Topological Gel Research Articles

Role of terminal beads in fracture of topological gels: A coarse-grained molecular dynamics study

Topological gels feature movable cross-linking sites which yield great deformation capability, but we show that their long-term performance would be undesirable. Because a long backbone chain has only 2 terminal beads to restrict sliding of rings, the entire chain would fail by just a small debonding. Chain segmentation by adding terminal beads is a promising strategy to improve durability. In this work, we develop a coarse-grained (CG) model of topological gels based on MARTINI force field, to simulate tensile behaviors of the gel. The developed CG model accurately predicts elastic modulus of typical topological gels with different crosslinking densities. Based on the model, we show that adding terminal beads to the chain can effectively improve mechanical properties of topological gels after initial debonding. With analyses on distribution of ring molecules along the chain and effective load-bearing chains during tensile deformation, mechanism of the improvement on durability is discussed.

Inside Back Cover: Synthesis of Topological Gels by Penetrating Polymerization Using a Molecular Net

Inside Back Cover: Synthesis of Topological Gels by Penetrating Polymerization Using a Molecular Net

Inside Back Cover: Synthesis of Topological Gels by Penetrating Polymerization Using a Molecular Net

Inside Back Cover: Synthesis of Topological Gels by Penetrating Polymerization Using a Molecular Net

Synthesis of Topological Gels by Penetrating Polymerization Using a Molecular Net

AbstractTopological gels possess structures that are cross‐linked only via physical constraints; ideally, no attractive intermolecular interactions act between their components, which yields interesting physical properties. However, most reported previous topological gels were synthesized based on supramolecular interlocked structures such as polyrotaxane, for which attractive intermolecular interactions are essential. Here, we synthesize a water‐soluble “molecular net” (MN) with a large molecular weight and three‐dimensional network structure using poly(ethylene glycol). When a water‐soluble monomer (N‐isopropylacrylamide) is polymerized in the presence of the MNs, the extending polymer chains penetrates the MNs to form an ideal topological MN gel with no specific attractive interactions between its components. The MN gels show unique physical properties as well a significantly high degree of swelling and high extensibility due to slipping of the physical cross‐linking. We postulate this method to yield a new paradigm in gel science with unprecedented physical properties.

Rotaxa-polymeric-gelation of acrylamides with vinyl-β-cyclodextrin

Previously, we have reported a single-step preparation method for a topological gel by copolymerizing isoprene and vinyl-modified β-cyclodextrin (V-β-CyD). In the prepared gel, the vinyl part of V-β-CyD consists a part of the copolymer chain, and the β-CyD part includes another copolymer chain, thus V-β-CyD works as a cross-linker. We call the procedure rotaxa-polymeric-gelation (RPG) because polymerization, rotaxane formation, and gelation proceed simultaneously. In this study, we applied acrylamide (AAm) instead of isoprene for RPG, and successfully obtained highly swellable hydrogel. By contrast, a combination of V-β-CyD and N-isopropylacrylamide (NIPAAm) was not suitable for RPG because NIPAAm and cavity of β-CyD were mismatched in size. In addition, we prepared three-component gels. Among of them, gels composed of NIPAAm, isoprene, and V-β-CyD showed thermo-responsiveness by reflecting the character of polyNIPAAm. These results provide an insight for usage of vinyl monomers for RPG strategy.

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.

Functionalization of Cyclodextrin Derivatives to Create Supramolecular Pharmaceutical Materials

