Properties of polyethylene glycol supported tetraarylporphyrin in aqueous solution and its interaction with liposomal membranes

5,10,15,20-Tetrakis(4-hydroxyphenyl)porphyrin was functionalized by covalent attachment of poly(ethylene glycol) (PEG) chains of various molecular weights, 350, 2000, and 5000 Da. The properties of PEG-functionalized tetraarylporphyrins in aqueous solution and their interactions with liposomes have been studied. Electronic absorption spectroscopy, dynamic light scattering, atomic force microscopy, and fluorescence quenching were used to monitor aggregation of porphyrin chromophores and behavior of the attached PEG chains in the aqueous solution. The tendency for aggregation of porphyrin chromophores in aqueous solution and the efficiency of fluorescence quenching by KI decrease with increasing length of PEG chain linked to the porphyrin ring. The experimental results indicate that polymer clusters are present in aqueous solution of all pegylated porphyrins. The interactions between the pegylated porphyrins and phosphatidylcholine liposomes in the aqueous solution were studied using the fluorescence methods. The apparent binding constants of porphyrin chromophores to liposomes were determined. The degree of binding was found to be dependent on the molecular weight of the attached polymer.     

Ligand-receptor interactions between surfaces: the role of binary polymer spacers

The interactions between a receptor-modified planar surface and a surface grafted with a bimodal polymer layer, where one of the polymer species is ligand functionalized, are studied using a molecular theory. The effects of changing the binding energy of the ligand-receptor pair, the polymer surface coverage, the composition, and molecular weight of both the unfunctionalized and ligand functionalized polymers on the interactions between the surfaces are investigated. Our findings show that bridging exists between the surfaces including when the molecular weight of the ligand-bearing polymer is smaller than that of the unfunctionalized polymer, even though the ligand is initially buried within the polymer layer. The distance at which the surfaces bind depends only on the molecular weight of the ligand-modified polymer, while the strength of the interaction at a given surface separation can be tuned by changing the molecular weight of the polymers, the total polymer surface coverage, and the fraction of ligated polymers. The composition of the bimodal layer alters the structure of the polymer layer, thereby influencing the strength of the steric repulsions between the surfaces. Our theoretical results show good agreement with experimental data. The present theoretical study can be used as guidelines for the design of surfaces with tailored abilities for tunning the binding strength and surface-ligand separation distances for polymer-grafted surfaces bearing specific targeting ligands.     

Optimization of functionalized polymer layers for specific targeting of mobile receptors on cell surfaces

The reversible binding between a planar polymer layer functionalized by ligands and a planar cell surface containing different densities of mobile receptors has been studied by Monte Carlo simulations. Using the acceptance-ratio method, the distance-dependent profiles for the average number of ligands bound to receptors, the total free energy for the polymer layer-cell surface interaction and the interaction force were obtained. Four main design parameters for the polymer layer were considered: the degree of functionalization, chain degree of polymerization, polymer grafting density and the binding energy for the ligand-receptor interaction. We found that an increase in the degree of functionalization or in the absolute energy of ligand-receptor binding results in a larger number of ligands bound to the receptors, lower free energy, and stronger attractive force. Polymer layers composed of shorter chains were found to exhibit a deeper and narrower free energy profile and a larger attractive force, while longer tethers can interact with the cell surface at a larger and broader range of separation distances, in agreement with experimental observations. Our simulation results show that the increase in polymer grafting density from the mushroom to brush regime enhances the ligand availability and results in a stronger attractive force, increases the maximum binding distance, but exhibits a shallower free energy minimum due to the smaller tolerance to compression for polymer layers with high grafting density. We used two measures of the polymer layer binding affinity to the cell surface: the free energy minimum, related to the equilibrium binding constant and the fraction of bound ligands. We found that the polymer layers with a smaller chain length and grafting density, larger degree of functionalization, and larger absolute binding energy exhibit both a larger equilibrium binding constant to the cell surface and a larger average number of bound ligands, except for high binding energies when the maximum level of binding is reached independently of polymer length and grafting density. We showed that high binding specificity can be achieved by the polymer layers with intermediate ligand-receptor binding energies or an intermediate number of ligands, as a larger binding energy or number of ligands ensures a high binding affinity but lacks specificity while a smaller binding energy or number of ligands provides inadequate affinity. We found that the results for polymer layers with different properties follow a similar pattern when both high binding affinity to cells with high receptor density and high binding specificity are considered. As a result, the optimal design of the polymer layers can be achieved by using several different strategies, which are discussed.     

