Molecular dynamics simulation of the phase behavior of lamellar amphiphilic model systems

Using efficient simplified continuum amphiphile models-AB dimers, A2B2 tetramers, and A4B4 octamers-as models for diblock copolymers, surfactants and liquid crystals, the phase behavior of the corresponding lamellar systems is investigated by large-scale parallelized dissipative particle dynamics type of molecular dynamics simulation. We not only observe the first order nature of the order-disorder transition, but also find that this first order phase transition becomes weaker as the block length of the models increases. The theoretically predicted chain stretching effect is reproduced as the like-monomer attractive potential well depth is increased. At the order-disorder transition point, a singular discontinuous jump in the stretching amplitude is accompanied with a distinctive orientation-induced stretching effect in the layer normal direction and an orientation induced-compressing effect in the layer plane directions. The configuration of the lamellar phase is very sensitive to the commensurability of its equilibrium layer spacing with the cell size. Too large layer spacings lead to the formation of a slightly tilted lamellar structure in the quasistatic-cooling process and to an undulation instability in the quasistatic-heating process. The relation between equilibrium layer spacing and like-monomer attraction is examined. (C) 2003 American Institute of Physics.