Monomer-surface Interaction Research Articles

Effect of polymer rigidity on the phase behaviour of polymer adsorption on to planar surface.

We study the process of a semiflexible polymer chain adsorption on to planar surface by the dynamic Monte Carlo (DMC) method, based on the 3D off-lattice model. Both the strength of attractive monomer–surface interaction (εa) and bending energy (b) have pronounced effect on the adsorption and shape of semiflexible polymer chain. The semiflexible polymer can just fully adsorb on to the surface at certain εa, which is defined as critical εa. The essential features of the semiflexible polymer adsorption on to surface are that (i) the critical εa increases with increase in b; (ii) the shape of the fully adsorbed semiflexible polymer chain is film-like toroid, and the toroid becomes more and more perfect with increase in b. In addition, the size of toroid and the number of turns of toroid can be controlled by the b and εa.

Revisiting polymer surface diffusion in the extreme case of strong adsorption.

Revisiting polymer surface adsorption with a level of quantification not possible at the time of earlier seminal contributions to this field, we employ fluorescence microscopy to quantify the in-plane diffusion of end-labeled polystyrene adsorbed onto quartz and mica from cyclohexane solution, mostly at 25 °C. Care is taken to prohibit a surface-hopping mechanism, and the experimental techniques are adapted to measurements that persist for up to a few days. The main conclusion is that we fail to observe a single Fickian diffusion coefficient: instead, diffusion displays a broad multicomponent spectrum, indicating that the heterogeneity of surface diffusion fails to average out even over these long times and over distances (∼600 nm, the diameter of a diffraction-limited spot) greatly exceeding the size of the polymer molecules. This holds generally when we vary the molecular weight, the surface roughness, and the temperature. It quantifies the long-believed scenario that strongly adsorbed polymer layers (monomer-surface interaction of more than 1k(B)T) intrinsically present diverse surface conformations that present heterogeneous environments to one another as they diffuse. Bearing in mind that in spite of adsorption from dilute solution the interfacial polymer concentration is high, ramifications of these findings are relevant to the interfacial mobility of polymer glasses, melts, and nanocomposites.

The interplay of the polyelectrolyte-surface electrostatic and non-electrostatic interactions in the polyelectrolytes adsorption onto two charged objects – A self-consistent field study

The continuum self-consistent field (SCF) theory is applied to the study of the adsorption of flexible polyelectrolytes (PEs) onto the surfaces of two parallel and infinitely long charged columns, taking into account the short-range monomer-surface non-Coulombic interaction. Due to the complex interplay between the electrostatic and surface interactions, very interesting PE adsorption behaviors in terms of the degree of charge compensation and the bridging chain conformation are found from the numerical solution of the SCF equations. The screening-enhanced salt effect and the permanent adsorption of PEs, irrespectively of the salt concentration, emerge in the presence of the monomer-surface non-electrostatic interaction. The numerical results reveal that, for relatively weak monomer-surface interactions, the degree of charge compensation decreases with increasing monomer-surface interaction. Numerical result shows that, for the strength of monomer-surface interaction above the desorption-adsorption critical value and in a salt-free solution, the total amount of the adsorbed PE chains is linearly proportional to the surface charge density in the high PE charge fraction regime.

A Comparative Spectroelectrochemical Study of the Redox Electrochemistry of N-(Polyvinylamine)-Substituted-o-Nitroaniline

The electrochemistry and spectroelectrochemistry of polyvinylamine (PVAm) and N-(polyvinylamine)-substituted-o-nitroaniline at a polycrystalline gold electrode in contact with different electrolyte solutions (0.1 M HClO4 and 0.1 M KClO4) have been investigated with cyclic voltammetry and surface enhanced Raman spectroscopy (SERS). Cyclic voltammograms recorded with in acidic electrolyte solution showed formation of N-(polyvinylamine)-o-phenylenediamine in the negative going potential scan; in neutral solution reduction proceeds only to N-(polyvinylamine)-o-phenylhydroxylamine. Vibrational modes are discussed and assigned to monomer-surface interactions of adsorbates at the gold surface in both electrolyte solutions. A variety of adsorption behaviors of these polymers at a gold surface is concluded; they apparently depend on the applied electrode potential, pH of electrolyte (acidic or neutral), type and mole percentage of the o-nitroaniline moiety. The adsorption at the gold surface is strongly enhanced with 12.7 mole% of o-nitroaniline moiety, respectively, at its PVAm-backbone. The direct coordination of these polymers with the gold surface occurred via the nitro-group of the o-nitroaniline moiety and primary amino-groups located at the polymer backbone.

Adsorption of star polymers: computer simulations

The behaviour of star polymers adsorbed on smooth surfaces is studied using coarse-grained bead-spring models and Langevin dynamics simulations. The conformational properties of a single adsorbed star polymer in good-solvent conditions are considered as functions of the functionality (number of arms) f, the number of monomers per arm N, and the monomer-surface interaction energy es. Four conformational regimes are identified: a linear-polymer regime; a two-dimensional star polymer regime; a sombrero regime; and a colloidal regime. The latter three correspond to regimes predicted theoretically by Halperin and Joanny [J. Phys. II (France), 1991, 1, 623–636]. Solvent effects are explored by dialing in effective attractions between the monomer beads; with decreasing solvent quality, the star polymers adopt more compact, globular structures. Good-solvent to bad-solvent quenches at finite surface coverages are considered; these correspond to established experimental protocols for adsorbing and then drying polymer sub-monolayers on surfaces. The structure of the polymer film is surveyed as a function of surface coverage, f, N, and es, in good-solvent and bad-solvent conditions. The simulated post-quench structures are in good qualitative agreement with those observed in atomic-force microscopy measurements, while the simulated pre-quench structures shed light on the microscopic mechanisms of film formation. This study draws together much of what is known about surface-adsorbed star polymers from theory, simulation, and experiment.