Impact of membrane fluidity on steric stabilization by lipopolymers

In this work, the impact of lipid lateral mobility on the steric interaction between membranes containing poly(ethylene glycol) (PEG) functionalized lipids was investigated using the surface force apparatus. The force-distance profiles show the presence of electrostatic and steric repulsion that arise from the presence of negatively charged PEG functionalized lipids. Fluid-phase bilayers have high lateral diffusion relative to gel-phase bilayers; however, a quantitative comparison of the interaction forces between membranes in these two different phase states demonstrates a reduced rate of diffusion in the fluid phase for the PEG-lipids under constrained geometries. Thus, the amount of polymer in the contact zone can be modulated and is reduced with fluid membranes; however, complete exclusion was not achieved. As a result, the steric repulsion afforded by PEG chains or binding affinity of ligated PEG chains can only be modestly tailored by the phase state of the liposome.     

Confinement free energy of surfaces bearing end-grafted polymers in the mushroom regime and local measurement of the polymer density

End-tethered polymer chains usually adopt mushroomlike structures on the surface when their density is low. The behaviors of these surface-attached hemicoils are described by existing polymer theory. Dolan and Edwards derived the free energy of a single polymer chain confined between two planar surfaces. Their theory was used to approximate the steric interaction free energy, E, of two identical surfaces bearing polymers in the mushroom regime and to compare with experimental data obtained from surface force measurements. However, because of a mislabeled plot in the original paper, experimental force profiles did not seem to fit the free energy approximation satisfactorily. We have correctly relabeled the involved plot and derived a new simple expression for E. In order to verify this expression, we have performed experiments on PEG45 polymers incorporated in lipid bilayers using a surface force apparatus. The measured force profiles are in perfect agreement with the prediction. We show that such measurements can be used to determine the local density of grafted polymer with good precision.     

Effects of ionic strength and surface charge on protein adsorption at PEGylated surfaces

PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers, allowing control of the PEG surface density, as well as the surface charge. PEG (MW 2 kDa) surface densities between 0 and 0.5 nm(-2) were obtained by changing the PEG to lysine-mer ratio in the PLL-g-PEG polymer, resulting in net positive, negative and neutral surfaces. Colloid probe atomic force microscopy (AFM) was used to characterize the interfacial forces associated with the different surfaces. The AFM force analysis revealed interplay between electrical double layer and steric interactions, thus providing information on the surface charge and on the PEG layer thickness as a function of copolymer architecture. Adsorption of the model proteins lysozyme, alpha-lactalbumin, and myoglobin onto the various PEGylated surfaces was performed to investigate the effect of protein charge. In addition, adsorption experiments were performed over a range of ionic strengths, to study the role of electrostatic forces between surface charges and proteins acting through the PEG layer. The adsorbed mass of protein, measured by optical waveguide lightmode spectroscopy (OWLS), was shown to depend on a combination of surface charge, protein charge, PEG thickness, and grafting density. At high grafting density and high ionic strength, the steric barrier properties of PEG determine the net interfacial force. At low ionic strength, however, the electrical double layer thickness exceeds the thickness of the PEG layer, and surface charges "shining through" the PEG layer contribute to protein interactions with PLL-g-PEG coated surfaces. The combination of AFM surface force measurements and protein adsorption experiments provides insights into the interfacial forces associated with various PEGylated surfaces and the mechanisms of protein resistance.     

