  A coarse-grained model is developed to allow large-scale molecular dynamics
(MD) simulations of a branched polyetherimide derived from two backbone
monomers [4,4'-bisphenol A dianhydride (BPADA) and m-phenylenediamine (MPD)], a
chain terminator [phthalic anhydride (PA)], and a branching agent
[tris[4-(4-aminophenoxy)phenyl] ethane (TAPE)]. An atomistic model is first
built for the branched polyetherimide. A systematic protocol based on
chemistry-informed grouping of atoms, derivation of bond and angle interactions
by direct Boltzmann inversion, and parameterization of nonbonded interactions
by potential of mean force (PMF) calculations via gas-phase MD simulations of
atomic group pairs, is used to construct the coarse-grained model. A six-pair
geometry, with one atomic group at the center and six replicates of the other
atomic group placed surrounding the central group in a NaCl structure, has been
demonstrated to significantly speed up the PMF calculations and partially
capture the many-body aspect of the PMFs. Furthermore, we propose a correction
term to the PMFs that can make the resulting coarse-grained model transferable
temperature-wise, by enabling the model to capture the thermal expansion
property of the polymer. The coarse-grained model has been applied to explore
the mechanical, structural, and rheological properties of the branched
polyetherimide.
