Supramolecular Elastomers with Movable Cross-Linkers Showing High Fracture Energy Based on Stress Dispersion

Strain-Induced Orientation of Host Rings that Determines the Sliding of Guest Polymers and Plasticity of Glassy Polyrotaxane

The unique motility of mechanically interlocked polymers enables their mechanical properties to profoundly transform. This property has been exploited less in glassy materials than in rubbery materials. This study demonstrated that in the glassy state the rings must orient before sliding and clarified the requisite structural changes by the synchrotron microbeam X-ray diffraction mapping of a ductile cyclodextrin (CD)-based glassy polyrotaxane. After inducing neck formation and propagation by uniaxial tension, the strain-localized area was scanned, elucidating how the CD orientation and its correlation distance change. As necking approaches and local strain increases, the CD rotational axis orients considerably in the tensile direction. Near the neck inflection point, polymer sliding triggers a sudden structural transformation, forming a phase-separated structure between the CDs and polymers that toughens the neck. This strain-induced orientation preceding sliding appears to facilitate sliding. In the rubbery state, host molecules can orient freely with the guest polymer orientation, but glassy materials must be designed to facilitate host orientation to enable guest sliding with minimum molecular friction.

Interaction of Cyclodextrins with Amphiphilic Alternating Cooligomers Possessing the Dense Triazole Backbone

The interaction of cyclodextrins (CDs) with structure-controlled polymers is expected to provide significant insights into macromolecular recognition. However, the interaction of CDs with structure-controlled polymers has been an underexamined issue of investigation. Herein, alternating amphiphilic cooligomers (oligoCnAH, where n denotes the carbon number of alkyl groups; n = 4, 8, and 12) were synthesized by copper(I)-catalyzed azide-alkyne cycloaddition polymerization of heterodimers of 4-azido-5-hexynoic acid (AH) derivatives carrying N-alkylamide and t-butyl (tBu) ester side chains, followed by hydrolysis of the tBu ester, to study the interaction of CDs with oligoCnAH by 1H NMR, nuclear Overhauser effect spectroscopy, and pulse-field-gradient spin-echo NMR. These NMR studies indicated that αCD interacted with oligoC4AH, αCD and βCD interacted with oligoC8AH, and all CDs interacted with oligoC12AH. Based on the equilibrium models proposed, the binding constants were evaluated for the binary mixtures, which showed interaction. Comparing the interactions of the CDs/oligoC12AH binary mixtures with those of the binary mixtures of CDs and alternating copolymers of sodium maleate and dodecyl vinyl ether (polyC12M), it is concluded that oligoC12AH forms less stable micelles than does polyC12M presumably because of the lower molecular weight, the hydrophilic amide groups in the side chain, and the longer interval between neighboring C12 groups in oligoC12AH.

Improvement in Cohesive Properties of Adhesion Systems Using Movable Cross-Linked Materials with Stress Relaxation Properties

A strong, tough, and stable adhesion system used in various environments must be developed. A long-lasting adhesion system should effectively perform in the following five aspects: adhesion strength, toughness, energy dissipation property, self-restoration property, and creep resistance property. However, these properties are difficult to balance using conventional adhesives. Here, a new topological adhesion system using single-movable cross-network (SC) materials [SC(DMAAm) Adh] was designed. 3-(Trimethoxysilyl) propyl acrylate was used as the anchor, N,N-dimethyl acrylamide (DMAAm) was used as the main chain monomer, and γ-cyclodextrin (γ-CD) units acted as movable cross-links. The movable cross-links provided SC(DMAAm) Adh with energy dissipation properties, thereby improving its toughness. The γ-CD units also acted as bulky stoppers that provided a high adhesion strength and self-restoration properties. Moreover, the combination of the movable cross-links and bulky stoppers provided creep resistance to SC(DMAAm) Adh. The performance of the adhesion systems under different mobilities of the polymer chains was examined by adjusting the water content. In proper water-containing states, all mechanical properties of SC(DMAAm) Adh were better than those of the adhesion systems using homopolymers [P(DMAAm) Adh] and polymers with covalent cross-linking points [CP(DMAAm) Adh].

