Nanostructure of polymer layers formed upon the adsorption from binary and ternary solutions

Thicknesses of nanolayers formed upon the adsorption from dilute and semidilute solutions of polystyrene, poly(butyl methacrylate), and their mixtures on the surface of solid SiO₂ are estimated on the basis of adsorption isotherms and atomic force microscopy measurements. It is established that the thickness of an adsorption layer is determined by the sizes of individual macromolecular coils and clusters arising in a solution. In the case of polymer blends, adsorption leads to the formation of mosaic structures with the alternation of polymeric components in the substrate plane; the characteristic size of a domain is ≈200 nm for each component. It is shown that adsorption layers formed on the surface of a silicon single crystal (covered with intact oxide) are fractal objects whose dimension depends on the nature of polymer and conditions of its adsorption.     

Polymer adsorption on a fractal substrate: numerical study

We study the adsorption of flexible polymer macromolecules on a percolation cluster, formed by a regular two-dimensional disordered lattice at critical concentration p(c) of attractive sites. The percolation cluster is characterized by a fractal dimension d(s) (p(c))=91/49. The conformational properties of polymer chains grafted to such a fractal substrate are studied by means of the pruned-enriched Rosenbluth method. We find estimates for the surface crossover exponent governing the scaling of the adsorption energy in the vicinity of transition point, φ(s)(p(c))=0.425±0.009, and for adsorption transition temperature, T(A)(p(c))=2.64±0.02. As expected, the adsorption is diminished when the fractal dimension of the substrate is smaller than that of a plain Euclidean surface. The universal size and shape characteristics of a typical spatial conformation which attains a polymer chain in the adsorbed state are analyzed as well.     

Atomic force and ultrasonic force microscopy investigation of adsorbed layers formed by two incompatible polymers: polystyrene and poly(butyl methacrylate)

Atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) have been used to study the properties of adsorption layers formed by two incompatible polymers, polystyrene and poly(butyl methacrylate), in the course of simultaneous adsorption on the surface of silica (naturally oxidized surface of a silicon wafer). The adsorption was performed from solutions containing both of the components in a common solvent (carbon tetrachloride) in dilute and semidilute concentration regimes. It was discovered that in both cases the structure of adsorption layers has a complex mosaic structure, the details of which depend on solution composition, on the solution concentration regime, and on the ratio of the components in the adsorption layer. The observed structural inhomogeneity on the length scale of approximately 200-500 nm (distribution of segment density revealed by UFM) appears as result of thermodynamic incompatibility in the system and is conditioned by changes in the conformation states of the adsorbed macromolecules in the route of competitive adsorption of the components. The adsorbed polymer films with thicknesses of approximately 20-500 nm appeared to have fractal properties and could be characterized with fractal dimensions dependent on the ratio of the components at the interface and the adsorption conditions.     

Effect of temperature on the structure of adsorption layers formed by adsorption of polymers in the transition region from dilute to semidilute solutions

The measurements of adsorption from solutions of polymers (poly(butyl methacrylate) and polystyrene) and their mixtures at various temperatures in the wide concentration region from dilute to semidilute solution have been made. The adsorption isotherms and fraction of bound segments confirm the existence of the transition concentration region near the critical concentration of the chain overlapping when the change of adsorption mechanisms take place. The effect of temperature on adsorption from the mixtures is different for both the polymers depending on the concentration regime. For PBMA, which adsorbs from the mixtures preferentially, the adsorption isotherms are of the same shape as for adsorption from binary solutions and are characterized by the presence of a small forepart in the transition region and by increasing adsorption with temperature. For less adsorbing PS by transition from dilute to semidilute regime the inversion of the temperature effect on adsorption is observed. These results are confirmed by the estimations of the parameters of preferential adsorption at various temperatures. The data on the fraction of bound segments for dilute regime corresponds to the extended conformation of chains at the surface. The transition to semidilute regime leads to the diminishing of the fraction of bound segments as a result of simultaneous adsorption of macromolecular aggregates. Values of adsorption layer thickness have been calculated for various solution regimes and concentration. The dependence of the adsorption layer thickness on the temperature and on the solution regime at which adsorption occurs was established.     

