Dependance of Poly(acrylic acid) Interfacial Adhesion on the Nanostructure of Electrodeposited ZnO Films

Abstract

Understanding the impact of the intricate morphology and surface chemistry of ZnO nanorod arrays on their interactions with polyelectrolyte polymers is crucial for the development of nascent ZnO-based adhesion-promoting materials. AFM-based single molecule force spectroscopy was applied for the analysis of the adsorption of poly­(acrylic acid) (PAA) on zinc oxide (ZnO) film covered stainless steel substrates in aqueous electrolytes at pH 7. Based on the electrodeposition process, the morphology of zinc oxide films could be varied ranging from platelet-like crystals to nanorods. This approach allowed for the morphology dependent analysis of macromolecular adsorption processes on complex ZnO nanostructures which have diverse applications in the field of adhesion-promoting thin films. The surface chemical composition, as determined by X-ray photoelectron spectroscopy, could be correlated to the AFM-based desorption studies. Only equilibrium desorption events (plateaus), centered at 42 pN, were observed on mirror polished, preconditioned stainless steel. However, for platelet-like ZnO films, the poly­(acrylic acid) desorption showed a mixture of rupture events (mean rupture forces of about 350 pN) and equilibrium desorption, while ZnO nanorod structures showed solely rupture events with mean rupture forces of about 1300 pN. These results indicate that simultaneous multiple ruptures of carboxylate–zinc bonds occur due to the macromolecular coordination of poly­(acrylic acid) to the ZnO nanorods. The analysis of the interfacial adhesion processes is further supported by the dwell time dependence of desorption processes.

Single-Molecule Desorption Studies of Poly(acrylic acid) at Electrolyte/Oxide/TiAlN Interfaces

“…The retraction of the cantilever from the surface leads to multiple rupture events in the retraction force–distance curve, Figure , which are attributed to the sequential breakage of multiple bonds formed between carboxylate moieties present in the PAA chain and the oxide surface during the approaching step. ,, This sequence of nonequilibrium multiple rupture forces is indeed associated with the desorption of immobilized PAA polymer chains (incorporating several anchoring carboxylate groups) irreversibly adsorbed onto the surface. , This condition of nonequilibrium process attributed to the desorption of PAA chains is acquired when, during the rupture, the lifetime of the bonds is comparable to the rate at which the cantilever is separated from the surface. Under these circumstances, the magnitude of the rupture force evolves with the applied loading rate according to a logarithm dependence, as discussed later.…”

“…This analysis gives local information about material properties, bond energies, and surface interactions. , Using SMFS measurements, relevant conclusions can be drawn about the strength of the bonds formed between polymer molecules and the substrate, depending on the specific chemical functionalities of the surface. Poly­(acrylic acid) (PAA) is a weak polyelectrolyte polymer bearing carboxylic units, which has been used to model polymer-to-surface interactions. − Recently, SMFS with PAA-modified AFM tips has been applied in our group for the characterization of the interfacial adhesion properties of metal oxide surfaces such as zinc oxide. , …”

“…19−21 Recently, SMFS with PAA-modified AFM tips has been applied in our group for the characterization of the interfacial adhesion properties of metal oxide surfaces such as zinc oxide. 17,22 Equilibrium and nonequilibrium desorption events were observed depending on the surface chemistry. 17,22 The measured interaction forces were mainly dependent on the surface orientation of the metal oxide and the pH of the electrolyte.…”

In situ backside Raman spectroscopy of zinc oxide nanorods in an atmospheric‐pressure dielectric barrier discharge plasma

“…ZnO-NRs were grown on stainless steel mesh by cathodic electrodeposition in a three-electrode setup as already described elsewhere. [33] First, the mesh was cleaned in an ultrasound bath of (sequentially) tetrahydrofuran, isopropanol, and ethanol. Afterward, the ZnO-NR deposition was carried out in an aqueous solution of 0.01 M hexamethylenetetramine and 0.01 M Zn(NO 3 ) 2 Á6H 2 O.…”

Surface modification of ZnMgAl-coated steel by dielectric-barrier discharge plasma

“…The adsorption of polyacrylic acid molecules to metal oxides such as Al 2 O 3 and ZnO has been deeply studied by several authors. 44,45 We assume that the reason for the increased adhesion forces in the case of acrylate based adhesive is that carboxylic groups form strong acid-base interactions with the surface metal ions due bidentate bindings. By comparison, the amine and alcohol functions of the epoxy amine adhesive form interfacial monodentate bonds.…”