Taiki Tominaga

Thermodynamic Interactions in Double-Network Hydrogels

Double-network hydrogels (DN-gels) prepared from the combination of a moderately cross-linked anionic polyelectrolyte (PE) and an uncross-linked linear polymer solution (NP) exhibit mechanical properties such as fracture toughness that are intriguingly superior to that of their individual constituents. The scheme of double-network preparation, however, is not equally successful for all polyelectrolyte/neutral polymer pairs. A successful example is the combination of poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PAMPS) cross-linked network and linear polyacrylamide (PAAm), which results in DN-gels with fracture strength under compression approaching that of articular cartilage ( approximately 20 MPa). Small-angle neutron scattering was used to determine the thermodynamic interaction parameters for PAMPS and PAAm in water as a first step to elucidate the molecular origin responsible for this superior property. Measurements on PAMPS/PAAm DN-gels and their solution blend counterparts indicate that the two polymers interact favorably with each other while in water. This favorable PAMPS/PAAm interaction given by the condition chi(PE-NP) < chi(PE-water) <chi(NP-water), where chi is the Flory-Huggins interaction parameter, is consistent with some of the salient features of the DN structure revealed by SANS, and it may also contribute to the ultimate mechanical properties of DN-gels.

Molecular Model for Toughening in Double-Network Hydrogels

A molecular mechanism is proposed for the toughness enhancement observed in double-network (DN) hydrogels prepared from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) polyelectrolyte network and poly(acrylamide) (PAAm) linear polymer. It is an extension of the phenomenological model set forth recently by Gong et al. ( Macromolecules 2007, 40, 6658- 6664 ). This mechanism rationalizes the changes in molecular structure of the DN gel constituents observed via in situ neutron scattering measurements, the composition dependence of the solution viscosity, and the thermodynamic interaction parameters of PAMPS and PAAm molecules obtained previously from neutron scattering studies. More specifically, this proposed mechanism provides an explanation for the observed periodic compositional fluctuations in the micrometer range induced by large strain deformation.

Effect of substrate adhesion and hydrophobicity on hydrogel friction

In this paper, the frictional behavior of a neutral hydrogel, polyvinyl alcohol (PVA), on smooth solid substrates with various levels of hydrophobicity have been investigated in water using a strain-controlled parallel-plate rheometer. For the sliding velocity dependence of friction, we detected a distinct friction transition on hydrophobic substrates that are strongly adhesive to the gel, while no clear transition was observed on hydrophilic substrates that are weakly adhesive to the gel. Even on the most hydrophobic substrate, the maximum frictional stress is approximately 1/10-1/5 of the gels elastic modulus under a large normal strain of 26%. Furthermore, the frictional stress on hydrophobic substrates in the high velocity region, larger than the transition, is much lower than that on hydrophilic ones. We attempted to explain these phenomena with the help of two models: a molecular model based on the thermal fluctuations occurring during adsorption-desorption of polymers and a continuum mechanics model based on elastic dewetting and forced wetting.

Friction of a soft hydrogel on rough solid substrates

We investigated the sliding friction between a soft hydrogel and rough and weakly adhesive solid substrates in a water environment. Polyvinyl alcohol (PVA) hydrogels of different elastic moduli and two sets of glass substrates with different contact angles to water, all of which varied in their surface roughness, were used. The friction measurement was performed by using a strain-controlled parallel-plate rheometer. With an increase in substrate roughness, the friction in the low velocity region increased slightly, while it decreased significantly above a critical velocity on a surface with a roughness larger than 1 μm. Below this critical velocity, the frictional stress changed with the glass substrate surface energy, while above this critical velocity, it was not sensitive to the glass substrate surface energy. The velocity-dependence of friction on rough surfaces is explained in terms of surface contact dynamics and is characterized by two velocities, i.e., vf and vdrainage. The former is determined by the cooperative diffusion constant of the gels, and the latter is by the surface roughness of the substrate and the normal pressure applied on the gel.

Self-repairing filamentous actin hydrogel with hierarchical structure.

