On the nanostructure of polymer layers formed by adsorption from binary and ternary solutions on a solid SiO2: a combined adsorption and atomic force microscopy (AFM) study

The thickness of nanolayers formed by adsorption from dilute and semi-dilute solutions on a solid SiO₂ surface has been estimated from adsorption isotherms and atomic force microscopy (AFM) measurements for polystyrene, poly(butyl methacrylate), and their mixtures. The thickness of the adsorption layers depends strongly on the adsorption conditions and is controlled by several features of the adsorbing entities. In a low-concentration regime of adsorption, the length of polymer chains and the nature of their interaction with the substrate are the most important factors controlling the adsorption process. Above the critical concentration C*, macromolecular clusters (aggregates of several overlapping chains) are formed in a solution as a result of polymer chains self-assembly. Therefore, the final adsorption layer thickness is determined mainly by the size of the clusters in this concentrated regime of adsorption. We also demonstrate that in the case of polymer mixtures, the adsorption leads to formation of mosaic structures with alternation of the polymeric components in plane of the substrate and a characteristic domain size of approximately 200 nm for each of the components. AFM study reveals that the adsorbed layers are fractal structures whose fractal dimensions depend on the type of the polymer and the adsorption process. We demonstrate therefore that the structure of nanolayers of polymers and their mixtures on the solid surface can be regulated by variation of the adsorption conditions.     

Chromatographic study of adsorption from binary and ternary solutions

Adsorption of benzene, anisole, and nitrobenzene on hydroxylated silica gel from binary and ternary solutions (adsorbate--n-hexane and adsorbate--n-hexane--terahydrofuran) was studied by HPLC. The equation that describes the adsorbate retention as a function of its concentration in binary and ternary solutions was proposed. The equation makes it possible to calculate the adsorption isotherms and adsorption equilibrium constants directly from chromatograms using the retention parameters and equilibrium concentrations of the adsorbate in the mobile phase.     

Reduction of friction at oxide interfaces upon polymer adsorption from aqueous solutions

Reduction of the interfacial friction for the contact of a silicon oxide surface with sodium borosilicate in aqueous solutions has been accomplished through the adsorption of poly(L-lysine)-graft-poly(ethylene glycol) on one or both surfaces. Spontaneous polymer adsorption has been achieved via the electrostatic attraction of the cationic polylysine polymer backbone and a net negative surface charge, present for a specific range of solution pH values. Interfacial friction has been measured in aqueous solution, in the absence of wear, and on a microscopic scale with atomic force microscopy. The successful investigation of the polymer-coated interfaces has been aided by the use of sodium borosilicate microspheres (5.1 microm diameter) as the contacting probe tip. Measurements of interfacial friction as a function of applied load reveal a significant reduction in friction upon the adsorption of the polymer, as well as sensitivity to the coated nature of the interface (single-sided versus two-sided) and the composition of the adsorbed polymer. These measurements demonstrate the fundamental opportunity for lubrication in aqueous environments through the selective adsorption of polymer coatings.     

Influence of the chain composition on the thermodynamic properties of binary and ternary polymer solutions

Thermodynamic properties of binary and ternary polymer solutions (one or two uncharged polymers in one solvent) were studied. Poly(vinyl pyrrolidone) (PVP), fully hydrolyzed poly(vinyl alcohol) (PVA) homopolymers, and water-soluble poly(vinyl alcohol-co-vinyl acetal, -vinyl propional, and -vinyl butiral) copolymers with various acetal content and chain structure, respectively, were used in the experiments. The hydrophilic/hydrophobic character of the PVA-based macromolecules and their compatibility with the PVP homopolymer were systematically regulated by changing the chemical structure of the copolymers (acetal content and/or length of side chains). The water activities in binary and ternary solutions of the chemically different polymers were determined by a gel-deswelling method developed here for ternary solutions. On the basis of the Flory-Huggins theory, the relevant solvent-segment and segment-segment pair interaction parameters (chi) have been calculated. The chi12 segment-solvent interaction parameters proved to be sensitive indicators for changes in the chemical structure of the copolymers. With increase of either the acetal content or the length of side chains in the copolymer, chi12 approached the value characteristic of a theta condition. No significant differences could be revealed in the segment-segment interaction parameters obtained for the PVP-copolymer mixtures with various acetal derivatives, when the chi12 and chi13 interaction parameters determined in binary solutions were used in the calculations for chi23. Determination of the parameters chi1,23 as suggested by Panayiotou, however, showed that increasing the acetal content or the length of the hydrophobic side chains in the copolymer resulted in a reduction in the interaction between the PVA "acetals" and PVP molecules.     

