今後のセミナー Upcoming seminars

(International Webinar on Gels and Networks)
Speaker: Dr. Wei Hong (Southern University of Science and Technology, China)
Date&Time: 15 Dec. (Wed.) 9:00-(BST, CET), 16:00-(CST), 17:00-(KST, JST日本時間)
Registration: Click Here
Title: Polymeric gel, where structure matters
Abstract (FLYER)
Composed mostly of small solvent molecules and loosely connected polymer networks, polymeric gels appear and feel like liquid solution. It is natural to ask the question: are the mechanical properties of a polymeric gel fully determined by its chemical composition, including the concentrations of monomer, solvent, crosslinkers, etc., or would the synthesis condition and process affect its network structure and further the macroscopic properties? Through carefully planned experiments, we systematically studied the mechanical properties of a set of compositionally identical hydrogels prepared through different processes. A strong dependence of the elastic properties of polymeric gels on the synthesis conditions was observed, and the results well captured by a set of scaling relations derived from the theory of semi-dilute solutions and the proposed network structures. Further, the fracture of polymeric gels was studied, and some intriguing relations between the intrinsic fracture energies under various loading and swelling conditions were identified.

過去のセミナー Past seminars

(International Webinar on Gels and Networks)
Speaker: Dr. U. Hyeok Choi (Inha University, South Korea)
Date: 19 Nov.(Fri.)
Title: Understanding Ion Transport and Relaxation Processes in Nanocomposite Polymer Electrolytes
Abstract (FLYER)
Polymer electrolytes are of great interest as materials in energy storage devices because of ion conducting polymers enabling good adherence to electrodes and excellent processability for being made into thin film. The key challenge facing the development of polymer electrolytes for energy storage applications is to achieve high mechanical performance without sacrificing the requisite ionic conductivity. This makes it possible to stop the formation of lithium dendrite, which is detrimental to the devices.
We prepared epoxy-based networked polymer electrolytes including Li salts with either plastic crystals or ionic liquids. The curing of a homogeneous mixture of epoxy and electrolyte could generate a two-phase system in which the epoxy phase was selected to provide mechanical strength and the electrolyte phase was selected to maximize ionic conductivity. To further introduce multifunctional properties, nanocomposite polymer electrolytes were also prepared by combining non-aqueous or aqueous gel polymer electrolytes with inorganic nanoparticles. We systematically conducted an investigation of the effect of electrolyte types and their concentration on the conductometric, dielectric, and rheological properties of the networked polymer electrolytes, using dielectric relaxation spectroscopy and oscillatory shear. These results were complemented by morphology studies in order to understand structure-property relations. Our study leads to insight regarding optimal design of multifunctional electrolytes for energy storage devices.

(International Webinar on Gels and Networks)
Speaker: Dr. Masao Doi (Beihang University, China/Nagoya University, Japan)
Date: 06 Oct.(Wed.)
Title: Diffusio-mechanical coupling - Elastic effects in the diffusion of gels and polymer solutions -
Abstract (FLYER) (MOVIE)
A gel placed in a solvent swells absorbing solvent from the surrounding. This process is a kind of diffusion since polymer molecules are diffusing into solvent, or solvent molecules are diffusing into polymer network. The diffusion in gels generally involves deformation of polymer network and is coupled with elasticity of the network. Such coupling is called diffusion-mechanical coupling. The diffusion-mechanical coupling is also important in the liquid state of polymers (polymer melts and solutions)., where polymer molecules form a temporary network by entanglement. In this talk, I will demonstrate how to handle the diffusion-mechanical coupling in elastic and viscoelastic materials. The topics to be discussed are (a) Swelling and bending dynamics of sheet-like gel: cooperative diffusion constant revisited. (b) Mechanical instability of a filament of gels and polymer solutions: power of Onsager principle. (c) Case II diffusion: complexity in diffusion.

(International Webinar on Gels and Networks)
Speaker: Dr. Daniel King (Hokkaido University, Japan)
Date: 09 Sep. (Thr.)
Title: Scaling the Toughness of Soft Materials to New Heights through Fiber Reinforcement
Abstract (FLYER) (MOVIE)
The toughest of all materials found in nature consist of soft/hard composite structures. Examples include ligaments, which are made up of rigid collagen fibrils within a soft extracellular matrix, and nacre, consisting of ceramic plates bound together by a soft protein-based glue. In both cases, the soft/hard nature and hierarchical design results in these composites having mechanical properties that are significantly better than the neat components. With an aim to develop soft materials that have extraordinary toughness, we have fabricated soft/hard composites based on the reinforcement of soft yet extremely viscoelastic elastomers with woven fabrics. To maximize energy dissipation, deformation must occur over a large area, and the components undergoing deformation must be extremely tough. To make robust, materials at small size-scale, fibers can be introduced to induce the fracture of matrix over an area much greater than the nominal crack path. Ultimately, we demonstrate that to make extremely tough fiber reinforced soft composites, three mechanical requirements exist: 1) A strong interface must exist between components, 2) the modulus of the fabric must exceed the modulus of the matrix by many orders of magnitude, and 3) the combined work to fracture of both components must be high. These criteria differ significantly from commercially utilized fiber reinforced polymers, and the materials introduced here represent the first case of all three requirements being achieved simultaneously.

