今後のセミナー Upcoming seminars

日時:3月2日(木)(2 Mar.2023(Thu.)), 15:00-16:30
場所:東京大学農学部5号館105号室(Room 105, 5th Building, School of Agriculture, Yoyoi Campus, The University of Tokyo) Map
講演者1:吉田 圭介 氏 (Mr. Keisuke Yoshida, Department of Physical Sciences, Ritsumeikan University)
(Shapes and mechanics of a heavy thin structure on a frictional substrate)
Thin structures can easily deform under gravity, involving buckling, wrinkling, draping, and folding. Their equilibrium shapes are determined by the balance between the stretching, bending, and gravitational forces. When a structure is on a rigid substrate, dry frictional contact may also come into play. In this talk, I will describe how the interplay between elasticity, geometry, gravity, and friction leads to a variety of morphologies of beams and sheets. I will first talk about how an elastic arch on a rigid frictional substrate deforms due to its own weight. We conducted the numerical simulations and theoretical analysis based on the Kirchhoff's rod theory and completely characterize the equilibrium behaviors of this system. I will then talk about shapes of a heavy thin sheet. We conducted a so-called "blister test", in which a thin sheet on a stiff and flat substrate is pushed at its center from below. As the vertical displacement is increased, we experimentally observed unusual pattern transitions and force responses. I will rationalize our experimental results by developing a scaling argument based on the Föppl–von Kármán 's plate theory.

講演者2:瀬戸 亮平 氏 (Prof. Ryohei Seto, Wenzhou Institute, University of Chinese Academy of Sciences, China)
(Migration in dense suspensions under inhomogeneous flow)
Suspension rheology is a classical subject. An idealized model system of suspensions is rather simple, i.e., a mixture of Newtonian-type liquid and many rigid spheres dispersed there. The surface of freely suspended particles give moving boundary conditions to the liquid. Such a system contains no freedom to store elastic energy like viscoelastic fluids. Therefore, it is natural to model suspensions within a viscous fluid framework.
However, it has caused some confusion in explaining complex rheological behaviors at higher volume fractions, such as shear thickening and normal stress differences, within conventional framework of viscous fluids. Recent works have progressed by breaking the standard approach and led to a situation where a basic consensus can be reached. The key has been to consider the mechanics of contacting particles rather than confine research to the hydrodynamic problems described above. Suspension rheology can now be reproduced by the Discrete Element Method, a mechanical simulation [1].
We extended the rheology simulations described above to understand how suspensions flow under inhomogeneous flow conditions. The simulation can reproduce the pressure-driven flow of a suspension through a channel. We have studied how the non-viscous fluid-like aspects of the suspension appear in such inhomogeneous flow situations. Solid volume fraction is no longer uniform as it flows through the channel. Particles migrate from the outside to the inside, and a jammed plug is formed in the middle of the channel. I will discuss how the pressure field and the mechanical contribution of normal stresses contribute to the phenomenon.

[1] R. Mari, R. Seto, J. F. Morris, and M. M. Denn. Shear thickening, frictionless and frictional rheologies in non-Brownian suspensions. J. Rheol., 58(6):1693–1724, 2014.
[2] C. Ness, R. Seto, and R. Mari. The physics of dense suspensions. Annu. Rev. Condens. Matter Phys., 13(1):97–117, March 2022.
[3] M. M. Denn and J. F. Morris. Rheology of non-brownian suspensions. Annu. Rev. Chem. Biomol. Eng., 5(1):203–228, 2014.

過去のセミナー Past seminars

Speaker: Dr. Joseph Gril (Université Clermont Auvergne, CNRS, Institut Pascal, France)
Date & Time: 7 Feb.2023(Tue.), 14:00-15:00
Place: Room 105, 5th Building, School of Agriculture, Yoyoi Campus, The University of Tokyo
Campus Map
Title: Engineering for the conservation of works of art: mechanical study of the wooden support of the Mona Lisa
Since its installation in the "Salle des Etats" in 2005, the poplar board on which the Mona Lisa is painted has been monitored by a Franco-Italian team of researchers in wood science and solid mechanics. Charged with assessing the fragility of the work while advising on its preventive conservation, the team has developed original methods over the course of annual examinations: optical measurements of the shape and the effect of mechanical actions; instrumented crosspieces to monitor the curvature and forces applied to the reverse side; measurement of the pressures exerted by the rebate of the frame... These data made it possible to build virtual models simulating the effects of the hygro-mechanical stresses to which the work is subjected, to demonstrate the low risk of deterioration provided that stable conservation conditions are maintained, and to envisage improvements to the framing system. This approach to conservation, based on the contribution of the engineering sciences and the attention paid to individual objects, has since undergone a remarkable expansion.

