Highly Stretchable, Ultratough, and Strong Polyesters with Improved Postcrystallization Optical Property Enabled by Dynamic Multiple Hydrogen Bonds

Influence of Number and Strength of Hydrogen Bonds on Fracture Property and Microscopic Mechanisms of Associative Hydrogen-Bonded Polymers via Molecular Dynamics Simulation

A coarse-grained model combining the acceptor-hydrogen-donor 3-body potential is first developed in this work to explore the fracture property and microscopic mechanisms of associative hydrogen-bonded polymers (AHBPs). Next, a triaxial deformation mode is performed to reveal that the fracture toughness of AHBPs initially improves and then is reduced as the number of active groups increases. By characterizing the stress decomposition, the strong hydrogen bond (HB) network improves the maximum stress but reduces the elongation. The destructive process of the HB network is described by quantifying the broken number and reduced energy of HBs. Interestingly, the strength of a single HB first decreases and then rises with strain. The initial decrease is mainly caused by the disruption of strong/moderate HBs, while the following rise is due to the further breakage of weak HBs and partial recovery of strong/moderate HBs. Meanwhile, the formed clusters of HBs due to the self-attraction act as the physical cross-links, whose evolution process is recorded by analyzing their number and size. Following it, the orientation degree and asphericity factor are calculated with strain to reflect the change in chain configuration, which is influenced by the HB network. Subsequently, the nucleation and growth process of voids is quantified. More than 90% of voids are nucleated in the polymer region, while others are in the HB region, which can be proved by the local elastic modulus and snapshots. The growth and coalescence rate of the voids can be suppressed by the strong HB network. Finally, the fracture toughness of AHBPs exhibits a continuous increase with improving strength of HBs due to the strong HB network. In summary, our work presents a clear and novel comprehension of the fracture property of AHBPs at the molecular scale.

Viscoelastic Response in Hydrous Polymers: The Role of Hydrogen Bonds and Microstructure

Water responsive polymers represent a remarkable group of soft materials, acting as a laboratory for diverse water responsive physical phenomena and cutting-edge biology-electronics interfaces. We report on peculiarly distinctive viscoelastic behaviors of the biobased water responsive polymer cellulose 10-undecenoyl ester, while biobased regenerated cellulose displays stronger hydroplastic behaviors. We discovered a novel hydrous deformation mechanism involving the stretching of hydrogen bonds mediated by hydroxyl groups and water molecules, serving as a crucial factor in accommodating deformations. In parallel, the microstructure of cellulose 10-undecenoyl ester with unique coexisting nanoparticles and a continuous phase of entangled chains is mechanically resilient in the anhydrous state but enhances structural stiffness in the hydrous state. This variation arises from a different hydration level within the hydrous microstructure. Such a fundamental discovery offers valuable insights into the connection between the microscopic physical properties that can be influenced by water and the corresponding viscoelastic responses, extending its applicability to a wide range of hygroscopic materials.

Dynamic Bonds: Adaptable Timescales for Responsive Materials

Dynamic bonds introduce unique properties such as self-healing, recyclability, shape memory, and malleability to polymers. Significant efforts have been made to synthesize a variety of dynamic linkers, creating a diverse library of materials. In addition to the development of new dynamic chemistries, fine-tuning of dynamic bonds has emerged as a technique to modulate dynamic properties. This Review highlights approaches for controlling the timescales of dynamic bonds in polymers. Particularly, eight dynamic bonds are considered, including urea/urethanes, boronic esters, Thiol-Michael exchange, Diels-Alder adducts, transesterification, imine bonds, coordination bonds, and hydrogen bonding. This Review emphasizes how structural modifications and external factors have been used as tools to tune the dynamic character of materials. Finally, this Review proposes strategies for tailoring the timescales of dynamic bonds in polymer materials through both kinetic effects and modulating bond thermodynamics.

