Polymer-Solid Interface Connectivity and Adhesion: Design of a Bio-Based Pressure Sensitive Adhesive

Abstract

Adhesion at polymer-solid interfaces was explored for a new bio-based pressure sensitive adhesive (PSA) in terms of sticker groups, φX, on the polymer phase, receptor groups, φY, on the solid surface, and the bond strength of the sticker-receptor X-Y acid-base interaction, χ. The polymer-solid interface restructuring models of Gong and Lee et al. were extended with new percolation models of entanglements and interface strength to determine the optimal sticker group concentration, φ*X. For the general case where φY and χ are constant, it is predicted that when φX < φ*X, that the critical peel energy behaves as G1c ∼ φX/φ*X and the locus of failure is adhesive between the polymer and the solid. However, when φX > φ*X, failure occurs cohesively in a polymer-polymer interface adjacent to the solid and the strength decreases as G1c ∼ φ*X/φX. The switch from adhesive to cohesive failure can be understood in terms of the changes in the chain conformations of the adhered chains and their decreasing interpenetration, Xi, with the bulk chains, via Xi ∼ 1/r, where r = χφXφY. The optimal value of φX which maximizes the adhesion and determines the mode of failure is given by φ*X ≈ 0.129/C∝, and for typical values of the characteristic ratio C∝ in the range 7–20, φ*X ≈ 1% mole fraction, corresponding to about 2 sticker groups per entanglement molecular weight, Me. This result was verified for a bio-based PSA synthesized from an acrylated high oleic fatty acid, which was copolymerized with maleic anhydride as the sticker group. The observed behavior is counterintuitive to the current wisdom for the effect of acid-based interactions on adhesion, where the strength is expected to increase with the number of X-Y contacts. The surprisingly low value of φ*X ≈ 1% sticker groups which maximizes the adhesion strength can now be readily calculated using the percolation model of entanglements and fracture.