Tetra-PEG Gel Research Articles

A mesoscale model for the micromechanical study of gels

Gels are comprised of polymer networks swelled by some interstitial solvent. They are under wide investigation by material scientists and engineers for their broad applicability in fields ranging from adhesives to tissue engineering. Gels’ mechanical properties greatly influence their efficacy in such applications and are largely dictated by their underlying microstructures and constituent-scale properties. Yet predictively mapping the local-to-global property functions of gels remains difficult due - in part - to the complexity introduced by solute-solvent interactions. We here introduce a novel, discrete mesoscale modeling method that preserves local solute concentration-dependent gradients in osmotic pressure through the Flory-Huggins mixing parameter, χ . The iteration of the model used here replicates gels fabricated from telechelically crosslinked star-shaped polymers and intakes χ , macromer molecular weight ( M w ), crosslink functionality ( f ), and as-prepared solute concentration ( ϕ * ) as its inputs, all of which are analogues to the control parameters of experimentalists. Here we demonstrate how this method captures solvent-dependent homogenization ( χ ≤ 0.5 ) or phase separation ( χ > 0.5 ) of polymer suspensions in the absence of phenomenological pairwise potentials. We then demonstrate its accurate, ab initio prediction of gel topology, isotropic swelling mechanics, and uniaxial tensile stress for a 10k tetra-PEG gel. Finally, we use the model to predict trends in the mechanical response and failure of multi-functional PEG-based gels over a range of M w and f , while investigating said trends’ micromechanical origins. The model predicts that increased crosslink functionality results in higher initial chain stretch (as measured at the equilibrated swollen state) for gels of the same underlying chain length, which improves modulus and failure stress but decreases failure strain and toughness.

Analysis of the sustained release ability of bevacizumab-loaded tetra-PEG gel

Multiple intravitreal injections, which are painful and costly, are often required in the treatment of retinal disorders. Therefore, a novel drug delivery system using hydrogels is currently being evaluated as an alternative. This study aimed to evaluate the ability of tetra-armed polyethylene glycol (tetra-PEG) gel for sustained release in vitro. Bevacizumab-loaded tetra-PEG gel and 5-Carboxyfluorescein N-succinimidyl ester (FAM-NHS)-labeled IgG-loaded tetra-PEG gel were prepared by mixing tetra-PEG with thiol termini (tetra-PEG-SH) solution, maleimide termini (tetra-PEG-MA) solution, and bevacizumab or FAM-NHS labeled IgG. The gels were prepared with three different polymer concentrations of 1.5%, 5%, and 10%, then an in vitro release study performed to assess the sustained release ability of the drug-loaded tetra-PEG gels. High performance liquid chromatography (HPLC) was used to test the structural stability of the bevacizumab released from the tetra-PEG gel. The binding of bevacizumab to tetra-PEG-SH or MA was assessed using SDS-polyacrylamide gel electrophoresis (PAGE). The bioactivity of released bevacizumab was tested using KDR/NFAT-RE HEK293 cells. In addition, in vitro degradation and swelling studies were also performed. The in vitro release analysis showed that the release of bevacizumab was slower in the 5% and 10% tetra-PEG gels than that of 1.5% tetra-PEG gels. Similarly, the release of FAM-NHS-labeled IgG was slowest in the 1.5%, 5%, and 10% tetra-PEG gels, in that order. The 5% and 10% tetra-PEG gels released bevacizumab and FAM-NHS-labeled IgG over a period of 1–2 weeks. Both bevacizumab and FAM-NHS-labeled IgG were not fully released in 2 weeks. HPLC analysis showed that the retention time of the samples released from the bevacizumab-loaded tetra-PEG gel was similar to that of the bevacizumab standard. The SDS-PAGE analysis showed that bevacizumab binds to tetra-PEG-MA. The bioactivity assay test revealed no decrease in the bioactivity of the released bevacizumab. In vitro degradation and swelling studies revealed that 1.5%, 5%, and 10% tetra-PEG gels expanded by approximately 1.4-, 2-, and 3-fold, respectively. Based on the results of the release and swelling tests, 5% tetra-PEG gels are considered good candidates for controlled release systems for therapeutic antibodies such as bevacizumab. The binding of PEG to the therapeutic antibodies may reduce the availability of therapeutic antibodies that can be released.