Mechano-Regulable and Healable Silk-Based Materials for Adaptive Applications

Mechanically adaptive materials responsive to environmental stimuli through changing mechanical properties are highly attractive in intelligent devices. However, it is hard to regulate the mechanical properties of most mechanically adaptive materials in a facile way. Moreover, it remains a challenge to achieve mechano-regulable materials with mechanical properties ranging from high strength to extreme toughness. Here, inspired by the reversible nanofibril network structure of skeletal muscle to achieve muscle strength regulation, we present a mechano-regulable biopolymeric silk fibroin (SF) composite through regulating dynamic metal-ligand coordination bonds by using water molecules as competitive regulators. Efficient interfacial hydrogen bonds between tannic acid-tungsten disulfide nanohybrids and the SF matrix endow the composite with high mechanical strength and self-healing ability. The resulting composite exhibits 837-fold change in Young's modulus (5.77 ± 0.61 GPa to 6.89 ± 0.64 MPa) after water vapor triggering, high mechanical properties (72.5 ± 6.3 MPa), and excellent self-healing efficiency (nearly 100%). The proof-of-concept ultraconformable iontronic skin and smart actuators are demonstrated, thereby providing a direction for future self-adaptive smart device applications.

Tailoring the crystallization of poly(l-lactide) via structural optimization of hydrogen-bonding segments with different aliphatic spacer lengths

A series of hydroxy-terminated oxalamide segments (OXA-n, HO-(CH2)n-NHCOCONH-(CH2)n-OH, n = 2, 4 and 6) were designed as initiators for ring-opening polymerization and then poly(l-lactide) with OXA-n in the middle (PLLAOXA-n) were synthesized.

Dynamic Nanoconfinement Enabled Highly Stretchable and Supratough Polymeric Materials with Desirable Healability and Biocompatibility

Lightweight polymeric materials are highly attractive platforms for many potential industrial applications in aerospace, soft robots, and biological engineering fields. For these real-world applications, it is vital for them to exhibit a desirable combination of great toughness, large ductility, and high strength together with desired healability and biocompatibility. However, existing material design strategies usually fail to achieve such a performance portfolio owing to their different and even mutually exclusive governing mechanisms. To overcome these hurdles, herein, for the first time a dynamic hydrogen-bonded nanoconfinement concept is proposed, and the design of highly stretchable and supratough biocompatible poly(vinyl alcohol) (PVA) with well-dispersed dynamic nanoconfinement phases induced by hydrogen-bond (H-bond) crosslinking is demonstrated. Because of H-bond crosslinking and dynamic nanoconfinement, the as-prepared PVA nanocomposite film exhibits a world-record toughness of 425 ± 31 MJ m-3 in combination with a tensile strength of 98 MPa and a large break strain of 550%, representing the best of its kind and even outperforming most natural and artificial materials. In addition, the final polymer exhibits a good self-healing ability and biocompatibility. This work affords new opportunities for creating mechanically robust, healable, and biocompatible polymeric materials, which hold great promise for applications, such as soft robots and artificial ligaments.

Functionalizing Ti3C2Tx for enhancing fire resistance and reducing toxic gases of flexible polyurethane foam composites with reinforced mechanical properties

Flexible polyurethane foam (FPUF) is the most used polyurethane, but the highly flammable characteristic limits its widespread usage. In this work, ZIF-8@Ti₃C₂T_xwas synthesized to reduce the heat and toxic gases of FPUF. Flame-retardant FPUF was characterized by cone calorimeter (Cone), thermogravimetric analysis/fourier-transform infrared spectroscopy (TG-FTIR), tensileand compression tests. Compared with pure FPUF, these results showed that the peak of heat release rate (PHRR), total heat release (THR), CO and HCN of FPUF6 decreased by 46%, 69%, 27% and 43.5%, respectively. Moreover, the tensile and compression strength of FPUF6 demonstrated a 52% and 130% increment, respectively. The superior dual metal catalytical charring-forming effect and physical barrier effect of ZIF-8@Ti₃C₂T_x were achieved. In summary, a simple and reliable strategy for preparing flame-retardant FPUF with reinforced mechanical and fire safety properties was provided.

Polymers with Dynamic Bonds: Adaptive Functional Materials for a Sustainable Future

Polymeric materials play critical role in many current technologies. Among them, adaptive polymeric materials with dynamic (reversible) bonds exhibit unique properties and provide exciting opportunities for various future technologies. Dynamic bonds enable structural rearrangements in polymer networks in specific conditions. Replacement of a few covalent bonds by dynamic bonds can enhance polymeric properties, e.g., strongly improve the toughness and the adhesive properties of polymers. Moreover, they provide recyclability and enable new properties, such as self-healing and shape memory effects. We briefly overview new developments in the field of polymers with dynamic bonds and current understanding of their dynamic properties. We further highlight several examples of unique properties of polymers with dynamic bonds and provide our perspectives for them to be used in many current and future applications.