Strategies toward biocompatible artificial implants: Grafting of functionalized poly(ethylene glycol)s to poly(ethylene terephthalate) surfaces

Epoxide and aldehyde end‐functionalized poly(ethylene glycol)s (PEGs) (M_w = 400, 1000, 3400, 5000, and 20,000) were grafted to poly(ethylene terephthalate) (PET) film substrates that contained amine or alcohol groups. PET‐PAH and PET‐PEI were prepared by reacting poly(allylamine) (PAH) and polyethylenimine (PEI) with PET substrates, respectively; PET‐PVOH was prepared by the adsorption of poly(vinyl alcohol) (PVOH) to PET substrates. Grafting was characterized and quantified by the increase of the intensity of the PEG carbon peak in the X‐ray photoelectron spectra. Grafting yield was optimized by controlling reaction parameters and was found to be substrate‐independent in general. Graft density consistently decreased as PEG chain length was increased. This is likely due to the higher steric requirement of higher molecular weight PEG molecules. Water contact angles of surfaces containing long PEG chains (3400, 5000, and 20,000) are much lower than those containing shorter PEG chains (400 and 1000). This indicates that longer PEG chains are more effective in rendering surfaces hydrophilic. Protein adsorption experiments were carried out on PET‐ and PEG‐modified derivatives using collagen, lysozyme, and albumin. After PEG grafting, the amount of protein adsorbed was reduced in all cases. Trends in surface requirements for protein resistance are: surfaces with longer PEG chains and higher chain density, especially the former, are more protein resistant; PEG grafted to surfaces containing branched or network polymers is not effective at covering the underlying substrate, and thus does not protect the entire surface from protein adsorption; and substrates containing surface charge are less protein‐resistant. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5389–5400, 2004     

Relationship between interfacial forces measured by colloid-probe atomic force microscopy and protein resistance of poly(ethylene glycol)-grafted poly(L-lysine) adlayers on niobia surfaces

Adsorbed layers of "comb-type" copolymers consisting of PEG chains grafted onto a poly(l-lysine) (PLL) backbone on niobium oxide substrates were studied by colloid-probe AFM in order to characterize the interfacial forces associated with coatings of varying architectures (PEG/PLL ratios and PEG chain lengths) and their relevance to protein resistance. The steric and electrostatic forces measured varied substantially with the architecture of the PLL-g-PEG copolymers. Varying the ionic strength of the buffer solutions enabled discrimination between electrostatic and steric-entropic contributions to the net interfacial force. For high PEG grafting densities the steric component was most prominent, but at low ionic strengths and high grafting densities, a repulsive electrostatic surface force was also observed; its origin was assigned to the niobia charges beneath the copolymer, as insufficient protonated amine groups in the PLL backbone were available for compensation of the oxide surface charges. For lower grafting densities and lower ionic strengths there was a substantial attractive electrostatic contribution arising from interaction of the electrical double layer arising from the protonated amine groups, with that of the silica probe surface (as under low ionic strength conditions, the electrical double layer was thicker than the PEG layer). For these PLL-g-PEG coatings the net interfacial force can thus be a markedly varying superposition of electrostatic and steric-entropic contributions, depending on various factors. The force curves correlate with protein adsorption data, demonstrating the utility of AFM colloid-probe force measurements for quantitative analysis of surface forces and how they determine interfacial interactions with proteins. Such characterization of the net interfacial forces is essential to elucidate the multiple types of interfacial forces relevant to the interactions between PLL-g-PEG coatings and proteins and to advance interpretation of protein adsorption or repellence beyond the oversimplified steric barrier model; in particular, our data demonstrate the importance of an ionic-strength-dependent minimum PEG layer thickness to screen the electrostatic interactions of charged interfaces.     