Monte Carlo simulation study of the kinetics of brush formation by irreversible adsorption of telechelic polymers onto a solid substrate

The irreversible adsorption of telechelic polymer chains from solution and melts onto solid substrates has been studied using the bond fluctuation Monte Carlo model. Complex brush formation kinetics dominated by diffusion of chains to the substrate at short times (diffusion-limited regime or DLR) and by penetration of chains through the maturing brush at longer times (penetration-limited regime or PLR) were observed. During the entire adsorption process, the rate of chain adsorption decreases monotonically with time. In the DLR, characterized by a maximum in the concentration of singly bound chains and a rapidly increasing fraction of doubly bound chains (loops), this decrease is due primarily to the depletion of free chains near the substrate and the formation of concentration gradients of free (nonadsorbed) chains in the bulk solution. The DLR and PLR are separated by an intermediate regime during which the brush becomes dominated by doubly bound chains and both penetration of the maturing brush and diffusion of chains to the brush surface play a role in determining the kinetics of brush growth. The PLR is characterized by steep gradients of free chains within the growing brush and the disappearance of concentration gradients for free chains in the bulk solution. In the PLR, the concentration of singly bound chains is low and decreases slowly while surface coverage and the fraction of doubly bound chains increase slowly. The rates of adsorption of new chains and the formation of loops in the PLR slow dramatically with increasing surface coverage and increasing chain length and less dramatically with decreasing bulk concentration.     

Interactions between inorganic nanoparticles and cellulose nanofibrils

Nanofibrillated cellulose (NFC) is increasingly utilized in materials and biomedical applications consequently increasing interest in the modification of its surface properties. Besides modification using polyelectrolytes and polysaccharides, NFC can be combined with solid particles enabling formation of fibril network loaded with particles. Use of particles enabling easy functionalization could be beneficial for the development of hybrid structures, and lead to preparation of nanocomposites and functional materials. In order to explore interactions related to preparation of such structures, the interactions between nanosized precipitated calcium carbonate (nanoPCC) and nanoclay particles and NFC were examined by observing adsorption of the particles on NFC substrate using a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) imaging. By a treatment with carboxymethylated cellulose (CMC), the anionicity of the NFC substrate could be increased, providing an additional tool to affect the interplay between NFC and the inorganic particles. For slightly cationic nanoPCC particles an increase in the anionicity of the NFC by the CMC treatment increased the affinity, while the opposite was true for anionic nanoclay. Additionally, for interactions between nanoclay and NFC, dispersion stability was an important factor. QCM-D was successfully used to examine the adsorption characteristics of nanoparticles although the technique is commonly used to study the adsorption of thin polymer layers. Distinct adsorption characteristics were observed depending on the nanoparticle used; nanoclay particles deposited as a thin layer, whereas nanoPCC particles formed clusters.     

Analogy in adsorption behavior of homopolyelectrolyte mixtures as compared to analogous diblock polyampholytes

The adsorption behavior of aqueous mixtures of the homopolyelectrolytes poly(methacrylic acid) (PMAA) and poly[(dimethylamino)ethyl methacrylate] (PDMAEMA) was investigated in comparison with the adsorption of the ampholytic diblock copolymer PMAA‐b‐PDMAEMA on silicon substrates. Ellipsometry was used to determine the amount of adsorbed homopolyelectrolyte and diblock polyampholyte. Furthermore, the topography of the adsorbed polymers was investigated with atomic force microscopy (AFM) and compared with the structures observed in aqueous solutions by dynamic light scattering (DLS). For all types of investigated polyelectrolytic mixtures or the single polyampholyte, the adsorption was strongly influenced by the pH of the polymer solution. Although single homopolyelectrolytes showed only one maximum in adsorption according to their charge, the mixtures made from these homopolyelectrolytes showed two or three maxima. The third maximum near the isoelectric point of the mixture was assigned to a new species formed by aggregation of the two homopolyelectrolytes. Altogether, the adsorption behavior of the polyelectrolytic mixtures was in between the behavior of the pure homopolyelectrolytes and the analogous polyampholytes and therefore understandable from both of these polymer species. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 338–345, 2002; DOI 10.1002/polb.10091     

