Self-healing material
Encyclopedia
Self-healing materials are a class of smart materials that have the structurally incorporated ability to repair damage caused by mechanical usage over time. The inspiration comes from biological systems, which have the ability to heal after being wounded. Initiation of cracks and other types of damage on a microscopic level has been shown to change thermal
, electrical, and acoustical properties, and eventually lead to whole scale failure of the material. Usually, cracks are mended by hand, which is difficult because cracks are often hard to detect. A material (polymers, ceramics, etc.) that can intrinsically correct damage caused by normal usage could lower production costs of a number of different industrial processes through longer part lifetime, reduction of inefficiency over time caused by degradation, as well as prevent costs incurred by material failure. For a material to be defined as self-healing, it is necessary that the healing process occurs without human intervention. Some examples shown below include healing polymers that are not "self-healing" polymers.
, and temperature.
From a macromolecular perspective, stress induced damage at the molecular level leads to larger scale damage called microcracks. A microcrack is formed where neighboring polymer chains have been damaged in close proximity, ultimately leading to the weakening of the fiber as a whole.
reporters such as DPPH
(2,2-Diphenyl-1-Picrylhydrazyl) and PMNB (Pentamethylnitrosobenzene.) When a bond is cleaved homolytically, two radical species are formed which can recombine to repair damage or can initiate other homolytic cleavages which can in turn lead to more damage.
, where mechanical stress cleaves two sigma bonds in a retro Diels-Alder reaction. This stress results in additional pi-bonded electrons as opposed to radical or charged moieties.
s, metal coordination
, and van der Waals forces. Mechanical stress in supramolecular polymers causes the disruption of these specific non-covalent interactions, leading to monomer separation and polymer breakdown.
is stable under normal condition, the reversible process usually requires an external stimulus for it to occur. For a reversible healing polymer, if the material is damaged by means such as heating and reverted to its constituents, it can be repaired or "healed" to its polymer
form by applying the original condition used to polymerize it.
containing the functional groups such as furan
or maleimide
form two carbon-carbon bonds in a specific manner and construct the polymer through DA reaction. This polymer, upon heating, breaks down to its original monomeric units via RDA reaction and then reforms the polymer
upon cooling or through any other conditions that were initially used to make the polymer. During the last few decades, two types of reversible polymers have been studied: (i) polymers where the pendant groups, such as furan
or maleimide
groups, cross-link through successive DA coupling reactions; (ii) polymers where the multifunctional monomers link to each other through successive DA coupling reactions.
In this type of polymer
, the polymer
forms through the cross linking of the pendant groups from the linear thermoplastics. For example, Saegusa et al. have shown the reversible cross-linking of modified poly(N-acetylethyleneimine)s containing either maleimide
or furancarbonyl pendant moideties. The reaction is shown in Scheme 3. They mixed the two complementary polymers to make a highly cross-linked material through DA reaction of furan and maleimide units at room temperature, as the cross-linked polymer
is more thermodynamically stable than the individual starting materials. However, upon heating the polymer to 80 °C for two hours in a polar
solvent
, two monomers were regenerated via RDA reaction, indicating the breaking of polymers. This was possible because the heating energy provided enough energy to go over the energy barrier and results in the two monomers. Cooling the two starting monomers, or damaged polymer
, to room temperature for 7 days healed and reformed the polymer
.
The reversible DA/RDA reaction is not limited to furan-meleimides based polymers as it is shown by the work of Schiraldi et al. They have shown the reversible cross-linking of polymers bearing pendent anthracene
group with maleimides. However, the reversible reaction occurred only partially upon heating to 250 °C due to the competing decomposition
reaction.
In this type of polymer, the DA reaction takes place in the backbone itself to construct the polymer, not as a link. For polymerization and healing processes of a DA-step-growth furan
-maleimide
based polymer (3M4F) were demonstrated by subjecting it to heating/cooling cycles. Tris-maleimide (3M) and tetra-furan (4F) formed a polymer through DA reaction and, when heated to 120 °C, de-polymerized through RDA reaction, resulting in the starting materials. Subsequent heating to 90–120 °C and cooling to room temperature healed the polymer, partially restoring its mechanical properties through intervention. The reaction is shown in Scheme 4.
. Under reducing condition, the disulfide
(SS) bridges in the polymer breaks and results in monomers, however, under oxidizing condition, the thiols (SH) of each monomer forms the disulfide bond
, cross-linking the starting materials to form the polymer. Chujo et al. have shown the thiol
-based reversible cross-linked polymer using poly(N-acetylethyleneimine). (Scheme 5)
healing (such as heat or light). Energy is introduced into the system to allow repolymerization to take place. This is not possible for all materials. Thermosetting polymers, for example, are not remoldable. Once they are polymerized (cured), decomposition
occurs before the melt temperature is reached. Thus, adding heat to initiate healing in the polymer is not possible. Additionally, thermosetting polymers cannot be recycled, so it is even more important to extend the lifetime of materials of this nature.