Poly(ethylene glycol)-conjugated phospholipids in aqueous micellar solutions: hydration, static structure, and interparticle interactions

By means of dielectric relaxation spectroscopy (DRS) and small-angle X-ray scattering (SAXS), we have investigated hydration behavior, solvent dynamics, and static structures of aqueous solutions of poly(ethylene glycol)-conjugated distearoyl phosphatidylethanolamine (DSPE-PEG) (molecular weight of PEG: M(PEG)= 2000, 5000, and 12,000 Da). A quantitative analysis of the bulk-water relaxation amplitude revealed the effective hydration number of a DSPE-PEG molecule per ethylene oxide monomer unit to be approximately 5.0-5.5, virtually independent of M(PEG). The overall hydration number of a DSPE-PEG molecule is ca. 20% higher than that of the corresponding normal PEG (without DSPE). This is attributed to both hydration of a charged head group of phosphoric acid in DSPE and a packing effect of PEG chains into micellar structures. The pair-distance distribution functions, p(r), extracted from the GIFT analysis of SAXS intensities show that the DSPE-PEGs form spherical-like micelles in water having the maximum diameter of approximately 16, 22, and 31 nm, respectively, for M(PEG) = 2000, 5000, and 12,000 Da and nearly identical aggregation numbers of 72 (+/-10%). The DSPE-PEG micelles behave as charged colloids whose interparticle interaction potential can be approximated by the screened Coulomb potential model. The extracted pair correlation function g(r) demonstrates that both electrostatic repulsion induced by the charged head group and excluded volume effects of the fully hydrated PEG layer contribute to repulsive interactions among the PEG-lipid micelles. This should be a key factor for the function of PEG lipids as a stabilizer of liposomes.     

Free energy of mixing of cross-linked polymer blends

Free energy of mixing of cross-linked polymer blends is derived, as a modification to the Flory-Huggins-de Gennes free energy functional for linear polymer blends. The latter arrives from the assumption of mean-field, short-range thermal interactions among ideal Gaussian chains. However, upon cross-linking a linear chain, the chain no longer remains Gaussian; new chain architectures belying the threadlike image of linear chains emerge. Fractal dimensions of these nonlinear chain clusters convene and command new entropic interactions. Topological constraints by cross-links introduce long-range nonequilibrium elastic forces. Relatively shorter range steric repulsions between fractal network surfaces may arrive if cross-linking is carried out inside the blend's thermodynamically unstable region. Modified free energy has been used to highlight experiments on phase instability of cross-linked polymer blends.     

Direct surface force measurements of polyelectrolyte multilayer films containing nanocrystalline cellulose

Polyelectrolyte multilayer films containing nanocrystalline cellulose (NCC) and poly(allylamine hydrochloride) (PAH) make up a new class of nanostructured composite with applications ranging from coatings to biomedical devices. Moreover, these materials are amenable to surface force studies using colloid-probe atomic force microscopy (CP-AFM). For electrostatically assembled films with either NCC or PAH as the outermost layer, surface morphology was investigated by AFM and wettability was examined by contact angle measurements. By varying the surrounding ionic strength and pH, the relative contributions from electrostatic, van der Waals, steric, and polymer bridging interactions were evaluated. The ionic cross-linking in these films rendered them stable under all solution conditions studied although swelling at low pH and high ionic strength was inferred. The underlying polymer layer in the multilayered film was found to dictate the dominant surface forces when polymer migration and chain extension were facilitated. The precontact normal forces between a silica probe and an NCC-capped multilayer film were monotonically repulsive at pH values where the material surfaces were similarly and fully charged. In contrast, at pH 3.5, the anionic surfaces were weakly charged but the underlying layer of cationic PAH was fully charged and attractive forces dominated due to polymer bridging from extended PAH chains. The interaction with an anionic carboxylic acid probe showed similar behavior to the silica probe; however, for a cationic amine probe with an anionic NCC-capped film, electrostatic double-layer attraction at low pH, and electrostatic double-layer repulsion at high pH, were observed. Finally, the effect of the capping layer was studied with an anionic probe, which indicated that NCC-capped films exhibited purely repulsive forces which were larger in magnitude than the combination of electrostatic double-layer attraction and steric repulsion, measured for PAH-capped films. Wherever possible, DLVO theory was used to fit the measured surface forces and apparent surface potentials and surface charge densities were calculated.     

