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

Adhesion at polymer-solid interfaces was explored for a new bio-based PSA in terms of sticker groups $\phi _{X }$ on the polymer, receptor groups $\phi _{Y}$ on the solid and the strength of the X-Y acid-base interaction, $\chi $. The polymer-solid interface models of Gong, Lee and Wool were extended with new percolation models of entanglements and interface strength to determine the optimal sticker group concentration $\phi $*$_{X}$. For the general case where $\phi _{Y}$ and $\chi $ are constant, it is predicted that when $\phi _{X}<\phi $*$_{X}$, the peel strength behaves as G$_{1c}\sim \phi _{X}$/$\phi $*$_{X}$ and the locus of failure is adhesive between the polymer and the solid. However, when $\phi _{X}>\phi $*$_{X}$, failure occurs cohesively in a polymer-polymer interface adjacent to the solid and the strength decreases as G$_{1c}\sim \phi $*$_{X}$/$\phi _{X}$. The switch from adhesive to cohesive failure can be understood in terms of the changes in the chain conformations of the adhered chains and their decreasing interpenetration X with the bulk chains, via X$\sim $1/r, where r = $\chi \phi _{X}\phi _{Y}$. The optimal value of $\phi _{X}$ which maximizes the adhesion and determines the mode of failure is given by $\phi $*$_{X}\approx $0.07/C$_{\propto }$, and for typical values of the characteristic ratio C$_{\propto }$ in the range 7-10, $\phi $*$_{X}\approx $1{\%} mole fraction, corresponding to about 2 sticker groups per critical entanglement length M$_{c}$. Supported by USDA