Experimental Comparison of Bond Lifetime and Viscoelastic Relaxation in Transient Networks with Well-Controlled Structures.

We demonstrate an experimental comparison of the bond lifetime, estimated using surface plasmon resonance (SPR), and the viscoelastic relaxation time of transient networks with well-controlled structures (dynamically cross-linked Tetra-PEG gel). SPR and viscoelastic measurements revealed that the temperature dependences of the two characteristic times are in agreement, while the viscoelastic response is delayed with respect to the lifetime by a factor of 2-3, dependent on the network strand length. Polymers cross-linked by temporary interactions form transient networks, which show fascinating viscoelasticity with a single relaxation mode. However, the molecular understanding of such simple viscoelasticity has remained incomplete because of the difficulty of experimentally evaluating bond lifetimes and heterogeneous structures in conventional transient networks. Our results suggest that bond dissociation and recombination both contribute to the macromechanical response. This report on direct bond-lifetime-viscoelastic-relaxation time comparison provides important information for the molecular design of transient network materials.

Stress relaxation in tunable gels.

Hydrogels are a staple of biomaterials development. Optimizing their use in e.g. drug delivery or tissue engineering requires a solid understanding of how to adjust their mechanical properties. Here, we present a numerical study of a class of hydrogels made of 4-arm star polymers with a combination of covalent and reversible crosslinks. This design principle combines the flexibility and responsivity associated with reversible linkers with stability provided by chemical crosslinks. In molecular dynamics simulations of such hybrid gel networks, we observe that the strength of the reversible bonds can tune the material from solid to fluid. We identify at what fraction of reversible bonds this tunability is most pronounced, and find that the stress relaxation time of the gels in this tunable regime is set directly by the average lifetime of the reversible bonds. As our design is easy to realize in the already widely-used tetraPEG gel setting, our work will provide guidelines to improve the mechanical performance of biomedical gels.

Polymer Gel Electrolyte Prepared by “Salting-In” Poly(ethylene glycol) into a Phosphonium-Based Ionic Liquid with a Lithium Salt

We report the synthesis of an ion gel electrolyte based on a tetra-arm poly(ethylene glycol) (TetraPEG) network in a phosphonium-based ionic liquid (IL) solution utilizing the “salting-in” effect with a Li salt. Poly(ethylene glycol) (PEG) chains are insoluble in phosphonium-based ILs (such as triethylpentylphosphonium bis(trifluoromethanesulfonyl)amide, [P2225][TFSA]); however, the addition of LiTFSA enabled complete solvation of TetraPEG into a [P2225][TFSA] solution. The Li salt and polymer concentration dependences for PEG solubility in a [P2225][TFSA] system were investigated, and the optimum concentrations for polymer gel electrolyte preparation were elucidated. The gelation reaction (Michael addition between tetra-maleimide-terminated and tetra-thiol-terminated PEGs) of TetraPEG prepolymers in LiTFSA/[P2225][TFSA] solutions was characterized via chemical reaction kinetics to determine the apparent gelation rate constant (kgel′), and the reaction efficiency (p) was estimated to be p ∼ 95% at the end of the reaction. The resulting nearly defect-free TetraPEG gel electrolyte polymer network showed high ionic conductivity, comparable to the corresponding solution electrolyte. The TetraPEG gel electrolyte containing LiTFSA/[P2225][TFSA] also possessed a wide electrochemical window (∼4.2 V vs Li/Li+), thus allowing the reversible Li deposition/dissolution reaction to be performed using typical metal negative electrodes.

Characterization of N-phenylmaleimide-terminated poly(ethylene glycol)s and their application to a tetra-arm poly(ethylene glycol) gel.

