Masayuki Tokita

Kinetics of water flow through a polymer gel

The water flow through the poly(acrylamide) gel under a constant water pressure is measured by newly designed apparatus. The Young modulus and Poisson’s ratio of the rod shape gels are measured by the uni-axial elongation experiments, which determine the longitudinal modulus independently from the water flow experiments. The time evolution of the water flow in the dilute gel is calculated based on the collective diffusion model of the polymer network coupled with the friction between the polymer network and the water. The calculated results are compared with the time evolution of the flow experiments, and the values of the longitudinal modulus and the friction coefficient are estimated. The estimated values are consistent with the results of our mechanical-response experiments and the light scattering experiments reported previously. We find that the time evolution of the water flow is well described by a single characteristic relaxation time predicted by our model for dilute gels.

Adhesive force between paired microdroplets coated with lipid monolayers

We created pairs of adhering water-in-oil microdroplets coated with lipid monolayers as model cells and studied the effects of the physicochemical properties of the lipids on the adhesive force ΔF. Four species of liquid-phase lipids were used: dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylcholine (DOPC), dipalmitoylphosphatidylcholine (DPPC), and dimyristoylphosphatidylcholine (DMPC). The dependence of ΔF on the choice of lipid was evaluated by independently measuring the interfacial tension at the oil–water interface, γ, and the contact angle between the adhering droplets, θ. It was found that a difference in size between the hydrophilic head and hydrophobic tail of the lipids results in an increase in γ. Hence, cone-shaped DOPE had a larger γ than did cylinder-shaped PC (γ: DOPE ≫ DMPC ∼ DPPC > DOPC). On the other hand, DMPC with the shortest tail length had the smallest θ among the lipids (θ: DOPC > DPPC > DOPE ≫ DMPC). Finally, it was found that ΔF drastically decreases when the carbon number of the alkyl chain in the tails is smaller than 16 (ΔF: DOPE > DOPC ∼ DPPC ≫ DMPC). Furthermore, using polyethylene glycol (PEG)-conjugated DOPE, we demonstrated that the conjugation of shorter PEG molecules (<750) to the head part of the DOPE changes its molecular shape to cylindrical, and thus its γ and ΔF become similar to those of the DOPC system.

Multiple patterns of polymer gels in microspheres due to the interplay among phase separation, wetting, and gelation.

Significance We investigate how microdroplet confinement affects pattern formation of a polymer blend in the liquid-and-gel coexisting phase, wherein interactions between the droplet surface and the polymers regulate wettability of the gelation polymer. The complete and partial wetting of the polymers produces two stable states: hollow microspheres and hemisphere microgels. In addition, gelation during phase separation produces various shapes as trapped states. The relation between capsule thickness and droplet size is changed by the dynamical coupling. Furthermore, multiple patterns with spherical asymmetric shapes are produced by the partial wetting and shape deformation along the phase boundaries between the sol/gel phases. These findings reveal a complex pattern formation arising from the interplay among the interfacial tensions, gel elasticity, and wetting in microspheres. We report the spontaneous patterning of polymer microgels by confining a polymer blend within microspheres. A poly(ethylene glycol) (PEG) and gelatin solution was confined inside water-in-oil (W/O) microdroplets coated with a layer of zwitterionic lipids: dioleoylphosphatidylethanolamine (PE) and dioleoylphosphatidylcholine (PC). The droplet confinement affected the kinetics of the phase separation, wetting, and gelation after a temperature quench, which determined the final microgel pattern. The gelatin-rich phase completely wetted to the PE membrane and formed a hollow microcapsule as a stable state in the PE droplets. Gelation during phase separation varied the relation between the droplet size and thickness of the capsule wall. In the case of the PC droplets, phase separation was completed only for the smaller droplets, wherein the microgel partially wetted the PC membrane and had a hemisphere shape. In addition, the temperature decrease below the gelation point increased the interfacial tension between the PEG/gelatin phases and triggered a dewetting transition. Interestingly, the accompanying shape deformation to minimize the interfacial area was only observed for the smaller PC droplets. The critical size decreased as the gelatin concentration increased, indicating the role of the gel elasticity as an inhibitor of the deformation. Furthermore, variously patterned microgels with spherically asymmetric shapes, such as discs and stars, were produced as kinetically trapped states by regulating the incubation time, polymer composition, and droplet size. These findings demonstrate a way to regulate the complex shapes of microgels using the interplay among phase separation, wetting, and gelation of confined polymer blends in microdroplets.

Phase behaviors of agarose gel

We present evidence for the existence of phase separation in the gel state of agarose having the mixture of water and methanol as the gel solvent. Firstly, the sol-gel transition line and the cloud point line are determined independently as a function of the concentration of agarose as well as the concentration of methanol in the mixed solvent by the quasi-equilibrium cooling of the solutions. Then the spinodal line is determined by quenching the solutions below the sol-gel transition line. We find that the spinodal line appears below the cloud point line and both lines are entirely buried below the sol-gel transition line in the aqueous agarose system. The concentration fluctuations are, therefore, frozen into the polymer network of agarose gel that promotes the opacity of the resultant gel. The structure of agarose gel is observed by the confocal laser scanning microscope (CLSM) imaging technique that reveals that the density fluctuations are grown up to micrometer scale in space. The phase separation boundary is found to shift to the higher temperature region than the sol-gel transition line when the concentration of methanol in the mixed solvent is increased. The results indicate that the position of the phase separation boundary in relative to the sol-gel transition line varies with the quality of solvent. These results are in agreement with the theory of the sol-gel transition in which both the divergence of the connectivity and the thermodynamic instability are taken into account.