. (Note: this is already a commonly utilized practice for strengthening materials. See Fiber-reinforced plastic.) The resulting porous network is filled with monomer
. When damage occurs in the material from regular use, the tubes also crack and the monomer is released into the cracks. Other tubes containing a hardening agent also crack and mix with the monomer, causing the crack to be healed.
In order for this process to happen at room temperature
, and for the reactants to remain in a monomeric state within the capsule, a catalyst is also imbedded into the thermoset. The catalyst lowers the energy barrier of the reaction
and allows the monomer to polymerize without the addition of heat. The capsules (often made of wax
) around the monomer and the catalyst are important maintain separation until the crack facilitates the reaction.
There are many challenges in designing this type of material
. First, the reactivity of the catalyst must be maintained even after it is enclosed in wax. Additionally, the monomer must flow at a sufficient rate (have low enough viscosity
) to cover the entire crack before it is polymerized, or full healing capacity will not be reached. Finally, the catalyst must quickly dissolve into monomer in order to react efficiently and prevent the crack from spreading further.
This process has been demonstrated with dicyclopentadiene
(DCPD) and Grubbs' catalyst
(benzylidene-bis(tricyclohexylphosphine)dichlororuthenium). Both DCPD and Grubbs' catalyst
are imbedded in a epoxy
resin
. The monomer
on its own is relatively unreactive and polymerization
does not take place. When a microcrack reaches both the capsule containing DCPD and the catalyst, the monomer
is released from the core-shell microcapsule and comes in contact with exposed catalyst, upon which the monomer undergoes ring opening metathesis polymerization (ROMP). The metathesis reaction
of the monomer
involves the severance of the two double bonds in favor of new bonds. The presence of the catalyst allows for the energy barrier (energy of activation) to be lowered, and the polymerization reaction
can proceed at room temperature. The resulting polymer
allows the epoxy
composite material
to regain 67% of its former strength.
Grubbs' catalyst
is a good choice for this type of system because it is insensitive to air and water, thus robust enough to maintain reactivity within the material. Utilizing a live catalyst is important to promote multiple healing actions. The major drawback is the cost. It was shown that using more of the catalyst corresponded directly to higher degree of healing. Ruthenium
is a quite costly, which makes it impractical for commercial applications.
On August 3, 2010, University of Wisconsin-Milwaukee researchers demonstrated an aluminum-fly ash composite material, a self-healing metal which uses a similar "balloon" capsule deposition process during casting. This material has been formed into prototype engine components for field research by use in diesel truck engines (see UWM Researchers Work to Develop Self-Healing Metal).
system containing microcapsules
. These microcapsules were filled with a (liquid) monomer
. If a microcrack occurs in this system, the microcapsule will rupture and the monomer will fill the crack. Subsequently it will polymerise
, initiated by catalyst particles (Grubbs
catalyst) that are also dispersed through the system. This model system of a self healing particle proved to work very well in pure polymers and polymer coatings.
A hollow glass fibre approach may be more appropriate for self-healing impact damage in fibre-reinforced polymer composite materials. Impact damage can cause a significant reduction in compressive strength with little damage obvious to the naked eye. Hollow glass fibres containing liquid healing agents (some fibres carrying a liquid epoxy monomer and some the corresponding liquid hardener) are embedded within a composite laminate. Studies have shown significant potential.
s (i.e. hydrogen bond
), which will disassociate at elevated temperatures. Healing of these supramolecullary based materials is accomplished by heating them and allowing the non-covalent bonds to break. Upon cooling new bonds will be formed and the material will potentially heal any damage. An advantage of this method is that no reactive chemicals or (toxic) catalysts are needed. However, these materials are not "autonomic" as they require the intervention of an outside agent to initiate a healing response.
design approaches being used in the development of self-healing systems for polymer composites.
Thermal
A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...
, electrical, and acoustical properties, and eventually lead to whole scale failure of the material. Usually, cracks are mended by hand, which is difficult because cracks are often hard to detect. A material (polymers, ceramics, etc.) that can intrinsically correct damage caused by normal usage could lower production costs of a number of different industrial processes through longer part lifetime, reduction of inefficiency over time caused by degradation, as well as prevent costs incurred by material failure. For a material to be defined as self-healing, it is necessary that the healing process occurs without human intervention. Some examples shown below include healing polymers that are not "self-healing" polymers.