Tetra-arm poly(ethylene glycol) (TetraPEG) gels are tough materials whose toughness originates from their uniform network structure. They can be formed by combining the termini of tetra-arm polymers via chemical reactions with high conversion efficiency, such as the Michael addition, condensations using an active ester group, and alkyne-azide cycloadditions. Herein, we report the synthesis of a tetra-PEG gel using a tetra-arm polymer with N-phenylmaleimide moieties at the polymer ends (tetra-N-aryl MA PEG) as a scaffold. Tetra-N-aryl MA PEG can be obtained via a simple maleimidation using the modification agent p-maleimidophenyl isocyanate (PMPI), which directly transforms the hydroxy groups at the polymer ends into reactive N-aryl maleimide groups in a one-pot reaction. The thus-obtained tetra-N-aryl MA PEG was fully characterized using high-performance liquid chromatography (HPLC), matrix-assisted laser desorption ionization time of flight mass spectrometry, and proton nuclear magnetic resonance spectroscopy. HPLC analysis not only demonstrated the high purity of tetra-N-aryl MA PEG and the full conversion of the hydroxy groups, but also provided an effective characterization method for N-aryl maleimide-based PEG using a simple protocol, which enables us quantitative analysis of functionalized polymers with different N-aryl maleimide numbers. Furthermore, we fabricated a TetraPEG gel via Michael addition of the obtained tetra-N-aryl MA and thiol-terminated TetraPEGs. Thus, this report presents the application of tetra-N-aryl MA PEG as an effective precursor to obtain a uniform network structure and a method for its characterization; these results should provide support for the development of functional molecules, soft materials, and further functional materials based on the uniform-network-structure concept.

Applications of Hydrogels with Special Physical Properties in Biomedicine.

As a polymer matrix containing a large amount of water, hydrogels have been widely used in many fields such as biology and medicine due to its similarity to extracellular matrix components, and its contact with blood, body fluids, and human tissue does not affect the metabolic processes of living organisms. However, due to the lack of unique physical properties of traditional polymer hydrogels, its further application in the high-end field is limited. With the progress of study, a series of hydrogels with special structures, such as double network hydrogel, composite hydrogel, Tetra-PEG gel, and topological gel, have improved the situation to a large extent. At the same time, the progress of research on the biocompatibility and biodegradability of hydrogels, which are expected to be used in biomedical fields, is also worthy of attention. This review introduces four such types of high-strength polymeric hydrogels and the mechanisms for improving their mechanical strength. Moreover, a discussion will be made around specific methods for imparting special physical properties to hydrogels and applications in the field of biomedicine such as cell culture, medical surgery, tissue engineering, and biosensing. At the end of the review, the main reasons and contradictions for the limits of the current applications are explained. An outlook on the future research in related fields and the importance of carrying out research in this area to promote medical progress are emphasized.

Cyclic Structures in Polymer Model Networks

Advances of the past ten years for synthesis, analysis, and modeling of polymer model networks are critically reviewed with a focus on control and analysis of cyclic structures and their impact on network properties. The hetero‐complementary coupling reaction of the tetra‐PEG gel architecture allows only for cyclic structures containing an even number of network strands, which eliminates the formation of several network defects including pending loops. Nuclear magnetic resonance (NMR) can be used to detect the amount of inactive chains and the smallest active cyclic structures of a model network. Network disassembly spectrometry (NDS) allows for a detection of pending loops and the smallest active cycles at high accuracy. The size distribution of cyclic structures in a polymer model network depends predominantly on the overlap number of strands and the effective junction functionality. The contribution of a network strand to modulus inside a model network may be approximated by the number of strands of the smallest cyclic structure where it is part of. These discoveries pave the way for future developments towards an improved prediction of the phantom modulus of real networks that consist entirely of finite cycles.

Diffusion Behavior of Water Molecules in Hydrogels with Controlled Network Structure

Diffusion behavior of particles in hydrogels is important both for the fundamental understanding of mass transport and for practical applications and has been investigated for a long time. There are three major theories describing the diffusion behavior of small particles in a polymer network: obstruction, hydrodynamic, and free volume theories. Although many researchers have examined these three theories, their applicability is still unclear due to ambiguity stemming from the heterogeneity of conventional hydrogels. Recently, we have developed a near-ideal hydrogel called Tetra-PEG gel that provides a unique possibility to correlate gel structure with properties. In this study, we measured the diffusion coefficient of water molecules (D) in Tetra-PEG gels by pulsed field gradient spin-echo 1H NMR. By comparing D and the correlation length of a polymer network (ξ) measured by small-angle neutron scattering, we observed an identity formula similar to the hydrodynamic theory (D/D0 = exp(−d/ξ), where D0 is t...

