Adhesion and Friction Properties of Fluoropolymer Brushes: On the Tribological Inertness of Fluorine

The effects of fluorination on the adhesion and friction properties of covalently bound poly­(fluoroalkyl methacrylate) polymer brushes (thickness ∼80 nm) were systematically investigated. Si(111) surfaces were functionalized with a covalently bound initiator via a thiol–yne click reaction to have a high surface coverage for initiator immobilization. Surface-initiated atom-transfer radical polymerization (SI-ATRP) was employed for the synthesis of four different fluoropolymer brushes (SPF<i>x</i>, where <i>x</i> = 0, 3, 7, or 17 F atoms per monomer), based on fluoroalkyl methacrylates. All polymer brushes were characterized with static contact angle measurements, X-ray photoelectron spectroscopy (XPS), and infrared absorption reflection spectroscopy (IRRAS). The polymer brushes exhibited an excellent hydrophobicity, with static water contact angles of up to 121° depending on the number of fluorine atoms per side chain in fluoroalkyl methacrylate. The degree of swelling was precisely studied by using ellipsometry in different solvents such as acetone, hexadecane, hexafluoroisopropanol, nonafluorobutyl methyl ether, and Fluorinert FC-40. The polymer brushes have shown nanoscale swelling behavior in all solvents except hexadecane. The grafting density decreased upon increasing fluorine content in polymer brushes from 0.65 chains/nm² (SPF0) to 0.10 chains/nm² (SPF17) as observed in Fluorinert FC-40 as a good solvent. Adhesion and friction force measurements were conducted with silica colloidal probe atomic force microscopy (CP-AFM) under ambient, dry (argon), and lubricating fluid conditions. SPF17 showed the lowest coefficient of friction 0.005 under ambient condition (RH = 44 ± 2%) and a further decrease with 50% under fluidic conditions. These polymer brushes also showed adhesion forces as low as 6.9 nN under ambient conditions, which further went down to 0.003 nN under fluidic conditions (Fluorinert FC-40 and hexadecane) at 10 nN force

Copper-Free Click Biofunctionalization of Silicon Nitride Surfaces via Strain-Promoted Alkyne–Azide Cycloaddition Reactions

Cu-free “click” chemistry is explored on silicon nitride (Si₃N₄) surfaces as an effective way for oriented immobilization of biomolecules. An ω-unsaturated ester was grafted onto Si₃N₄ using UV irradiation. Hydrolysis followed by carbodiimide-mediated activation yielded surface-bound active succinimidyl and pentafluorophenyl ester groups. These reactive surfaces were employed for the attachment of bicyclononyne with an amine spacer, which subsequently enabled room temperature strain-promoted azide–alkyne cycloaddition (SPAAC). This stepwise approach was characterized by means of static water contact angle, X-ray photoelectron spectroscopy, and fluorescence microscopy. The surface-bound SPAAC reaction was studied with both a fluorine-tagged azide and an azide-linked lactose, yielding hydrophobic and bioactive surfaces for which the presence of trace amounts of Cu ions would have been problematic. Additionally, patterning of the Si₃N₄ surface using this metal-free click reaction with a fluorescent azide is shown. These results demonstrate the ability of the SPAAC as a generic tool for anchoring complex molecules onto a surface under extremely mild, namely ambient and metal-free, conditions in a clean and relatively fast manner

Adhesion and Friction Properties of Polymer Brushes: Fluoro versus Nonfluoro Polymer Brushes at Varying Thickness

A series of different thicknesses of fluoro poly­(2,2,2-trifluoroethyl methacrylate) and its analogous nonfluoro poly­(ethyl methacrylate) polymer brushes were prepared via surface-initiated ATRP (SI-ATRP) on Si(111) surfaces. The thiol-yne click reaction was used to immobilize the SI-ATRP initiator with a high surface coverage, in order to achieve denser polymer brushes (grafting density from ∼0.1 to 0.8 chains/nm²). All polymer brushes were characterized by static water contact angle measurements, infrared absorption reflection spectroscopy, and X-ray photoelectron spectroscopy. Adhesion and friction force measurements were conducted with silica colloidal probe atomic force microscopy (CP-AFM) under ambient and dry (argon) conditions. The fluoro poly­(2,2,2-trifluoroethyl methacrylate) polymer showed a decrease in adhesion and friction with increasing thickness. The analogous nonfluoro poly­(ethyl methacrylate) polymer brushes showed high adhesion and friction under ambient conditions. Friction coefficients down to 0.0057 (ambient conditions) and 0.0031 (dry argon) were obtained for poly­(2,2,2-trifluoroethyl methacrylate) polymer brushes with 140 nm thickness, which are the lowest among these types of polymer brushes