Enhancement of the mechanical properties of organic-inorganic hybrid elastomers by introducing movable and reversible crosslinks

Organic-inorganic materials have been widely utilized in various fields as multifunctional materials. Poly(dimethyl siloxane) (PDMS), a typical inorganic polymer, has industrially appealing functions, such as transparency, biocompatibility, and gas permeability; however, it has poor mechanical properties. We incorporated organic-inorganic hybrid elastomers (PDMS-γCD-AAl⊃P(EA-HEMA) (x)) with movable crosslinks, and we utilized hydrogen bonds as reversible crosslinks. The organic polymer poly ethyl acrylate-r-hydroxy ethyl methacrylate (P(EA-HEMA)) penetrated the cavity of triacetylated γ-cyclodextrin (γCD), which was introduced into the side chains of PDMS, and it compounded with PDMS at the nanoscale. Structural studies involving visual and X-ray scattering measurements revealed that movable crosslinks improved the compatibility levels of PDMS and acrylate copolymers. However, macroscopic phase separation occurred when the number of reversible crosslinks increased. Furthermore, studies on the mobility levels of acrylate copolymers and movable crosslinks indicated that the relaxation behaviour of PDMS-γCD-AAl⊃P(EA-HEMA) (x) changed with changing numbers of reversible crosslinks. Introducing reversible crosslinks improved the Young's modulus and toughness values. The movable and reversible crosslinks between the organic and inorganic polymers contributed to the high elongation properties. The design of PDMS-γCD-AAl⊃P(EA-HEMA) (x) incorporated cooperatively movable and reversible crosslinks to achieve high compatibility of immiscible polymers and to control the mechanical properties.

Highly Stretchable Stress-Strain Sensor from Elastomer Nanocomposites with Movable Cross-links and Ketjenblack

Practical applications like very thin stress-strain sensors require high strength, stretchability, and conductivity, simultaneously. One of the approaches is improving the toughness of the stress-strain sensing materials. Polymeric materials with movable cross-links in which the polymer chain penetrates the cavity of cyclodextrin (CD) demonstrate enhanced strength and stretchability, simultaneously. We designed two approaches that utilize elastomer nanocomposites with movable cross-links and carbon filler (ketjenblack, KB). One approach is mixing SC (a single movable cross-network material), a linear polymer (poly(ethyl acrylate), PEA), and KB to obtain their composite. The electrical resistance increases proportionally with tensile strain, leading to the application of this composite as a stress-strain sensor. The responses of this material are stable for over 100 loading and unloading cycles. The other approach is a composite made with KB and a movable cross-network elastomer for knitting dissimilar polymers (KP), where movable cross-links connect the CD-modified polystyrene (PSCD) and PEA. The obtained composite acts as a highly sensitive stress-strain sensor that exhibits an exponential increase in resistance with increasing tensile strain due to the polymer dethreading from the CD rings. The designed preparations of highly repeatable or highly responsive stress-strain sensors with good mechanical properties can help broaden their application in electrical devices.

Bioinspired Ultra Tear-Resistant Elastomer with a Slidable Double-Network Structure

Tear resistance is of vital importance in the fabrication and application of synthetic soft materials. However, the paradox of simultaneously improving the tearing energy and elasticity remains a huge challenge for conventional approaches. Here, inspired by the skin, we successfully constructed an extraordinary tear-resistant, superelastic elastomer by the introduction of nanosized polycyclodextrin into the elastomer network to form a slidable interpenetrate double network structure. The tearing energy of the SDEP elastomer is up to 274 KJ/m², which is comparable to metals and alloys and increased more than 100 times compared with the chemically cross-linked elastomer. The fracture strain exceeded 3300%, which is hardly achieved by other materials with high tearing energy. This comprehensive improvement of antitearing and super elasticity property was achieved by (i) a slide ring effect to dissipate energy and blunt a crack tip; (ii) straightening and reorientation of the slidable double network to deflect the advancing of a crack tip; (iii) a double network sharing the load. These results provide a novel strategy to fabricate elastic, tear-resistant soft material, which may contribute to the practical application as tear-resistant flexible electronics and irregular-shaped stretchable devices.