Some aspects of AFM nanomechanical probing of surface polymer films

We analyzed how the approach developed for the microindentation of non-uniform elastic solids can be adapted to analyze the atomic force microscopy (AFM) probing of ultrathin (1-100 nm thick) polymer films on a solid substrate, as well as polymer films with a multilayered structure. We suggested that recent Johnson's modification of the contact mechanics model that included a viscoelastic contribution could also be utilized to analyze rate-dependent loading data for polymer surfaces. The graded model proposed for microindentation experiments was modified allowing to account not only for variable elastic moduli within different layers but also for the gradient of properties between layers within a transition zone. Two examples of a recent application of this model for molecularly thick hyperbranched polymer monolayers (<3 nm thick) and tri-layered polymer films (20-40 nm thick) tethered to a solid substrate were presented and discussed. In both cases, complex shapes of both loading curves and elastic modulus depth profiles obtained from experimental AFM data were successfully fitted by the graded model with realistic structural parameters.     

Laser reflectometry: Examination of the influence of some assumptions and approximations to calculate the adsorption from the reflectometric signal

The adsorption density of a polymer was calculated from the output of a reflectometric instrument, using the model described by Dijt et al. taking into account a step layer profile of the adsorbed film, in order to weight the influence of several experimental parameters (refractive index and thickness of the substrate, concentration of the polymer solution) and hypothesis (thickness of the adsorbed layer, linear dependence of the output on the surface excess). Calculated data were backed with measurements in an impinging jet cell, using two adsorption systems: the cationic poly(allyl amine, HCl) on SiO₂ and the anionic poly(styrene sulfonate) on a polyamide 6-6 covered silicon substrate. We established the limit of a linear response of the reflectometric signal versus adsorption density, which depends on the refractive index of the substrate and on the concentration of the polymer solution. Calculations showed that an increase in the assumed adsorbed film thickness between 5 and 20 nm caused a decrease of about 13% in the calculated surface excess at the plateau of the isotherm for PSS. At concentrations of the polymer solution above 1 or 2 g L⁻¹, we found experimental circumstances where the reflectometric output was negative or null, although adsorption was taking place. This was an effect of the high refractive index of the solution.     

Temperature effect on adsorption properties of silica-polyacrylic acid interface

The temperature influence (15–35 °C) on the adsorption mechanism and conformation of nonionic polymers (polyethylene glycol (PEG), polyethylene oxide (PEO) and polyvinyl alcohol (PVA)) on the zirconium dioxide surface was examined. The applied techniques (spectrophotometry, viscosimetry, potentiometric titration and microelectrophoresis) allowed characterization of the changes in structure and thickness of polymer adsorption layers with the increasing temperature. The rise of temperature favours more stretched conformation of polymer chains on the ZrO₂ surface, which results in higher adsorption and thicker adsorption layer. Moreover, these conformational changes of adsorbed macromolecules affect the electric (solid surface charge density) and electrokinetic (zeta potential) properties of the zirconia–polymer interface. The obtained data indicate that the polyvinyl alcohol adsorption has a greater influence on zirconia properties in comparison to that of PEG and PEO. It is due to the presence of acetate groups in the PVA macromolecules (degree of hydrolysis 97.5%), which undergo dissociation.     

Fibronectin adsorption onto polyelectrolyte multilayer films

The Layer-by-layer deposition of positively and negatively charged macromolecular species is an ideal method for constructing thin films incorporating biological molecules. We investigate the adsorption of fibronectin onto polyelectrolyte multilayer (PEM) films using optical waveguide lightmode spectroscopy (OWLS) and atomic force microscopy (AFM). PEM films are formed by adsorption onto Si(Ti)O2 from alternately introduced flowing solutions of anionic poly(sodium 4-styrenesulfonate) (PSS) and cationic poly(allylamine hydrochloride) (PAH). Using OWLS, we find the initial rate and overall extent offibronectin adsorption to be greatest on PEM films terminated with a PAH layer. The polarizability density of the adsorbed protein layer, as measured by its refractive index, is virtually identical on both PAH- and PSS-terminated films; the higher adsorbed density on the PAH-terminated film is due to an adsorbed layer of roughly twice the thickness. The binding of monoclonal antibodies specific to the protein's cell binding site is considerably enhanced to fibronectin adsorbed to the PSS layer, indicating a more accessible adsorbed layer. With increased salt concentration, we find thicker PEM films but considerably thinner adsorbed fibronectin layers, owing to increased electrostatic screening. Using AFM, we find adsorbed fibronectin layers to contain clusters; these are more numerous and symmetric on the PSS-terminated film. By considering the electrostatic binding of a segmental model fibronectin molecule, we propose a picture of fibronectin adsorbed primarily in an end-on-oriented monolayer on a PAH-terminated film and as clusters plus side-on-oriented isolated molecules onto a PSS-terminated film.     

