Tutorial: Molecular Dynamics Simulations of Microstructure and Transport Phenomena in Glassy Polymers

Molecular Dynamics Simulations of Helium Permeation in Polyimides with a Bulky Dianhydride and a Fluorinated Diamine

“…Our so‐called “hybrid PMC‐MD single‐chain sampling” method has now been extensively reported in the literature and systematically validated 27, 34, 35, 41, 54–60. In addition, it has been found to be reproducible27, 41 and very efficient computationally. Once generated by PMC‐MD single‐chain sampling at 700 K (i.e., slightly above their glass‐transition temperature), uncorrelated polyimide chains are randomly orientated and distributed in a periodic cubic MD box in order to give an initial density close to the experimental value 27.…”

“…The force‐field for polyimides has been extensively described elsewhere27, 41 and will only be briefly outlined. As often seen in realistic polymer simulations, the very high‐frequency modes, such as bond stretching, are removed using rigid constraints,42 which allows for the use of an integration time step of Δ t = 10 −15 s in the MD algorithm.…”

“…In this approach, the configurational phase‐space of the polymer is sampled using pivot Monte Carlo (PMC) moves52 for rotatable torsions, while standard MD algorithms are used to explore the various oscillatory modes of the chains. The energy criterion is based on Flory's “local energy approximation”;53 that is, that polymer configurations in the pure melt can be described by considering isolated molecules with only a certain number of specific near‐neighbour intramolecular interactions; in the case of polyimides, this amounts to four backbone bonds on either side of a given atom 41, 54. Our so‐called “hybrid PMC‐MD single‐chain sampling” method has now been extensively reported in the literature and systematically validated 27, 34, 35, 41, 54–60.…”

“…Once generated by PMC‐MD single‐chain sampling at 700 K (i.e., slightly above their glass‐transition temperature), uncorrelated polyimide chains are randomly orientated and distributed in a periodic cubic MD box in order to give an initial density close to the experimental value 27. To remove the huge overlap energies of the initial structures, the van der Waals excluded‐volume is introduced very progressively during short MD simulations under constant‐volume and temperature conditions ( NVT ) 41, 54, 58–60. Unphysical spearings and interlockings61 are avoided by temporarily placing “phantom atoms” at the centre‐of‐coordinates of each six‐membered or five‐membered ring upon completion of the generation procedure 41, 54.…”

“…To remove the huge overlap energies of the initial structures, the van der Waals excluded‐volume is introduced very progressively during short MD simulations under constant‐volume and temperature conditions ( NVT ) 41, 54, 58–60. Unphysical spearings and interlockings61 are avoided by temporarily placing “phantom atoms” at the centre‐of‐coordinates of each six‐membered or five‐membered ring upon completion of the generation procedure 41, 54. While it has been reported that the fast introduction of a full interaction potential for initially non‐interacting bead‐springs chains leads to increases of the average intramolecular distances in the intermediate length scale,62 this effect appears to be very limited in stiffer more realistic chains, despite fairly poor statistics.…”