Masami Kawaguchi

Polymer adsorption at solid-liquid interfaces

Abstract This review focuses on the theoretical and experimental advances over the last decade in polymer adsorption at solid-liquid interfaces. The following aspects are discussed: (1) theoretical developments using several approaches; (2) details of the newly developing experimental techniques; (3) experimental results obtained using these techniques with various systems, including homopolymers, block copolymers, and polyelectrolytes.

Sequential polymer adsorption: competition and displacement process

Abstract This review focuses on recent advances, both theoretical and experimental, in the competitive and displacement adsorption of polymer chains on liquid-solid interfaces. It discusses the following: 1) competitive and displacement adsorption of the same chemical chains of different lengths, 2) displacement of preadsorbed polymer chains by low molecular weight displacer molecules, and 3) competitive and displacement adsorption of chemically different chains.

Competitive and displacement adsorption of polyelectrolyte and water-soluble nonioic polymer at the silica surface

Abstract Individual, competitive, and displacement adsorption of poly(ethylene oxide) (PEO) with a narrow molecular weight distribution and poly(4-vinyl- N-n -propylpyridinium bromide) (PVPP) with complete quaternization onto a nonporous silica (Aerosil 130) from an aqueous KBr solution, pH 4, at 25°C was studied as a function of KBr concentration and molecular weight of PEO. In individual adsorption, adsorbed amount of both PEO and PVPP increases with increasing KBr concentration due to a decrease in excluded volume effects of polymer chains. In competitive adsorption, PEO adsorbs preferentially over PVPP due to the strong hydrogen bonds between PEO and silanol groups. In displacement adsorption PVPP molecules are desorbed from the silica surface by addition of PEO molecules. Both competitive adsortion and exchangeability slightly increase with increasing molecular weight of PEO.

Concentration dependence of surface pressure of polyether monolayers at the air-water interface

Abstract Surface pressure (π) of monolayers of two polyethers, i.e., poly(ethylene oxide) and poly(tetrahydrofuran) at the air-water interface have been measured as a function of surface polymer concentration (Γ) at 22°C using the surface balance with the sensitivity of 0.03 dyn cm −1 . Poly(ethylene oxide) and poly(tetrahydrofuran) with sharp molecular weight distributions were used. Both polyether monolayers showed the condensed-type surface pressure-area isotherm. At the lower polymer concentration the surface pressure was interpreted by the virial expansion form and the number average molecular weight can be determined by extrapolation of π /Γ RT to Γ = 0, showing very good agreement with the weight average molecular weight. It was found that the air-water interface at 22°C corresponds to a good solvent condition for both polyethers. At the intermediate polymer concentration the exponent for the concentration dependence of surface pressure was in excellent agreement with that predicted from the scaling concepts.

Dispersion Method for Safety Research on Manufactured Nanomaterials

Nanomaterials tend to agglomerate in aqueous media, resulting in inaccurate safety assessment of the biological response to these substances. The present study searched for suitable dispersion methods for the preparation of nanomaterial suspensions. Titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles were dispersed in a biocompatible dispersion medium by direct probe-type sonicator and indirect cup-type sonicator. Size characterization was completed using dynamic light scattering and transmission electron microscopy. A series of dispersion time and output power, as well as two different particle concentrations were tested. Microscopic contamination of metal titanium that broke away from the tip of the probe into the suspension was found. Size of agglomerated nanoparticles decreased with increase in sonication time or output power. Particle concentration did not show obvious effect on size distribution of TiO2 nanoparticles, while significant reduction of secondary diameter of ZnO was observed at higher concentration. A practicable protocol was then adopted and sizes of well-dispersed nanoparticles increased by less than 10% at 7 d after sonication. Multi-walled carbon nanotubes were also well dispersed by the same protocol. The cup-type sonicator might be a useful alternative to the traditional bath-type sonicator or probe-type sonicator based on its effective energy delivery and assurance of suspension purity.

