**4. Active self-healing based on intrinsic methodology**

Structurally dynamic polymers are a macromolecular system in which dynamic bonds are responsible for the restructuring of molecular architecture upon to external stimuli. Reversible bonding chemistry (i.e., covalent and non-covalent) is used to design structurally dynamic polymer systems produced to respond on macroscopic changes of material's architecture. Repeatable damage healing is demonstrated by various non-covalent interactions (reversible physical bonding), the covalent chemistries (reversible chemical bonding), and recently by their varied combination. Dynamic bonds are sensitive to specific stimulus and selectively undergo reversible bonding and de-bonding under equilibrium conditions. Intrinsic healing systems are programmed to respond to macroscopic damages. Basically, it is active methodology; healing is achieved by dynamic bonding of the polymer matrix. Intrinsic healing has specific properties of certain materials, such as molecular structures and chemical or physical bonds. The intrinsic self-healing requires human/external intervention to perform in most of the cases. The healing is possible via temporary local mobility enhancement of polymeric chains. Various modes of energy (e.g., temperature, static load, UV) are critical factors for the mobility of polymeric chains. Some chemical principles with multiple chemistries are responsible for macroscale healing. Reversible supramolecular interactions are low-energy interactions and based on hydrogen bonding, ionomer bonding, π-π interactions, or metal coordination. Two other categories of intrinsic healing systems combining physical and chemical approaches can be included: shape memory polymers and polymer blends.

### **4.1 Thermodynamic covalent bonding-based Diels-Alder (DA) and Retro-DA (r-DA) reactions**

Cross-linked polymer networks have superior mechanical properties and thermal as well as chemical resistance as compared to their uncross-linked and linear analog. But, due to high cross-linked density, these systems are rigid and susceptible to mechanical damage. By incorporation of some dynamic covalent functionality into matrix backbone and/or in side chains, we achieved stimuli-responsive systems. Dynamic cross-linked systems have improved service life and energy efficiency and resist to foreign object impact. Basically, thermoreversible bonding is a more useful technique to load-bearing structures. It is accomplished by reversible chemical reactions upon an external stimulus. Typical dynamic bond chemistry is belonging to disulfide [56], hindered urea [57], and alkoxyamine [58] that are having flexible bonding units. A polymer (3M4F) system demonstrated self-repairing by subjecting it to heating/cooling cycles [18] shown in **Figure 7**.

In contrast, cycloaddition reaction is an efficient method to design carbon-carbon linkage without the use of catalyst. An electron-rich diene and electron-poor dienophile species play a key role to succeed DA/r-DA cycloaddition reaction. A product of DA reaction is known as DA adduct. DA adduct is the mixture of endo- and exodiastereomers. Due to the temperature of the r-DA reaction, the exo-diastereomer is a major adduct. DA adducts have norbornane-type covalent functionality which is slightly weaker than other covalent linkages in matrix. Upon excess mechanical loading, the excess stress is transferred to weak bonding of adducts, and de-bonding

**29**

**Figure 7.**

*Self-Healing Polymer Composites for Structural Application*

*Reversible cross-linked furan-maleimide-based polymer network [18].*

occurs. Upon elevated temperature, DA adducts can be dissociated into corresponding diene and dienophile moieties through r-DA reaction within the cross-linked system. Upon further cooling, re-bonding is preceded into DA adduct by reaction between corresponding diene and dienophile units. Due to thermal reversibility of DA reactions, they are frequently applied to production of remendable and recyclable materials [17–20]. The 4 + 2 cycloaddition Diels-Alder reaction is the most studied thermally controlled reaction and belongs to the group of "click reactions" that are famous for flexibility and well-organized, and selective chemical synthesis [21]. By utilizing r-DA and following DA reactions, covalent network displays a dissociation and reformation through void-filling thermoreversible process upon controlled heating. Chemically, on the basis of utilized form of diene and dienophile functionalities for DA/r-DA reaction, these systems are classified into three categories: furan-maleimide polymer systems, dicyclopentadiene-based systems, and anthracene-functionalized polymer systems. Graphene nanosheet-functionalized polyurethane-based composite has shown infrared (IR) laser-assisted self-healing, which is advantageous to flexible electronics [59]. Most of the DA cross-linked network is fabricated through stepgrowth poly-addition or coupling reactions of polyfurans and polymaleimides. Furan and maleimide pair is highly reactive for cycloaddition reaction and low-temperature shifting of DA/r-DA equilibrium because these moieties exist in s-cis conformation, which offered rigid system favorable for DA reaction. Furan group acts as diene, and maleimide acts as dienophile based on DA/r-DA reaction chemistry. These reactions still need an external heat source to initiate the healing process. Various healing

