Reversible adhesion between a hydrogel and a polymer brush

Guillaume Sudre,Ludovic Olanier,Dominique Hourdet,Yvette Tran,Costantino Creton

doi:10.1039/c2sm25868d

Abstract

We have developed a new experimental methodology to investigate the adhesive properties of hydrogels on solid surfaces under fully immersed conditions. The method, based on contact mechanics, provides time-resolved reproducible and quantitative data on the work of adhesion between a hydrogel at swelling equilibrium and a planar surface grafted with responsive brushes. We used poly(N,N-dimethylacrylamide) (PDMA) and polyacrylamide (PAM) as model gels and poly(acrylic acid) (PAA) as pH dependent polymer brush. The effect of pH, contact time and debonding velocity on adhesive interactions was specifically investigated. As expected from molecular interactions, we found that adhesion increased as the pH decreased and this was attributed to the formation of hydrogen bonds at the interface. Surprisingly, however, the buildup of adhesion increased slowly with the time of contact up to one hour and depended markedly on debonding velocity despite the very elastic nature of the hydrogels. Furthermore, the maximum pH where adhesion was observed was significantly higher for the couple PAM–PAA than for the couple PDMA–PAA, in contrast with the onset of molecular interactions in dilute solutions.

Highlights

Adhesion of elastic soft materials on solid surfaces in air has been the subject of decades of investigations and key results can be found in reviews,[1,2,3] this is not the case for elastic soft materials under water
EFFECTS OF ENVIRONMENTAL CHANGES ON THE ADHESION One of the main goals of the study was to investigate the effect of a change in pH of the water on the adhesive interactions between the gel and the brush grafted on the silicon wafer. These experiments were carried out with a time of contact of 300 s, an average contact compressive stress of 4 kPa and a debonding velocity equal to 10 μm s‐1 for all the adhesion tests of this paper
It is important to note that all experiments reported here are perfectly reproducible for a given gel–brush pair

Summary

Introduction

Adhesion of elastic soft materials on solid surfaces in air has been the subject of decades of investigations and key results can be found in reviews,[1,2,3] this is not the case for elastic soft materials under water. Competition between adhesive interactions and interactions with water molecules significantly complicates the problem. Such problems can be investigated with hydrophobic systems or with hydrophilic systems. A very common soft hydrophilic material which displays a kind of entropic elasticity common in rubbers is the polymer‐based hydrogel. Most hydrogels are essentially networks of hydrophilic polymers which are highly swollen with water. When immersed, they swell to an equilibrium volume, controlled by a balance between entropy of mixing and conformational entropy, and behave as an incompressible solid with rubber elasticity. More generally interactions, of such soft materials on solid surfaces is of great interest since their role as a sponge makes them suitable to deliver hydrophilic cargo to specific locations (controlled release) and provide texture in food for example

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