Polymer Brush Coating and Adhesion Technology at Scale.

Creating strong joints between dissimilar materials for high-performance hybrid products places high demands on modern adhesives. Traditionally, adhesion relies on the compatibility between surfaces, often requiring the use of primers and thick bonding layers to achieve stable joints. The coatings of polymer brushes enable the compatibilization of material surfaces through precise control over surface chemistry, facilitating strong adhesion through a nanometer-thin layer. Here, we give a detailed account of our research on adhesion promoted by polymer brushes along with examples from industrial applications. We discuss two fundamentally different adhesive mechanisms of polymer brushes, namely (1) physical bonding via entanglement and (2) chemical bonding. The former mechanism is demonstrated by e.g., the strong bonding between poly(methyl methacrylate) (PMMA) brush coated stainless steel and bulk PMMA, while the latter is shown by e.g., the improved adhesion between silicone and titanium substrates, functionalized by a hydrosilane-modified poly(hydroxyethyl methacrylate) (PHEMA) brush. This review establishes that the clever design of polymer brushes can facilitate strong bonding between metals and various polymer materials or compatibilize fillers or nanoparticles with otherwise incompatible polymeric matrices. To realize the full potential of polymer brush functionalized materials, we discuss the progress in the synthesis of polymer brushes under ambient and scalable industrial conditions, and present recent developments in atom transfer radical polymerization for the large-scale production of brush-modified materials.

Growth of a Surface-Tethered, All-Carbon Backboned Fluoropolymer by Photoactivated Molecular Layer Deposition.

The synthesis of an all-carbon backboned fluoropolymer using photoactivated molecular layer deposition (pMLD) is developed. pMLD is a vapor-phase, layer-by-layer, organic thin film synthesis method utilizing UV light, allowing for the creation of materials previously unavailable via thermal MLD. The carbon backbone is achieved by reacting an iodine-containing fluorocarbon monomer (1,4-diiodooctafluorobutane) and a diene monomer (1,5-hexadiene) under UV irradiation in a step-growth polymerization sequence. The polymerization occurs with a growth rate of 1.3 Å/cycle, forming a copolymer containing hydrocarbon and fluorocarbon segments. X-ray photoelectron spectroscopy (XPS) was used to confirm the formation of new carbon-carbon bonds and quantify the final film composition. In situ XPS thermal annealing experiments confirm the film stability up to 400 °C. The ability to pattern the fluoropolymer on a surface is demonstrated using a photomask, suggesting that these films could be incorporated into photolithographic processes. Together, these results demonstrate that pMLD can be used to synthesize carbon backboned films with photopatterning ability, expanding the available chemistries and potential applications of MLD polymers.

Controlled electropolymerisation of a carbazole-functionalised iron porphyrin electrocatalyst for CO2 reduction.

Using a one-step electropolymerisation procedure, CO2 absorbing microporous carbazole-functionalised films of iron porphyrins are prepared in a controlled manner. The electrocatalytic reduction of CO2 for these films is investigated to elucidate their efficiency and the origin of their ultimate degradation.

High- versus low-quality graphene: a mechanistic investigation of electrografted diazonium-based films for growth of polymer brushes.

Electrografting using aryldiazonium salts provides a fast and efficient technique to functionalize commercially available 3-5 layered graphene (vapour-deposited) on nickel. In this study, Raman spectroscopy is used to quantify the grafting efficiency of cyclic voltammetry which is one of the most versatile, yet simple, electrochemical techniques available. To a large extent the number of defects/substituents introduced to the basal plane of high-quality graphene by this procedure can be controlled through the sweeping conditions employed. After extended electrografting the defect density reaches a saturation level ( ∼ 10(13) cm(-2)) which is independent of the quality of the graphene expressed through its initial content of defects. However, it is reached within fewer voltammetric cycles for low-quality graphene. Based on these results it is suggested that the grafting occurs (a) directly at defect sites for, in particular, low-quality graphene, (b) directly at the basal plane for, in particular, high-quality graphene, and/or (c) at already grafted molecules to give a mushroom-like film growth for all films. Moreover, it is shown that a tertiary alkyl bromide can be introduced at a given surface density to serve as radical initiator for surface-initiated atom transfer radical polymerization (SI-ATRP). Brushes of poly(methyl methacrylate) are grown from these substrates, and the relationship between polymer thickness and sweeping conditions is studied.

Electron transport through a diazonium-based initiator layer to covalently attached polymer brushes of ferrocenylmethyl methacrylate.

A versatile method based on electrografting of aryldiazonium salts was used to introduce covalently attached initiators for atom transfer radical polymerization (ATRP) on glassy carbon surfaces. Polymer brushes of ferrocenylmethyl methacrylate were prepared from the surface-attached initiators, and these films were thoroughly analyzed using various techniques, including X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRRAS), ellipsometry, and electrochemistry. Of particular interest was the electrochemical characterization of the electron transfer through the diazonium-based initiator layer to the redox centers in the polymer brush films. It was found that the apparent rate constant of electron transfer decreases exponentially with the dry-state thickness of this layer. To investigate the electron transfer in the brushes themselves, scanning electrochemical microscopy (SECM) was applied, thereby allowing the effect from the initiator layer to be excluded. The unusual transition feature of the approach curves recorded suggests that an initial fast charge transfer to the outermost-situated ferrocenyl groups is followed by a slower electron transport involving the neighboring redox units.

On surface-initiated atom transfer radical polymerization using diazonium chemistry to introduce the initiator layer.

This work features the controllability of surface-initiated atom transfer radical polymerization (SI-ATRP) of methyl methacrylate, initiated by a multilayered 2-bromoisobutyryl moiety formed via diazonium chemistry. The thickness as a function of polymerization time has been studied by varying different parameters such as the bromine content of the initiator layer, polarity of reaction medium, ligand type (L), and the ratio of activator (Cu(I)) to deactivator (Cu(II)) in order to ascertain the controllability of the SI-ATRP process. The variation of thickness versus surface concentration of bromine shows a gradual transition from mushroom to brush-type conformation of the surface anchored chains in both polar and nonpolar reaction medium. Interestingly, it is revealed that very thick polymer brushes, on the order of 1 µm, can be obtained at high bromine content of the initiator layer in toluene. The initial polymerization rate and the overall final thickness are higher in the case of nonpolar solvent (toluene) compared to polar medium (acetonitrile or N,N-dimethylformamide). The ligand affects the initial rate of polymerization, which correlates with the redox potentials of the pertinent Cu(II)/Cu(I) complexes (L = Me(6)TREN, PMDETA, and BIPY). It is also observed that the ability of polymer brushes to reinitiate depends on the initial thickness and the solvent used for generating it.