Effects of Amphiphilic Copolymers Bearing Short Blocks of Lipid-Mimetic Units on the Membrane Properties and Morphology of DSPC Liposomes

Amphiphilic, nonionic diblock copolymers based on poly(ethylene glycol) (PEG 2000–5000), comprising short blocks of lipid-mimetic units, where tested for their ability to afford steric stabilization of distearoylphosphathydilcholine:cholesterol liposomes. The copolymers bear 1–4 lipid-mimetic anchors per copolymer chain. Effects on liposomes size depend on copolymer type and content. Cryo-TEM reveals well-separated, intact, predominantly spherical liposomes at copolymer contents up to 5 mol%. A “flat” liposomes fraction occurs upon incorporation of above 7.5 mol% of copolymers bearing 2 or 4 lipid anchors. 5,6-carboxyfluorescein assay indicates lower leakage of stabilized vs. plain liposomes up to concentration 7.5 mol%. Leakage from liposomes with higher copolymer concentration is insignificantly greater.     

Atomic force microscopy study of the interaction between adsorbed poly(ethylene oxide) layers: effects of surface modification and approach velocity

The interaction forces between layers of the triblock copolymer Pluronic F108 adsorbed onto hydrophobic radio frequency glow discharge (RFGD) thin film surfaces and hydrophilic silica, in polymer-free 0.15 M NaCl solution, have been measured using the atomic force microscope (AFM) colloid probe technique. Compression of Pluronic F108 layers adsorbed on the hydrophobic RFGD surfaces results in a purely repulsive force due to the steric overlap of the layers, the form of which suggests that the PEO chains adopt a brush conformation. Subsequent fitting of these data to the polymer brush models of Alexander-de Gennes and Milner, Witten, and Cates confirms that the adsorbed Pluronic F108 adsorbs onto hydrophobic surfaces as a polymer brush with a parabolic segment density profile. In comparison, the interaction between Pluronic F108 layers adsorbed on silica exhibits a long ranged shallow attractive force and a weaker steric repulsion. The attractive component is reasonably well described by van der Waals forces, but polymer bridging cannot be ruled out. The weaker steric component of the force suggests that the polymer is less densely packed on the surface and is less extended into solution, existing as polymeric isolated mushrooms. When the surfaces are driven together at high piezo ramp velocities, an additional repulsive force is measured, attributable to hydrodynamic drainage forces between the surfaces. In comparing theoretical predictions of the hydrodynamic force to the experimentally obtained data, agreement could only be obtained if the flow profile of the aqueous solution penetrated significantly into the polymer brush. This finding is in line with the theoretical predictions of Milner and provides further evidence that the segment density profile of the adsorbed polymer brush is parabolic. A velocity dependent additional stepped repulsive force, reminiscent of a solvation oscillatory force, is also observed when the adsorbed layers are compressed under high loads. This additional force is presumably a result of hindered drainage of water due to the presence of a high volume fraction of polymer chains between the surfaces.     

