Movable Cross-linking 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.

Mechanical properties of composite materials with low-covered polyrotaxane and plasma-surface-modified hexagonal boron nitride

An effective method for fabricating flexible materials containing thermally conductive fillers, such as hexagonal boron nitride (hBN), utilizes plasma surface modification and polyrotaxane (PR) with movable cross-linking points. This study demonstrated that lowering the coverage rates of PR composites increased the mobility of the cross-linking points. Plasma surface modification improved the dispersion of hBN and enabled homogeneous deformation of the PR composites owing to movable cross-linking points. Studying the deformation with a model including a finite extension effect indicated that the sliding range in the low-covered PR composites was extended compared to that in the high-covered PR composites, resulting in a smaller Young’s modulus, longer extension, and higher toughness.

Preparation of dual-cross network polymers by the knitting method and evaluation of their mechanical properties

Bulk copolymerization of alkyl acrylates and cyclodextrin (CD) host monomers produced a single movable cross-network (SC). The CD units acted as movable crosslinking points in the obtained SC elastomer. Introducing movable crosslinks into a poly(ethyl acrylate/butyl acrylate) copolymer resulted in good toughness (Gf) and stress dispersion. Here, to improve the Young’s modulus (E) and Gf of movable cross-network elastomers, the bulk copolymerization of liquid alkyl acrylate monomer swelling in SC gave another type of movable cross-network elastomer with penetrating polymers (SCPs). Moreover, the bulk copolymerization of alkyl acrylate and the CD monomer in the presence of SC resulted in dual cross-network (DC) elastomers. The Gf of the DC elastomer with a suitable weight % (wt%) of the secondary movable cross-network polymer was higher than those of the SCP or SC elastomers. The combination of suitable hydrophobicity and glass transition of the secondary network was important for improving Gf. Small-angle X-ray scattering (SAXS) indicated that the DC elastomers exhibited heterogeneity at the nanoscale. The DC elastomers showed a significantly broader relaxation time distribution than the SC and SCP elastomers. Thus, the nanoscale heterogeneity and broader relaxation time distribution were important to increase Gf. This method to fabricate SCP and DC elastomers with penetrating polymers would be applicable to improve the Gf of conventional polymeric materials.

Synergetic improvement in the mechanical properties of polyurethanes with movable crosslinking and hydrogen bonds.

Polyurethane (PU) materials with movable crosslinking were prepared by a typical two-step synthetic process using an acetylated γ-cyclodextrin (TAcγCD) diol compound. The soft segment of PU is polytetrahydrofuran (PTHF), and the hard segment consists of hexamethylene diisocyanate (HDI) and 1,3-propylene glycol (POD). The synthesized PU materials exhibited the typical mechanical characteristics of a movable crosslinking network, and the presence of hydrogen bonds from the urethane bonds resulted in a synergistic effect. Two kinds of noncovalent bond crosslinking increased the Young's modulus of the material without affecting its toughness. Fourier transform infrared spectroscopy and X-ray scattering measurements were performed to analyze the effect of introducing movable crosslinking on the internal hydrogen bond and the microphase separation structure of PU, and the results showed that the carbonyl groups on TAcγCD could form hydrogen bonds with the PU chains and that the introduction of movable crosslinking weakened the hydrogen bonds between the hard segments of PU. When stretched, the movable crosslinking of the PU materials suppressed the orientation of polymer chains (shish-kebab orientation) in the tensile direction. The mechanical properties of the movable crosslinked PU materials show promise for future application in the industrial field.

Effect of movable crosslinking points on mechanical properties in composite materials of large amount of plasma-surface-modified boron nitride and slide-ring elastomer

The slide-ring (SR) effect in composite materials is demonstrated by comparing the mechanical properties of plasma-modified boron nitride (p-BN)/SR elastomers and p-BN/fixed crosslinked (FC) elastomer composite materials that include a large amount of plasma-surface-modified BN (∼56 vol%). The toughness of p-BN/SR was ∼7 times that of p-BN/FC. Such SR effect was not observed in the raw BN/SR composite material but only in the p-BN/SR composite material. Time-domain nuclear magnetic resonance measurements suggested that the plasma-surface modification of BN formed crosslinking points to the SR and suppressed the decrease in the molecular mobility of SR in the composite materials.

Rotaxane-Based Difunctional Nitrile N-Oxide Crosslinker: Synthesis and Direct Introduction of Movable Crosslinking Points into Ethylene-Propylene-Butadiene Monomer (EPDM) Rubber.

