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1. Stretchable materials of high toughness and low hysteresis

   https://doi.org/10.1073/pnas.1821420116  

   Wang, Zhengjin ; Xiang, Chunping ; Yao, Xi ; Le Floch, Paul ; Mendez, Julien ; Suo, Zhigang ( March 2019 , Proceedings of the National Academy of Sciences)

   In materials of all types, hysteresis and toughness are usually correlated. For example, a highly stretchable elastomer or hydrogel of a single polymer network has low hysteresis and low toughness. The single network is commonly toughened by introducing sacrificial bonds, but breaking and possibly reforming the sacrificial bonds causes pronounced hysteresis. In this paper, we describe a principle of stretchable materials that disrupt the toughness–hysteresis correlation, achieving both high toughness and low hysteresis. We demonstrate the principle by fabricating a composite of two constituents: a matrix of low elastic modulus, and fibers of high elastic modulus, with strong adhesion between the matrix and the fibers, but with no sacrificial bonds. Both constituents have low hysteresis (5%) and low toughness (300 J/m²), whereas the composite retains the low hysteresis but achieves high toughness (10,000 J/m²). Both constituents are prone to fatigue fracture, whereas the composite is highly fatigue resistant. We conduct experiment and computation to ascertain that the large modulus contrast alleviates stress concentration at the crack front, and that strong adhesion binds the fibers and the matrix and suppresses sliding between them. Stretchable materials of high toughness and low hysteresis provide opportunities to the creation of high-cycle and low-dissipation soft robots and soft human–machine interfaces.

    

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2. Multiscale stress deconcentration amplifies fatigue resistance of rubber

   https://doi.org/10.1038/s41586-023-06782-2  

   Steck, Jason ; Kim, Junsoo ; Kutsovsky, Yakov ; Suo, Zhigang ( December 2023 , Nature)

   Rubbers reinforced with rigid particles are used in high-volume applications, including tyres, dampers, belts and hoses1. Many applications require high modulus to resist excessive deformation and high fatigue threshold to resist crack growth under cyclic load. The particles are known to greatly increase modulus but not fatigue threshold. For example, adding carbon particles to natural rubber increases its modulus by one to two orders of magnitude1,2,3, but its fatigue threshold, reinforced or not, has remained approximately 100 J m−2 for decades4,5,6,7. Here we amplify the fatigue threshold of particle-reinforced rubbers by multiscale stress deconcentration. We synthesize a rubber in which highly entangled long polymers strongly adhere with rigid particles. At a crack tip, stress deconcentrates across two length scales: first through polymers and then through particles. This rubber achieves a fatigue threshold of approximately 1,000 J m−2. Mounts and grippers made of this rubber bear high loads and resist crack growth over repeated operation. Multiscale stress deconcentration expands the space of materials properties, opening doors to curtailing polymer pollution and building high-performance soft machines. 

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3. Composites retard hydrolytic crack growth

   https://doi.org/10.1016/j.eml.2021.101433  

   Jiao, Quan ; Shi, Meixuanzi ; Yin, Tenghao ; Suo, Zhigang ; Vlassak, Joost J. ( October 2021 , Extreme Mechanics Letters)

   Degradable polymers are under intense development for sustainability and healthcare. Evidence has accumulated that the chemical reaction that decomposes a polymer an also grow a crack. Even under a small load, the crack speed can be orders of magnitude higher than the overall rate of degradation, leading to premature failure. Here, we demonstrate that a crack slows down markedly in a composite of two degradable materials. In a homogeneous degradable material, the stress concentrates at the crack tip, so that a relatively small applied stretch induces a high stress and a high rate of reaction. The fracture behavior of a composite that consists of two degradable materials, a stiff material for the fibers and a compliant material for the matrix, with strong adhesion between both, is different: The soft matrix blunts the crack and distributes the stresses at the crack tip over a long length of the fibers. The same rate of reaction requires a larger applied stretch. This stress de-concentration retards crack growth in the composite. We demonstrate this concept using a composite made of stiff polydimethylsiloxane (PDMS) fibers in a soft PDMS matrix. In the presence of water molecules in the environment, siloxane bonds in the PDMS hydrolyze, causing hydrolytic crack growth. We show that a hydrolytic crack grows much more slowly in a PDMS composite than in homogeneous PDMS, and may even arrest in the composite. It is hoped that this concept will contribute to the development of degradable materials that resist premature failure. 

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4. Direct ink writing of tough, stretchable silicone composites

   https://doi.org/10.1039/d2sm00923d  

   Mo, Chengyang ; Yin, Rui ; Raney, Jordan R. ( October 2022 , Soft Matter)

   In this work, we report 3D printable soft composites that are simultaneously stretchable and tough. The matrix of the composite consists of polydimethylsiloxane containing octuple hydrogen bonding sites, resulting in a material significantly tougher than conventional polydimethylsiloxane. Short glass fibers are also added to the material. Prior to solvent evaporation, the material possesses a viscoelastic yield stress making it suitable for printing via direct ink writing. We mechanically characterize the printed composite, including fracture tests. We observe robust crack deflection and delay of catastrophic failure, leading to measured toughness values up to 2 00 000 J m −2 for specimens with 5 vol% glass fibers. The printed composites exhibit an unprecedented combination of stiffness, stretchability, and toughness. 

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5. Melt-Spun Nanocomposite Fibers Reinforced with Aligned Tunicate Nanocrystals

   https://doi.org/10.3390/polym11121912  

   Redondo, Alexandre ; Chatterjee, Sourav ; Brodard, Pierre ; Korley, LaShanda T. ; Weder, Christoph ; Gunkel, Ilja ; Steiner, Ullrich ( December 2019 , Polymers)

   The fabrication of nanocomposite films and fibers based on cellulose nanocrystals (P-tCNCs) and a thermoplastic polyurethane (PU) elastomer is reported. High-aspect-ratio P-tCNCs were isolated from tunicates using phosphoric acid hydrolysis, which is a process that affords nanocrystals displaying high thermal stability. Nanocomposites were produced by solvent casting (films) or melt-mixing in a twin-screw extruder and subsequent melt-spinning (fibers). The processing protocols were found to affect the orientation of both PU hard segments and the P-tCNCs within the PU matrix and therefore the mechanical properties. While the films were isotropic, both the polymer matrix and the P-tCNCs proved to be aligned along the fiber direction in the fibers, as shown using SAXS/WAXS, angle-dependent Raman spectroscopy, and birefringence analysis. Tensile tests reveal that fibers and films, at similar P-tCNC contents, display Young’s moduli and strain-at-break that are within the same order of magnitude, but the stress-at-break was found to be ten-times higher for fibers, conferring them a superior toughness over films. 

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