Atomistic Modeling of Alumina/Epoxy Adhesion
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Atomistic Modeling of Alumina/Epoxy Adhesion F.O. Valega Mackenzie1 and B. J. Thijsse Delft University of Technology, Dept. Materials Science & Engineering, Mekelweg 2, 2628 CD Delft, The Netherlands. [email protected] ABSTRACT In this work we report a specialized reactive force field (ReaxFF) developed for the study of alumina/epoxy interfaces. Force field parameters were obtained by fitting the reactions of small clusters and separate components of epoxies on alumina surfaces in the alpha phase. We also introduce a procedure to obtain crosslinked epoxies based on a proximity criterion to drive reactions and induce crosslinking. Properties of the resulting polymer, like the coefficient of thermal expansion, are found to be of the same order of magnitude as in experiments. Molecular dynamics was used to calculate the adhesion between these polymers and different alumina surfaces: Al2O3-deficient, Al-terminated, O-terminated, 12% and 75% hydroxylated. Typical values for strong adhesion are about 0.70 J/m2 which compare well with previously reported works. The role of defects is also studied. INTRODUCTION Oxide/polymer interfaces are of paramount importance in many industrial applications, e.g. corrosion-preventing coatings and chip packaging. Understanding how these interfaces respond when subjected to real life conditions such as compressive stress or humidity requires a better insight of what happens at the atomistic scale. Current experimental techniques lack a consistent way to characterize atomic adhesion at the interfaces of different materials. There are number of qualitative tests that aim to establish a result based on comparison under similar conditions, e.g. the tape and peel tests. More quantitative approaches, e.g. four point bending and indentation, can also be used. However, results are difficult to compare and it is not evident what really happens at the atomistic scale. Molecular modeling techniques are a great alternative to simulate many adhesion and delamination mechanisms. It has been common practice to use force field to model the manner in which atoms interact at the interfaces. In general force fields offer a good way to deal with dissimilar materials. In the case of polymers, they define the connectivity between the atoms and their interactions. Force fields also include non-bonded van der Waals and Coulomb terms so that ionic compounds can be modeled as well. Additionally the non-bonded terms present for each atomic species make possible to bring ionic and polymeric compounds together. Unfortunately classical force fields are limited to systems where the atom connectivity remains intact, which is an essential deficiency for the study of adhesion, wear, plasticity, delamination, etc., where bonds between atoms break and are re-established with different partners. In order to circumvent this and other complications it is possible to describe force field interactions based on a bond order formulation, instead of connectivity. Several schemes have been proposed in literature like t
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