Rheological and Interfacial Properties of Silicone Oil Emulsions Stabilized by Silica Particles

Emulsions stabilized by colloidal particles, namely Pickering emulsions were prepared by mixing silicone oil with silica particles dispersed in the continuous water phase as functions of silica concentration and silicone oil viscosity. Characteristics of the resulting oil dispersed in water (O/W) emulsions were determined by the measurements of adsorbed amounts of the silica particles, oil droplet size, and some rheological responses, such as hysteresis loop, stress-strain sweep curve, and dynamic viscoelastic moduli. Oil droplets were found to be stabilized by the formation of a fractal structure of the silica particles dispersed in the continuous water phase surrounding the oil droplets. The oil droplet size decreased with an increase in the silica concentration, whereas it increased with increasing the viscosity of the silicone oil. The resulting emulsions showed an ideal elastic matter at the smaller deformation since their stress-strain sweep curves were satisfied with Hookes law. On the other hand, at the larger deformation the emulsions showed thixotropic behavior and such thixotropy much more pronounced with an increase in the silica concentration, due to partially breaking the fractal structure of the silica particles.

Viscous fingering patterns in polymer solutions

Abstract Characteristics of viscous fingering patterns in polymer solutions were investigated by radially pushing air in a radial Hele-Shaw cell containing hydroxypropyl methyl cellulose (HPMC) solutions. Low and high molecular weight HPMC samples were used. An increase in molecular weight easily yielded chain entanglements, which led to a strong shear thinning and an increase in elasticity. For the low molecular weight HPMC solutions, only the dense-branching patterns were observed over the entire injection pressure range. On the other hand, for the high molecular weight HPMC solutions, the resulting patterns showed a systematic change from dense-branching to skewering patterns through splitting patterns as the injection pressure increased. The characteristic quantities of pattern growth, such as the finger width and tip velocity, were evaluated. Darcys law was compared with the measured tip velocity. HPMC solutions provided a linear relation between the measured tip velocity and the ratio of the injection pressure to effective viscosity at a shear rate imposed to HPMC solutions.

Rheological properties of silica suspensions in polymer solutions

Abstract When silica suspensions were prepared by mixing the silica particles with a polymer solution, adsorption of the polymer chains on the silica surfaces occurred and this should strongly influence the stability of the silica suspensions and their rheological properties. This review focuses on the results of recent experimental advances in rheological properties of silica suspensions in polymer solutions in terms of polymer adsorption phenomena.

Characterization of paraffin oil emulsions stabilized by hydroxypropyl methylcellulose

To study the relationship between emulsion stability and polymer emulsifier concentration, the preparation of paraffin oil emulsions by hydroxypropyl methylcellulose (HPMC) was carried out with HPMC concentrations below the overlapping concentration (C(*)) of HPMC. The stability of the emulsions incorporating HPMC was investigated by measuring the creaming velocity, volume fraction of emulsified paraffin oil, oil droplet size, and some rheological responses such as the stress-strain sweep curve and strain and frequency dependences of dynamic viscoelastic moduli. The paraffin oil was almost emulsified by HPMC above C(*)/20: the volume fraction of paraffin oil in the emulsion was higher than 0.72. Increasing in the HPMC concentration led to decreases in both the average oil droplet size and creaming velocity and an increase in the yield stress. All emulsions behaved as solid-like viscoelastic matter. Additionally, the measured dynamic storage moduli were compared with those calculated from a relationship based on functions of the volume fraction of oil in the emulsions and Laplace pressure; good agreement between the measured and calculated moduli was obtained. On the other hand, at HPMC concentrations below C(*)/50, the emulsified paraffin oil became unstable and the oil and the HPMC solution eventually separated.

Surface dilational moduli of latex-particle monolayers spread at air-water interface.

Latex particles prepared by radical dispersion polymerization of styrene and diacetone acrylamide (DAAM) in the presence of potassium persulfate as an initiator were spread as particle monolayers at the air-water interface. The surface pressure isotherms of the latex-particle monolayers are almost reversible during the compression-expansion cycles. The surface dilational moduli of the latex-particle monolayers at a fixed surface pressure of 20 mN/m and a fixed frequency of 10 mHz are almost independent of the strain. Moreover, at a fixed strain of 10% and fixed surface pressures of 10 or 15 mN/m the surface dilational moduli of the latex particle monolayers were measured as a function of frequency. The Lissajous orbits of the latex-particle monolayers exhibit positive hysteresis loops for all surface pressure ranges examined. The crossover between the magnitude of the surface elastic modulus and the magnitude of the surface viscous modulus occurs between the frequencies of 10 and 12 mHz and beyond the frequency of 12 mHz the former is larger than the latter. Such crossover indicates that the strain response of the latex-particle monolayers behave changes from liquid-like viscoelastic behavior to solid-like behavior with increasing frequency.