Isamu Kaneda

Hydration, phase separation and nonlinear rheology of temperature-sensitive water-soluble polymers.

The collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating and the phase diagrams of aqueous PNIPAM solutions with a very flat lower critical solution temperature (LCST) phase separation line are theoretically studied on the basis of cooperative dehydration (simultaneous dissociation of bound water molecules in a group of correlated sequence), and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods. The transition becomes sharper with the cooperativity parameter σ of hydration. The reentrant coil-globule-coil transition and cononsolvency in a mixed solvent of water and methanol are also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves (LCST cononsolvency) with the mol fraction of methanol due to the competition is calculated and compared with the experimental data. Aqueous solutions of hydrophobically modified PNIPAM carrying short alkyl chains at both chain ends (telechelic PNIPAM) are theoretically and experimentally studied. The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association (association-induced phase separation), and separate from the coil-globule transition line. Associated structures in the solution, such as flower micelles, mesoglobules, and higher fractal assembly, are studied by ultra small-angle neutron scattering with theoretical modeling of the scattering function. Dynamic-mechanical modulus, nonlinear stationary viscosity, and stress build-up in start-up shear flows of the associated networks are studied on the basis of the affine and non-affine transient network theory. The molecular conditions for thickening, strain hardening, and stress overshoot are found in terms of the nonlinear amplitude A of the chain tension and the tension-dissociation coupling constant g.

Stress Buildup under Start-Up Shear Flows in Self-Assembled Transient Networks of Telechelic Associating Polymers†

The nonaffine transient network theory is used to study the time development of the shear and normal stresses under start-up shear flows in networks formed by self-assembled telechelic, hydrophobically modified water-soluble polymers. The initial slope, strain hardening, and overshoot of the shear stress are studied in detail in relation to the nonlinear tension-elongation curve of the elastically active chains in the network. The condition for the occurrence of strain hardening (upward deviation of the stress from the reference curve defined by the linear moduli) is found to be gamma > gammac(A), where gamma is the shear rate, gamma(c) is its critical value for strain hardening, and A is the amplitude of the nonlinear term in the tension of a chain. The critical shear rate gamma(c) is calculated as a function of A. It is approximately 6.3 (in the time unit of the reciprocal thermal dissociation rate) for a nonlinear chain with A = 10. The overshoot time t(max) when the stress reaches a maximum and the total deformation gamma(max) = gamma(t max) accumulated before the peak time are obtained in terms of the molecular parameters of the polymer chain. The maximum deformation gamma(max) turns out to depend weakly upon the shear rate gamma. The first and second normal stress differences are also studied on the basis of the exact numerical integration of the theoretical model by paying special attention to their overshoot, undershoot, and sign change as a function of the shear rate. These theoretical results are compared with recent rheological experiments of the solutions of telechelic hydrophobically modified poly(ethylene oxide)s carrying short branched alkyl chains (2-decyl-tetradecyl) at both ends.

Solvent transportation behavior of mechanically constrained agarose gels

The mode of transport of solvent from compressed agarose gels that were quenched at various temperatures was investigated. The compression load and the volume of the agarose gels decreased with time during compressive restraint. The decrease in volume was induced by squeezing of the solvent from the gel by compressive restraint. Relaxation of the compression load and the decrease in volume of the gels could be analyzed using a stretched exponential function; moreover, interestingly, both time constants were coupled. It is proposed that relaxation of the compression load was induced by solvent transportation (squeezing out). The hydrogel prepared at just below the sol–gel coexisting temperature (Tgel) exhibited distinguishing behavior, characterized by an increase in the rate of the solvent transportation. It is postulated that micro-phase separation occurred in the sample quenched close to Tgel due to spinodal decomposition, with consequent formation of relatively large solvent paths in the gel. Turbidity measurements and small angle light scattering data supported this postulate.

Stress Growth in Transient Polymer Networks under Startup Shear Flow

We study the stress growth in transient networks of telechelic associating polymers under the startup shear flow within the framework of our recent nonaffine transient network theory. We show that a transient strain hardening takes place due to the nonlinearity in the tension-elongation relation of the main chain. We calculate the critical shear rate \(\gamma\dot,\) c for the strain hardening as a function of the chain nonlinearity, and the overshoot time t max as a function of the shear rate \(\gamma\dot,\) . The theoretical results are compared with experimental data on aqueous solutions of a hydrophobically modified ethoxylated urethane (HEUR). The experimental results are consistent with the theoretical prediction. By detailed comparisons, we find that the effect of the polydispersity in the chain length significantly affects the transient stress because the nonlinear stretching effect of shorter polymer chains appears at smaller strains.