Methylene Blue Dimerization Does Not Interfere in Surface-Area Measurements of Kaolinite and Soils

AbstractMethylene blue (MB) was adsorbed from aqueous solutions onto a kaolinite and four soil samples to determine the effects of MB dimerization on the measured surface area. Adsorption isotherms were prepared using four adsorbing solutions containing, respectively, 9, 46, 71, and 83% of MB molecules in the dimeric state. Langmuir-type isotherms were obtained in each case. The results indicate that equilibration occurs quickly. The aggregation state of MB molecules at the surface does not depend on the aggregation state in the initial adsorbing solutions, but on the final equilibrium concentration of MB. A comparison with the specific surface area measured by adsorption of ethylene glycol monoethyl ether indicates that MB adsorbs as a monomer, regardless of the aggregation number in solution. This result occurs owing to the strength of monomer-surface and monomer-monomer interactions. If monomer-surface interactions are favored, the MB dimer adsorbs in the monomeric form. If monomer-monomer interactions are favored, dimer adsorption may occur. The visible spectra of adsorbed molecules indicated that MB was present at the surface as a mixture of monomeric and dimeric species. These results suggest that dimers are formed in the contact region between two aggregating particles.

Adsorption of branched polymers on Sierpinski-type fractal of base ; exact results

We have studied the branched polymers situated on the two-dimensional modified Sierpinski gasket lattice of base b =3in the presence of monomer-monomer and monomer-surface interactions. We have determined critical properties of this model and found that some surface crossover exponents differed much from that in the case of the standard SG.

Equilibrium behavior of confined triblock copolymer films

Using a two-dimensional self-consistent field calculation, we determine the equilibrium morphology of thin films of ABC triblock copolymers confined between hard, smooth plates. The B segment is chosen to be the central block and all the blocks are incompatible. The chains microphase-segregate into a lamellar phase, with the stripes either perpendicular or parallel to the walls. When all the monomer-surface interactions are identical, the perpendicular orientation has the lowest free energy. When a repulsion is introduced between the surface and the A and C monomers. The surface interactions further stabilize the perpendicular orientation. At strong surface interactions, the morphology of the perpendicular structure is controlled by the overall thickness of the molten layer. In comparing diblocks to triblocks as candidates for forming laterally patterned films, our work indicates that triblocks possess distinct advantages over diblocks. First, no special effort needs to be taken to establish neutral surfaces. Second, the film does not have to be confined between two substrates. Thus, triblocks can be used to fabricate patterned polymer surfaces, which can be used for novel optical or electronic applications.

Equilibrium behavior of confined triblock copolymer films

Using a two-dimensional self-consistent field calculation, we determine the equilibrium morphology of thin films of ABC triblock copolymers confined between hard, smooth plates. The B segment is chosen to be the central block and all the blocks are incompatible. The chains microphase-segregate into a lamellar phase, with the stripes either perpendicular or parallel to the walls. When all the monomer-surface interactions are identical, the perpendicular orientation has the lowest free energy. When a repulsion is introduced between the surface and the A and C monomers. The surface interactions further stabilize the perpendicular orientation. At strong surface interactions, the morphology of the perpendicular structure is controlled by the overall thickness of the molten layer. In comparing diblocks to triblocks as candidates for forming laterally patterned films, our work indicates that triblocks possess distinct advantages over diblocks. First, no special effort needs to be taken to establish neutral surfaces. Second, the film does not have to be confined between two substrates. Thus, triblocks can be used to fabricate patterned polymer surfaces, which can be used for novel optical or electronic applications.

Continuously varying crossover exponent for adsorption of linear polymers on fractals.

In this work we study the problem of physical (reversible) adsorption of a linear polymer in a good solvent in the case when the container of the polymer-solvent system is taken to be the Mandelbrot-Given (MG) model of a percolation cluster and the plane-filling (PF) fractal lattice. We accept the self-avoiding random walk as a model of the linear polymer, and, in addition, we assume that the adsorption energy pertaining to the bonds that lie in the layer next to the adsorbing boundary depends on their directions and is different from the adsorption energy for the bonds that belong to the boundary. Under these conditions we have found, in the case of both fractals (MG and PF), that the crossover exponent \ensuremath{\varphi} (associated with the number of adsorbed monomers) continuously varies with the parameter that measures the monomer-surface interaction along the bonds that are perpendicular to the adsorbing boundary. We discuss this result and its relevance to the understanding of the validity (violation) of the universality hypothesis in the case of critical phenomena on fractals. (c) 1995 The American Physical Society

Competition between self-attraction and adsorption in branched polymers situated on a fractal lattice

The authors have studied the branched polymer model, situated on the Sierpinski gasket lattice, with competing monomer-monomer and monomer-surface interactions. They have determined critical properties of this model within an exact renormalization group approach. Their results reveal that the corresponding model system possesses a rich phase diagram that is typical for polymer surface physics.

Critical exponents for surface interacting self-avoiding lattice walks. II. The square lattice

Critical exponents for the two-dimensional version of adsorption of a polymer chain at an interface are estimated as δa≃1.9 and ζ≃1.5 from the exact enumeration of self-avoiding walks up to 21 terms on the square lattice, where δa and ζ are defined from the respective dependences of the free energy and the thickness of adsorption layer on monomer–surface interaction; ζ is to be estimated to be about unity for three-dimensional lattices. These values are different from the scaling predictions of de Gennes: δa=4 and ζ=3 in two dimensions, and ζ=1.5 in three dimensions. An assumption in the scaling treatment is examined by using the exact enumeration data in order to explain the discrepancy.