The Strength of an Adhesive Weak Boundary Layer

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Abstract
The strength of model adhesive joints composed of different thicknesses of low (10,300 narrow distribution) molecular weight polystyrene sandwiched between high molecular weight poly(methyl methacrylate) has been studied. The joints model a polymer-to-polymer adhesive bond across a low-strength boundary layer. As an appraisal of strength, the fracture toughness was measured by driving a cleavage wedge into the specimens along the polystyrene layer and analyzing the results with Kanninen's equation. The fracture toughness for both crack growth initiation K Ic and arrest K Ia was essentially that for bulk poly(methyl methacrylate) until the polystyrene layer exceeded 1 μm, at which K Ic and K Ia fell by approximately 85%. Examination of the fracture surfaces of specimens with polystyrene layers less than 1 μm thick revealed that the fracture path was predominantly in the poly(methyl methacrylate). Possible reasons for this are discussed.