Swelling induced debonding of thin hydrogel films grafted on silicon substrates

TL;DR

This study investigates how swelling stresses cause thin hydrogel films grafted on silicon to delaminate, revealing a threshold thickness for debonding influenced by grafting density and modeled through nonlinear fracture mechanics.

Contribution

It introduces a detailed experimental and theoretical analysis of swelling-induced debonding in hydrogel films, linking grafting density to delamination behavior and fracture energy.

Findings

Delamination occurs at a threshold thickness that increases with grafting density.

Debonding velocity decreases as grafting density increases.

A nonlinear fracture mechanics model explains the swelling-driven crack propagation.

Abstract

We report on the delamination of thin ($\approx \mu$m) hydrogel films grafted to silicon substrates under the action of swelling stresses. Poly(dimetylacrylamide) (PDMA) films are synthesized by simultaneously cross-linking and grafting preformed polymer chains onto the silicon substrate using a thiol-ene reaction. The grafting density at the film/substrate interface is tuned by varying the surface density of reactive thiol-silane groups on the silicon substrate. Delamination of the films from well controlled line defects with low adhesion is monitored under a humid water vapor flow ensuring full saturation of the polymer network. A propagating delamination of the film is observed under the action of differential swelling stresses at the debonding front. A threshold thickness for the onset of this delamination is evidenced which is increasing with grafting density while the debonding velocity is also observed to decrease with an increase in grafting density. These observations are discussed within the framework of a nonlinear fracture mechanics model which assumes that the driving force for crack propagation is the difference between the swelling state of the bonded and delaminated parts of the film. Using this model, the threshold energy for crack initiation was determined from the measured threshold thickness and discussed in relation to the surface density of reactive thiol groups on the substrate.