systems based on reversible covalent bonding are presented in **Table 2**.

If the amount of damage is microscopic, capsule-based or intrinsic systems may be the best option. But, macroscopic damaged volume and vascular-based systems are

*DOI: http://dx.doi.org/10.5772/intechopen.82420*

*Functional Materials*

polymers and polymer blends.

**(r-DA) reactions**

a porous conductive wire defrosts the system by internal heating, and further healing reactions are proceeded [16]. The concept may be used to develop self-healing in

Structurally dynamic polymers are a macromolecular system in which dynamic

bonds are responsible for the restructuring of molecular architecture upon to external stimuli. Reversible bonding chemistry (i.e., covalent and non-covalent) is used to design structurally dynamic polymer systems produced to respond on macroscopic changes of material's architecture. Repeatable damage healing is demonstrated by various non-covalent interactions (reversible physical bonding), the covalent chemistries (reversible chemical bonding), and recently by their varied combination. Dynamic bonds are sensitive to specific stimulus and selectively undergo reversible bonding and de-bonding under equilibrium conditions. Intrinsic healing systems are programmed to respond to macroscopic damages. Basically, it is active methodology; healing is achieved by dynamic bonding of the polymer matrix. Intrinsic healing has specific properties of certain materials, such as molecular structures and chemical or physical bonds. The intrinsic self-healing requires human/external intervention to perform in most of the cases. The healing is possible via temporary local mobility enhancement of polymeric chains. Various modes of energy (e.g., temperature, static load, UV) are critical factors for the mobility of polymeric chains. Some chemical principles with multiple chemistries are responsible for macroscale healing. Reversible supramolecular interactions are low-energy interactions and based on hydrogen bonding, ionomer bonding, π-π interactions, or metal coordination. Two other categories of intrinsic healing systems combining physical and chemical approaches can be included: shape memory

**4.1 Thermodynamic covalent bonding-based Diels-Alder (DA) and Retro-DA** 

by subjecting it to heating/cooling cycles [18] shown in **Figure 7**.

Cross-linked polymer networks have superior mechanical properties and thermal as well as chemical resistance as compared to their uncross-linked and linear analog. But, due to high cross-linked density, these systems are rigid and susceptible to mechanical damage. By incorporation of some dynamic covalent functionality into matrix backbone and/or in side chains, we achieved stimuli-responsive systems. Dynamic cross-linked systems have improved service life and energy efficiency and resist to foreign object impact. Basically, thermoreversible bonding is a more useful technique to load-bearing structures. It is accomplished by reversible chemical reactions upon an external stimulus. Typical dynamic bond chemistry is belonging to disulfide [56], hindered urea [57], and alkoxyamine [58] that are having flexible bonding units. A polymer (3M4F) system demonstrated self-repairing

In contrast, cycloaddition reaction is an efficient method to design carbon-carbon linkage without the use of catalyst. An electron-rich diene and electron-poor dienophile species play a key role to succeed DA/r-DA cycloaddition reaction. A product of DA reaction is known as DA adduct. DA adduct is the mixture of endo- and exodiastereomers. Due to the temperature of the r-DA reaction, the exo-diastereomer is a major adduct. DA adducts have norbornane-type covalent functionality which is slightly weaker than other covalent linkages in matrix. Upon excess mechanical loading, the excess stress is transferred to weak bonding of adducts, and de-bonding

aerostructure at high altitude having low temperature.