Adsorption of lipid-functionalized poly(ethylene glycol) to gold surfaces as a cushion for polymer-supported lipid bilayers

Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, the properties of a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N- [3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) adsorbed from ethanol and water solutions onto gold surfaces were studied using a variety of surface-sensitive techniques. X-ray photoelectron spectroscopy showed that the PEG molecules are tethered to the gold surface via thiolate bonds. When adsorbed from water, ethanol, or their mixtures, reflection-absorption infrared spectroscopy showed that amorphous PEG layers with disordered DSPE alkyl chains were formed, independent of adsorption time or solution concentration. On the basis of advancing and receding water and hexadecane contact angles on the lipopolymer films, the DSPE lipid groups appear to segregate from the PEG layer and become exposed at the surface of the polymer films. Swelling observed in surface plasmon resonance experiments and the large contact angle hysteresis observed indicate that highly swellable, mobile films capable of molecular rearrangements are formed. The self-assembling and amorphous properties of these PEG layers make them ideal candidates as polymer cushions for polymer-supported lipid bilayers. The DSPE surface concentration can be controlled, to a limited degree, by varying the adsorption time of DSPE-PEG-PDP from ethanol. A more effective strategy is to coadsorb DSPE-PEG-PDP with a non-lipid-functionalized PEG-PDP from an ethanol/water mixture, which allows the PEG thickness and density to remain constant while decreasing the density of DSPE groups.     

Low-temperature synthesis and characteristics of fractal graphene layers

Large scale fractal graphene layers are obtained by complex method of liquid phase exfoliation and self-organization. Atomic force microscopy (AFM) is used to study the surface properties of formed layers and to assess their thickness. Surface potential of graphene and potential transition between the graphene and substrate is measured by Kelvin probe method. The influence of the effect of dielectric confinement on the optical properties of graphene is discussed in this work. Raman scattering spectra were used for structural analysis and assessment of the level of defects. Current-voltage characteristics of graphene ribbons were measured and discussed for different number of layers.     

BSA adsorption on bimodal PEO brushes

BSA adsorption onto bimodal PEO brushes at a solid surface was measured using optical reflectometry. Bimodal brushes consist of long (N=770) and short (N=48) PEO chains and were prepared on PS surfaces, applying mixtures of PS(29)-PEO(48) and PS(37)-PEO(770) block copolymers and using the Langmuir-Blodgett technique. Pi-A isotherms of (mixtures of) the block copolymers were measured to establish the brush regime. The isotherms of PS(29)-PEO(48) show hysteresis between compression and expansion cycles, indicating aggregation of the PS(29)-PEO(48) upon compression. Mixtures of PS(29)-PEO(48) and PS(37)-PEO(770) demonstrate a similar hysteresis effect, which eventually vanishes when the ratio of PS(37)-PEO(770) to PS(29)-PEO(48) is increased. The adsorption of BSA was determined at brushes for which the grafting density of the long PEO chains was varied, while the total grafting density was kept constant. BSA adsorption onto monomodal PEO(48) and PEO(770) brushes was determined for comparison. The BSA adsorption behavior of the bimodal brushes is similar to the adsorption of BSA at PEO(770) monomodal brushes. The maximum of BSA adsorption at low grafting density of PEO(770) can be explained by ternary adsorption, implying an attraction between BSA and PEO. The contribution of primary adsorption to the total adsorbed amount is negligible.     

Comparison of adsorption affinity of polyacrylic acid for surfaces of mixed silica–alumina

The influence of solution pH (in the range 3–9) on the adsorption of polyacrylic acid (PAA) on the mixed silica–alumina surface (SA-3: SiO₂ 97 %–Al₂O₃ 3 % and SA-96: SiO₂ 4 %–Al₂O₃ 96 %) was investigated. The following methods were applied in experiments: spectrophotometry, viscosimetry, potentiometric titration, and microelectrophoresis, which enable determination of adsorbed amount of the polymer, thickness of its adsorption layers, surface charge density, and zeta potential of solid particles in the presence and absence of PAA, respectively. The obtained results indicate that rise of solution pH causes the decrease of PAA adsorption and the increase of its adsorption layer thickness on surfaces of both solids. Moreover, significantly higher adsorption of polyacrylic acid was obtained on the SA-96 surface. This is a result of more favorable electrostatic interactions occurring between the adsorbing polymer chains and the SA-96 surface and formation of a greater number of adsorbate-adsorbent connections through hydrogen bridges.     

