (1953)]. The increase in the speed of the equilibration is found to be dependent on the interactions (i.e., the polymer being simulated) and the molecular weight of the chains. In addition, other factors not considered, such as the density, would also have a significant effect. The algorithm is an extension of the conventional configurational-bias method adapted to the regrowth of interior segments of chain molecules. Appropriate biasing probabilities for the trial moves as outlined by Jain and de Pablo for the configurational-bias scheme of chain ends, suitably modified for the interior segments, are utilized [T. S. Jain and J. J. de Pablo, in Simulation Methods for Polymers, edited by M. Kotelyanskii and D. N. Theodorou (Marcel Dekker, New York, 2004), pp. 223-255]. The biasing scheme satisfies the condition of detailed balance and produces efficient sampling with the correct equilibrium probability distribution of states. The method of interior regrowth overcomes the limitations of the original configurational-bias scheme and allows for the simulation of polymers of higher molecular weight linear chains and ring polymers which lack chain ends.