Quantitative Evaluation of Homogeneous Hydrogel Surface Wettability

A hydrogel is constructed of polymers that are crosslinked to form a three-dimensional network structure with water absorbed within. When using hydrogel as a biomaterial, wettability is one of the important physical properties as it largely affects how protein and cells can adhere on its surface. In previous studies aiming to evaluate the wettability of hydrogel, the affect was evaluated by changing the composition and side chain of the polymer chain constituting the network structure for existing hydrogels such as agarose and gelatin. However, the existing network structure of hydrogel has spatial heterogeneity and topological heterogeneity, and therefore it has been said to be difficult to independently determine which parameter of the network structure was truly affecting wettability. In this research, in order to solve this problem, we aimed to independently and quantitatively evaluate the effect of each parameter on wettability by using the Tetra-PEG gel.Tetra-PEG gel is a type of gel that is composed of two symmetrical tetra-branched structure of Tetra-amine-terminated PEG (Tetra-PEG-NH2) and Tetra-OSu-terminated PEG (Tetra-PEG-OSu). Gelation is done simply by mixing equal amounts of these polymer solutions. As our first attempt, we controlled the polymer volume fraction of the hydrogel and the molecular weight between the crosslinking points by adjusting the concentration of the polymer solution and the molecular weight of the polymer, respectively. To evaluate the effect of charged groups, p-tuned Tetra-PEG gel having ion pairs of positive and negative charged groups was prepared. On the other hand, r-tuned Tetra - PEG gel was prepared to evaluate the effect of the presence of ionic pairs on wettability. Positive or negative charged groups were introduced into the system by changing the mixing ratio of Tetra-PEG-NH2 and Tetra-PEG-Osu, and by so causing either polymer to excessively exist within the hydrogel.Wettability was evaluated through static contact angle measurement. 2 uL of water droplets were dropped onto the surface of the hydrogel, and the contact angle immediately after the dropping was measured. The contact angle measurement results for 5k, 10k, 20k, and 40k Tetra-PEG gel are shown in Fig. 1. For 5k Tetra - PEG gel, contact angle showed values less than 30 degrees, and contact angle showed a decrease with increase in volume fraction. However, contact angle for 5k polymer suddenlyd increase at volume fraction of 0.11.To give a logical and consistent explanation to this result, we introduced the idea of mobility of the polymer chains into the discussion of how the wettability of the hydrogel was determined. When the polymer molecular weight is large, the polymer chain is largely movable, so the PEG chain is bound in the bulk due to the intermolecular force with the water molecules in the gel . As a result, the surface exhibited a hydrophobic property. On the other hand, when the polymer molecular weight is small like 5k Tetra-PEG gel, it is considered that the surface was kept relatively hydrophilic because the polymer chain was bound near the surface and pulled the droplet inwards. Furthermore, in the region of low polymer molecular weight such as 5k Tetra-PEG gel, it is considered that if the volume fraction increases, the polymer chain becomes difficult to move and the hydrophilicity also increases. Fig. 2 shows the results of contact angle measurement when the charged group concentration was changed. At polymer volume fractions 0.066, 0.081, and 0.096, the contact angle decreased with an increase in the charged group concentration, but decrease in contact angle could not be confirmed at polymer volume fraction of 0.50. In addition, it was confirmed from Fig. 3 that the contact angle of r-tuned Tetra-PEG gel was lower than that of p-tuned Tetra-PEG gel.These results are thought to be because the charged group acted as a hydrophilic group for water molecules which are polar molecules. Wettability did not decrease with p-tuned Tetra-PEG gel with a volume fraction of 0.050, because there were too few charged groups (5 mol/m3) per volume and was not sufficient to visibly affect the surface characteristics. In addition, since the p-tuned Tetra-PEG gel in which the ion pair exists exhibits lower wettability than the r-tuned Tetra-PEG gel, the affinity between the polymer and water decreases due to the presence of the ion pair. As a conclusion, it was revealed that polymer molecular weight greatly affects the mobility of the polymer chain and the wettability, and the wettability is increased by the introduction of the charged group, and when the ion pair is present, wettability decreases. Figure 1

