Ab initio study of the tensile behavior of single polyimide molecular chain

The Energetics of Large Deformations of a Single Polyimide Molecular Chain: DFT and MO Calculations

“… a) Relationship between the energy E ( d ) and displacement d under uniaxial compressive loading conditions, b) Buckling behavior under uniaxial compressive loading conditions ( ε z = −8.0%) obtained from DFT/LDA calculations 10. The arrow shows the inflection point of the energy/displacement curves.…”

“… a) The equilibrium structure and the supercell of one repeating unit of PMDA‐PDA obtained from DFT/LDA calculations 10. b) The equilibrium structure of two repeating unit of PMDA‐PDA obtained from MP2/6‐31G calculations.…”

“…The molecular structure of one repeating unit of PMDA‐PDA was obtained from the wide‐angle X‐ray diffraction (WAXD) measurements data reported by Kim et al14 The equilibrium atomic structure of the single molecular chain is then obtained by relaxation of the atomic configuration with various calculation methods. We have already calculated the relaxed atomic structure and supercell using first‐principle calculations, as shown in Figure 1 10. The first‐principle calculations were performed with the Vienna ab initio Simulation Package (VASP),16–18 based on the DFT method with LDA (DFT/LDA).…”

“…For the purpose of investigating the large deformation energetics of a molecular chain including fracture behavior, the best candidate is a calculation from first principles. In a previous study, we investigated the tensile (compressive) behavior of a single polyimide {poly[( p ‐phenylenepyromellitimide)‐ co ‐( p ‐phenylenediamine]; PMDA‐PDA} molecular chain using first‐principle calculations based on density functional theory (DFT) with a local density approximation (LDA), and found that the six‐membered ring of the PDA group deforms the most up to an engineering strain of 7.0% under uniaxial tensile deformations and the carbon–nitrogen bonds between the p ‐phenylenepyromellitimide (PMDA) and p ‐phenylenediamine (PDA) groups break at the point where fracture of the molecular chain occurs 10. However, to conduct MD simulations based on first‐principle calculations, we need to investigate twisting and bending deformations between PMDA and PDA groups in the molecular chain in more detail to use as reference data.…”

“…For example, in the commonly‐used general Amber force field (GAFF)8 and Dreiding generic force field (Dreiding) 9. However, the harmonic type of potential energy expression is insufficient to investigate the large deformation of polymer for the following reasons:10 firstly, a harmonic potential energy expression can only be valid for small deformations, in which a linear response can be expected; and, secondly, realistic bond‐breaking behavior in molecular chains, which should be an important piece of information for developing molecular level‐devices, can only be introduced artificially. Therefore, it is impossible to evaluate the precise strength of a dense polymer using the harmonic type of potential energy expression.…”