Polymer breakdown
From a molecular perspective, traditional polymers yield to mechanical stress though cleavage of sigma bonds. While newer polymers can yield in other ways, traditional polymers typically yield through homolytic or heterolytic bond cleavage. The factors that determine how a polymer will yield include: type of stress, chemical properties inherent to the polymer, level and type of solvationSolvation
Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
, and temperature.
From a macromolecular perspective, stress induced damage at the molecular level leads to larger scale damage called microcracks. A microcrack is formed where neighboring polymer chains have been damaged in close proximity, ultimately leading to the weakening of the fiber as a whole.
Homolytic bond cleavage
Polymers have been observed to undergo homolytic bond cleavage through the use of radicalRadical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
reporters such as DPPH
DPPH
DPPH is a common abbreviation for an organic chemical compound 2,2-diphenyl-1-picrylhydrazyl. It is a dark-colored crystalline powder composed of stable free-radical molecules...
(2,2-Diphenyl-1-Picrylhydrazyl) and PMNB (Pentamethylnitrosobenzene.) When a bond is cleaved homolytically, two radical species are formed which can recombine to repair damage or can initiate other homolytic cleavages which can in turn lead to more damage.
Heterolytic bond cleavage
Polymers have also been observed to undergo heterolytic bond cleavage through isotope labeling experiments. When a bond is cleaved heterolytically, cationic and anionic species are formed which can in turn recombine to repair damage, can be quenched by solvent, or can react destructively with nearby polymers.Reversible bond cleavage
Certain polymers yield to mechanical stress in an atypical, reversible manner. Diels-Alder-based polymers undergo a reversible cycloadditionCycloaddition
A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction.Cycloadditions are usually described by the...
, where mechanical stress cleaves two sigma bonds in a retro Diels-Alder reaction. This stress results in additional pi-bonded electrons as opposed to radical or charged moieties.
Supramolecular breakdown
Supramolecular polymers are composed of monomers that interact non-covalently. Common interactions include hydrogen bondHydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s, metal coordination
Coordination
Coordination is the act of coordinating, making different people or things work together for a goal or effect to fulfill desired goals in an organization.Coordination is a managerial function in which different activities of the business are properly adjusted and interlinked.Coordination may also...
, and van der Waals forces. Mechanical stress in supramolecular polymers causes the disruption of these specific non-covalent interactions, leading to monomer separation and polymer breakdown.
Reversible healing polymers
Reversible systems are polymeric systems that can revert to the initial state whether it is monomeric, oligomeric, or non-cross-linked. Since the polymerPolymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
is stable under normal condition, the reversible process usually requires an external stimulus for it to occur. For a reversible healing polymer, if the material is damaged by means such as heating and reverted to its constituents, it can be repaired or "healed" to its polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
form by applying the original condition used to polymerize it.
Diels-Alder and Retro-Diels-Alder
Among the examples of reversible healing polymers, the Diels-Alder (DA) reaction and its Retro-Diels-Alder (RDA) analogue seems to be very promising due to its thermal reversibility. In general, the monomerMonomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
containing the functional groups such as furan
Furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....
or maleimide
Maleimide
Maleimide is the chemical compound with the formula H2C22NH . This unsaturated imide is an important building block in organic synthesis. The name is a contraction of maleic acid and imide, the -CNHC- functional group...
form two carbon-carbon bonds in a specific manner and construct the polymer through DA reaction. This polymer, upon heating, breaks down to its original monomeric units via RDA reaction and then reforms the polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
upon cooling or through any other conditions that were initially used to make the polymer. During the last few decades, two types of reversible polymers have been studied: (i) polymers where the pendant groups, such as furan
Furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....
or maleimide
Maleimide
Maleimide is the chemical compound with the formula H2C22NH . This unsaturated imide is an important building block in organic synthesis. The name is a contraction of maleic acid and imide, the -CNHC- functional group...
groups, cross-link through successive DA coupling reactions; (ii) polymers where the multifunctional monomers link to each other through successive DA coupling reactions.
