مواد پلیمری خودترمیم: مروری بر پیشرفت های اخیر Self-healing polymeric materials: A review of recent developments

Polymers and structural composites are used in a variety of applications, which include transport vehicles (cars, aircrafts, ships, and spacecrafts), sporting goods, civil engineering, and electronics. However, these materials are susceptible to damage induced by mechanical, chemical, thermal, UV radiation, or a combination of these factors [1]. This could lead to the formation of microcracks deep within the structure where detection and external intervention are difficult or impossible. The presence of the microcracks in the polymer matrix can affect both the fiber- and matrixdominated properties of a composite. Riefsnider et al. [2] have predicted reductions in fiberdominated properties such as tensile strength and fatigue life due to the redistribution of loads caused by matrix damage. Chamis and Sullivan [3] and more recently, Wilson et al. [4] have shown that matrix-dominated properties such as compressive strength are also influenced by the amount of matrix damage. Jang et al. [5] and Morton and Godwin [6] extensively studied impact response in toughened polymer composites and found that matrix cracking causes delamination and subsequent fiber fracture. In the case of a transport vehicle, the propagation of microcracks may affect the structural integrity of the polymeric components, shorten the life of the vehicle, and potentially compromise passenger safety. With polymers and composites being increasingly used in structural applications in aircraft, cars, ships, defence and construction industries, several techniques have been developed and adopted by industries for repairing visible or detectable damages on the polymeric structures. However, these conventional repair methods are not effective for healing invisible microcracks within the structure during its service life. In response, the concept of self-healing polymeric materials was proposed in the 1980s [7] as a means of healing invisible microcracks for extending the working life and safety of the polymeric components. The more recent publications in the topic by Dry and Sottos [8] in 1993 and then White et al. [9] in 2001 further inspired world wide interests in these materials [10]. Examples of such interests were demonstrated through US Air force [11] and European Space Agency [12] investments in self-healing polymers, and the strong presence of polymers at the First International Conference on Self-healing Materials organized by the Delft University of Technology of the Netherlands in February 2007.  Conceptually, self-healing polymeric materials have the built-in capability to substantially recover their load transferring ability after damage. Such recovery can occur autonomously or be activated after an application of a specific stimulus (e.g. heat, radiation). As such, these materials are expected to contribute greatly to the safety and durability of polymeric components without the high costs of active monitoring or external repair. Throughout the development of this new range of smart materials, the mimicking of biological systems has been used as a source of inspiration [13]. One example of biomimetic healing is seen in the vascular-style bleeding of healing agents following the original self-healing composites proposed by Dry and Sottos [8]. These materials may also be able to heal damage caused by insertion of other sensors/ actuators, cracking due to manufacturing-induced residual stresses, and fiber de-bonding. An ideal self-healing material is capable of continuously sensing and responding to damage over the lifetime of the polymeric components, and restoring the material’s performance without negatively affecting the initial materials properties. This is expected to make the materials safer, more reliable and durable while reducing costs and maintenance. Successful development of self-healing polymeric materials offers great opportunities for broadening the applications of these lightweight materials into the manufacture of structural and critical components. Healing of a polymeric material can refer to the recovery of properties such as fracture toughness, tensile strength, surface smoothness, barrier properties and even molecular weight. Due to the range of properties that are healed in these materials, it can be difficult to compare the extent of healing. Wool and O’Connor [14] proposed a basic method for describing the extent of healing in polymeric systems for a range of properties (Eqs. (1)–(۴)). This approach has been commonly adopted as discussed in later sections, and has been used as the basis for a non-property-specific method of comparing ‘‘healing efficiency’’ (Eq. (5)) of different selfhealing polymeric systems