Supramolecular polymer networks with high compressibility and fast self-recovery

Highly compressible supramolecular polymer networks are highly desirable but are seldom reported. In a recent work published in Nature Materials by Scherman et al., glass-like high-performance supramolecular polymer networks are successfully developed by means of slow-dissociative non-covalent cross-linkers. The resulting supramolecular polymer networks demonstrate superior compressive strength of up to 100 MPa and a rapid room-temperature self-recovery (<120 s). Highly compressible supramolecular polymer networks are highly desirable but are seldom reported. In a recent work published in Nature Materials by Scherman et al., glass-like high-performance supramolecular polymer networks are successfully developed by means of slow-dissociative non-covalent cross-linkers. The resulting supramolecular polymer networks demonstrate superior compressive strength of up to 100 MPa and a rapid room-temperature self-recovery (<120 s). Supramolecular polymer networks (SPNs), a fascinating class of soft materials, are three-dimensional structures composed of transiently cross-linked linear macromolecules connected by non-covalent bonds.1Xia D. Wang P. Ji X. Khashab N.M. Sessler J.L. Huang F. Functional supramolecular polymeric networks: the marriage of covalent polymers and macrocycle-based host-guest interactions.Chem. Rev. 2020; 120: 6070-6123Crossref PubMed Scopus (319) Google Scholar Unlike covalent bond cross-linked polymer networks, SPNs integrate the features of chemical and physical networks, and the marriage of the multifunctionality of chemosynthetic polymer networks and the flexibility of physical cross-linking endows the resultant SPNs with remarkable reversibility2Qin B. Zhang S. Sun P. Tang B. Yin Z. Cao X. Chen Q. Xu J.-F. Zhang X. Tough and multi-recyclable cross-linked supramolecular polyureas via incorporating noncovalent bonds into main-chains.Adv. Mater. 2020; 32: e2000096Crossref PubMed Scopus (127) Google Scholar and stimuli responsiveness.3Gao Z. Han Y. Chen S. Li Z. Tong H. Wang F. Photoresponsive supramolecular polymer networks via hydrogen bond assisted molecular tweezer/guest complexation.ACS Macro Lett. 2017; 6: 541-545Crossref Scopus (33) Google Scholar Furthermore, SPNs can be customized to meet specific requirements.4Löwenberg C. Balk M. Wischke C. Behl M. Lendlein A. Shape-memory hydrogels: evolution of structural principles to enable shape switching of hydrophilic polymer networks.Acc. Chem. Res. 2017; 50: 723-732Crossref PubMed Scopus (206) Google Scholar Indeed, SPNs have been applied in many fields, including the fabrication of self-healing materials,5Wang C. Wu H. Chen Z. McDowell M.T. Cui Y. Bao Z. Self-healing chemistry enables the stable operation of silicon microparticle anodes for high-energy lithium-ion batteries.Nat. Chem. 2013; 5: 1042-1048Crossref PubMed Scopus (938) Google Scholar memory retention materials,6Zhao R. Zhao T. Jiang X. Liu X. Shi D. Liu C. Yang S. Chen E.-Q. Thermoplastic high strain multishape memory polymer: side-chain polynorbornene with columnar liquid crystalline phase.Adv. Mater. 2017; 29: 1605908Crossref Scopus (54) Google Scholar and drug-delivery devices.7Zhang S. Bellinger A.M. Glettig D.L. Barman R. Lee Y.-A.L. Zhu J. Cleveland C. Montgomery V.A. Gu L. Nash L.D. et al.A pH-responsive supramolecular polymer gel as an enteric elastomer for use in gastric devices.Nat. Mater. 2015; 14: 1065-1071Crossref PubMed Scopus (239) Google Scholar However, the construction of highly compressible and rapidly recoverable SPNs is highly challenging. A significant breakthrough in the field of SPNs was recently reported in Nature Materials. Scherman and coworkers successfully developed glass-like SPNs with excellent compressive strength of up to 100 MPa by introducing non-covalent cross-linkers with a dissociation rate constant kd < 1 s−1.8Huang Z. Chen X. O’Neill S.J.K. Wu G. Whitaker D.J. Li J. McCune J.A. Scherman O.A. Highly compressible glass-like supramolecular polymer networks.Nat. Mater. 2022; 21: 103-109Crossref PubMed Scopus (69) Google Scholar Notably, no fracture was observed even when the SPNs were compressed at 93% strain, and these SPNs possessed fast room-temperature self-recovery capacity on short timescales of <120 s. In their work, the cross-link design of the prepared SPNs was based on CB[8]-mediated ternary complexation (Figure 1A), and the kd of the dynamic cross-link could be regulated by manipulating the molecular structure of the second guest (RBVI). Specifically, the SPNs were obtained via the photopolymerization of the slow-dissociative cross-linker 5FBVI-CB[8]-RBVI, and the dynamic viscoelasticity of these networks was evaluated using oscillatory rheology. The SPNs constructed using NVI or BVI cross-linkers containing CB[8] showed higher viscoelasticity than those prepared using the cross-linkers without CB[8] (Figure 1B). Meanwhile, at 20°C, the time-temperature superposition (TTS) and time-kinetics superposition experiments indicated that the cross-linkers (diMeBVI, ClBVI, BVI, and CNBVI) with higher kd exhibited a performance similar to that of NVI with a lower kd at 40–80°C (Figures 1C and 1D). This implies that the room-temperature dynamic viscoelasticity of the SPNs could be effectively regulated by modifying the cross-linker structure instead of raising the temperature (Figure 1). The compressibility and self-recovery of these glass-like SPNs were also investigated using representative compressive testing equipment (Figure 1E). It was found that the compressive strength of the SPNs increased gradually with a decrease in kd, and the SPN cross-linked by the slowest dissociative 5FBVI-CB[8]-NVI showed the highest compressive strength of up to 100 MPa. Moreover, the SPN cross-linked with 2.5 mol% 5FBVI-CB[8]-NVI achieved a maximum compressive strength of 1.04 GPa at a total monomer concentration (CM) of 6.0 M with no fracture at 93% strain (Figure 1F). Clearly, the compressive performance of these slow-dissociative SPNs is far better than those of numerous materials, including elastomers, gels, and bovine cartilage. To demonstrate the highly reversible compressibility of the 5FBVI-CB[8]-NVI-cross-linked SPN, a cuboid specimen with the length, width, and thickness of 70, 50, and 6 mm, respectively, was produced for a car-compression experiment (Figure 1G). A car weighing 1200 kg was placed on top of the specimen for 1 min, and the specimen was repeatedly compacted by the car 16 times. No fracture or irreversible deformation was observed because of the complete self-recovery of the SPN. Scherman et al. have successfully constructed a series of glass-like SPNs with highly compressible and excellent room-temperature self-recovery performances by developing slow-dissociative cross-links, which find potential applications in the fields of soft robotics, artificial muscles, tissue engineering, and wearable bioelectronics. This study was financially supported by the National Natural Science Foundation of China (22061018), the Natural Science Foundation for Distinguished Young Scholars of Jiangxi Province (20212ACB213003), and the Academic and Technical Leader Plan of Jiangxi Provincial Main Disciplines (20212BCJ23004). The authors declare no competing interests.