Thermodynamic Analysis of Polymer Gel Elasticity

Elastomer and polymer gel have a three-dimensional polymer network structure and the characteristic of being soft and well extended (rubber elasticity). Many researches on rubber elasticity have been carried out mainly with elastomer since 1950's. Entanglement of network chains dominates the physical properties because elastomer contains almost no solvent, thus it is difficult to investigate the correlation between the structure and the physical properties in detail. On the other hand, polymer gel is a dilute system coexisting with solvent, thus it is possible to design various network structures with controlled entanglement. Recently, we developed Tetra-PEG gel, which is formed by AB-type polycondensation of tetra-armed prepolymers. Tetra-PEG gel has a highly homogeneous network structure, which is suitable for verifying the theory of polymer gel. In the previous study, Phantom network model, which is a rubber elasticity model, was verified. Phantom network model assumes that the crosslinks fluctuate with deformation, and elastic modulus (G) is described as G = ξk B T Here, k B is the Boltzmann constant, T is absolute temperature, and ξ is the number densities of independent cycles surrounded by the mesh chain. In tree-like approximation, ξ can be calculated from polymer concentration, polymer molecular weight, and connection probability between prepolymers. This equation indicates that one cycle has the influence of the thermal energy k B T on the elasticity. As a result of the verification using Tetra-PEG gel, it was confirmed for the first time in the world that the connection probability dependence of the elastic modulus and the calculated ξ (ξ th) was the same. This indicates that Phantom network model can describe the elasticity of polymer gel. However, the polymer concentration (C) dependence of them was different as shown in Figure 2. Here, the polymer concentration was normalized by the overlapping concentration (C*). The polymer concentration is a concept inherent to polymer gel, and it has not been considered so much in conventional researches. Therefore, we returned to thermodynamics, which is the origin of rubber elasticity models, and experimentally verified it. The elastic modulus from Helmholtz free energy is described as the following equation. G = U” + S”T The first term (U”) is referred to as energy elasticity, and the second term (S”T) as entropy elasticity. From this equation, it is shown that elastic modulus can be separated into energy elasticity and entropy elasticity by measuring temperature dependence of elastic modulus. Classical rubber elasticity theory ignores the energy elasticity and predicts that the elastic modulus is proportional to temperature (G ~ T). It has been experimentally confirmed that the proportional relationship is roughly correct in the case of elastomer. On the other hand, the elastic modulus of Tetra-PEG gel was not proportional to temperature, and the negative energy elasticity whose value cannot be ignored was measured (Figure 1). This result may indicate an essential difference between elastomer and polymer gel. Therefore, we further investigated the energy elasticity and entropy elasticity using Tetra-PEG gel with various network structures. When only the connection probability was changed, energy elasticity was proportional to entropy elasticity (U”~ S”T). Since in previous study it was shown that the connection probability dependence of the elastic modulus and ξ was the same, we found that the dependence of the entropy elasticity and the energy elasticity was also the same as ξ. Furthermore, the entropy elasticity had the same polymer concentration dependence as ξ, which shows that the entropy elasticity follows Phantom network model except the value. Therefore, this result also shows the different concentration dependence of the elastic modulus and ξ is derived from the energy elasticity (Figure 2). In the poster presentation, I will talk about the cause of energy elasticity and the effect of solvent quality of polymer gel, too. Figure 1

Diffusion Behavior of Water Molecules in Hydrogels with Controlled Network Structure