**4. Active self-healing based on intrinsic methodology**

**28**

**Figure 7.** *Reversible cross-linked furan-maleimide-based polymer network [18].*

occurs. Upon elevated temperature, DA adducts can be dissociated into corresponding diene and dienophile moieties through r-DA reaction within the cross-linked system. Upon further cooling, re-bonding is preceded into DA adduct by reaction between corresponding diene and dienophile units. Due to thermal reversibility of DA reactions, they are frequently applied to production of remendable and recyclable materials [17–20]. The 4 + 2 cycloaddition Diels-Alder reaction is the most studied thermally controlled reaction and belongs to the group of "click reactions" that are famous for flexibility and well-organized, and selective chemical synthesis [21]. By utilizing r-DA and following DA reactions, covalent network displays a dissociation and reformation through void-filling thermoreversible process upon controlled heating. Chemically, on the basis of utilized form of diene and dienophile functionalities for DA/r-DA reaction, these systems are classified into three categories: furan-maleimide polymer systems, dicyclopentadiene-based systems, and anthracene-functionalized polymer systems. Graphene nanosheet-functionalized polyurethane-based composite has shown infrared (IR) laser-assisted self-healing, which is advantageous to flexible electronics [59]. Most of the DA cross-linked network is fabricated through stepgrowth poly-addition or coupling reactions of polyfurans and polymaleimides. Furan and maleimide pair is highly reactive for cycloaddition reaction and low-temperature shifting of DA/r-DA equilibrium because these moieties exist in s-cis conformation, which offered rigid system favorable for DA reaction. Furan group acts as diene, and maleimide acts as dienophile based on DA/r-DA reaction chemistry. These reactions still need an external heat source to initiate the healing process. Various healing systems based on reversible covalent bonding are presented in **Table 2**.

If the amount of damage is microscopic, capsule-based or intrinsic systems may be the best option. But, macroscopic damaged volume and vascular-based systems are


#### **Table 2.**

*Various self-healing systems based upon reversible covalent bonding.*

efficient which allow large amounts of healing agent to be transported to the damage site. The aforementioned self-healing techniques address the repairs, mitigations, crack growth, and various damage conditions in polymer matrix. These techniques have advantages and limitations specific to applications that are summarized as follows:


#### **4.2 Supramolecular noncovalent interaction-based self-healing**

In materials, microscopic damages are repaired by extrinsic technology in which foreign species play a lead role in the healing process. These techniques respond to damages autonomically or stimulus-assisted phenomenon and take a shorter time to recover the strength of materials. But in the case of macroscopic damages, these extrinsic techniques are poorer in performance. Additionally, structurally covalent dynamic polymers are also requested with additional heat to clear microdamages. To overcome these issues, some significant research in the field of supramolecular systems is focused which respond to damages autonomously and recovered mechanical integrity without the addition of foreign reactive species and human intervention. In high-performance materials, macroscopic and energetic damage events are usual. These damages are healed by physical interactions. The physical interactions are noncovalent in nature and cause autonomic healing due to inherent origin. These recover about fully mechanical properties but take longer time. These noncovalent interactions recover mechanical properties almost completely but take longer time. These interactions are reversible subjected to the thermodynamic equilibrium and show additional impacts such as environmental-dependent switch properties, easy processability

**31**

**Table 3**.

**Table 3.**

bonding, and π-π stacking.

*4.2.1 Supramolecular chemistry based on H-bonding*

*Self-Healing Polymer Composites for Structural Application*

and self-healing behavior as compared to traditional polymers. Reversible bonding can be used to design supramolecular healable polymers and composites which respond to external stimuli such as heat [62], pressure [63], water [64], or light [65]. Various supramolecular interaction-based self-healing systems are shown in

Polystyrene grafted with poly(acrylate amide)

SupraPolix BV

pyrenyl end groups

pyrenyl end groups

acid)/Na + ion (EMAA)

