Dopamine modification of interfaces between polymers and metals

Abstract

Increasingly in science inspiration is drawn from Nature that provides many outstanding examples of adhesive strategies. One of them is the adhesion of marine organisms, which present strong binding to virtually all inorganic and organic surfaces in aqueous environments in which most synthetic adhesives function poorly. The attachment occurs by secreting adhesive proteins that harden and cross-link in situ. Previous studies of these functionally unique proteins have revealed the presence of an unusual amino acid, 3,4-dihydroxy-L-phenylalanine (DOPA), which is responsible for the cohesive and adhesive strength of this natural glue and gives to marine organisms the ability to bond with numerous substrates including glass, Teflon, wood, concrete, plastics, metals, biological cell lines, bone, teeth, and others. Although the precise mechanism for assembly of the proteins is not understood, the obtained knowledge broadens biomimetic strategies for synthesizing new practical adhesives. In this PhD thesis our efforts to understand the adherence behavior of dopamine-based promoters on metals are described. Firstly a simple DOPA protein mimics – dopamine was used to coat metal substrates and the properties of dopamine and polydopamine coatings on metals were studied in detail. Dip coating from aqueous dopamine hydrochloride solutions at pH 4 and pH 8 was performed on gold (having nor or hardly any native oxide) and aluminium (having a native aluminium oxide layer). The formed coatings were analyzed with surface sensitive techniques dealing with coating composition and the oxidation state, as well as with mechanical measurements characterising the adhesion of coatings. The outcomes of these studies emphasized a need to design a better defined system. As a result two model molecules, N-stearoyldopamine and 4-stearylcatechol, were synthesized. These molecules contain a catechol residue that is capable of binding to metals and metal oxides and a hydrophobic alkyl-chain that gives an ability to coordinate molecules on surfaces using the Langmuir-Blodgett preparation technique. By using these catechol-containing molecules the role of amide functionality on packing, orientation and interaction with the metal surface could be compared. Attention was devoted in studying the adsorption behavior onto gold and aluminum oxide in terms of monolayer characteristics. The organization of Langmuir-Blodgett monolayers on a molecular scale was examined by several surface sensitive techniques. The experimental work was supported with molecular dynamic simulations on gold in order to get a more complete understanding of the monolayer configuration near the interface. Our study shows that within the monolayer the catechols functions as a surface anchor on gold and the alkyl-chains appear to be tilted within the monolayer. Moreover, the irregularity of the 4-stearylcatechol film on gold leads to micelle type structures that are caused by the absence of the amide functionality. On the contrary, from the molecular simulations it appeared that for both types of molecules parallel orientations of the catechols with the gold are also present. However, hydrogen bonds formed between the amide functionality and the catechol hydroxyl groups have a profound influence on the structure and regularity on the adsorbed layer. Attempts were made to quantify the adhesion strength of the anchoring catechols on metal oxide. A covalently bound top-coating was used to determine the adhesion of an Nlinoleoyldopamine primer on an aluminum alloy. Several application conditions were tested for the top-coating and the primer. The resulting tensile strength values of the topcoating having an N-linoleoyldopamine monolayer primer formed by the Langmuir-Blodgett technique showed an obvious improvement in adhesion whereas a negative impact on adhesion occurred when the primer was not applied in a uniform and controlled manner. According to the results of this work and the literature data, synthetic adhesives inspired by mussel adhesive proteins can be used successfully to improve the adhesive properties between polymers and metals. Consequently, the outcomes of the research will give an input in the development of useful synthetic polymer adhesives that exhibit similar wet adhesive capabilities as mussel foot proteins.