Imagine a material that can heal itself after being damaged. Sounds like science fiction, right? But it’s not! Enter thermoplastic polyurethane (TPU), a versatile material that’s making waves in the world of self-healing materials. TPU is already used in everything from shoes and car parts to medical devices, but its ability to repair itself is taking it to a whole new level.
Inspired by Nature:
The concept of self-healing materials is inspired by nature. Think of a cut on your skin that heals itself, or a tree that can repair damage from a storm. TPU mimics this natural process, using internal mechanisms or external stimuli to repair itself after being damaged.
The Power of Non-Covalent Bonds:
The key to TPU’s self-healing abilities lies in the use of dynamic non-covalent bonds. These bonds are weaker than traditional covalent bonds, allowing the material to break and reform easily. This flexibility allows TPU to repair itself without compromising its overall structure.
Hydrogen Bonding: A Powerful Force:
One of the most promising approaches to self-healing TPU involves the use of hydrogen bonds. These bonds are relatively weak, but they can be used to create a network of interconnected molecules that can easily break and reform, allowing the material to heal itself.
Metal Coordination Bonds: A Versatile Solution:
Another exciting approach involves the use of metal coordination bonds. These bonds are highly versatile and can be tailored to create specific self-healing properties. By carefully designing the metal complexes, researchers can create TPU materials that can heal themselves in a variety of ways.
The Future of Self-Healing TPU:
The development of self-healing TPU materials is still in its early stages, but the potential is enormous. Imagine a world where car parts can repair themselves after a minor accident, or where shoes can automatically mend themselves after a tear. These are just a few examples of the incredible possibilities that self-healing TPU offers.
A Revolution in Material Science
Self-healing TPU is a revolutionary material that’s changing the way we think about materials science. By mimicking nature’s ability to heal itself, TPU offers a sustainable and innovative solution to a wide range of challenges. As research continues, we can expect to see even more amazing applications of this incredible material, making the world a more resilient and sustainable place.
In the past two decades, the self-healing properties of TPU have received great attention and have made substantial progress. The self-healing properties are inspired by nature, and self-healing composite materials have the ability to autonomously repair and restore their original properties through intrinsic mechanisms or in response to external stimuli.
In the self-healing process, polymer materials usually must meet four repair conditions, namely, precise positioning of damaged polymer sites , optimal mobility of material properties , and effective time responsiveness of intermolecular interactions.
01 Dynamic non-covalent bonding system in self-healing TPU
The application of dynamic non-covalent bonds in self-healing TPU materials is a booming research field. The incorporation of dynamic non-covalent bonds reduces the bond energy compared to dynamic covalent bonds and allows the restoration of original material properties without affecting the primary structure.
In recent years, significant progress has been made in integrating multiple self-healing mechanisms into TPU composites, improving self-healing efficiency while maintaining its inherent mechanical properties [2]. The following introduces the research on self-healing TPU elastomers in hydrogen bonding systems and metal ligand bonding systems.
1.1 Hydrogen bonding system
The use of hydrogen bonds to enhance the tensile properties of self-healing TPU elastomers has attracted widespread attention. Since there are a large amount of C, O, N, H and other elements in the primary chain of TPU molecules, the damaged surface of TPU shows self-healing ability within a specific time range, and the demand for bond energy during the TPU repair process is low, Intermolecular hydrogen bonding contacts are uninterrupted.
Wu et al. [3] used a synthetic TPU prepared by a two-step method , using polycaprolactone diol (PCL) and polytetrahydrofuran (PTMG) as the soft segment, and isophorone diisocyanate (IPDI) as the hard segment, 1 ,4-butanediol (BDO) as chain extender. TPU/PPy nanocomposites were prepared by solution blending method. After breaking the composite film, applying near-infrared (NIR) irradiation to the broken area can restore more than 30% of the mechanical strength within 80 seconds .
Yao et al. [4] introduced a new technology that uses T-shaped chain extenders with bisamide hydrogen bonds in the side chains to extend PU prepolymers and build multifunctional, high-rigidity, and higher toughness of supramolecular PU. Mobile side chain hydrogen bonds provide the flexibility to adjust the stiffness of PU, including highly stiff and ductile elastomers (Young’s modulus, 105.87 MPa; tear energy, 27 kJm−2). In addition, dynamic side chains with multiple H bonds can self-heal autonomously at room temperature (25 °C) .
1.2 Metal coordination key system
Metal coordination bonds play a vital role in non-covalent bond systems and exhibit excellent performance. Metal-ligand bonds have thermodynamically stable structures, but they have limitations such as dynamic instability.
We can design metal coordination bonds to achieve the desired effect. Changing the metal center and ligands can create tunable metal complexes and improve the suitability to provide electrical, optical, magnetic and other properties to polymers. Systematic research on self-healing polymers based on metal-ligand bonds has been ongoing since 2011 .
Pan et al. developed a self-healing TPU elastomer that uses cerium (III) trifluoromethanesulfonate as the metal center and phloem as the ligand. They synthesized a self-healing TPU elastomer with excellent self-healing and mechanical properties. Self-healing TPU elastomer. The best sample (PUPC-2) has high tensile strength (5.17 MPa), high tensile strain (1415%), excellent toughness (35.1 MJ·m−3), and the healing rate is close to 100% at room temperature for 48 hours.
Wang et al. developed a feasible method to prepare internal self-healing PU using multi-metal-DAP coordination bonds. By utilizing the coordination ability of DAP with various metal ions, the polymer has good mechanical properties and good self-healing ability.
Due to the dynamic properties of DAP-Fe and DAPsingle bondTb, the tensile strength and fracture strain of PU-DAP/Fe and PU-DAP/Tb reached a complete healing rate of 100%.
02 Challenges and development trends of self-healing polymers
Both dynamic covalent and non-covalent bonds are components of self-healing systems, and each system has distinct advantages and disadvantages.
Dynamic covalent bonding offers excellent controllability and mechanical properties, but it may encounter energy consumption and stability challenges. Dynamic non-covalent bonding, on the other hand, exhibits fast response , negligible energy consumption , and versatility; however, it has limitations in terms of durability and mechanical properties.
Therefore, the following needs to be done:
(1) Multi-system coordination : By implementing multiple self-healing mechanisms, such as integrating reversible and non-reversible covalent bonds, a more efficient and comprehensive self-healing effect can be achieved. This multi-system coordination not only enhances self-healing capabilities but also broadens its applicability under various environmental conditions.
(2) Environmental protection : The future development of self-healing TPU materials will focus on environmental protection and sustainability. Researchers will try to find renewable, degradable or environmentally friendly material components to reduce the use of harmful substances and their impact on the environment.
In addition, research will focus on optimizing the self-healing process , reducing energy consumption and waste generation, and promoting the sustainable development of self-healing material technology. In the process of creating self-healing composites, thoughtful combinations and designs must be pursued, considering the advantages and disadvantages of both types of combinations, dynamic covalent and dynamic non-covalent bonds, depending on specific requirements. This principle also applies to designing recyclable materials , ensuring they can be easily reprocessed, for example, using a twin-screw extruder.