Formation of antifouling functional coating from deposition of a zwitterionic-co-nonionic polymer via “grafting to” approach

We develop a new process for the preparation of synergistic antifouling functional coatings on gold surfaces via a “grafting to” approach. The strategy includes a synthetic step of polymer brushes that consist of poly (ethylene glycol) (PEG) and zwitterionic side chains via a typical reversible-addition fragmentation chain transfer (RAFT) polymerization process, and a subsequent deposition of the polymer brushes onto a gold substrate. The presence of PEG and zwitterion chains on these polymer brush-coated gold surfaces has been proved to have a synergistic effect on the final antifouling property of the coating. PEG chains lower the electrostatic repulsion between zwitterionic polymer chains and increase their graft density on gold surfaces, while zwitterionic polymer effectively improves the antifouling property that is offered by PEG chains alone. Protein adsorption and cell attachment assays tests are conducted to confirm that this copolymer layer on gold surface has a pronounced resistance against proteins such as Bovine serum albumin and Lysozyme. Importantly, the antifouling property can be systematically adjusted by varying the molar ratio of PEG to zwitterionic chains in the final coating copolymer.     

Direct measurement of shear forces between polymer-bearing surfaces sliding past each other

We have used a recently developed surface force balance to measure, with extreme sensitivity, both lateral and normal forces between interacting surfaces, for the case of simple liquids and particularly with surface-attached polymers. The presence of polymers on the surfaces reduces drastically the force required to maintain them in sliding motion, under a given normal load, relative to the bare surface case. We believe this is due to the long range steric repulsion which can sustain a large normal load while maintaining a very fluid interfacial layer. The effect is much more marked for end-tethered chains in a good solvent than for adsorbed chains in a θ-solvent. This is attributed to the different extents of interpenetration of the compressed polymer layers.     

Surface presentation of bioactive ligands in a nonadhesive background using DOPA-tethered biotinylated poly(ethylene glycol)

We have developed surfaces for the selective presentation of biotinylated peptides and proteins in a background that resists nonspecific protein adsorption; controlled amounts of biotinylated poly(ethylene glycol) (MW 3400 Da; PEG3400) anchored to titanium-dioxide-coated surfaces via an adhesive tri-peptide sequence of L-3,4-dihydroxyphenylalanine (DOPA3-PEG3400-biotin; DPB) were incorporated within a DOPA3-PEG2000 background. Using optical waveguide lightmode spectroscopy, we found that the amounts of sequentially adsorbed NeutrAvidin and singly biotinylated molecules increased proportionally with the amount of DPB in the surface. Biotinylated peptides (MW approximately 2000 Da) were able to fill all three of the remaining avidin-binding sites, while only one molecule of biotinylated PEG5000 or stem cell factor bound to each avidin. The resulting biotin-avidin-biotin linkages were stable for prolonged periods under continuous perfusion, even in the presence of excess free biotin. Hematopoietic M07e cells bound to immobilized peptide ligands for alpha5beta1 (cyclic RGD) and alpha4beta1 (cylic LDV) integrins in a DPB-dose-dependent manner, with near-maximal binding to cylic LDV for surfaces containing 1% DPB. Multiple ligands were adsorbed in a controlled manner by incubating NeutrAvidin with the respective ligands in the desired molar ratio and then adding the resulting complexes to DPB-containing surfaces. Cell adhesion to surfaces containing both cylic LDV and cyclic RGD increased in an additive manner compared to that for the individual ligands. The bioactivity of adsorbed biotinylated stem cell factor was retained, as demonstrated by DPB-dose-dependent M07e cell adhesion and ERK1/2 activation.     

Colloid probe AFM investigation of interactions between fibrinogen and PEG-like plasma polymer surfaces

