The prediction of adhesion between polymer matrices and silane-treated glass surfaces in filled composites

Extending the database and the analysis of work reported earlier, the practical adhesion between a glass filler, modified by various silane coupling agents, and different polymeric matrices is measured and compared with predictions based on a generalized thermodynamic criterion. The criterion used is the magnitude of the (negative) molar Gibbs free energy of mixing, (-DeltaG(mix))(0.5), for a pseudo-solution consisting of equal molar amounts of the repeat units of the polymer matrix and the organo-functional group of the silane coupling agent. It is computed using the group contribution method of UNIFAC, in which molar volumes, molar areas, and molar interaction energies are constructed from contributions of the functional groups which make up the molecules of the solution. Measurements leading to the values of the adhesion strength are carried out using the single-particle composite method, in which a rectangular polymer specimen containing a single silane-treated glass bead is subjected to increasing uni-axial tensile stress until interfacial failure, as observed using a microscope, occurs at one of the poles of the sphere. Earlier work reported a good correlation between the local stress at the pole computed from such measurements and the value of (-DeltaG(mix))0.5 computed using UNIFAC for ton different organo-silane-modified spheres imbedded in a poly(vinyl butyral) matrix. The present work extends the database to two additional matrices. viz. poly(methyl methacrylate) and poly(ethyl methacrylate). In addition, elastic fracture-mechanics theory is used to deduce specific adhesion energies in all cases. The relative values of the latter are all found to be in good correlation with the predictive thermodynamic criterion.