Molecular recognition is useful in creating functional supramolecular materials. Non-covalent bond formations, such as host-guest interactions, hydrogen bonding, and electrostatic interaction, are effective tools for introducing various functions and properties into materials. This review focuses on such macroscopic functions as selective molecular adhesion, self-healing, toughness, and the actuation of supramolecular polymeric materials-materials which have potential in pharmaceutical development. These functions have been achieved using reversible bonding between cyclodextrins (CDs; cyclic host molecules) and guest molecules. For example, macroscopic adhesions between host-modified hydrogels and guest-modified hydrogels have been investigated. CD-modified hydrogels were found to show selective adhesion to a guest hydrogel with an appropriate molecular size for the CD cavity, indicating that the host-guest complex formation between the gels led to the adhesive behavior. Surprisingly, polymeric materials having host-guest cross-linking points show both high toughness and flexibility, unlike conventional covalently cross-linked materials. These materials also exhibited self-healing properties, capable of repairing damage to the materials. Furthermore, the supramolecular materials demonstrated macroscopic rapid expansion and contraction driven by external stimuli under wet or semi-dry conditions, in which the supramolecular gels vary the cross-linking density between the polymers accordingly. Different topological gels are able to vary the length of the polymer chain between cross-linking points to show large deformation. Both types of actuators were found to exhibit externally stimulated flexing behaviors. This review summarizes recent advancements in the development of these supramolecular materials, which appear to be promising new components in pharmaceutical science.

Single-step preparation of topological gels using vinyl-modified β-cyclodextrin as a figure-of-six cross-linker

Herein, we have proposed a single-step preparation of topological gels using vinyl-modified β-cyclodextrin (V-β-CyD) and isoprene. Copolymerization of V-β-CyD and isoprene in an aqueous solution resulted in gelation due to V-β-CyD acting as a novel type of copolymer chain cross-linker. The vinyl moiety of V-β-CyD becomes a part of the copolymer, while the β-CyD moiety of V-β-CyD simultaneously incorporates the isoprene component of the copolymer. V-β-CyD is capable of two different modes of cross-linking at each end, i.e., chemically bonding and mechanically interlocking. Due to the shape of the cross-linking point, we refer to it as figure-of-six cross-linking. Nuclear magnetic resonance analysis showed that the gel contained V-β-CyD and isoprene in an approximately 1:0.3 stoichiometry. The relatively high content of β-CyD was reflected in the character of the gel; the gel swelled in dimethylformamide which is a good solvent of β-CyD. A fluorometric analysis using 6-(p-toluidino)-2-naphthalenesulfonic acid showed that the appended β-CyD was able to accommodate guest molecules. Introduction of an additional vinyl monomer into the gel was also successful. Addition of 4-vinylphenylboronic acid to the preparation procedure yielded a sugar-responsive gel that swelled in the presence of d-fructose.

Preparation of a photo- and thermo-responsive topological gel from anthracene-modified polyrotaxanes.

A topological gel was formed from anthracene-modified polyrotaxanes (An-PRs) under UV irradiation, and the gel can turn back to a sol under thermal treatment due to the dimerization between the anthracene units and the dissociation of the formed dimer.

Stimuli-responsive polymeric materials functioning via host–guest interactions

Molecular recognition is essential for realizing functional supramolecular materials. Non-covalent host–guest interactions are an effective tool to introduce switching and functional properties into materials. This review focuses on the achievement of selective molecular adhesion, self-healing, toughness, and actuation properties. These functions have been achieved by reversible bond formation with cyclodextrins (CDs). Self-healing materials with host–guest interactions involving CDs have been used to achieve redox-responsive healing properties and healing efficiency. Furthermore, the materials, which undergo self-healing by chemical and physical mechanisms, exhibit rapid and efficient self-healing properties under semi-dry conditions. To prepare a supramolecular actuator using host–guest complexes, two approaches have been introduced. The first is the functionalization of a supramolecular gel actuator by changing the cross-linking density, and the second is the functionalization of a topological gel actuator by changing distances between the cross-linking points. Both actuators exhibit contractive bending behavior. This review summarizes advancements within the past 10 years in supramolecular materials that function via the chemical mechanism of host–guest interactions and the physical mechanism of the sliding motion of ring molecules.