Interaction forces between surfaces designed to be protein resistant and fibrinogen (Fg) were investigated in phosphate-buffered saline with colloid probe atomic force microscopy. The surfaces of the silica probes were coated with a layer of fibrinogen molecules by adsorption from the buffer. The technique of low-power, pulsed AC plasma polymerization was used to make poly(ethylene glycol) (PEG)-like coatings on poly(ethylene teraphthalate) by using diethylene glycol vinyl ether as the monomer gas. The degree of PEG-like nature of the films was controlled by use of a different effective plasma power in the chamber for each coating, ranging from 0.6 to 3.6 W. This produced a series of thin films with a different number of ether carbons, as assessed by X-ray photoelectron spectroscopy. The interaction force measurements are discussed in relation to trends observed in the reduction of fibrinogen adsorption, as determined quantitatively by (125)I radio-labeling. The plasma polymer coatings with the greatest protein-repelling properties were the most PEG-like in nature and showed the strongest repulsion in interaction force measurements with the fibrinogen-coated probe. Once forced into contact, all the surfaces showed increased adhesion with the protein layer on the probe, and the strength and extension length of adhesion was dependent on both the applied load and the plasma polymer surface chemistry. When the medium was changed from buffer to water, the adhesion after contact was eliminated and only appeared at much higher loads. This indicates that the structure of the fibrinogen molecules on the probe is changed from an extended conformation in buffer to a flat conformation in water, with the former state allowing for stronger interaction with the polymer chains on the surface. These experiments underline the utility of aqueous surface force measurements toward understanding protein-surface interactions, and developing nonfouling surfaces that confer a steric barrier against protein adsorption.     

Cation-induced collapse of low-molecular-weight polyacrylic acid in the dispersion of barium titanate

Observations on the steric layers formed by the adsorption of low-molecular-weight polyacrylic acid (PAA) were taken using the colloidal probe method in an atomic force microscope. The effects of divalent barium ions and of monovalent potassium ions at varying concentrations were observed on the repulsive interaction profiles. High ionic concentrations screened double-layer forces to small distances, whereby the acting forces were reduced to steric interactions. De Gennes scaling theory was used to model the effect of electrolyte on an aqueous barium titanate system, which was stabilized with PAA. The brush model was found to represent the force curves better than the mushroom model. The collapse of PAA layers with increasing salt approximated a grafted polymer brush in monovalent electrolyte, but the addition of barium ions caused markedly less steric collapse. It is suggested that the formation of a Ba(2+)-PAA complex in the adsorbed layer increases its compressibility parameter.     

Decomposing bridging adhesion between polyelectrolyte layers into single molecule contributions

A novel approach to analyze the force response of multiple polymer strands, which are bridged between two surfaces, is proposed. The response of single polymer strands is experimentally accessible by measuring the force upon separation of two polymer-coated surfaces with the atomic force microscope. Our approach is based on the decomposition of the stretching and desorption sequence into contributions of independently bridged chains and of the elimination of loops formed on the opposite surface during contact. This approach was applied to investigate the bridging adhesion of surfaces coated with poly(vinylamine) (PVA). The force response of single PVA molecules was described on the basis of a recently proposed model, which accounts for the discrete chain character of the polymer at higher extension forces. As exemplary results, we determined the length distributions of the individual chains and the loop number distribution of these bridging chains on the polyelectrolyte-coated surfaces. The former were compared with scaling theories of polymer adsorption.     

Forces between thiolate-modified gold surfaces in a melt of end-functionalized polymers

To better understand surface forces across polymer melts, we measured the force between two chemically well-defined solid surfaces in a melt of polymer chains with a functional end group. As for surfaces, we used self-assembed monolayers (SAMs) of alkyl thiols with different end groups (methyl, amino, and hydroxyl) on gold. The polymer was a hydroxyl-terminated polyisoprene. To measure the force, an atomic force microscope was used. Between methyl-terminated SAMs, a weak and short-range repulsion was detected. Between hydroxyl or amino-terminated SAMs, a strong and long-range repulsion was observed up to distances of 16 nm. This indicates that the hydroxyl group of the polymer binds to the hydroxyl or amino groups of the SAMs. It forms a brush-like structure, which leads to steric repulsion. On amino-terminated SAMs, force-versus-distance curves on approach and retraction were monotonically repulsive and reversible. With hydroxyl-terminated SAMs, a jump was observed on approach when the load exceeded a certain threshold. On retraction, an adhesion had to be overcome. We interpret the jump as a rupture of the polymer layer. It indicates that the kinetics of bond and brush formation is faster on OH-SAMs than on NH2-SAMs.