Time-Dependent Flow Properties of Transient Hydrogels with Temporal Network Junctions

The nonlinear rheological behavior under startup shear flows in aqueous solutions of telechelic hydrophobically ethoxylated urethane carrying branched alkyl end-chains, 2-decyl-tetradecyl, (referred to as C24-HEUR) was studied by strain-controlled rheomery. Unusual stress upturn (known as strain hardening), followed by the stress overshoot, was observed for 3 wt% aqueous solution above a critical shear rate. The phenomenon is explained by our recent transient network theory in terms of nonlinear chain stretching occurring during the stress build-up. Upon addition of glycerol, the relaxation time was shortened, while the equilibrium modulus increased with the concentration of glycerol. The stress upturn disappeared above a certain value of the glycerol concentration, strongly suggesting that glycerol affects the dynamics of the transient network through the interaction with the micellar junctions.

Rheology Control Agents for Cosmetics

“Usage feeling” is one of the most important characteristics of cosmetics, particularly skin-care products. Because the usage feeling of cosmetics is strongly related to their rheological properties, the ingredients, which affect the rheological properties of the product, are key factors in the development of skin-care products. In this chapter, two types of novel rheology control agents for cosmetics that have been developed are described. The first is hydrophobically ethoxylated urethane, which contains a relatively large C24 hydrophobe. This telechelic polymer forms a transient network structure in aqueous systems. Because the transient network structure is due to physical interactions, it is easily destroyed and reconstructed. This interesting physical property creates a unique usage feeling for use in skin-care products. The second agent is a water-swellable microgel that was polymerized in a W/O microemulsion system. Although water-soluble polymers are widely used in cosmetics as viscosity thickeners, the thickeners often suffer a serious problem, so-called spinability, which is due to the entanglement of polymer chains. In contrast, microgels avoid such problems. The details of these rheology control agents, their syntheses, physicochemical properties, and rheological properties are reviewed.

Polymerization of a water‐swellable microgel by a novel inverse microemulsion polymerization and its application as a viscosity thickener for cosmetics

A water-swellable microgel was polymerized in the inverse micelle of a nonionic surfactant, namely a confined space. The microgel was polymerized using 2-acrylamido-2-methylpropanesulfonic acid, dimethyl-acrylamide, and methylene-bis-acrylamide. To determine a suitable polymerization system that appeared W/O microemulsion at around 65 degrees, the phase diagram of the pseudo-ternary system was studied. The microgel polymerized in the system was isolated by a reprecipitating method; consequently the samples were obtained as powder. The rheological properties of the microgel were studied after re-dispersing the powder sample in an aqueous medium. The viscosity-thickening effect of the crosslinked microgel was higher than that of the non-crosslinked sample. Moreover, a suitable crosslinking ratio was confirmed. The viscosity thickening effects of the microgel under various pH, salt concentration, and alcohol concentration conditions were also investigated. The study revealed that the microgel can be considered as a multi-purpose viscosity thickener for cosmetics.

A High Performance Hair Treatment to Enhance Hair Strength Based on a Novel Self Cross-Linking Polymer

毛髪にハリコシを付与する方法として毛髪自体の育成を助ける生物学的アプローチと, 外部からある基剤を塗布して補強する物理的アプローチが考えられる。前者は軟毛の悩みを根本的に解決できる可能性があるが, 効果が得られるまでに長期間を要する, 効果に個人差がある, などの課題がある。一方, 後者は即効性はあるものの通常洗髪などにより効果が失われるなど, 持続性に乏しいため毎日の適用が必要となる。そこで即効性に優れ, しかもその効果が長期的に持続するハリコシ向上トリートメントの開発を目的として検討を行った。本基剤に求められる基本的条件として, 1. 優れた耐水性, 2. 1本1本の毛髪を独立してコートできること, の2点が挙げられる。そこで耐水性の付与を目的としてアルコキシシランの縮合反応によるネットワーク形成を利用し, 極性をコントロールすることで毛髪への付着性を向上させ, 1本ずつの毛髪を均一な皮膜でコーティング可能な基剤とした。さらに毛髪上に形成される皮膜の強度, 平滑性を検討し, 優れたハリコシ付与効果と良好な使用性を併せ持つへアトリートメントとすることに成功した。