Surface and Thin Film Characteristics of Poly(tetrafluoroethylene) Melts from Molecular Dynamics Simulations

Abstract

The surface characteristics of thin films of poly(tetrafluoroethylene) (PTFE) melts were investigated by molecular dynamics simulations, employing the recent explicit atomic force field, including partial charges, which was fine-tuned to the experimental PVT data, the chain conformation and the crystal structure (Macromolecules 2003, 36, 5331). Surface tension was calculated from the virial equation of the pressure tensor, taking into full account of the long-range correction terms, in good agreement with experimental data. Compared with polymethylenes of the same chain length, PTFEs have larger vacuum/liquid interface thicknesses concomitant with lower surface tensions. In the surface region, the chain conformations remain unperturbed, but the chain backbone segments tend to be oriented parallel to the surface whereas the chain-end segments tend to be perpendicular to the surface. As compared with polymethylenes, the orientation of PTFE segments in the surface is found to persist deeper into the film due to the larger intermolecular orientational correlation length in PTFE melts than in polymethylene melts.

Interface characteristics of polystyrene melts in free-standing thin films and on graphite surface from molecular dynamics simulations

“…MD simulations were performed on melts of atactic polystyrene (PS) at 450 K, which is well above its experimental T g of 373 K, following the simulation methods described in detail in the previous studies of polymethylene films [40] and polytetrafluorene (PTFE) films [49]. The simulation box has 20 molecules and each molecule has 15 repeating units, with the stereochemical configuration of each repeat unit chosen randomly as meso or racemic [48,55].…”

“…The earlier studies showed the general features of polymer thin films and interfaces, but they had critical limitations in that they were unable to include accurate information about the detailed chemical structures of various polymers and their interactions at various surfaces. Such shortcomings have been improved by recent advances in computational capability and simulation algorithms, allowing large scale simulations on the polymer/vacuum [49] and the polymer/solid [50][51][52] interfaces with detailed consideration of explicit atomistic structures and interactions.…”

Surface films of short fluorocarbon–hydrocarbon diblocks studied by molecular dynamics simulations: Spontaneous formation of elongated hemimicelles

“…The mechanism leading to these structures as well as the structures by themselves are observed and characterized for the first time at atomic level. A few MD simulations on PFA or related molecules have been carried out in the past by several authors [15][16][17][18][19][20]. The closest approximations to the present work are the studies by Kim and Shin [16,17], based on 10-nslong simulations for F12H18 at several temperatures using implicit solvent.…”

Liquid-vapor equilibria and interfacial properties of n-alkanes and perfluoroalkanes by molecular simulation

“…Hariharan and Harris 23 presented a UA-based study in which their model could not capture surface-tension differences between decane and perfluorodecane. References [24][25][26] to that work 23 seem to suggest that this is caused by an inherent limitation of the UA method. However, the results in this work demonstrate that this is not the case; rather, the issue is with the specific UA model used and its parameterization.…”