講演者:和田 浩史 先生(立命館大学理工学部物理科学科)
アブストラクト: 我々の身の回りには、セル状あるいは泡状の固体があふれている。 これらのスカスカな構造体は、そのかるさに反して硬く強くまたしなやかである。 コルク、 木材、海綿、骨梁などは天然の機能性材料の代表例である。工業的には、金属、 高分子、ガラス、セラミックなどを材料として、天然素材の優れた特性を模倣す る機能性素材が数多く開発されてきた。これらの自然および人工素材は「セル構 造体」と総称され、工学分野において長く深い研究の歴史を持つ。 セル状の構造を持つ薄いパネルは、ある方向に曲げるとそれと直交する方向 にも曲率を生成する。いわゆるサンドイッチ構造(航空機や電車のボディを担う 構造)のデザインにおいて、やっかいな問題として知られている性質である。と ころで、中身の詰まった均一な弾性薄板(たとえばプラスチック製の下敷)を曲 げると円筒面になるのに、単層のセル構造からなる板を曲げると双曲面や球面に 変形するのは、そもそもなぜなのだろうか?私たちは、このきわめて単純な疑問 に対して、物理学の立場からちゃんと筋の通った答えを準備したいと考え、本研 究を進めてきた。正確にいうと、均一な薄膜の曲げにおいても、曲面になる場合 と円筒面になる場合がある。この問題は、Sir H. Lambによる古典的理論によって 130年も前に完全に解明されている。我々は、 正方格子のセル構造からなる単層の板 (さしあたって「メタプレート」と呼ぶもの)に注目し、その面外弾性に対する Kirchhoffの薄板理論を考える。そして、 Lambの古典理論に則って、セル構造 からなる薄膜においてはなぜ曲面構造が容易 に出現するのかという問いに対する (ある意味、とても平明な)結論を導く。3D プリンタによる模型造形とその力学応答 の計測、および有限要素解析を組み合わ せて、この理論的な結論を検証する。

(International Webinar on Gels and Networks)
Speaker: Dr. Tristan Baumberger (Institut des Nanosciences de Paris/Sorbonne Université, Paris, France)
Date: 18 Feb.2022(Fri.)
Title: Environmental control of crack growth in physical hydrogel
Abstract (FLYER)
The synergistic association of a fluid phase and an elastic network is the key to numerous applications of polymer hydrogels as food or cosmetic products, drug delivery vectors, wound dressings or scaffolds for tissue regeneration. The biphasic structure of hydrogels enables easy exchange of fluid or solute via the liquid continuum which exists between the constitutive solvent and a wet physiological environment [1]. It is the essence of poroelasticity that, in addition to purely osmotic forces, stresses acting on the network are able to drive the solvent flow. Such a transport mode is especially efficient in the crack tip zone where large stretching ratios prevail. Crack growth dynamics in gels is found very sensitive to environmental modifications [2]. This somewhat rejuvenates the old engineering topic known as stress corrosion of metals and glasses. For instance, solvent draining results in stress transfer to the network, therefore affects the energy release rate; it is predicted to be responsible for delayed rupture of hydrogels.
In this seminar I will rather review cases where advected environmental solutes directly affect the non-covalent crosslink resistance in the cohesive zone, therefore weakening or strengthening the fracture energy. This may result in environment-driven speeding-up or slowing-down and eventually pinning of cracks in physical hydrogels:
1. Non-binding ions make egg-box opening in alginate gels easier [3]
2. Nanoparticles clog and dehydrate the cohesive zone of gelatin gels [4]
3. Charged nanoparticles are efficient transient crosslinks
for oppositely charged networks [4].
[1] See L. Corté, Creating physical adhesion at hydrogel-tissue interfaces, International webinar on gels and networks: Friday January 28th 2022. https://youtu.be/z8uBFPyU3wU
[2] T. Baumberger, & O. Ronsin (2020). Mechanics of Soft Materials, 2(1), 1-38.
[3] T. Baumberger & O. Ronsin (2010) Biomacromolecules 11, 1571
[4] O. Ronsin, I. Naassaoui, A. Marcellan, and T. Baumberger (2019) Phys. Rev. Lett., 123, 158002