2 π-π interaction Polydiimide/poly(siloxane) with

3 Ionomers Poly(ethylene-co-methacrylic

*Typical supramolecular interactions and relevant self-healing polymer systems.*

Ureidopyrimidinone bond—

Copolyimide/poly(amide) with

**Healing system Ref. Remarks**

DCPD/DNE/epoxy systems [22] Adhesion promoter

used

[24] Self-assembly

[25, 26] Flexible and self-

[66] Thermoreversible system

[27–30] Self-sealing shooting range targets, tires

[23] Polyvalent H-bonding sites

mechanism

supporting material

Stimuli responsiveness [67] and a high diffusion rate of oligomeric components [68] are the main characteristic of supramolecular polymers which make them for rapid and controllable healing system. Oligomers are low-molecularweight species that make aggregates by self-assembling and perform rheological or mechanical properties similar as polymers. The reversible non-covalent interactions can be possible by hydrogen bonding, ionic bonding, metal-ligand

H-bonding is the most popular route to achieve supramolecular polymers. Upon heat, interactions between the polymeric chains are decreased and reassembled upon cooling, and finally the non-covalent cross-linking recovered the strength and mechanical integrity. To achieve sufficient cross-linking density in polymer, a high association constant between repeating units is needed. The association constant and a reversible interaction have a reverse relation. In contrast, at the less association constant, better reversibility is achieved but having smaller assemblies and poor mechanical properties. However, individual supramolecular polymers are suffered by low mechanical strength. The mechanical strength of supramolecular system is enhanced by an increase in the number of non-covalent interaction [22] and by reinforcement with nanofiller [26]. Significantly, the interactions of matrix fiber are increased by the presence of hydrogen bond accepting functionality in matrix polymer blend. Adhesion promoter increases the amount of H-bonding of the matrix system [22]. These promoters are known as co-healing agents shown in **Figure 8**. On fracture of capsules, the DCPD monomer penetrates the networks of epoxy matrix and reacts with embedded catalyst. The formation of interpenetrated network is initiated by polymerization of DCPD which would strengthen the matrix—poly(DCPD) surface. Higher strength is obtained by entangled

*DOI: http://dx.doi.org/10.5772/intechopen.82420*

**S.N. Nature of** 

1 Hydrogen bonding

**interactions**


#### **Table 3.**

*Functional Materials*

**S.N. Healable systems Healing** 

1 Dicyclopentadiene-based polymers

5 Thiol-modified poly

**Table 2.**

6 Polystyrene-based block copolymer

[N-acetylethylene-imine]

*Various self-healing systems based upon reversible covalent bonding.*

(i) These have slow healing rate.

depending on the application.

(iv) Complexity of vascular networks is a challenge.

ing to space exploration upon material puncture healing.

**4.2 Supramolecular noncovalent interaction-based self-healing**

efficient which allow large amounts of healing agent to be transported to the damage site. The aforementioned self-healing techniques address the repairs, mitigations, crack growth, and various damage conditions in polymer matrix. These techniques have advantages and limitations specific to applications that are summarized as follows:

**mechanism**

2 3M4F polymer DA/r-DA [18] Multivalent star-shaped

3 2ME4F polymer 4 + 2 DA reaction [20] Solvent-free synthesis 4 Anthracene-based polymers DA/r-DA [19] Polymer backbone

> Redox-reversible hydrogel system

Thiol-disulfide linkage

**Ref. Remarks**

functionalized

functionalized along itself

(side-chain modification)

(backbone modification)

systems

[60] Thiol-disulfide system

[61] Thiol-disulfide system

DA/r-DA [17] Polymer side chains

(ii) Additional requirement of heat/light in intrinsic systems can be good or bad

(iii) The use of foreign inserts in matrix leads to detrimental effects on matrix.