Cross-linked polymers
In this type of polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
, the polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
forms through the cross linking of the pendant groups from the linear thermoplastics. For example, Saegusa et al. have shown the reversible cross-linking of modified poly(N-acetylethyleneimine)s containing either maleimide
Maleimide
Maleimide is the chemical compound with the formula H2C22NH . This unsaturated imide is an important building block in organic synthesis. The name is a contraction of maleic acid and imide, the -CNHC- functional group...
or furancarbonyl pendant moideties. The reaction is shown in Scheme 3. They mixed the two complementary polymers to make a highly cross-linked material through DA reaction of furan and maleimide units at room temperature, as the cross-linked polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
is more thermodynamically stable than the individual starting materials. However, upon heating the polymer to 80 °C for two hours in a polar
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
, two monomers were regenerated via RDA reaction, indicating the breaking of polymers. This was possible because the heating energy provided enough energy to go over the energy barrier and results in the two monomers. Cooling the two starting monomers, or damaged polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
, to room temperature for 7 days healed and reformed the polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
.
The reversible DA/RDA reaction is not limited to furan-meleimides based polymers as it is shown by the work of Schiraldi et al. They have shown the reversible cross-linking of polymers bearing pendent anthracene
Anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon consisting of three fused benzene rings. It is a component of coal-tar. Anthracene is used in the production of the red dye alizarin and other dyes...
group with maleimides. However, the reversible reaction occurred only partially upon heating to 250 °C due to the competing decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
reaction.
Polymerization of multifunctional monomers
In this type of polymer, the DA reaction takes place in the backbone itself to construct the polymer, not as a link. For polymerization and healing processes of a DA-step-growth furan
Furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....
-maleimide
Maleimide
Maleimide is the chemical compound with the formula H2C22NH . This unsaturated imide is an important building block in organic synthesis. The name is a contraction of maleic acid and imide, the -CNHC- functional group...
based polymer (3M4F) were demonstrated by subjecting it to heating/cooling cycles. Tris-maleimide (3M) and tetra-furan (4F) formed a polymer through DA reaction and, when heated to 120 °C, de-polymerized through RDA reaction, resulting in the starting materials. Subsequent heating to 90–120 °C and cooling to room temperature healed the polymer, partially restoring its mechanical properties through intervention. The reaction is shown in Scheme 4.
Thiol-based polymers
The thiol-based polymers have disulfide bonds that can be reversibly cross-linked through oxidation and reductionReduction
Reduction, reduced, or reduce may refer to:- Chemistry :* Reduction, part of a reduction-oxidation reaction where oxygen is being removed from a compound.** Reduced gas, a gas with a low oxidation number...
. Under reducing condition, the disulfide
Disulfide
In chemistry, a disulfide usually refers to the structural unit composed of a linked pair of sulfur atoms. Disulfide usually refer to a chemical compound that contains a disulfide bond, such as diphenyl disulfide, C6H5S-SC6H5....
(SS) bridges in the polymer breaks and results in monomers, however, under oxidizing condition, the thiols (SH) of each monomer forms the disulfide bond
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
, cross-linking the starting materials to form the polymer. Chujo et al. have shown the thiol
Thiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...
-based reversible cross-linked polymer using poly(N-acetylethyleneimine). (Scheme 5)
Autonomic polymer healing
Thus far, all of the examples on this page require an external stimulus to initiate polymerPolymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
healing (such as heat or light). Energy is introduced into the system to allow repolymerization to take place. This is not possible for all materials. Thermosetting polymers, for example, are not remoldable. Once they are polymerized (cured), decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
occurs before the melt temperature is reached. Thus, adding heat to initiate healing in the polymer is not possible. Additionally, thermosetting polymers cannot be recycled, so it is even more important to extend the lifetime of materials of this nature.
Hollow tube approach
For the first method, fragile glass capillaries or fibers imbedded within a composite materialComposite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
. (Note: this is already a commonly utilized practice for strengthening materials. See Fiber-reinforced plastic.) The resulting porous network is filled with monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
. When damage occurs in the material from regular use, the tubes also crack and the monomer is released into the cracks. Other tubes containing a hardening agent also crack and mix with the monomer, causing the crack to be healed.
Microcapsule healing
This method is similar in design to the hollow tube approach. Monomer is encapsulated and embedded within the thermosetting polymer. When the crack reaches the microcapsule, the capsule breaks and the monomer bleeds into the crack, where it can polymerize and mend the crackIn order for this process to happen at room temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, and for the reactants to remain in a monomeric state within the capsule, a catalyst is also imbedded into the thermoset. The catalyst lowers the energy barrier of the reaction
Reaction
Reaction may refer to:Response to another event*Adverse drug reaction*Chemical reaction*Emotional reaction*Light reaction*Nuclear reaction*TNA Reaction, a documentary show of TNA behind scenes.*Reaction , as defined by Newton's third law...
and allows the monomer to polymerize without the addition of heat. The capsules (often made of wax
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
) around the monomer and the catalyst are important maintain separation until the crack facilitates the reaction.