(International Webinar on Gels and Networks)
Speaker: Dr. Robert Style (Swiss Federal Institute of Technology in Zürich, Switzerland)
Date: 08 Jul.(Thr.)
Title: Controlling phase separation with polymer networks
Abstract (FLYER) (MOVIE)
Nature has incredible control of phase separation. As a good example, some birds use phase separation to make feathers with vibrant blue colors that arise because they contain highly monodisperse, densely packed air bubbles that have a size around the wavelength of light. The birds must have exquisite control over the size of these bubbles, as even a 10nm change in their diameter will cause the color to change. However, replicating such a process in the lab is extremely difficult, as we must contend with factors such as coalescing of domains and Ostwald ripening, which result in polydisperse materials.
I will explain how performing phase separation inside of gels, or inside of glassy polymer networks, gives us much better control over the phase separation process. This allows us to create large pieces of material with uniform color by phase separating simple components – i.e. without the need for any dye molecules. I will also show how we can also control when and where phase separation occurs by tuning the mechanical properties of the polymer network, and talk about how this is relevant to protein phase separation inside living cells.

(International Webinar on Gels and Networks)
Speaker: Dr. Jasper Van der Gucht (Wageningen University, Netherlands)
Date: 10 Jun (Thu.)
Title: Non-linear mechanics and failure of (double) fiber gels
Abstract (FLYER) (MOVIE)
Fracture of materials typically occurs via the nucleation and propagation of cracks. Most polymer materials are brittle, and fracture occurs abruptly, without significant softening prior to failure. The origin of this brittle failure lies in the strong stress concentration at defects and crack tips. Recent simulations, however, show that mechanical failure may occur in a completely different way in sparsely connected fiber networks [1,2]. When deformed, such networks show a very heterogeneous stress distribution with emerging force chains. The continuous formation and rupture of these force chains suppresses stress concentration and can thereby prevent crack nucleation, leading to a continuous percolation-like failure. Here, we show extensive computer simulations [2] that unveil how the failure of fiber networks depends on connectivity, and on properties of the individual fibers. We show that the damage is largest and most diffuse for networks close to the mechanical rigidity point (or isostatic point); however, for large systems we find that eventually the network always breaks by crack nucleation, especially when the rupture threshold of the fibers is large. This allows us to extract a critical length scale that determines the type of failure in these systems. We show how these regimes can be tuned and discuss how they are relevant for biological fiber networks, such as collagen tissue, and for experimental work on reconstituted collagen networks [3].
We then consider double networks consisting of fibers embedded in a soft polymer matrix [4]. The double network structure toughens the network significantly, and leads to a transition from brittle to ductile failure. Our simulations show different regimes of failure and allow us to pinpoint microscopic mechanisms responsible for toughening of double networks and to explain experimental findings [5].
[1] L. Zhang, D. Z. Rocklin, L. M. Sander, and X. Mao, Phys. Rev. Materials 1, 052602(R) (2017).
[2] S. Dussi, J. Tauber, J. van der Gucht, Physical Review Letters 124, 018002 (2020).
[3] F. Burla, J. van der Gucht, et al. PNAS 117, 8326 (2020).
[4] F.Burla, J. Tauber, S. Dussi, J. van der Gucht, G.H. Koenderink, Nature Physics, 15, 549 (2019).
[5] J. Tauber, S. Dussi, J. van der Gucht, Phys. Rev. Mat. 4, 063603 (2020).

