Molecular dynamics simulations to compute the bulk response of amorphous PMMA

The results of molecular dynamics computations and corresponding values of laboratory measurements are compared to assess the efficacy of the numerical method. Details are presented on the modeling process, including the selection of an appropriate force field derived from separate computations on isotactic (crystalline) PMMA together with commensurate experimental data. It is found that in addition to typical energy minimization and temperature annealing cycles to establish equilibrium models, it is advantageous to also subject the model samples to a cycle of a relatively large pressure excursion (GPa's), to further improve the equilibrium state. Although the computations are limited to small samples in a physical sense (three polymer chains with fifty monomer units per chain per unit cell containing 2256 atoms), it appears that the primary limitation of the comparison with experimental data rests in the very short times (picoseconds). Estimates based on the time-temperature superposition principle do not overcome this difficulty, but may, on the other hand, signify limitations of the time-temperature trade-off as normally practiced.