(International Webinar on Gels and Networks)
Speaker: Dr. Laurent Corté (MINES ParisTech/ESPCI Paris, France)
Date: 28 Jan.2022(Fri.)
Title: Creating physical adhesion at hydrogel-tissue interfaces
Abstract (FLYER)
The fixation of hydrogels to soft biological tissues is of outmost interest for biomedical applications but it is highly challenging due to the fragile and wet nature of both hydrogels and tissues. Here, we explore how physical mechanisms occurring at hydrogel-tissue interfaces can be exploited to design bioadhesive hydrogels. For that, ex vivo and in vivo experiments were devised to measure the adhesion between model polyethylene glycol hydrogel films and the surface of porcine livers. In a first study, we find that a transition from a lubricated contact to an adhesive contact is governed by the transport of liquid across the tissue-hydrogel interface. We show that this transition corresponds to a competition between wetting and draining of the interface, which is well described by a simple model taking into account the microanatomy of tissues. This effect explains the strong decrease in adhesion observed between ex vivo and in vivo conditions and suggests routes to improve adhesion using superabsorbent systems. In a second study inspired by the pioneering works by Leibler and coworkers, we investigate how tissue-hydrogel adhesion can be created using particles that bridge the interface by adsorbing on both gels and tissues. In particular, we find that in the presence of blood, the particle bridging effect combined to the procoagulant properties of silica nanoparticles enhances the adhesion strength by inducing the rapid formation of a clot at the interface. These results and methods could contribute to the design of more predictive adhesion tests and of strategies to tailor the biointegration of hydrogel based-devices.

(International Webinar on Gels and Networks)
Speaker: Dr. Shingo Tamesue (Utsunomiya University, Japan)
Date&Time: 11 Jan.2022(Tue.)
Title: Adhesive hydrogel systems and self-healing hydrogels utilizing “intercalation” of layered inorganic compounds.
Abstract (FLYER)
In this webinar, I introduce the adhesive hydrogel systems and the self-healing hydrogel materials we reported recently. These systems and materials were formed by the “intercalation” of layered inorganic compounds. Mica, layered double hydroxide, and other layered inorganic compounds can entrap cations, anions, and molecules between their interlayers. These properties are called “intercalation”. The intercalation properties of layered compounds are used for creating stabilizers of paints. The driving force of “Intercalation” is not a usual ionic interaction working between cations and anions, and hence the materials formed by “Intercalation” have unique properties. Firstly, we developed adhesive hydrogel systems using “intercalation”. The pieces of the hydrogels having ionic groups in their gel networks adhered successfully by applying the aqueous dispersion of layered inorganic compounds, mica, onto the surface of the gel pieces. As the second topic, I introduce self-healing hydrogels utilizing intercalation. A cationic polymer was synthesized and mixed with mica, which can entrap various cations between its interlayers, in water. Afterward, the specimen hydrogelated due to the cross-linking of the cationic polymers via the intercalation. We evaluated the self-healing property of the hydrogels.

(International Webinar on Gels and Networks)
Speaker: Dr. Wei Hong (Southern University of Science and Technology, China)
Date: 15 Dec.2021(Wed.)
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.

(International Webinar on Gels and Networks)
Speaker: Dr. U. Hyeok Choi (Inha University, South Korea)
Date: 19 Nov.2021(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.2021(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.2021(Thu.)
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.2021(Thu.)
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 2021(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 2021(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.2021(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.2021(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.2021(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.2021(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.2021(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.2020(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.2020(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.