Light scattering in binary and ternary (polymer) solutions with specific interactions

Concentration dependence of basic light-scattering quantities in three binary low-molecular and two ternary polymer solutions was studied. Depolarization ratio alone may serve as a sensitive measure of presence of specific interactions. Experimental data of anisotropic scattering in low-molecular mixtures were in harmony with the idea that the parameters determining molecular orientation should be considered as continuous probability variables; in the case of polymer solutions specific interactions do not influence the anisotropic scattering.The presence of specific interactions with the formation of a minimum on the course of isotropic scattering is characterized. Transition from one solution structure to another with a local maximum on the course of composition light scattering is indicated.     

Phase equilibria of binary and ternary polymer solutions using modified UNIQUAC-based local composition model

Journal of Thermal Analysis and Calorimetry - In the present research, a modification on UNIQUAC activity coefficient model was done based on the local composition concept. The model was applied...     

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.     

Saturated adsorption at the surface of binary solutions

Summary In order to examine the relationship between the saturated adsorption and the bulk critical phenomena (micelle formation, phase separation, solubility etc.), the surface tension was measured on the aqueous solutions of homologous sodium alkyl sulfates and pri-alkyl-alcohols. Adsorption isotherms for the solutions which did not possess any critical phenomenon, did not exhibit the saturation, whereas one for the solutions with some critical phenomenon displayed the saturation before the occurence of such a phenomenon in these solutions. From the thermodynamic interpretation, it was found for the alcohol solutions that the activity at the surface in the saturated region reaches the same value as in the bulk after phase separation.

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Zusammenfassung An wäßrigen Lösungen von homologen Natrium alkylsulfaten und Alkyl-Alkoholen wurde die Oberflächenspannung gemessen, um die Beziehung zwischen der Sättigungsadsorption und den kritischen Erscheinungen (Mizellbildung, Phasentrennung, Löslichkeit usw.) zu erklären. Die Adsorptionsisothermen für Lösungen, welche keine kritischen Erscheinungen haben, zeigen keine Sättigung, während diejenigen von Lösungen mit kritischen Erscheinungen vor dem Auftreten derartiger Phänomene eine Sättigung erreichen. Die Aktivität der Alkohole erreicht in der Oberflächenphase des Sättigungsbereiches den gleichen Wert wie in der Volumenphase nach der Phasentrennung

     

Switching and structuring of binary reactive polymer brush layers

Switchable binary polymer brushes grafted to Si-wafers were prepared from hydrophilic and hydrophobic polymer components. When exposed to solvents, either the hydrophobic or the hydrophilic component extends in to the liquid phase, depending on the polarity of the solvent. The hydrophilic component was poly-2-vinylpyridine; the hydrophobic component was made photocrosslinkable in that a polystyrene copolymer containing a photodimerizing chromophore was used. In this system surfaces differing in water contact angle between 60° and 100° can be produced by variation of the solvent. The chromophore was phenylindene, which forms crosslinks upon direct UV-irradiation. Therefore, the polystyrene component can be fixed in the extended or collapsed state. It will be shown that by irradiation through an appropriate mask, surfaces can be structured and the structures fixed. In both the systems structural patterns differing in surface properties were produced and fixed photochemically.     