(International Webinar on Gels and Networks)
Speaker: Dr. Shingo Matsukawa (Tokyo University of Marine Science and Technology, Japan)
Date: 12 May (Wed.)
Title: Network structures of polysaccharide gels from viewpoints of microscopic and macroscopic aspects
Abstract (FLYER) (MOVIE)
Macroscopic measurements on physical properties of food hydrocolloids provide useful information about formation of networks and network structures. For a deeper understanding, measurements of microscopic properties are instructive to give insights into mobilities and structures in nano and molecular levels. NMR measurements gives the information of molecular mobility, that is, relaxation times of polysaccharide reflect the flexibility of chains and relaxation times for water reflect the motion of water molecules and also polysaccharide chains through the chemical exchanging between water proton and labile proton on the chains. Moreover, the diffusion coefficients of probe polymers give the information about the mobility of molecules and the structure of the hydrocolloids. Furthermore, nano-particle tracking provides information on the local viscoelasticity of polysaccharide gels. The diffusion of particles by the Brownian forces can be used to probe the spatial heterogeneity of physical properties during the gelation, which gives the information about the phase separated structures in mixed polysaccharides gels. The results were supported by a simulation about a simulation of particles diffusion considering the heterogeneity of the gels.

(International Webinar on Gels and Networks)
Speaker: Dr. Kenji Urayama (Kyoto Institute of Technology, Japan)
Date: 21 Apr.(Wed.)
Title: Multiaxial deformation and crack growth of elastomers and gels
Abstract (FLYER) (MOVIE)
The large deformation behavior of elastomers and gels are conventionally examined by simple uniaxial deformation, but uniaxial deformation is only a special one among accessible deformations. Biaxial deformation varying independently the two orthogonal strains covers a wide range of strain, providing definite basis for comprehensive understanding of large deformation behavior. Understanding of the crack propagation phenomena is also crucial especially in practical applications. Their large deformability and viscoelasticity result in unique features in crack growth phenomena.
In this talk, we introduce our recent studies using biaxial stretching measurements; the stress-softening behavior (Mullins effect) of DN gels and filled elastomers with different physical origins; the unusual behavior of liquid crystal elastomers to equalize the orthogonal true stresses under unequal biaxial strain. We also reveal the properties of the crack growth with subsonic and supershear speeds, and the effects of stress softening and biaxial stretching on the crack growth for filled elastomers and gels.

(International Webinar on Gels and Networks)
Speaker: Dr. Youn Soo Kim (Pohang University of Science and Technology (POSTECH), Korea)
Date: 26 Mar.(Fri.)
Title: Improved network formation in polyelectrolyte complex hydrogels via suppression of micellization
Abstract (FLYER)
Physical hydrogels are consisted of three-dimensional polymer networks formed by dynamic crosslinking. However, the inherently low mechanical properties due to the weak bond strength of the non-covalent bond limit their applicability. Recently, increasing attention has been paid to the preparation of physical hydrogels with polyelectrolyte complex (PEC). PEC hydrogels, composed of oppositely charged polyelectrolytes, are an important class of polymer materials that are widely used in many applications, such as membranes, medical prosthetic, antistatic coatings, environmental signals to the sensors, drug delivery systems, and protein separation. Although PEC hydrogels can exhibit unique functions like sol-gel transition and self-healing, they generally exhibit insufficient mechanical strength or low water holding capacity due to the weak intermolecular bonds. In general, PEC hydrogels can be easily obtained using ABA triblock copolymers, where A block is a charged block and B block is a hydrophilic neutral block. At critical gelation concentrations, ABA triblock copolymers form three-dimensional polymer networks through the formation of self-assembled micelles, especially flower-type micelles. In this case, the loop-shaped polymers do not contribute to the network connection at all, which reduces the efficiency of gel formation. Here, we propose a novel BABAB pentablock copolymer that shows direct network formation rather than loop formation through inhibition of micellization. The mechanism of the directly formed polymer network as well as the rheological properties of our hydrogels will be discussed.

(International Webinar on Gels and Networks)
Speaker: Dr. Xiang Li (ISSP, The University of Tokyo, Japan)
Date: 15 Feb.(Mon.)
Title: Nanostructures of polymer gels; towards highly homogeneous gels
Abstract (FLYER)
Fabrication of ordered nanostructure is a crucial step in a wide range of fields, although the typical size of these objects is still limited to a scale of µm even with current technologies. Polymer gels are a familiar soft material consisting of a nanoporous three-dimensional network that is readily prepared in a large scale, in principle unlimited. However, application of gels as a nanostructured object is obstructed by the fact that gel networks inevitably have a significant level of defects including dangling ends, loops, entanglements, and nonuniform pore sizes, as a result of the fully stochastic gelation reaction.
In this study, we break this preconception: we present a simple but yet universal scheme to fabricate polymer gels with a highly ordered network. Our strategy is to bring a geometric constraint into the pregel solution so that the space is always uniformly filled with the starting polymer units throughout the gelation reaction. This gelation framework is known as “bond-percolation” in the classical percolation theory.