Plasma Micro-Nanotextured, Scratch, Water and Hexadecane Resistant, Superhydrophobic, and Superamphiphobic Polymeric Surfaces with Perfluorinated Monolayers

Superhydrophobic and superamphiphobic toward superoleophobic polymeric surfaces of polymethyl methacrylate (PMMA), polyether ether ketone (PEEK), and polydimethyl siloxane (PDMS) are fabricated in a two-step process: (1) plasma texturing (i.e., ion-enhanced plasma etching with simultaneous roughening), with varying plasma chemistry depending on the polymer, and subsequently (2) grafting of self-assembled perfluorododecyltrichlorosilane monolayers (SAMs). Depending on the absence or not of an etch mask (i.e., colloidal microparticle self-assembly on it), random or ordered hierarchical micro-nanotexturing can be obtained. We demonstrate that stable organic monolayers can be grafted onto all these textured polymeric surfaces. After the monolayer deposition, the initially hydrophilic polymeric surfaces become superamphiphobic with static contact angles for water and oils >153°, for hexadecane >142°, and hysteresis <10° for all surfaces. This approach thus provides a simple and generic method to obtain superamphiphobicity on polymers toward superoleophobicity. Hydrolytic and hexadecane immersion tests prove that superamphiphobicity is stable for more than 14 days. We also perform nanoscratch and post nanoscratch tests to prove the scratch resistance of both the texture and the SAM and demonstrate lower coefficient of friction of the SAM compared to the uncoated surface. Scanning electron microscope observation after the nanoscratch tests confirms the scratch resistance of the surfaces

Effect of Internal Heteroatoms on Level Alignment at Metal/Molecular Monolayer/Si Interfaces

Molecular monolayers at metal/semiconductor heterointerfaces affect electronic energy level alignment at the interface by modifying the interface’s electrical dipole. On a free surface, the molecular dipole is usually manipulated by means of substitution at its external end. However, at an interface such outer substituents are in close proximity to the top contact, making the distinction between molecular and interfacial effects difficult. To examine how the interface dipole would be influenced by a single atom, internal to the molecule, we used a series of three molecules of identical binding and tail groups, differing only in the inner atom: aryl vinyl ether (<b>PhO</b>), aryl vinyl sulfide (<b>PhS</b>), and the corresponding molecule with a CH₂ groupallyl benzene (<b>PhC</b>). Molecular monolayers based on all three molecules have been adsorbed on a flat, oxide-free Si surface. Extensive surface characterization, supported by density functional theory calculations, revealed high-quality, well-aligned monolayers exhibiting excellent chemical and electrical passivation of the silicon substrate, in all three cases. Current–voltage and capacitance–voltage analysis of Hg/PhX (X = C, O, S)/Si interfaces established that the type of internal atom has a significant effect on the Schottky barrier height at the interface, i.e., on the energy level alignment. Surprisingly, despite the formal chemical separation of the internal atom and the metallic electrode, Schottky barrier heights were not correlated to changes in the semiconductor’s effective work function, deduced from Kelvin probe and ultraviolet photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather, these changes correlated well with the ionization potential of the surface-adsorbed molecules. This is interpreted in terms of additional polarization at the molecule/metal interface, driven by potential equilibration considerations even in the absence of a formal chemical bond to the top Hg contact

Hydrolytic and Thermal Stability of Organic Monolayers on Various Inorganic Substrates

A comparative study is presented of the hydrolytic and thermal stability of 24 different kinds of monolayers on Si(111), Si(100), SiC, SiN, SiO₂, CrN, ITO, PAO, Au, and stainless steel surfaces. These surfaces were modified utilizing appropriate organic compounds having a constant alkyl chain length (C₁₈), but with different surface-reactive groups, such as 1-octadecene, 1-octadecyne, 1-octadecyltrichlorosilane, 1-octadecanethiol, 1-octadecylamine and 1-octadecylphosphonic acid. The hydrolytic stability of obtained monolayers was systematically investigated in triplicate in constantly flowing aqueous media at room temperature in acidic (pH 3), basic (pH 11), phosphate buffer saline (PBS) and deionized water (neutral conditions), for a period of 1 day, 7 days, and 30 days, yielding 1152 data points for the hydrolytic stability. The hydrolytic stability was monitored by static contact angle measurements and X-ray photoelectron spectroscopy (XPS). The covalently bound alkyne monolayers on Si(111), Si(100), and SiC were shown to be among the most stable monolayers under acidic and neutral conditions. Additionally, the thermal stability of 14 different monolayers was studied in vacuum using XPS at elevated temperatures (25–600 °C). Similar to the hydrolytic stability, the covalently bound both alkyne and alkene monolayers on Si(111), Si(100) and SiC started to degrade from temperatures above 260 °C, whereas on oxide surfaces (e.g., PAO) phosphonate monolayers even displayed thermal stability up to ∼500 °C