Disjoining pressure of thin layers of binary solutions

Theory of dispersion forces and thermodynamics was used for the calculation of the adsorption and disjoining pressure of a binary solution interlayer between two identical and two dissimilar plates. Disjoining pressure isotherms were obtained for liquid interlayers between solids, for wetting and free films of solutions. The conditions were determined under which the overlapping of the diffuse adsorption layers of the solute can provide the main contribution to the interaction and make the disjoining pressure positive. It has been shown by numerical methods that the disjoining pressure isotherms of thin interlayers of solutions may intersect the thickness axis, and that the repulsion forces appear at small distances. Thus, disperse systems may be stabilized in the presence of binary, nonionic solutions.     

Ultrathin binary grafted polymer layers with switchable morphology

Polymer surface layers comprised of mixed chains grafted to a functionalized silicon surface with a total layer thickness of only 1-3 nm are shown to exhibit reversible switching of their structure. Carboxylic acid-terminated polystyrene (PS) and poly (butyl acrylate) (PBA) were chemically attached to a silicon surface that was modified with an epoxysilane self-assembled monolayer by a "grafting to" routine. While one-step grafting resulted in large, submicron microstructures, a refined, two-step sequential grafting procedure allowed for extremely small spatial dimensions of PS and PBA domains. By adjusting the grafting parameters, such as concentration of each phase and molecular weight, very finely structured surfaces resulted with roughly 10-nm phase domains and less than 0.5-nm roughness. Combining the glassy PS and the rubbery PBA, we implemented a design approach to fabricate a mixed brush from two immiscible polymers so that switching of the surface nanomechanical properties is possible. Post-grafting hydrolysis converted PBA to poly(acrylic acid) to amplify this switching in surface wettability. Preliminary tribological studies showed a difference in wear behavior of glassy and rubbery surface layers. Such switchable coatings have practical applications as surface modifications of complex nanoscale electronic devices and sensors, which is why we restricted total thickness for potential nanoscale gaps.     

Kinetics of critical fluctuations in ternary polymer solutions

The time evolution of the static structure factor following a sudden temperature jump ΔT is investigated in the case of a symmetric mixture of two polymers and a solvent in the optical theta conditions. The two reverse processes corresponding to ΔT > 0 and ΔT < 0 are considered, and the kinetics of approach towards the final states are investigated both in the Rouse limit and in the presence of hydrodynamic interactions.     

Determination of activity coefficients of binary adsorption solutions

Conclusions A method was prepared for calculating the activity coefficients of the components of adsorption solutions from specified curves, corresponding to constant values of the integral of the Gibbs equation, when binary gas mixtures are adsorbed on microporous adsorbents.Translated from Izvestiya Akademiya Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 166–168, January, 1972.     

Investigation of excess enthalpies of selected binary and ternary solutions and mixtures

The excess enthalpies were investigated for 12 binary and 4 ternary systems, including mixtures and solutions of electrolytes and non-electrolytes. The excess enthalpies of mixtures and integral heats of solutions were measured with an isoperibol calorimeter at 35 °C. Heats of fusion and heat capacities as functions of temperature were measured with a Perkin Elmer Corp., DSC-2. Integral heats, heats of fusion and heat capacities allow investigations of excess enthalpies of solutions. For modelling of the experimental results, the modified Redlich-Kister equation was used with good success.

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Zusammenfassung Für 12 binäre und temäre Systeme, darunter Mischungen und Lösungen von Elektrolyten und Nichtelektrolyten, wurden die Exzessenthalpien untersucht. Die Exzessenthalpien der Mischungen und die integralen Lösungsenthalpien wurden mit einem isoperibolen Kalorimeter bei 35 °C gemessen. Die Schmelzwärmen und spezifischen Wärmekapazitäten in Abhängigkeit von der Temperatur wurden mit einem DSC 2 der Fa. Perkin-Elmer bestimmt. Die integralen Wärmen, Schmelzwärmen und Wärmekapazitäten erlauben, die Exzessenthalpien der Lösungen zu untersuchen. Die Versuchsergebnisse lassen sich erfolgreich mit modifizierten Redlich-Kister-Gleichungen modellieren.