Hydrogels have attracted attention as biomaterials because of the similar property to biological soft tissues. One of the applications of hydrogels for biomaterials is drug release carrier. For the application, it is important to understand and to control the diffusion behavior of molecules in hydrogels. There are three major models describing diffusion behavior of small molecules in the polymer solution. First is the Ogston model, in which polymer network is considered to behave as an obstacle preventing the diffusion of particles. Second is the hydrodynamic model, in which the existence of polymer increases the effective viscosity and decreases diffusion speed of particles. Third is the free volume model, in which particles diffuse passing through free volume of solvent and polymer. Although the validity of these models has been investigated by many researchers, there is no clear experimental evidence to support any models. Thus, even now, many models are left without validation. The muddling of models was assisted by the heterogeneity in conventional hydrogels. To understand the diffusion behavior of small molecules in a hydrogel, the validation of these models using a homogeneous hydrogel is necessary. Recently, we have developed a homogeneous hydrogel, Tetra-PEG gel, by combining two kinds of tetra-armed pre-polymers. It has been revealed that Tetra-PEG gel has an extremely homogeneous network structure, and network structure parameters (polymer volume fraction : φ 0, molecular weight between crosslinks : M w, binding ratio : p) of Tetra-PEG gel can be controlled independently. In this study, we investigated the effect of network structures on the self-diffusion coefficient of water molecules (D) in Tetra-PEG gels and in pure water (D 0) by Pulsed field-gradient spin-echo Nuclear Magnetic Resonance (PGSE-NMR). The values of D/D 0 decreased as an exponential function with an increase in φ 0 (0 - 0.15), and was independent of M w (10k, 20k g/mol) (Figure 1). This result well agreed with three models, and the difference among three models was negligible. On the other hand, D/D 0 decreased with a decrease in p (0 - 1) (Figure 1). This result was not reproduced by any three models, suggesting these models were not valid to describe diffusion behavior of small molecules in a hydrogel. Here, we tested another model proposed by M. Tokita et al. In the model, the diffusion of probe molecules is mainly determined by the correlation length of the polymer network (blob size : ξ) and the hydrodynamic radius of the probe molecules (R h) as, D/D 0 = exp(-R h/ξ). In Figure 2, D/D 0 in Tetra-PEG gels were plotted against 1/ξ. The values of xwere estimated by our previous small angle neutron scattering experiment. As shown in the figure, all the data including p-tune results fall onto a guide showing D/D 0 = 0.97exp(-2.8/ξ). The experimental values of the front coefficient (0.97) and R h (2.8Å) were close to the theoretical value (1.0) and the hydrodynamic radius of a water molecule (3.0Å), respectively. These results suggest that Tokita’s model is the correct model to describe the diffusion behavior of small molecules in a hydrogel. Figure 1

(Keynote) Preparation and Structural Investigation of Linear and 4-Arm Poly(ethylene glycol) (2 x 4 PEG) Gels

Gel is a 3D polymer network holding solvent molecules inside its network. There are two major heterogeneities in gels. One is spatial heterogeneity, and the other is topological heterogeneity. The spatial heterogeneity is introduced by random cross-linking, so the network structure has both sparse and dense region. The spatial heterogeneity is observed as excess scattering in a light, X-ray, or neutron scattering experiment. On the other hand, the topological heterogeneity is related to the presence of dangling chains, loop structure, and entanglements inside the network, and is investigated by viscoelasticity and/or swelling experiments, or by other means, such as NMR. Because of these inhomogeneities, gels have seldom been applied to structural materials because of their poor mechanical properties. Hence, realization of ideal model networks free from heterogeneities has been strongly anticipated. Sakai et al. succeeded in fabrication of near ideal polymer networks called “Tetra-PEG gel”.[1] A Tetra-PEG gel is synthesized from two complementary end-functionalized poly(ethylene glycol) (PEG) which is able to react each other. It is pointed out that the Tetra-PEG gel has insignificant small spatial homogeneity and negligible topological heterogeneity, respectively by small angle scattering[2] and mechanical test.[3] Recently, we extended the above-mentioned method and succeeded in fabrication of multiple model network gels consisting of alternating tetra-functional poly(ethylene glycol)s (PEGs) and bis-functional linear PEGs (2 x 4 gels), where the molecular weight of linear PEG was varied from 200 Da to 20000 Da while that of tetra-functional PEG component was fixed to be 20000 Da.[4] The same level of homogeneities in the network structure as those of Tetra-PEG gels has been confirmed by dynamic light scattering (DLS) and dynamic viscoelastic measurements. Advantages of 2 x 4 gels are that one can independently tune the cross-link density (by the concentration of tetra-functional PEG) and the total polymer concentration. By using a series of 2 x 4 gels, we examined the concentration/cross-link density dependence of the correlation blob, the elastic blob, and geometric blob. We will discuss the scaling rules of these blob sizes as a function of polymer/cross-linker concentrations. Sakai, T. et al, Macromolecules 2008, 41, 5379.Matsunaga, T. et al, Macromolecules 2009, 42, 1344.Akagi, Y. et al, Macromolecules 2010, 43, 488.Tsuji, Y. et al., Gels 2018, 4, 50.