(v) Do not address the ballistic or hypervelocity impacts, which are great promis-

In materials, microscopic damages are repaired by extrinsic technology in which foreign species play a lead role in the healing process. These techniques respond to damages autonomically or stimulus-assisted phenomenon and take a shorter time to recover the strength of materials. But in the case of macroscopic damages, these extrinsic techniques are poorer in performance. Additionally, structurally covalent dynamic polymers are also requested with additional heat to clear microdamages. To overcome these issues, some significant research in the field of supramolecular systems is focused which respond to damages autonomously and recovered mechanical integrity without the addition of foreign reactive species and human intervention. In high-performance materials, macroscopic and energetic damage events are usual. These damages are healed by physical interactions. The physical interactions are noncovalent in nature and cause autonomic healing due to inherent origin. These recover about fully mechanical properties but take longer time. These noncovalent interactions recover mechanical properties almost completely but take longer time. These interactions are reversible subjected to the thermodynamic equilibrium and show additional impacts such as environmental-dependent switch properties, easy processability

**30**

*Typical supramolecular interactions and relevant self-healing polymer systems.*

and self-healing behavior as compared to traditional polymers. Reversible bonding can be used to design supramolecular healable polymers and composites which respond to external stimuli such as heat [62], pressure [63], water [64], or light [65]. Various supramolecular interaction-based self-healing systems are shown in **Table 3**.

Stimuli responsiveness [67] and a high diffusion rate of oligomeric components [68] are the main characteristic of supramolecular polymers which make them for rapid and controllable healing system. Oligomers are low-molecularweight species that make aggregates by self-assembling and perform rheological or mechanical properties similar as polymers. The reversible non-covalent interactions can be possible by hydrogen bonding, ionic bonding, metal-ligand bonding, and π-π stacking.

#### *4.2.1 Supramolecular chemistry based on H-bonding*

H-bonding is the most popular route to achieve supramolecular polymers. Upon heat, interactions between the polymeric chains are decreased and reassembled upon cooling, and finally the non-covalent cross-linking recovered the strength and mechanical integrity. To achieve sufficient cross-linking density in polymer, a high association constant between repeating units is needed. The association constant and a reversible interaction have a reverse relation. In contrast, at the less association constant, better reversibility is achieved but having smaller assemblies and poor mechanical properties. However, individual supramolecular polymers are suffered by low mechanical strength. The mechanical strength of supramolecular system is enhanced by an increase in the number of non-covalent interaction [22] and by reinforcement with nanofiller [26]. Significantly, the interactions of matrix fiber are increased by the presence of hydrogen bond accepting functionality in matrix polymer blend. Adhesion promoter increases the amount of H-bonding of the matrix system [22]. These promoters are known as co-healing agents shown in **Figure 8**. On fracture of capsules, the DCPD monomer penetrates the networks of epoxy matrix and reacts with embedded catalyst. The formation of interpenetrated network is initiated by polymerization of DCPD which would strengthen the matrix—poly(DCPD) surface. Higher strength is obtained by entangled

**Figure 8.** *Supramolecular interactions with adhesion promoter [22].*

networks, and H-bonding will also improve overall bonding strength and healing efficiency. H-bonding-incorporated supramolecular system is shown in **Figure 8**.

The reinforcement of cellulose nanocrystals or cellulose nanowhiskers into polymer matrix such as poly(ethylene oxide-co-epichlorohydrin) [69] and low-density polyethylene [70] has shown improved stiffness corresponding to parental matrix materials.

### *4.2.2 Metal-ligand supramolecular polymers*

Optical and photo physical properties of metal complexes offer to design advanced materials. Reversible behavior of metal ion and ligand bond in metalligand complexes attract the research community to design stimuli-responsive materials. Metallo-supramolecular polymers have low-molecular-weight species known as telechelic. These are attached with ligand end group through metal-ion linkage. These polymers can be healed upon contact of light [65, 71]. During the whole healing mechanism, supramolecular interactions and light-heat conversion happen subsequently. The temporary disentanglement of metal-ligand motifs is possible when excited electronically upon contact with UV, and further, heat energy is released. Subsequently, the average molecular weight and viscosity of system are decreased, and defect healing is resulted. Local damages can also be recovered just upon light exposure. A metallo-hydrogel based on histidine and Zn2+ ions is designed using coordination-driven self-assembly [72]. The hydrogel formation is instantaneous, and it exhibits stimuli-responsive behavior with respect to pH, heat, and external chemicals.

