Published: July 20, 2011 r2011 American Chemical Society 16451 dx.doi.org/10.1021/jp203480q | J. Phys. Chem. C 2011, 115, 16451–16460 ARTICLE pubs.acs.org/JPCC Interphase Formation during Curing: Reactive Coarse Grained Molecular Dynamics Simulations Karim Farah,* Fr  ed  eric Leroy, Florian M€ uller-Plathe, and Michael C. B€ ohm Eduard-Zintl-Institut f€ ur Anorganische und Physikalische Chemie and Center of Smart Interfaces, Technische Universit € at Darmstadt, Petersenstrasse 20, D-64287 Darmstadt, Germany I. INTRODUCTION The ability of polymer materials to adhere to solid substrates has been intensively exploited in the design of polymer matrix composites, in coating processes, and in the adhesive joining of surfaces. 1À11 Polymer adhesion onto solid surfaces originates from interactions spanning the range from weak van der Waals forces to chemical bond formation with surface constituents. 1 Adhesion is generally accompanied by a modiﬁcation of the polymer properties near the adhesiveÀsubstrate interface region. PolymerÀsubstrate interactions determine the polymer properties in these particular domains denoted as “interphases”. 1À11 They are considered as important regions inﬂuencing the durability of adhesiveÀsubstrate systems. Thus, intense experimental eﬀorts have been devoted to the characterization of interphases. 1À11 The development of experimental techniques for their investigation, however, is rather challenging. Measurements are particularly diﬃcult for so-called “closed” joint conﬁgurations in which the adhesive part is covered by two solid surfaces. 1 “Open” joints oﬀer an easier experimental access; these are polymer ﬁlms coating only one surface. Properties are then measured for diﬀerent ﬁlm thicknesses. The interphase behavior is observed for thin polymer ﬁlms while bulk properties are found for thick ones. 1,3,6,7,11,12 Epoxies and polyurethanes belong to two popular classes of polymers for coating as well as for the adhesive joining of surfaces. 1,5,7,8,10À12 These adhesives are frequently prepared as reactive liquid mixtures of monomers and linkers. In the initial step, the surface is coated with the reactive mixture. Subse- quently, the cross-linking reaction is initiated and an adhesive polymer network is generated. 7,11,12 The interphase properties of epoxy and polyurethane adhesives cured onto various substrates have been a subject of detailed experimental investigations. Wehlack et al. 1 described the formation of organometallic bonds near the interface region of gold, aluminum, and copper coated by epoxy and polyurethane network ﬁlms. Bockenheimer et al. 11 analyzed the inﬂuence of mechanically pretreated aluminum surfaces that have been blasted either with alumina grit or glass beads on the formation of epoxy ﬁlms. They showed a correlation between the nature of the surface pretreatment and the degree of cross-linker conversion in the resulting polymer ﬁlms. Both the network morphology and the mobility of the particles at the interface were related to the surface pretreatment. With the help of scanning force microscopy and energy dispersive X-ray analysis, Chung et al. 7 revealed a mechanical interphase for epoxy resins cured with amine compounds on a copper surface. Received: April 14, 2011 Revised: July 15, 2011 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) simula- tions. Particular attention has been directed to segregation phe- nomena 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. B 2010, 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.