(International Webinar on Gels and Networks)
Speaker: Dr. Yvette Tran (ESPCI Paris, France)
Date: 05 Feb.(Fri.)
Title: Responsive hydrogel thin films: design and functionalities
Abstract (FLYER) (MOVIE)
Surface-attached hydrogel films are actual novel alternative to brushes and layer-by-layer assemblies as polymer thin layers. We have recently developed a simple and versatile approach to synthesize reliable and reproducible films with thickness widely ranging from a few nanometers to several micrometers. Surface-attached hydrogel films show very interesting responsive properties: they reversibly modify their thickness with temperature by absorbing/expulsing water with high amplitude change (the change is four-fold or more); the transition is sharp and rapid (within a few degrees around the transition temperature and below one second); hydrogels with adjustable internal architectures can be built such as multilayer hydrogel films, nanocomposite hydrogel films, micro-patterns of hydrogels. I will show that the tailoring of surface-attached hydrogels with well-controlled chemistry allows to face new challenges in various areas. This approach of polymer thin layers makes possible a fine characterization of mechanical properties (friction and adhesion) of hydrogel films in water. Temperature-responsive hydrogels are also used as actuators in microfluidic devices. Moreover, they are suitable for the development of modulable Bragg mirrors with high spectroscopic shift.

(International Webinar on Gels and Networks)
Speaker: Dr. Evelyne van Ruymbeke (Universite catholique de Louvain, Belgium)
Date: 29 Jan.(Fri.)
Title: Rouse processes in the linear viscoelastic response of transient polymer networks
Abstract (FLYER) (MOVIE)
These last years, several works have shown that combining two different dynamics within the same polymer system can lead to very interesting viscoelastic properties. These samples can be, for example, transient networks (dual or interpenetrated) which combine supramolecular and disentanglement dynamics, or which combine two different supramolecular dynamics, governed by different sticker lifetimes. In the present work, we analyze the linear viscoelastic response of several transient networks to discuss their (partial) Rouse relaxation. While the relaxation of simple transient networks built from unentangled chains is well described by a sticky Rouse process, we investigate how this model can be extended to unentangled networks governed by two different reversible junctions. We also discuss the presence of Rouse relaxation in the case of entangled double dynamics networks, mainly due to Constraint Release process induced by the fast relaxing component, and see how it can be quantified.

(International Webinar on Gels and Networks)
Speaker: Dr. Takamasa Sakai (Univ. Tokyo, Japan)
Date: 27 Nov.(Fri.)
Title: Developments in the Fundamental Physics of Polymer Gels and Application as Biomaterials
Abstract (MOVIE)
Hydrogel is a polymeric network swollen with a large amount of water, and is capable of water-mediated exchange of materials with the outside world. Thus, hydrogels are considered to be very useful as biomaterials because of their very similar composition and properties to those of biological soft tissues. Many hydrogels degrade and dissolve after swelling in vivo due to various factors. The swelling pressure of a hydrogel is defined as the difference between the osmotic pressure that drives swelling, and the elastic pressure that resists swelling. Therefore, it is essential to correctly understand and control the elastic and osmotic pressures of hydrogels for their social implementation. The physical properties of polymer gels have been generally predicted by analogy with polymeric single chains, rubber elasticity and polymeric solutions. However, our recent studies using Tetra-PEG gel, which is a model network gel, have revealed that the elastic and osmotic pressures, which are very elementary properties of gels, cannot be explained by existing theories. We will first discuss these fundamentals, and then development of an artificial vitreous body.

(International Webinar on Gels and Networks)
Speaker: Dr. Takayuki Kurokawa (Hokkaido Univ., Japan)
Date: 9 Nov.(Mon.)
Title: Activity Measurement of Polyelectrolyte in Hydrogels by Microelectrode Technique
Abstract (MOVIE)
Even though some techniques, osmotic pressure, fluorescent indicators, conductance, are adopted to measure the electric potential of polyelectrolyte hydrogels, they usually provide indirect methods to monitor the potential values and show the average data. On the other hand, streaming potential or zeta potential, or contact method, they only declare the surface properties of polyelectrolyte gels and sometimes invalid to give the quantitative measurement. In addition, microelectrode technique (MET) is an effective method to study the electric potential of polyelectrolyte hydrogels. The potential curves show that the MET can quantitatively measure the spatial distribution of Donnan potential of polyelectrolyte hydrogels. Unlike traditional osmotic pressure method that only give average potential values of hydrogels, MET can accurately detect the depth profile of potential of hydrogels from surface to bulk.