Functioning via host–guest interactions

Molecular recognition is essential for realizing functional supramolecular materials. Non-covalent host–guest interactions are an effective tool to introduce switching and functional properties into materials. This review focuses on the achievement of selective molecular adhesion, self-healing, toughness, and actuation properties. These functions have been achieved by reversible bond formation with cyclodextrins (CDs). Self-healing materials with host–guest interactions involving CDs have been used to achieve redox-responsive healing properties and healing efficiency. Furthermore, the materials, which undergo self-healing by chemical and physical mechanisms, exhibit rapid and efficient self-healing properties under semi-dry conditions. To prepare a supramolecular actuator using host–guest complexes, two approaches have been introduced. The first is the functionalization of a supramolecular gel actuator by changing the cross-linking density, and the second is the functionalization of a topological gel actuator by changing distances between the cross-linking points. Both actuators exhibit contractive bending behavior. This review summarizes advancements within the past ten years in supramolecular materials that function via the chemical mechanism of host–guest interactions and the physical mechanism of the sliding motion of ring molecules.

From topological gels to slide‐ring materials

ABSTRACTA novel type of gel, called a topological gel, has been recently developed with a supramolecular architecture with topological characteristics. In the topological gel, polymer chains with bulky end groups are neither covalently crosslinked as in chemical gels nor attractively interacting as in physical gels but are topologically interlocked by figure‐eight shaped crosslinks. Hence, these crosslinks can pass along the polymer chains freely to equalize the tension of the threading polymer chains similarly to pulleys; this is called the pulley effect. This concept can be applied not only to gels but also to a wide variety of polymeric materials without solvents. Then, polymeric materials with movable crosslinks are referred to as slide‐ring materials (SRMs) in a wider sense. Here, we review the synthesis, structure, physical properties, and applications of topological gels and SRMs. In particular, slide‐ring elastomers show remarkable scratch‐proof properties for application to coating materials for automobiles, cell phones, mobile computers, golf clubs, and so on. © 2014 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40509.

Pillar[5]arene-based nonionic polyrotaxanes and a topological gel prepared from cyclic host liquids.

We synthesize nonionic polymer-based polyrotaxanes in solvent systems based on pillar[5]arene cyclic host liquids. Using a reactive liquid, pillar[5]arene enabled synthesis of a nonionic topological gel.

Synthesis of polyrotaxanes from acetyl-β-cyclodextrin

Polyrotaxanes are intermediary products in the synthesis of topological gels. They are created by inclusion complex formation of hydrophobic linear macromolecules with cyclodextrins or their derivatives. Then, pairs of cyclodextrin molecules with covalently linkage were practically forming the nodes of the semi-flexible polymer network. Such gels are called topological gels and they can absorb huge quantities of water due to the net flexibility allowing the poly(ethylene oxide) chains to slide through the cyclodextrin cavities, without being pulled out altogether. For polyrotaxane formation poly(ethylene oxide) was used like linear macromolecules. There are hydroxyl groups at poly(ethylene oxide) chains, whereby the linking of the voluminous molecules should be made. To avoid the reaction of cyclodextrin OH groups with stoppers, they should be protected by, e.g., acetylation. In this work, the acetylation of the OH groups of β-cyclodextrin was performed by acetic acid anhydride with iodine as the catalyst. The acetylation reaction was assessed by the FTIR and HPLC method. By the HPLC analysis was found that the acetylation was completed in 20 minutes. Inserting of poly(ethylene oxide) with 4000 g/mol molecule mass into acetyl-β-cyclodextrin with 2:1 poly(ethylene oxide) monomer unit to acetyl-β-cyclodextrin ratio was also monitored by FTIR, and it was found that the process was completed in 12 h at the temperature of 10°C. If the process is performed at temperatures above 10°C, or for periods longer than 12 hours, the process of uncontrolled hydrolysis of acetate groups was initiated.

Slide-ring materials using topological supramolecular architecture

We have recently developed a novel type of gel called slide-ring gel or topological gel by using the supramolecular architecture with topological characteristics. In this gel, polymer chains with bulky end groups exhibit neither covalently cross-links as in chemical gels nor attractive interactions as in physical gels but are topologically interlocked by figure-of-eight cross-links. Hence, these cross-links can pass along the polymer chains freely to equalize the tension of the threading polymer chains similarly to pulleys; this is called pulley effect. The slide-ring gel is a new cross-linking concept for the polymer network as well as a real example of a slip-link model or sliding gel, which was previously considered only theoretically. Here we review the synthesis, structure, and mechanical properties of the slide-ring gels with freely movable cross-linking junctions based primarily on our recent studies. The pulley effect of the slide-ring gels has been recently confirmed by mechanical measurements, small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), quasi-elastic light scattering (QELS), etc. This concept can be applied to not only gels but also to a wide variety of polymeric materials without solvents. In particular, the slide-ring elastomer shows remarkable scratch-proof properties to be applied to coating materials for automobile, cell phone, mobile computer, fishing rod, golf club and so on.