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.

Time-strain inseparability in multiaxial stress relaxation of supramolecular gels formed via host-guest interactions

Supramolecular hydrogels utilizing host-guest interactions (HG gels) exhibit large deformability and pronounced viscoelasticity. The inclusion complexes between β-cyclodextrin (host) and adamantane (guest) units on the water-soluble polymers form transient bonds. The HG gels show significant stress relaxation with finite equilibrium stress following the step strain. The stress relaxation process reflects the detachment dynamics of the transient bonds which sustain the initial stress, while the finite equilibrium stress is preserved by the permanent topological cross-links with a rotaxane structure. Nonlinear stress relaxation experiments in biaxial stretching with various combinations of two orthogonal strains unambiguously reveal that time and strain effects on stress are not separable. The relaxation is accelerated for a short time frame (<10² s) with an increase in the magnitude of strain, whereas it is retarded for a longer time window with an increase in the anisotropy of the imposed biaxial strain. The time-strain inseparability in the HG gels is in contrast to the simple nonlinear viscoelasticity of a dual cross-link gel with covalent and transient cross-links in which the separability was previously validated by the same assessment. We currently interpret that the significant susceptibility of the detachment dynamics to the deformation type results from the structural characteristics of the HG gels, i.e., the host and guest moieties covalently connected to the network chains, the considerably low concentrations (<0.1 M) of these moieties, and the slidability of the permanent rotaxane cross-links.

Visualization of Solvent-Induced Structure Evolution in Cyclodextrin Polyrotaxane Gels

Cyclodextrin (CD)-based polyrotaxanes (PR) are widely used to construct high-mechanical-performance materials because of the high degree of conformational freedom. However, strong hydrogen bonds between CDs greatly limit the application of CD-PR in the preparation of ductile neutral hydrogels. In this work, spiropyrane (SP) into α-CD-based PR is introduced to "visualize" the segment motion of the network in neutral water. The aggregation-induced cohesion and critical factors for the force transmission are disclosed. This system offers a new approach for the fundamental research for the complicated topologically cross-linked structures, which is important for the design of CD-PR-based biocompatible soft materials.

Additional crystalline structures of syndiotactic polystyrene composites with acetylated cyclodextrin

Addition of acetylated cyclodextrin to syndiotactic polystyrene forms additional crystalline structures based on molecular recognition.

Extremely Rapid Self-Healable and Recyclable Supramolecular Materials through Planetary Ball Milling and Host-Guest Interactions

The host-guest interaction as noncovalent bonds can make polymeric materials tough and flexible based on the reversibility property, which is a promising approach to extend the lifetime of polymeric materials. Supramolecular materials with cyclodextrin and adamantane are prepared by mixing host polymers and guest polymers by planetary ball milling. The toughness of the supramolecular materials prepared by ball milling is approximately 2 to 5 times higher than that of supramolecular materials prepared by casting, which is the conventional method. The materials maintain their mechanical properties during repeated ball milling treatments. They are also applicable as self-healable bulk materials and coatings, and they retain the transparency of the substrate. Moreover, fractured pieces of the materials can be re-adhered within 10 min. Dynamic mechanical analysis, thermal property measurements, small-angle X-ray scattering, and microscopy observations reveal these behaviors in detail. Scars formed on the coating disappear within a few seconds at 60 °C. At the same time, the coating shows scratch resistance due to its good mechanical properties. The ball milling method mixes the host polymer and guest polymer at the nano level to achieve the self-healing and recycling properties.