Relationship Between Molecular Contact Thermodynamics and Surface Contact Mechanics

Measurements have been made of the adhesion and friction forces between organic monolayers in heptane/acetone mixtures using an atomic force microscope (AFM). It has been found that the contact mechanics are best modeled by treating the friction force as the sum of a load-dependent term (attributed to “molecular plowing”) and an area-dependent term attributed to shearing (adhesion). The relative contributions of plowing and shearing are determined by the coefficient of friction, μ, and the surface shear strength τ. The transition from adhesion- to load-determined friction is controlled by the solvation state of the surface: solvated surfaces represent a limiting case in which the shear term approaches zero, and the friction-load relationship is linear, while in other circumstances, the friction-load relationship is nonlinear and consistent with Derjaguin–Muller–Toporov mechanics. A striking correlation has been observed between the concentration-dependence of the association constant (<i>K</i>_a) for the formation of 1:1 hydrogen-bonded complexes and the pull-off force <i>F</i>_a and surface shear strength τ for the same molecules when one partner is immobilized by attachment to an AFM probe and the other is adsorbed to a surface. Analysis of the concentration-dependence of <i>F</i>_a and τ enables the prediction of <i>K</i>_S with remarkably high precision, indicating that for these hydrogen bonding systems, the tip–sample adhesion is dominated by the H-bond thermodynamics. For mixed monolayers, H-bond thermodynamics dominate the interaction even at very low concentrations of the H-bond acceptor. Even for weakly adhering systems, a nonlinear friction-load relationship results. The variation in τ with the film composition is correlated very closely with the variation in <i>F</i>_a. However, the coefficient of friction varies little with the film composition and is invariant with the strength of tip–sample adhesion, being dominated by molecular plowing and, for sufficiently large concentrations of hydroxyl terminated adsorbates, the disruption of intramonolayer hydrogen bonding interactions

Nanoscale Contact Mechanics of Biocompatible Polyzwitterionic Brushes

Friction force microscopy has been used to demonstrate that biocompatible, lubricious poly­(2-(methacryloyloxy)­ethylphosphorylcholine) (PMPC) brushes exhibit different frictional properties depending on the medium (methanol, ethanol, 2-propanol, and water; the latter also with different quantities of added salt). The chemical functionalization of the probe (amine-, carboxylic acid-, and methyl-terminated probes were used) is not as important as the medium in determining the contact mechanics. For solvents such as methanol, where the adhesion between AFM probe and PMPC brushes is negligible, a linear friction–load relationship is observed. In contrast, the friction–load plot is nonlinear in ethanol or water, media in which stronger adhesion is measured. For ethanol, the data indicate Johnson–Kendall–Roberts (JKR) mechanics, whereas the Derjaguin–Muller–Toporov (DMT) model provided a good fit for the data acquired in water. Contact mechanics on zwitterionic PMPC brushes immersed in aqueous solutions of varying ionic strength followed the same trend, with high adhesion energies being correlated with a nonlinear friction–load relationship. These results can be rationalized by treating the friction force as the sum of a load-dependent term, attributed to molecular plowing, and an area-dependent shear term. In a good solvent for PMPC such as methanol, the shear term is negligible and the sliding interaction is dominated by molecular plowing. However, the adhesion energy is significantly larger in water and ethanol and the shear term is no longer negligible

Generic Methods for Micrometer- And Nanometer-Scale Surface Derivatization Based on Photochemical Coupling of Primary Amines to Monolayers of Aryl Azides on Gold and Aluminum Oxide Surfaces