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The author expresses his gratitude to the leader of the Teaching and Research TEAM, Prof. Dr. H. Schuberth, and to Dr. G. Figurski, for help and discussions.     

The onset of three-phase separation in ternary polymer solutions

A theoretical study is made of three-phase separation in a ternary solution consisting of a pure solvent and two homologous monodisperse polymers with relative chain lengths P₁ and P₂. The Flory—Huggins interaction parameter X is assumed to depend linearly on the volume fraction ? of the polymer mixture: χ = χ₀(T) + k(T)_?. For a given P₁ the ratio of P₂ to P₁ for the onset of three-phase separation increases with an increase in k, first gradually and then sharply. Loci of critical points for this condition on the χ₀ vs. ? diagram are determined for a series of fixed values of P₁ and their significance is discussed.     

Vapour pressure osmometry determination of water activity of binary and ternary aqueous (polymer + polymer) solutions

Precise water activity measurements at T = 308.15 K were carried out on several binary (water + polymer) and ternary {water + polymer (1) + polymer (2)} systems using the vapour pressure osmometry (VPO) technique. Polymers were polyethylene glycol 400 (PEG400), polyethylene glycol 6000 (PEG6000), polypropylene glycol 400 (PPG400), polyvinylpyrrolidone (PVP) and dextran (DEX). The water activity results obtained were used to calculate the vapour pressure of solutions as a function of concentration and the segment-based local composition models, NRTL and Wilson, were used to correlate the experimental water activity values. It was found that, for the polymer concentration range studied here, the values of the water activity obtained for the binary (water + polymer) solutions decrease in the order DEX > PVP > PEG6000 > PPG400 > PEG400. Furthermore, water activities of solutions of each polymer in the aqueous solutions of (5, 10, 15 and 20)% (w/w) other polymers investigated were also measured at T = 308.15 K. The ability of polymer (1) in decreasing the water activity of binary {water + polymer (2)} solutions was discussed on the basis of the (polymer + water) and {polymer (1) + polymer (2)} interactions.     

Ethanol, acetic acid, and water adsorption from binary and ternary liquid mixtures on high-silica zeolites

Adsorption isotherms were measured for ethanol, acetic acid, and water adsorbed on high-silica ZSM-5 zeolite powder from binary and ternary liquid mixtures at room temperature. Ethanol and water adsorption on two high-silica ZSM-5 zeolites with different aluminum contents and a high-silica beta zeolite were also compared. The amounts adsorbed were measured using a recently developed technique that accurately measures the changes in adsorbent/liquid mixture density and liquid concentration. This technique allows the adsorption of each compound in a liquid mixture to be measured. Adsorption data for binary mixtures were fit with the dual-site extended Langmuir model, and the parameters were used to predict ternary adsorption isotherms for each compound with reasonable accuracy. In ternary mixtures, acetic acid competed with ethanol and water for adsorption sites and reduced ethanol adsorption more than it reduced water adsorption.     

Bulk heterojunction polymer solar cells based on binary and ternary blend systems

Polymer solar cells (PSCs) were fabricated using a ternary blend film consisting two conjugated polymers and a soluble fullerene derivative as the donor and acceptor materials, respectively. And, to compare ternary blend system, the single‐component copolymers consisting of the repeating units of each of the copolymers, used in ternary blend solar cells, were designed and synthesized for use as the electron donor materials in binary blend solar cells. We systematically investigated the field‐effect carrier mobilities and the optical, electrochemical, and photovoltaic properties of the copolymers. Under optimized conditions, the binary blend polymer systems showed power conversion efficiencies (PCEs) for the PSCs in the range 3.87–4.16% under AM 1.5 illumination (100 mW cm^?2). All polymers exhibited similar PCEs that did not depend on the ratio of repeating units. The binary blend solar cell containing a single‐component copolymer as the electron donor material performed better than the ternary blend solar cell in this work. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011