Similarity between Gel and Semi-Dilute Solution

A polymer gel in the air is in equilibrium state, as a closed system. However, when the gel is immersed in a solvent, the volume changes automatically toward the new equilibrium as an open system. The equilibrium is determined as the balance of elastic pressure and osmotic pressure according to Flory-Rehner theorem. Conventional polymer gels swell in good solvent because osmotic pressure is larger than elastic pressure. The swelling brings some drawbacks in actual applications of polymer gels. For example, swelling of hydrogels in vivo compress the surrounding tissues, and causes severe adverse reactions. Thus, understanding and control over swelling is an urgent issue in material science. On the other hand, gel swelling has been investing in the field of polymer physics, especially focusing on the equilibrium-swollen condition. Based on the scaling rules on elastic pressure (Πel) and osmotic pressure (Πos), both pressures balance in the equilibrium-swollen state as, Πel ≈ Πos ~ C 3ν/3ν-1. Here, ν is a parameter indicating the interaction between polymer and solvent. Previous researches have shown that the scaling relationship is consistent with experiments. Based on the consistency, some papers concluded the validity of C* theorem which predicted that a polymer gel maintains a concentration proportional to the overlapping concentration (C*) of polymer solution of the corresponding network strand of the gel. However, these agreements only suggest thatΠos of polymer gel is the same form with that of semi-dilute solution, as pointed out by Urayama. Polymer gels swell toward equilibrium keeping semi-diluted, even if the concentration of the polymer gels is below C* of the network strands. Once polymer gels are fabricated, the networks of the polymer gels behave as polymers in the semi-dilute solution. In the case of gels fabricated from prepolymers such as tetra-PEG gel, sol-gel transition can occur even if the initial polymer concentration (C 0) is far below C*. As for conventional gelling system from small molecular monomer and crosslinker, the initial condition is inherently below C*. Then, whether or not polymer gels are “universally” semi-diluted is an inevitable question. If any polymer gels are semi-diluted system, it is important to understand how the gel from diluted prepolymer solution reaches semi-diluted system during gelation. Despite of the importance, these issues have never been investigated. This situation is in good contrast with viscoelastic change during gelation, which is fully understood based on Winter-Chambon’s criterion. In this research, we measured the Πos of gelling system from diluted prepolymer solutions. Gelation process was reproduced statically using the tetra-PEG system that can control the reaction conversion by controlling mixing ratio of AB-type prepolymer solutions, and Πos was measured sequentially. As a result, we discovered that Πos of critical clusters, which are the polymeric clusters just before the sol-gel transition, determined that of corresponding gels. In addition, the molecular weights of critical clusters depend on C 0 when gelling from the diluted systems which simply suggested that sol-gel transition requires more growth of polymeric clusters at lower C 0. By integrating the result of previous study about fractal dimension of the critical clusters, similarity between gelation thresholds and the C* condition was derived. Based on this suggestion, the picture of gelation from the diluted solution is as follows. Given that the system is initially diluted, overlapping of prepolymers is not preferred at beginning (FIG. a). As the branching reaction proceeds (FIG. b), polymeric clusters grow, and C* of the system decreases. As a result, although C 0 is constant, the system approaches the overlapping condition. In this process, Πos decreases due to the decrease in translational entropy of clusters. The system reaches the overlapping condition, just before sol-gel transition (FIG. c). At the sol-gel transition point, viscosity and stiffness show drastic changes, because they strongly relate to connectivity. In contrast, Πos does not show any apparent changes most likely due to the concentration fluctuation at their overlapping condition is not strongly influenced by further crosslinking. As a result, Πos of gels corresponds to the osmotic pressure just before sol-gel transition point. Figure 1

Electrochemical Properties of a TetraPEG-based Gel Electrolyte Containing a Nonflammable Fluorinated Alkyl Phosphate for Safer Lithium-ion Batteries

We report a nonflammable gel electrolyte based on two key soft materials: 1) a homogeneous polymer network prepared from tetra-armed poly(ethylene glycol) (TetraPEG) and 2) a tris(2,2,2-trifluoroethyl)phosphate (TFEP)-based electrolyte solution. At the equilibrium swelling state, the concentration of the polymer in the TetraPEG gel was relatively low, at only 5.6 wt.%. The TetraPEG gel exhibits a wide electrochemical window of ∼5.0 V and thus works well as an electrolyte for a positive electrode, i.e., a lithium iron phosphate (LiFePO4).

Decisive test of the ideal behavior of tetra-PEG gels.

