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Materials Science, Composites
The self-healing of damages that occur during the operation of the corresponding structures makes it possible to extend the service life of the latter, and in this case, the problem of saving non-renewable resources is simultaneously solved. Two strategies are considered: (a) creating reversible crosslinks in the thermoplastic and (b) introducing a healing agent into cracks. Bond exchange reactions in network polymers (a) proceed as a dissociative process, in which crosslinks are split into their constituent reactive fragments with subsequent regeneration, or as an associative process, the limiting stage of which is the interaction of the reactive end group and the crosslink.
- self-healing materials
- self-healing polymers
- covalent adaptable networks
1. Introduction
The avalanche-like growth in the production and consumption of synthetic polymeric materials, inherent in recent decades, has forced humankind look for ways to dispose of these in case of damage. In particular, the use of thermosetting epoxy resins results in a large amount of waste. Disposal methods, such as landfills and incineration, lead to serious environmental pollution and the waste of resources. In order not to multiply the pollution of the planet, polymers should be repaired or degraded. The first path leads to the development of self-healing methods [1,2,3,4,5,6], and at the same time, the problem of saving non-renewable resources is solved; the second path leads to the creation of materials that are biodegradable or destroyed by light, moisture, and similar climatic factors. According to S. van der Zwaag [7], for twenty centuries, the development of materials has been reduced to the prevention of damage, that is, materials must be designed and manufactured in such a way as to delay the formation and propagation of damage that occurs during the operation of products as much as possible. Currently, “an alternative approach to 20 centuries of materials science” is gaining momentum, namely, the development of self-healing polymers and polymer composites that can heal in response to damage, regardless of their localization in the material and the time of occurrence. The phenomenon of healing is defined as the restoration of the original mechanical properties of the damaged material.
The idea of extending the service life of composite materials through self-healing is largely borrowed from biological systems in which self-healing is an ordinary event (for example, the healing of human skin or tree bark) [8,9,10,11]. As an implementation of this idea, composites with microvascular networks have been developed [12,13,14]. Attention is drawn to self-healing coatings, an integral property of which is the ability to heal damage [13,14,15,16,17]. In particular, such coatings were developed to ensure the corrosion resistance of metal products [15,18,19,20,21].
2. Methods of Self-Healing of Polymeric Materials
Thermoplastic polymers are able to heal when the surfaces of cracks connect, and the polymer chains are entangled again [22]. One of the simple ways to heal cracked or scratched coatings is to apply solvents or heat to the damaged area, which allows for the surface to be wetted and healed, as well as moving and entangling the polymer chains [23]. The healing of weakly crosslinked meshes also occurs at a temperature 10–20° above their glass transition temperature (Tg). This was demonstrated in the work of [24], where epoxy coatings were prepared by curing diglycidyl ether of bisphenol A (DGEBA) with a mixture of diamine and monoamine in the presence of 0.05–1 phr of carbon black as a photothermal filler. It was shown that an increase in the content of the latter or in the intensity of IR irradiation reduced the healing time.
This approach is unsuitable for thermoplastics and complex multicomponent composites. An analysis of the functional and structural properties of self-healing network polymers [25,26,27] led to the conclusion that their development is reduced to two strategies [8,11,28]: (a) using the reversibility of crosslinks [10,29,30] and (b) introducing a healing agent into cracks [12,31,32,33]. In the latter case, a specific method of vascular self-healing is distinguished [34,35,36,37,38]. In addition, polymer spacers are used in composites [39,40,41].
The first (intrinsic) approach assumes that self-healing occurs due to the chemical bonds of the polymer material itself, which can heal the structure after damage under the influence of external factors such as heat, ultraviolet light, or chemicals. In other words, self-healing networks heal due to their inherent (intrinsic) properties, namely, crosslinks can migrate along the polymer chains between different positions without the risk of structural damage or the loss of material properties.