Photoresponsive Slide‐Ring Gel

Slide-ring topological gels with mobile cross-linking units are a new class of soft materials that exhibit distinct features differing from those of conventional chemically or physically cross-linked gels. Photoresponsive behavior arising from the dynamic nature of the cross-linkers is demonstrated for a slide-ring gel prepared by adding azobenzene units to the mobile α-cyclodextrin units of a poly(ethylene oxide)-based polyrotaxane (see figure).

Novel Cross-Linking Concept of Polymer Network: Synthesis, Structure, and Properties of Slide-Ring Gels with Freely Movable Junctions

We have recently developed a novel type of gel called the slide-ring gel or topological gel that is different from physical and chemical gels by using the supramolecular architecture with topological characteristics. In this gel, polymer chains with bulky end groups exhibit neither covalently cross-links as in chemical gels nor attractive interactions as in physical gels but are topologically interlocked by figure-of-eight cross-links. Hence, these cross-links can pass along the polymer chains freely to equalize the tension of the threading polymer chains similarly to pulleys; this is called the pulley effect. The slide-ring gel is a new cross-linking concept for the polymer network as well as a real example of a slip-link model or sliding gel, which was previously considered only theoretically. In this study, we review the synthesis, structure, and mechanical properties of the slide-ring gels with freely movable cross-linking junctions based primarily on our recent studies. The pulley effect of the slide-ring gels has been recently confirmed by mechanical measurements, small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), quasi-elastic light scattering (QELS), etc. This concept can be applied to not only gels but also to a wide variety of polymeric materials without solvents.

Gelation as Studied by Proton Multiple-Quantum NMR

A novel NMR approach for the study of gelation in flexible polymer systems is presented, and the data are compared to results from rheological and light scattering experiments. Proton multiple-quantum NMR, also applicable at low field and amenable to high-throughput experimentation, represents a simple and robust yet powerful method that allows one to detect the topological gel point, quantify the gelation kinetics, and characterize composition, microstructure, and dynamics of the forming and the final gel. Applications to end-linked and statistically cross-linked poly(dimethylsiloxane) chains in the bulk, as well as statistically cross-linked poly- (styrene) chains in solution are presented, and good agreement as well as complementarity between the results from the different methods is found.

Slide-ring gel: Topological gel with freely movable cross-links

The structure of “slide-ring (SR) gel” was investigated by means of small angle neutron scattering (SANS). The SR gel was synthesized by coupling of α -cyclodextrin molecules on polyrotaxane chains. A cross-linking point of SR gel has a shape of “figure-of-eight”. Two-dimensional SANS patterns for uniaxially stretched SR gels showed a “normal” butterfly pattern. This result indicates that the cross-links slide along the polymer chain so as to minimize the local strain as a “pulley”. The pulley effect was found to be significant to reduce spatial inhomogeneities inherent in polymer gels.

Sliding mode of cyclodextrin in polyrotaxane and slide-ring gel

The sliding modes of cyclic molecules in polyrotaxane and of cross-linkingjunctions in topological gel or slide-ring gel were investigated by quasi-elasticlight scattering. We found that a polyrotaxane sparsely includingα-cyclodextrin showed the sliding mode in solution other than the self- and cooperative-diffusionmodes whereas the sliding mode was not observed in a polyrotaxane densely includingα-cyclodextrin. After gelation of the sparse polyrotaxane, the self-diffusion mode of thepolyrotaxane disappeared but the sliding mode was still observed. This indicated that thefigure-of-eight cross-links in the slide-ring gel slid in the polymer network, passing throughthe polymer chains.