A Simple Technique to Measure the Friction Coefficient between Polymer Network of Hydrogel and Water

A simple apparatus to control water flow through a hydrogel at a fixed temperature was designed, in which the hydrogel was mechanically constrained in a glass microcapillary at gelation. An evaluation of the friction between the polymer network of polyacrylamide gels and the solvent water measured by this simple technique is presented here. The effects of the experimental conditions, the pressure applied to the solvent, the temperature and the gel size (the length and the cross-sectional area), on the friction coefficients were examined. The results agreed well with the model of the flux of water flow in a capillary based on the Hagen–Poiseuille equation.

Transport Phenomena in Gel

Gel becomes an important class of soft materials since it can be seen in a wide variety of the chemical and the biological systems. The unique properties of gel arise from the structure, namely, the three-dimensional polymer network that is swollen by a huge amount of solvent. Despite the small volume fraction of the polymer network, which is usually only a few percent or less, gel shows the typical properties that belong to solids such as the elasticity. Gel is, therefore, regarded as a dilute solid because its elasticity is much smaller than that of typical solids. Because of the diluted structure, small molecules can pass along the open space of the polymer network. In addition to the viscous resistance of gel fluid, however, the substance experiences resistance due to the polymer network of gel during the transport process. It is, therefore, of importance to study the diffusion of the small molecules in gel as well as the flow of gel fluid itself through the polymer network of gel. It may be natural to assume that the effects of the resistance due to the polymer network of gel depends strongly on the network structure. Therefore, detailed study on the transport processes in and through gel may open a new insight into the relationship between the structure and the transport properties of gel. The two typical transport processes in and through gel, that is, the diffusion of small molecules due to the thermal fluctuations and the flow of gel fluid that is caused by the mechanical pressure gradient will be reviewed.

Structural Transition of Non-ionic Poly(acrylamide) Gel

We discuss the structure of the opaque poly(acrylamide) gel that is synthesized at higher mole fractions of the cross-linking agent above 0.2. The structure of the opaque gel is analyzed by the small angle neutron scattering technique. The fractal analysis of the scattering function yields that the polymer network of the opaque poly(acrylamide) gel can be seen as a mass fractal of the fractal dimension of about DM ∼ 2.7 when the mole fraction of the cross-linker is higher than 0.3. On the other hand, much larger exponents are obtained in the lower concentration region of the cross-linker less than 0.3. It suggests that the polymer network of the gel behaves as a surface fractal of the fractal dimension of DS ∼ 2.5. The structure of the polymer network changes from the surface fractal to the mass fractal at the mole fraction of the cross-linker is 0.3 when the mole fraction of the cross-linker is increased from 0.2 to 0.5. The structure of the gel is also observed by using the confocal laser scanning microscope. The fractal analysis of the confocal images indicate that the fractal dimension of the two dimensional distribution of the colloidal particles in the cross section of the colloidal aggregate is found to be about 1.7.

The Effective Surface Roughness Scaling of the Gelation Surface Pattern Formation

The surface pattern formation on a gelation surface is analyzed using an effective surface roughness. The spontaneous surface deformation on DiMethylAcrylAmide (DMAA) gelation surface is controlled by temperature, initiator concentration, and ambient oxygen. The effective surface roughness is defined using 2-dimensional photo data to characterize the surface deformation. Parameter dependence of the effective surface roughness is systematically investigated. We find that decrease of ambient oxygen, increase of initiator concentration, and high temperature tend to suppress the surface deformation in almost similar manner. That trend allows us to collapse all the data to a unified master curve. As a result, we finally obtain an empirical scaling form of the effective surface roughness. This scaling is useful to control the degree of surface patterning. However, the actual dynamics of this pattern formation is not still uncovered.

Transport in and Through Gel

Gel is a state of matter that classified into the solid because it consists of the three-dimensional cross-linked polymer network. It, however, shows some liquid-like properties since it also contains a considerable amount of fluid. According to such a characteristic structure, many substances can pass the gel. In many separation technologies, therefore, gel is used as a molecular sieve. Although the gel plays many important roles in the separation technologies, the detailed roles played by the gel in the transport phenomena is not well understood yet. The transport phenomena in the gel are necessary to be clarified. In this chapter, we discuss tow transport phenomena that is related to the gel. The one is the friction of the gel against the liquid that flows through the gel, and the other is the resistance of the gel for the diffusional translation of the substances in the gel.

Dynamics of Spinodal Decomposition in a Ternary Gelling System

The phase diagram and phase transitions of the ternary system of gelatin, water and poly(ethylene glycol) oligomers were studied as a function of the weight fraction of gelatin and the weight fraction and molecular weight of poly(ethylene glycol) oligomers. It was found that both phase separation and the sol-gel transition occur in this ternary system. The relative position of the phase separation line and the sol-gel transition line depends on the weight fraction and the molecular weight of the poly(ethylene glycol) oligomer that coexists in the solution. All aspects of the phase diagram are sensitive to the molecular weight of the poly(ethylene glycol) oligomer. Since the phase separation line crosses the sol-gel transition line in the phase space that is created by the temperature and the weight fraction of gelatin, the phase space is typically divided into four regions, where each region corresponds to a definite phase. The transitions between mutual phases were studied using the light-scattering technique.