Polymers containing such dynamic bonds are known as covalent adaptable networks (CANs). Bond exchange reactions in network polymers proceed according to one of two mechanisms [42,43]: a dissociative process, in which crosslinks are split into their constituent reactive fragments with subsequent regeneration [44]; or an associative process, in which a pendant reactive group enters into substitution reactions with an existing crosslink. These mechanisms are realized as interchain exchange reactions [45], in particular, transesterification [46,47] and metathesis [48,49,50]. In any case, the topological structure in CAN is restructured, which is triggered by pressure and temperature and leads to the healing of damages (Figure 1).
Figure 1. Dissociative (a) and associative (b) CANs based on exchange reactions and occurring, respectively, with or without a temporary loss of crosslink density. Reproduced from [43]. with permission from the Royal Society of Chemistry.
Associative CANs belong to the family of dynamic polymers [51] called dynamers [52,53], a term introduced by J.-M. Lehn for polymers, which also includes supramolecular polymers. The dynamic properties of the latter are achieved through intermolecular interactions such as hydrogen bonds, metal–ligand (M–L) coordination, and π–π interactions [39,41,54,55]. The exchange of non-covalent bonds is also used as a dissociative method for solving problems in the self-healing of polymeric materials [51,56].
The second (extrinsic) method consists of the release of healing agents from containers (hollow glass fibers, capsules, microvessels), which break under the action of a crack propagating inside the material and lead to its healing [32,33]. Most of the healing action takes place at room temperature, but sometimes heating is required to improve healing efficiency. Efficiency is measured quantitatively as the ratio of any property of the healed material to the original, expressed as a percentage.
As a rule, the coating is more difficult to heal with the intrinsic method than its bulk counterpart. This fact is explained by the fact that the small thickness of the coating and strong adhesive bond with the substrate limit the mobility of polymer networks, which is necessary to eliminate the damage [37]. Basically, polymer coatings are considered [37,56,57] as autonomous systems that use the external healing method capable of restoring their bulk integrity or functional properties without any external physical intervention. However, the use of containers in thin layers of coatings is limited by the size of the inclusions. Thus, in the work of [20], nanocapsules with a diameter of 100–800 nm were used.
Some systems use nanoparticles (NPs) that spontaneously migrate into cracks formed because of polymer damage [22,58,59,60]. If CdS NPs are small (3 nm) compared to the radius of gyration (Rg) of the polymer, the constraints on the chain configuration are small, and therefore the entropy cost (ΔS) for incorporating NPs into the polymer matrix is low. However, with larger (5.2 nm) particles (comparable to Rg), ΔS increases, due to which NPs will be more easily pushed out of the matrix into an open crack [61].
Self-healing materials can be repaired by intrinsic methods only after crack closure, that is, their recovery consists of two stages: crack closure and healing. Therefore, the mechanisms of closure and the chemical process of the restoration of polymer structures should be especially considered [62]. It should be noted that self-healing is different from self-adhesion. In the first case, the contacting surfaces are not in equilibrium with respect to the reactive groups, and the second case, the process is in equilibrium. Extrinsic methods require considering the relationship between the choice of the location of functional containers and the localization of stress in the composite [63].
The introduction of thermoplastic polymers, such as copolymers of ethylene with methyl acrylate or methacrylic acid, increases the interlayer fracture toughness of the composites, although it causes a decrease in the interlayer shear strength. Thus, to create a three-dimensional self-healing fiber system that also provides high fracture toughness, filaments from ethylene-methacrylic acid copolymer were sewn into an epoxy carbon fiber laminate [64]. The method of healing with thermoplastics requires the application of heat [65].
Note that despite the development of self-healing methods, it would not hurt to have indicators in polymers and composite materials that can detect mechanical damage and the need for healing before the damage becomes catastrophic. This diagnostic is especially important for the method of intrinsic self-healing, in order to determine the moment at which it is necessary to stimulate the process with pressure and temperature. Research [66] has offered an efficient way to measure the occurrence and propagation of damage by evaluating electrical resistance using embedded networks of carbon nanotubes. The applied deformation leads to an increase in the resistance in these materials due to the piezoresistance of individual NPs and an increase in the tunneling distance between the particles. Damage in the form of matrix cracking breaks electrical contacts, leading to an even more pronounced effect on the electrical resistance value.
This entry is adapted from the peer-reviewed paper 10.3390/polym14245404
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