Optimum Polymer - Solid Interface Design for Adhesion Strength: Carboxylation of Polybutadiene and Mixed Silanes Surface Modification of Aluminum Oxide

To understand the optimum design of polymer-solid interfaces for adhesion strength, model polymer-solid interfaces of carboxylated polybutadiene(cPBD) adhered to mixed silane modified Al₂O₃ surfaces were examined. The cPBD, having various ?COOH sticker group concentration φ(X) (0 ～ 10 mol%), was synthesized through high-pressure carboxylation of PBD, while Al₂O₃ surfaces were modified to have various -NH₂ density, φ(Y) (0 ～ 100 mol%), using self-assembly of mixed amine- and methyl-terminated silanes. The coadsorption kinetic model of the two silanes was analyzed through X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and dynamic contact angle (DCA), which gave the capability of controlling the receptor concentration of aluminum oxide surfaces. The polymer surface chain responses after exposure to various media were understood by measuring contact angle changes of various probe liquids. T-peel tests of the model polymer–solid interfaces, as a function of time and sticker and receptor group concentrations showed much longer time dependence than the characteristic time of a bulk polymer chain. Additionally, the classical equation of interface failure was re-examined to see the effects of deformation rate, annealing temperature, and annealing time. A simple scaling analysis of free energy of an adsorbed polymer on a solid surface was extended to predict the adhesion potential of the model polymer–solid interfaces. From the experiments and theory of adhesive vs. cohesive failure, it was found that there existed an optimum product value r* = φ(X)φ(Y)χ of sticker concentration φ(X), receptor concentration φ(Y), and their interaction strength χ, which was approximately 150 cal/mol for this polymer–solid interface. Below or above this optimum product value r*, the fracture energy of polymer-solid interfaces, G _IC, was less than its optimal value, G _lc*.