Mesoscale analysis of segmental dynamics in microphase-segregated polyurea

Abstract

Polyurea is an elastic co-polymer which possesses a very complex nanometer-scale microstructure consisting of (high glass-transition temperature, T _g) hydrogen-bonded discrete hard domains dispersed within a (low T _g) contiguous soft matrix. A number of experimental investigations reported in the open literature clearly established that (a) polyurea has an unusually high capacity for shock mitigation and (b) this ability of polyurea is related to its segmental dynamics (the same process which is responsible for the rubbery-to-glassy transition). Due to the fact that the segmental dynamics in question involves a large number of atoms with coordinated motion and, hence, is associated with nanosecond to microsecond characteristic times, it cannot be generally analyzed using all-atom molecular dynamics techniques. To overcome this problem, mesoscale coarse-grain simulation methods are employed in this study. Within the all-atomic simulation methods, the material is modeled as a collection of constituent atom-size particles. Within the mesoscale methods, on the other hand, this atomistic description of the material is replaced with a collection of coarser particles/beads which account for the collective degrees of freedom of the constituent atoms. Consequently, before the mesoscale methods could be employed to polyurea, all-atom molecular analyses had to be used to determine the basic properties (i.e., mass and radius) of the beads and to parameterize the mesoscale bonding and non-bonding forcefield functions. The mesoscale analyses were then used to (a) obtain critical information regarding the material microstructure and its evolution (from an initially fully blended homogeneous state) and (b) the segmental dynamics in the microsegregated state of the material.