Intelligent materials with adaptive adhesion properties based on comb-like polymer brushes

We investigated the adaptive adhesion properties of comb-like random copolymer brushes made of poly(ethylene glycol) (PEG)-poly(dimethylsiloxane) (PDMS) grafted on flat and rough substrates. The properties of the brush layers were investigated using ARXPS, contact angle, electrokinetics, null ellipsometry, and adhesion measurements. It was found that hydrophobic PDMS segments segregate at the brush topmost layer in the dry state. However, hydrophilic PEG chains segregate at the brush topmost layer in the wet state. The adhesion properties of fabricated materials were tested using the AFM colloid probe technique and probe tack tester. It was found that the adhesive properties depend strongly on the mechanical properties (stiff/soft) and chemical functionality (hydrophobicity/hydrophilicity) of the applied adhesion tester as well as on the chemical composition, surface roughness, and thickness of the brush. In particular, hydrophobic PDMS and hydrophilic PEG adhere more strongly to hydrophobically modified and hydrophilic native colloid probes, respectively. Thick brushes are more adhesive than thin ones, and brushes grafted to flat substrates are stickier than those grafted to rough substrates when measured with a hard AFM probe. Unlike the results of adhesion measurements performed using hard AFM probes, the PDMS surface probed by soft pressure-sensitive adhesives (PSA) is almost nonadhesive. However, PEG is strongly adhesive, and the adhesion increases with the PEG fraction in the brush when probed by both hydrophilic and hydrophobic soft adhesives. The surfaces roughness also has a considerable effect on adhesion. Contrary to the adhesion measurements performed by hard AFM colloid probes, the adhesion of rough surfaces measured with a soft PAA or SIS tack tester is greater than that on the corresponding flat one.