The objective of this work is to investigate the thermodynamic and scattering behavior of tetra-poly(ethylene glycol) (PEG) gels. Complementary measurements, including osmotic swelling pressure, elastic modulus, and small angle neutron scattering (SANS), are reported for a series of tetra-PEG gels made from different molecular weight precursor chains at different concentrations. Analysis of the osmotic swelling pressure vs polymer volume fraction curves makes it possible to separate the elastic and mixing contributions of the network free energy. It is shown that in tetra-PEG gels these free energy components are additive. The elastic term varies with the one-third power of the polymer volume fraction and its numerical value is equal to the shear modulus obtained from independent mechanical measurements. The mixing pressure of the cross-linked polymer is slightly smaller than that of the corresponding solution of the uncross-linked polymer of infinite molecular weight but it exhibits similar dependence on the polymer concentration. The observed deviation between the osmotic mixing pressures of the gel and the solution can be attributed to the presence of small amount of structural inhomogeneities frozen-in by the cross-links. SANS reveals that the scattering response of tetra-PEG gel is mainly governed by the thermodynamic concentration fluctuations of the network, i.e., the contribution from static inhomogeneities to the SANS signal is small.

Preparation of a High-Strength Hydrogel with Slidable and Tunable Potential Functionalization Sites

A hydrogel with tunable potential functionalization sites has been successfully prepared. As potential functionalization sites, (2-hydroxypropyl)-α-CDs (Hy-α-CDs) were introduced into the network of tetrahedron-like poly(ethylene glycol) (tetra-PEG) gel through supramolecular chemistry. In the stage of complexation, poly-pseudo-rotaxane consisting of tetra-PEG macromonomer and Hy-α-CD formed in pregel solution. The dynamic complexation process and the structure of the poly-pseudo-rotaxane were investigated by NMR experiment. In the stage of gelation, some cross-linking reactions were completed through click chemistry. The structures and mechanical properties of the resultant hydrogels were characterized by ATR-FTIR, XPS, SEM, and compression test in detail. The number of Hy-α-CD introduced into the hydrogel is related closely to the structure of the poly-pseudo-rotaxane and can be controlled easily by tuning the feed ratio. Anthracene as an example of function was introduced into the hydrogel through Hy-α...

Development of a polymer-based easy-to-fabricate micro-free-flow electrophoresis device

Since 1990s, micro-free-flow electrophoresis (µFFE) devices have been developed to allow for smaller sample volume and reagent consumption. To solve several technical problems involving the generation of electrolysis gas on the electrodes, most of the µFFE devices reported in the past were fabricated using elaborate micromachining process on silicon or glass substrates. However, high-cost micromachining processes were required and these were not suitable for mass production. In this paper, we report a polymer-based easy-to-fabricate µFFE device using a poly(methyl methacrylate-co-styrene), P(MMA-co-S), substrate and tetra-PEG gel for preventing the invasion of electrolysis gas into the separation chamber. In the separation experiment using a mixture of rhodamine B and sulforhodamine B, the resolution increased linearly with the increase of the applied voltages up to 50 V, whereas a deviation from the linear relation was observed above 50 V, which is possibly the Joule heating. These results indicate that this device could be applicable to separation of biological samples.

High-performance gel electrolytes with tetra-armed polymer network for Li ion batteries

An organo gel with only 6 wt % tetra-armed poly(ethylene glycol), TetraPEG, was prepared and applied as a novel gel electrolyte for Li ion batteries (LIBs). The TetraPEG gel electrolyte containing 1.0 M LiPF6 in binary or ternary mixtures, i.e., EC + DEC and EC + DEC + TFEP (EC: ethylene carbonate, DEC: diethyl carbonate and TFEP: tris(2,2,2-trifluoroethyl)phosphate showed high ionic conductivity required for the use in LIB systems. The TetraPEG gel based on ternary EC + DEC + TFEP system acts as a nonflammable gel electrolyte at the TFEP content higher than 20 vol%. In cyclic voltammetry and charge/discharge cycling tests, the TetraPEG gel electrolytes showed good reversibility for a graphite negative electrode.

Experimental verification of homogeneity in polymer gels

Tetra-PEG gel has highly suppressed heterogeneity, which is considered inherent to conventional polymer gels. In this review, we show a series of experimental results on Tetra-PEG gels, and discuss the homogeneity. Tetra-PEG gel is useful as a model system for examining the model predicting physical properties of polymer gels.