Incorporation of rotaxane scaffolds into the crosslinking points of polymer networks significantly affects their rheological and mechanical properties. The present study involves the synthesis of a new rotaxane-type crosslinker containing nitrile N-oxide functional groups on both the axle and wheel components. The prepared crosslinker is highly reactive; however, it can be isolated and applied in the crosslinking reaction of a commercially important polymer, namely ethylene-propylene-butadiene monomer rubber (EPDM), in the absence of additives and catalysts. Tensile tests reveal that compared to a network containing conventional crosslinking points, both breaking strength and strain of the network structure prepared herein are improved due to the incorporation of movable crosslinking points. The synthesized network structure also exhibits five times higher fracture energy. The developed post-crosslinking methodology for the direct introduction of movable crosslinking points into pre-formed polymers will be valuable in the production of rotaxane materials for various applications.

Radical Cyclic [3]Daisy Chains

Summary Mechanically interlocked molecules (MIMs) that undergo controllable internal motions of their component parts in more than one dimension are rare entities in the molecular world. Cyclic [2]daisy chains ([c2]DCs) are a class of MIMs that have been identified as prototypes for molecular muscles. It remains, however, a challenge to synthesize [cn]DCs with n > 2 selectively and efficiently. Herein, we report the design and synthesis of [c3]DCs employing radical and anionic templates. Two mechanically interlocked [c3]DCs with 18 positive charges were obtained in >90% yields. One [c3]DC displayed good air stability in its radical cationic form, while the other underwent reversible “co-conformational” switching between open macrocyclic and closed trisarm-shaped forms under electrochemical control. These findings provide not only two-dimensional MIMs with attractive electronic and switchable properties, but also a starting point for the design of extended molecular arrays, which could become the forerunners of adjustable molecular nets and breathable molecular membranes.

Preparation of hydrophilic polymeric materials with movable cross-linkers and their mechanical property

Hydrophilic polymeric materials with movable cross-linkers were obtained by the bulk copolymerization of a hydrophilic cyclodextrin monomer (γCDAAmMe) and liquid acrylamide monomers. A 2D 1H-NOESY NMR study revealed that the main chains penetrated the CD ring units, which acted as movable cross-linking points. Copolymerization of γCDAAmMe with N,N-dimethyl acrylamide (DMAA) as a small main chain monomer results in hydrophilic movable cross-linked materials (pDMAA-γCDAAmMe (x)). On the other hand, using 4-acryloylmorpholine (ACMO) or N,N-dimethylaminopropyl acrylamide (DMAPAA) as a bulky main chain monomer disfavours the formation of movable cross-links due to steric hindrance between γCDAAmMe and the side chains. In the water content range of approximately 40–80 wt%, the Young's moduli of pDMAA-γCDAAmMe (1)s are similar to those of pDMAA-MBAAm (1). However, the fracture energy of movable cross-linked materials is higher than that of than materials covalently cross-linked with N,N′–methylenebis(acrylamide) (MBAAm). An addition of water enhanced the mobility of the main chain polymers, allowing the movable cross-linking points.

Visualization of the slide-ring effect: a study on movable cross-linking points using mechanochromism.

To evaluate the 'slide-ring' effect in a rotaxane cross-linked network, we incorporated mechanochromophores into static and rotaxane cross-linking points and compared the mechanochromisms exhibited by the obtained polymers. This novel strategy reveals a molecular-level phenomenon with visible color changes as well as enabling quantitative electron paramagnetic resonance measurements.

Physical and Adhesion Properties of Supramolecular Hydrogels Cross-linked by Movable Cross-linking Molecule and Host-guest Interactions

A supramolecular hydrogel was prepared by the introduction of host-guest interactions between polyrotaxane (PRx) having guest parts and poly(acrylamide) with cyclodextrin hosts. Although the chemic...

