6 research outputs found
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 âmolecular plowingâ) and an area-dependent
term attributed to shearing (adhesion). The relative contributions
of plowing and shearing are determined by the coefficient of friction,
ÎŒ, and the surface shear strength Ï. 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 DerjaguinâMullerâToporov
mechanics. A striking correlation has been observed between the concentration-dependence
of the association constant (<i>K</i><sub>a</sub>) for the
formation of 1:1 hydrogen-bonded complexes and the pull-off force <i>F</i><sub>a</sub> and surface shear strength Ï 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><sub>a</sub> and Ï
enables the prediction of <i>K</i><sub>S</sub> with remarkably
high precision, indicating that for these hydrogen bonding systems,
the tipâsample 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 Ï with the film composition is correlated very
closely with the variation in <i>F</i><sub>a</sub>. However,
the coefficient of friction varies little with the film composition
and is invariant with the strength of tipâsample 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 polyÂ(2-(methacryloyloxy)Âethylphosphorylcholine)
(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 frictionâload
relationship is observed. In contrast, the frictionâload plot
is nonlinear in ethanol or water, media in which stronger adhesion
is measured. For ethanol, the data indicate JohnsonâKendallâRoberts
(JKR) mechanics, whereas the DerjaguinâMullerâToporov
(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 frictionâload 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 oligoÂ(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 30â250 nm diameter were prepared via reversible
additionâfragmentation 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 polyÂ(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 âŒâ10 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 âŒ1.5 ÎŒm<sup>2</sup> s<sup>â1</sup>, which are comparable to those observed for lipid bilayers on glass
substrates