Protein PEGylation attenuates adsorption and aggregation on a negatively charged and moderately hydrophobic polymer surface

Covalent grafting of poly(ethylene glycol) chains to proteins ("PEGylation") is emerging as an effective technique to increase the in vivo circulation time and efficacy of protein drugs. PEGylated protein adsorption at a variety of solid/aqueous interfaces is a critical aspect of their manufacture, storage, and delivery. A special category of block copolymer, PEGylated proteins have one or more water-soluble linear polymer (PEG) blocks and a single globular protein block that each exert distinct intermolecular and surface interaction forces. We report the impact of PEGylation on protein adsorption at the interface between aqueous solutions and solid films of poly(lactide-co-glycolide) (PLG), a moderately hydrophobic and negatively charged polymer. Using the model protein lysozyme with controlled degrees of PEGylation, we employ total internal reflection fluorescence techniques to measure adsorption isotherms, adsorption reversibility, and the extent of surface-induced aggregation. Lysozyme PEGylation reduces the extent of protein adsorption and surface-induced aggregation and increases the reversibility of adsorption compared to the unconjugated protein. Results are interpreted in terms of steric forces among grafted PEG chains and their effects on protein-protein interactions and protein orientation on the surface.     

Adsorption in Mixtures with Competing Interactions

A binary mixture of oppositely charged particles with additional short-range attraction between like particles and short-range repulsion between different ones in the neighborhood of a substrate preferentially adsorbing the first component is studied by molecular dynamics simulations. The studied thermodynamic states correspond to an approach to the gas–crystal coexistence. Dependence of the near-surface structure, adsorption and selective adsorption on the strength of the wall–particle interactions and the gas density is determined. We find that alternating layers or bilayers of particles of the two components are formed, but the number of the adsorbed layers, their orientation and the ordered patterns formed inside these layers could be quite different for different substrates and gas density. Different structures are associated with different numbers of adsorbed layers, and for strong attraction the thickness of the adsorbed film can be as large as seven particle diameters. In all cases, similar amount of particles of the two components is adsorbed, because of the long-range attraction between different particles.     

Orientation effects of PTFE nanolayers upon the nematic 5CB

Planar nematic layers of 5CB oriented by 'sliding on' nanolayers of PTFE were studied by electro-optic methods. Deposited layers were characterized by AFM and spectroscopic ellipsometry. It was found that at 100°C presumably single PTFE chains (4 nm thickness) are deposited. By ellipsometry measurements c. 0.1° pretilt angle of the nematic layer was determined. In planar nematic layers low frequency flexoelectric splay oscillations were excited. An overall 1/f shape of the oscillation spectrum was found in the range 1 to 1000 Hz, giving no evidence of a surface viscosity effect in this range.     

Interaction forces between particles stabilized by a hydrophobically modified inulin surfactant

The adsorption isotherm of a hydrophobically modified inulin (INUTEC SP1) on polystyrene (PS) and poly(methyl methacrylate) (PMMA) particles was determined. The results show a high affinity isotherm for both particles as expected for a polymeric surfactant adsorption. The interactions forces between two layers of the hydrophobically modified inulin surfactant adsorbed onto a glass sphere and plate was determined using a modified atomic force microscope (AFM) apparatus. In the absence of any polymer, the interaction was attractive although the energy of interaction was lower than predicted by the van der Waals forces. The results between two layers of the adsorbed polymer confirms the adsorption isotherms results and provides an explanation to the high stability of the particles covered by INUTEC SP1 at high electrolyte concentration. Stability of dispersions against strong flocculation could be attributed to the conformation of the polymeric surfactant at the solid/liquid interface (multipoint attachment with several loops) which remains efficient at Na(2)SO(4) concentration reaching 1.5 mol dm(-3). The thickness of the adsorbed polymer layer in water determined both by AFM and rheology measurements, was found to be about 9 nm.