(Invited) Sliding Dynamics and Mechanical Properties of Slide-Ring Gels

To overcome the trade-off relationship between stiffness and toughness of conventional polymer gels with covalent cross-links, introducing dynamical cross-links into polymer network is a promising strategy. A typical example of dynamically cross-linked gels is slide-ring gel, in which polymer chains are cross-linked by rings and the cross-linking points can slide along polymer chains. When slide-ring gels are deformed, the network structure of slide-ring gels is homogenized due to the sliding of the ring molecules, which causes their high deformability [1]. We have recently performed crack propagation tests on slide-ring gels and found that their fracture energy is dominated by the sliding distance of the movable cross-links [2]. In this work, we investigate the relationship between the macroscopic fracture toughness of slide-ring gels and nano-scale sliding dynamics of rings on polymer chains. Slide-ring gels are prepared by cross-linking ring molecules of polyrotaxanes, necklace-like supramolecules consisting of cyclodextrins and polyethylene glycol. The fracture energy of slide-ring gels keeps constant at strain rates below a certain value, and decreases at a high strain rate, where the movable cross-linking points cannot slide and behave like covalent cross-links. From the strain rate dependence of the fracture energy, we have estimated the time scale for the sliding dynamics of cross-links in slide-ring gels.

Slide-ring shape memory polymers with movable cross-links

The development of shape memory materials to achieve recoverable high-strain capacities at high concentration of the crosslinker is still a great challenge due to their mutual exclusiveness. Here, slide-ring shape memory polymers (SMPs) with movable cross-links were prepared by varying the amount of the polyrotaxane cross-linkers. The slide-ring SMPs not only exhibit a combination of high strength and toughness, but also outstanding recoverable high-strain capacities and fast shape recovery. The elongation of the slide-ring SMPs at 1mol% crosslinker was as high as 881%, closer to physically cross-linked SMPs. As the growing deformation strain from 100% to 800%, the slide-ring SMPs still showed excellent shape memory performance with >90% shape fixity and shape recovery. The comparative analysis among movable cross-linking, chemical cross-linking and physical cross-linking SMPs in a different way revealed that the sliding effect of the polyrotaxane cross-linking is responsible for outstanding recoverable high-strain capacities of slide-ring SMPs. This design may provide a promising strategy for brittle shape memory materials to obtain high-strain capacities, such as epoxy resin and polylactic acid.

Dielectric elastomer actuators using Slide-Ring Material® with increased permittivity

The inclusion of high permittivity nanoparticles in elastomeric materials for dielectric elastomer actuators (DEAs) is one promising method to achieve large strain at relatively low applied voltages. However, the addition of these nanoparticles tends to increase the stiffness of the elastomer and disturbs the actuation of the DEA. This is attributed to restriction of the chain motion in the elastomer by the nanoparticles. Slide-Ring Material® (SRM) is a cross-linked polymeric material with freely movable cross-linking sites. The internal stresses in this structure are dramatically homogenized by the pulley effect; therefore, the restriction of chain motion due to the nanoparticles is expected to be significantly reduced. We have employed SRM as a host elastomer for a DEA with the addition of ferroelectric BaTiO3 (BT) nanoparticles. The effects of BT addition on the permittivity, stiffness and viscosity of the SRM–BT nanocomposites, and the actuation strain of DEAs using SRM were evaluated. The permittivity of the nanocomposites increased linearly with the concentration of BT and reached 3.6 times that for pure SRM at 50 wt%. The elastic modulus and the viscosity remained almost constant up to 20 wt% and then decreased above this concentration. The actuation strain of a planar actuator using SRM and 50 wt% BT was four times larger than that of the DEA with pure SRM.

Polymer with Movable Cross-linking Points Using Cyclic Moiety as Cross-linking Agent

Polymer with Movable Cross-linking Points Using Cyclic Moiety as Cross-linking Agent

Synthesis and Property of Temperature-Responsive Hydrogel with Movable Cross-Linking Points

A novel water-soluble cyclic macromonomer, AAm-c-PEG, based on a cyclic poly(ethylene glycol) (PEG) was prepared. Radical copolymerizations of N-isopropylacrylamide (NIPAAm) with the cyclic macromonomer were carried out to obtain mechanically cross-linked poly(NIPAAm) hydrogels with movable cross-linking points. For comparison, chemically cross-linked PEG-grafted poly(NIPAAm) hydrogels were prepared by radical copolymerization of NIPAAm with a PEG-based linear macromonomer, AAm-l-PEG, using methylenebis(acrylamide) (MBAAm) as a cross-linking agent. Shrinking experiment of these hydrogels revealed poly(NIPAAm) gels with movable cross-linking points exhibited faster volume shrinking than those of chemically cross-linked ones. Further, threading efficiency for cyclic PEG segment was estimated by measuring the swelling ratio in isopropyl alcohol (IPA) which is a nonsolvent for PEG.

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.

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.