A series of aryl azide terminated thiols and phosphonic acids has been synthesized, and used to prepare self-assembled monolayers on (respectively) gold and aluminum oxide surfaces. The rates of photoactivation were determined using contact angle measurement and X-ray photoelectron spectroscopy (XPS). The behavior of a diazirine functionalized aryl thiol was also studied. The rates of activation were found to be similar for all five adsorbates. However, the extent of photochemical coupling of a primary amine was significantly greater for the aryl azides than for the diazirine. A range of primary amines was successfully coupled to all of the azides with high yield. Little difference in reactivity was observed following perfluorination of the aromatic ring. Micrometer-scale patterns were fabricated by carrying out exposures of the aryl azide terminated SAMs through a mask submerged under a film of primary amine. Contrasting amines could be introduced to unreacted regions in a subsequent maskless step. A scanning near-field optical microscope was used to fabricate nanopatterns. Exposure of the azides to irradiation at 325 nm in air enabled selective deactivation of azides. The surrounding surface was functionalized with a primary amine in a maskless process; when a protein-resistant oligo­(ethylene glycol) functionalized amine was used it was possible to produce protein nanopatterns, by adsorbing protein to features defined using near-field exposure

Adsorption of Aldehyde-Functional Diblock Copolymer Spheres onto Surface-Grafted Polymer Brushes via Dynamic Covalent Chemistry Enables Friction Modification

Dynamic covalent chemistry has been exploited to prepare numerous examples of adaptable polymeric materials that exhibit unique properties. Herein, the chemical adsorption of aldehyde-functional diblock copolymer spherical nanoparticles onto amine-functionalized surface-grafted polymer brushes via dynamic Schiff base chemistry is demonstrated. Initially, a series of cis-diol-functional sterically-stabilized spheres of 30–250 nm diameter were prepared via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization. The pendent cis-diol groups within the steric stabilizer chains of these precursor nanoparticles were then oxidized using sodium periodate to produce the corresponding aldehyde-functional spheres. Similarly, hydrophilic cis-diol-functionalized methacrylic brushes grafted from a planar silicon surface using activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) were selectively oxidized to generate the corresponding aldehyde-functional brushes. Ellipsometry and X-ray photoelectron spectroscopy were used to confirm brush oxidation, while scanning electron microscopy studies demonstrated that the nanoparticles did not adsorb onto a cis-diol-functional precursor brush. Subsequently, the aldehyde-functional brushes were treated with excess small-molecule diamine, and the resulting imine linkages were converted into secondary amine bonds via reductive amination. The resulting primary amine-functionalized brushes formed multiple dynamic imine bonds with the aldehyde-functional diblock copolymer spheres, leading to a mean surface coverage of approximately 0.33 on the upper brush layer surface, regardless of the nanoparticle size. Friction force microscopy studies of the resulting nanoparticle-decorated brushes enabled calculation of friction coefficients, which were compared to that measured for the bare aldehyde-functional brush. Friction coefficients were reasonably consistent across all surfaces except when particle size was comparable to the size of the probe tip. In this case, differences were ascribed to an increase in contact area between the tip and the brush-nanoparticle layer. This new model system enhances our understanding of nanoparticle adsorption onto hydrophilic brush layers

Strong coupling in molecular systems: a simple predictor employing routine optical measurements

We provide a simple method that enables readily acquired experimental data to be used to predict whether or not a candidate molecular material may exhibit strong coupling. Specifically, we explore the relationship between the hybrid molecular/photonic (polaritonic) states and the bulk optical response of the molecular material. For a given material this approach enables a prediction of the maximum extent of strong coupling (vacuum Rabi splitting), irrespective of the nature of the confined light field. We provide formulae for the upper limit of the splitting in terms of the molar absorption coefficient, the attenuation coefficient, the extinction coefficient (imaginary part of the refractive index) and the absorbance. To illustrate this approach we provide a number of examples, we also discuss some of the limitations of our approach

New Poly(amino acid methacrylate) Brush Supports the Formation of Well-Defined Lipid Membranes

A novel poly­(amino acid methacrylate) brush comprising zwitterionic cysteine groups (PCysMA) was utilized as a support for lipid bilayers. The polymer brush provides a 12-nm-thick cushion between the underlying hard support and the aqueous phase. At neutral pH, the zeta potential of the PCysMA brush was ∼−10 mV. Cationic vesicles containing >25% DOTAP were found to form a homogeneous lipid bilayer, as determined by a combination of surface analytical techniques. The lipid mobility as measured by FRAP (fluorescence recovery after photobleaching) gave diffusion coefficients of ∼1.5 μm² s^–1, which are comparable to those observed for lipid bilayers on glass substrates