Interphase Formation during Curing: Reactive Coarse Grained Molecular Dynamics Simulations

Abstract

In the present work we have studied the influence of surface-induced interphases on the properties of a cross-linked polymer adhesive by reactive molecular dynamics (RMD) simulations. Particular attention has been directed to segregation phenomena leading to changes in the interphase dimension. To investigate the present systems, we have extended a recently developed RMD method from a simple irreversible polymerization [J. Phys. Chem. B2010, 114, 13656] to a curing process in the presence of a planar surface with tunable adsorption characteristics. The RMD simulations have been performed in coarse grained (CG) resolution for the constituents and the surface layer. The reactive mixture contains so-called initiator beads with connectivity one, monomer beads with connectivity two, as well as linker beads with connectivity four which render possible the formation of percolating network structures. The RMD simulations are controlled by a capture radius which defines the influence sphere of the CG beads in the reactive processes and by a delay time between two connectivity altering steps. A number of reactive starting mixtures have been prepared to analyze mass density distributions, local mole fraction profiles, the bond orientation relative to the surface, as well as average chain lengths as a function of the separation from the surface. To estimate the influence of the tunable surface on the derived quantities, we have correlated RMD data derived in the presence of the surface with data from pure bulk simulations. Adsorption selectivity has been modeled via a preferential surface potential for one of the components. On the basis of calculated mole fractions, we have evaluated the excess chemical potential for the linkers at the surface by Boltzmann inversion. The extension of the density fluctuations is not changed in the transition from the starting mixture to the cured network. For other quantities different interphase properties are influenced by the type of the system (starting mixture, cured system) and by the surface potentials. The RMD results have been adopted to identify key parameters in curing processes that are influenced by an attractive surface.