#### *4.2.3 Supramolecular π-π interaction-assisted self-healing*

A thermally triggered reversible network is achieved based on π-π stacking interactions in which end-capped π-electron-deficient groups interact with π-electronrich aromatic backbone. The chain-folding co-polyimide (electron deficient) and pyrenyl (electron rich) end-capped polyamide chains have such π-π interactions [66]. The driving forces for producing tough, stable, healable homogeneous blend of elastomers are interpolymeric π-π stacking complexes [73]. At higher temperatures, the disengagement of the supramolecular (π-π stacking and hydrogenbonding) interactions is possible, which leads to change in the apparent molecular

**33**

*Self-Healing Polymer Composites for Structural Application*

refurbishing these supramolecular interactions.

*4.2.4 Supramolecular self-healing ionomers*

**Figure 9.**

*presented by Fall [27].*

weight of the homogeneous noncovalent polymer blend and further rapid change in viscosity with temperature. A polymer blend recovered the mechanical strength by

*Schematic diagram of ionomeric healing upon ballistic impact based on order-disorder theory of healing* 

Ionomers are polymers in which the bulk properties are governed by ionic interactions in discrete regions of the material [74]. Ionomers contain up to 15% ionic groups and respond instantaneously and autonomously in absence of external species, and additional heat or other stimuli make ionomers unique. Various bonding interactions such as ionic, dipole-dipole, or ion-dipole bonds are key factors to develop self-healing systems based on ionomers. Ballistic healing proceeds through combination of an elastic response (i.e., attain pure shape) and a viscous response (secondary polymer flow and chain entanglement) of intrinsic aggregates. Initially after impact, projectile transfers some of the impact energy to ionomer system which melts the matrix, and rest kinetic energy is stored elastically to movement, and the projectile is ejected and leaves behind the matrix with some melted portion. Finally, the hole is sealed and recovered some mechanical properties followed by crystallization and reaggregation of ionomers at the damage site. Healing process in ionomers is multistep. Initially upon high-energy impacts, the local deformation proceeded to less ordered melt state and resultant the projectile is ejected. After that, complex aggregates are formed via recrystallization of interdiffusion of intermolecular interactions. It is a quick process that happens within seconds to hours. In the second step, restructuring of physical cross-linking leads the final stage of healing; it takes longer duration as usually days to months. Some of commercial products such as React-A-Seal, Surlyn, and Nucrel have EMAA as base matrix [27]. It is a copolymer of ethylene and a vinyl monomer with an acidic group. EMAA

ionomer exposed self-healing upon ballistic impact (**Figure 9**) [27, 28].

During whole mechanism "free volume" plays a great role which provides enough mobility to polymer chain rearrangement and interdiffusion. Besides, many other factors, such as impact energy, nature of ionic groups, and counterions, the neutralization degree, increased temperature during impact, the content of ionic groups and dielectric constant, and so on, also play a key role to succeed self-healing by ionomers. To enhance the healing efficiency of ionomers composites, conductive

*DOI: http://dx.doi.org/10.5772/intechopen.82420*

*Self-Healing Polymer Composites for Structural Application DOI: http://dx.doi.org/10.5772/intechopen.82420*

#### **Figure 9.**

*Functional Materials*

**Figure 8.**

networks, and H-bonding will also improve overall bonding strength and healing efficiency. H-bonding-incorporated supramolecular system is shown in **Figure 8**. The reinforcement of cellulose nanocrystals or cellulose nanowhiskers into polymer matrix such as poly(ethylene oxide-co-epichlorohydrin) [69] and low-density polyethylene [70] has shown improved stiffness corresponding to parental matrix materials.