A chemically cross-linked filamentous actin (F-actin) gel consisting of globular actin (G-actin) as repeating units was prepared. The F-actin gel was cross-linked by covalent bonds, and the main chain is represented by the self-assembly of G-actin with a high-ordered hierarchical structure. The gel exhibited good mechanical performance with a storage modulus >1 kPa and undergoes reversible sol-gel transitions in response to changes in the salt concentration (chemical-induced sol-gel transition) as well as to shear strain (mechanical-induced sol-gel transition). Therefore, the gel exhibits self-repairing ability through dynamic polymerization and depolymerization across the structure hierarchies under repeated shear stress.

The Performance of TOF near Backscattering Spectrometer DNA in MLF, J-PARC

The time-of-flight (TOF) type near-backscattering spectrometer (n-BSS), DNA, with Si crystal analyzers was built and started operation in 2012 at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). DNA is the first n-BSS with pulse shaping chopper installed at a spallation pulsed neutron source. It offers currently the highest energy-resolution of about 2.4 micro eV by operating a pulse shaping double-disk chopper at 225 Hz whose phase is optimized to the narrowest slit of 10 mm width. Energy resolution can be flexibly compromised with intensity during experiment by using two type slits with different widths and changing the copper frequency. An example of measurement with high energy-resolution under the condition that the pulse shaping chopper was operated is shown, where the limited measurable energy range was widely expanded by multi incident energy band technique. The experimental data demonstrate extremely high signal-to-noise ratio (~10 5 ) of this spectrometer.

The Design and q Resolution of the Small and Wide Angle Neutron Scattering Instrument (TAIKAN) in J-PARC

J-PARC Center, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan Research Center for Neutron Science and Technology, Comprehensive Research Organization for Science and Society (CROSS), Tokai, Ibaraki 319-1106, Japan J-PARC Center, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan Research Reactor Institute, Kyoto University (KURRI), Kumatori, Osaka 590-0494, Japan

Surfactant-induced friction reduction for hydrogels in the boundary lubrication regime

We studied the ability of surfactants to reduce friction by boundary lubrication for a bulk hydrogel sliding on a solid surface in an aqueous solution. A piece of negatively charged polyelectrolyte hydrogel was slid across solid surfaces with various levels of hydrophobicity, using a strain-controlled parallel-plate rheometer in water. A dramatic reduction in the sliding friction, especially in the low velocity region, was detected by the addition of a surfactant to the water medium. This friction reduction was only observed in gel-solid friction but not in solid-solid friction, indicating that the soft and wet nature of the gel surface was crucial for this surfactant-induced friction reduction. This phenomenon reveals that surfactants can remain at the gel-mated interface, thus preventing direct interfacial interaction between the sliding surfaces, and significantly decreasing the frictional stress. The reported dramatic reduction in friction highlights the frictional characteristics of soft and wet hydrogel materials.

Structural Approaches on the Toughness in Double Network Hydrogels

Most hydrogels are mechanically too weak to be used as any load bearing devices. We have overcome this problem by synthesizing hydrogels with a double network (DN) structure. Despite the presence of 90% water in their composition, these tough gels exhibit a fracture stress of 170 kg/cm2, similar to that of cartilage. The relation between their mechanical strength and structure for a wide range of conditions should be analyzed to apprehend the origin of the toughness of the DN-gels. We recently reported some experi- mental results obtained by dynamic light scattering and small angle neutron scattering. Some new experimental results obtained by neutron scattering in both deformed and undeformed conditions provided for a new under- standing of the origin of toughness. We review the studies on the structure of DN-gels towards understanding of the toughness origin. Studies on DN-gels for biomedical applications are also described.

Dynamical Behavior of Hydration Water Molecules between Phospholipid Membranes

The dynamical behavior of hydration water sandwiched between 1,2-dimyristyl-sn-glycero-3-phosphocholine (DMPC) bilayers was investigated by quasi-elastic neutron scattering (QENS) in the range between 275 and 316 K, where the main transition temperature of DMPC is interposed. The results revealed that the hydration water could be categorized into three types of water: (1) free water, whose dynamical behavior is slightly different from that of bulk water; (2) loosely bound water, whose dynamical behavior is 1 order of magnitude slower than that of the free water; and (3) tightly bound water, whose dynamical behavior is comparable with that of DMPC molecules. The number of loosely bound and tightly bound water molecules per DMPC molecule monotonically decreased and increased with decreasing temperature, respectively, and the sum of these water molecules remained constant. The number of free water molecules per DMPC molecule was constant in the measured temperature range.