Optical and photo physical properties of metal complexes offer to design advanced materials. Reversible behavior of metal ion and ligand bond in metalligand complexes attract the research community to design stimuli-responsive materials. Metallo-supramolecular polymers have low-molecular-weight species known as telechelic. These are attached with ligand end group through metal-ion linkage. These polymers can be healed upon contact of light [65, 71]. During the whole healing mechanism, supramolecular interactions and light-heat conversion happen subsequently. The temporary disentanglement of metal-ligand motifs is possible when excited electronically upon contact with UV, and further, heat energy is released. Subsequently, the average molecular weight and viscosity of system are decreased, and defect healing is resulted. Local damages can also be recovered just upon light exposure. A metallo-hydrogel based on histidine and Zn2+ ions is designed using coordination-driven self-assembly [72]. The hydrogel formation is instantaneous, and it exhibits stimuli-responsive behavior with respect to pH, heat, and

A thermally triggered reversible network is achieved based on π-π stacking interactions in which end-capped π-electron-deficient groups interact with π-electronrich aromatic backbone. The chain-folding co-polyimide (electron deficient) and pyrenyl (electron rich) end-capped polyamide chains have such π-π interactions [66]. The driving forces for producing tough, stable, healable homogeneous blend of elastomers are interpolymeric π-π stacking complexes [73]. At higher temperatures, the disengagement of the supramolecular (π-π stacking and hydrogenbonding) interactions is possible, which leads to change in the apparent molecular

*4.2.2 Metal-ligand supramolecular polymers*

*Supramolecular interactions with adhesion promoter [22].*

*4.2.3 Supramolecular π-π interaction-assisted self-healing*

**32**

external chemicals.

*Schematic diagram of ionomeric healing upon ballistic impact based on order-disorder theory of healing presented by Fall [27].*

weight of the homogeneous noncovalent polymer blend and further rapid change in viscosity with temperature. A polymer blend recovered the mechanical strength by refurbishing these supramolecular interactions.

#### *4.2.4 Supramolecular self-healing ionomers*

Ionomers are polymers in which the bulk properties are governed by ionic interactions in discrete regions of the material [74]. Ionomers contain up to 15% ionic groups and respond instantaneously and autonomously in absence of external species, and additional heat or other stimuli make ionomers unique. Various bonding interactions such as ionic, dipole-dipole, or ion-dipole bonds are key factors to develop self-healing systems based on ionomers. Ballistic healing proceeds through combination of an elastic response (i.e., attain pure shape) and a viscous response (secondary polymer flow and chain entanglement) of intrinsic aggregates. Initially after impact, projectile transfers some of the impact energy to ionomer system which melts the matrix, and rest kinetic energy is stored elastically to movement, and the projectile is ejected and leaves behind the matrix with some melted portion. Finally, the hole is sealed and recovered some mechanical properties followed by crystallization and reaggregation of ionomers at the damage site. Healing process in ionomers is multistep. Initially upon high-energy impacts, the local deformation proceeded to less ordered melt state and resultant the projectile is ejected. After that, complex aggregates are formed via recrystallization of interdiffusion of intermolecular interactions. It is a quick process that happens within seconds to hours. In the second step, restructuring of physical cross-linking leads the final stage of healing; it takes longer duration as usually days to months. Some of commercial products such as React-A-Seal, Surlyn, and Nucrel have EMAA as base matrix [27]. It is a copolymer of ethylene and a vinyl monomer with an acidic group. EMAA ionomer exposed self-healing upon ballistic impact (**Figure 9**) [27, 28].

During whole mechanism "free volume" plays a great role which provides enough mobility to polymer chain rearrangement and interdiffusion. Besides, many other factors, such as impact energy, nature of ionic groups, and counterions, the neutralization degree, increased temperature during impact, the content of ionic groups and dielectric constant, and so on, also play a key role to succeed self-healing by ionomers. To enhance the healing efficiency of ionomers composites, conductive


#### **Table 4.**

*Photoinduced damage-healing system.*

and magnetic fillers are added [29]. In addition to plasticizer such as zinc stearate ionic domain, the matrix properties are intact [75].

## **5. Photochemically induced healing chemistry**

Photoinduced healing is currently demanding because of rapid, eco-friendly concept of healing of polymer matrix composites. It is induced by the application of a strong light irradiation. To damage healing, the foreign healing inserts, catalyst, or additional heat is neither required to comply with the healing process. Such system based on photochemical reactions is developed (**Table 4**).