There are many challenges in designing this type of material
Material
Material is anything made of matter, constituted of one or more substances. Wood, cement, hydrogen, air and water are all examples of materials. Sometimes the term "material" is used more narrowly to refer to substances or components with certain physical properties that are used as inputs to...
. First, the reactivity of the catalyst must be maintained even after it is enclosed in wax. Additionally, the monomer must flow at a sufficient rate (have low enough viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
) to cover the entire crack before it is polymerized, or full healing capacity will not be reached. Finally, the catalyst must quickly dissolve into monomer in order to react efficiently and prevent the crack from spreading further.
This process has been demonstrated with dicyclopentadiene
Dicyclopentadiene
Dicyclopentadiene, abbreviated DCPD, is a chemical compound with formula C10H12. At room temperature, it is a white crystalline solid with a camphor-like odor. Its energy density is 10,975 Wh/l....
(DCPD) and Grubbs' catalyst
Grubbs' catalyst
Grubbs' Catalyst is a transition metal carbene complex named after Robert H. Grubbs, the chemist who first synthesized it. There are two generations of the catalyst, as shown on the right. In contrast to other olefin metathesis catalysts, Grubbs' Catalysts tolerate other functional groups in the...
(benzylidene-bis(tricyclohexylphosphine)dichlororuthenium). Both DCPD and Grubbs' catalyst
Grubbs' catalyst
Grubbs' Catalyst is a transition metal carbene complex named after Robert H. Grubbs, the chemist who first synthesized it. There are two generations of the catalyst, as shown on the right. In contrast to other olefin metathesis catalysts, Grubbs' Catalysts tolerate other functional groups in the...
are imbedded in a epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
. The monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
on its own is relatively unreactive and polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
does not take place. When a microcrack reaches both the capsule containing DCPD and the catalyst, the monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
is released from the core-shell microcapsule and comes in contact with exposed catalyst, upon which the monomer undergoes ring opening metathesis polymerization (ROMP). The metathesis reaction
Reaction
Reaction may refer to:Response to another event*Adverse drug reaction*Chemical reaction*Emotional reaction*Light reaction*Nuclear reaction*TNA Reaction, a documentary show of TNA behind scenes.*Reaction , as defined by Newton's third law...
of the monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
involves the severance of the two double bonds in favor of new bonds. The presence of the catalyst allows for the energy barrier (energy of activation) to be lowered, and the polymerization reaction
Reaction
Reaction may refer to:Response to another event*Adverse drug reaction*Chemical reaction*Emotional reaction*Light reaction*Nuclear reaction*TNA Reaction, a documentary show of TNA behind scenes.*Reaction , as defined by Newton's third law...
can proceed at room temperature. The resulting polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
allows the epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
to regain 67% of its former strength.
Grubbs' catalyst
Grubbs' catalyst
Grubbs' Catalyst is a transition metal carbene complex named after Robert H. Grubbs, the chemist who first synthesized it. There are two generations of the catalyst, as shown on the right. In contrast to other olefin metathesis catalysts, Grubbs' Catalysts tolerate other functional groups in the...
is a good choice for this type of system because it is insensitive to air and water, thus robust enough to maintain reactivity within the material. Utilizing a live catalyst is important to promote multiple healing actions. The major drawback is the cost. It was shown that using more of the catalyst corresponded directly to higher degree of healing. Ruthenium
Ruthenium
Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...
is a quite costly, which makes it impractical for commercial applications.
On August 3, 2010, University of Wisconsin-Milwaukee researchers demonstrated an aluminum-fly ash composite material, a self-healing metal which uses a similar "balloon" capsule deposition process during casting. This material has been formed into prototype engine components for field research by use in diesel truck engines (see UWM Researchers Work to Develop Self-Healing Metal).
Liquid-based healing agents
Completely autonomous synthetic self-healing material was reported in 2001 on example of an epoxyEpoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
system containing microcapsules
Micro-encapsulation
Micro-encapsulation is a process in which tiny particles or droplets are surrounded by a coating to give small capsules many useful properties. In a relatively simplistic form, a microcapsule is a small sphere with a uniform wall around it...
. These microcapsules were filled with a (liquid) monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
. If a microcrack occurs in this system, the microcapsule will rupture and the monomer will fill the crack. Subsequently it will polymerise
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
, initiated by catalyst particles (Grubbs
Robert H. Grubbs
Robert Howard Grubbs is an American chemist and Nobel laureate.As he noted in his official Nobel Prize autobiography, "In some places, my birthplace is listed as Calvert City and in others Possum Trot [NB: both in Marshall County]...
catalyst) that are also dispersed through the system. This model system of a self healing particle proved to work very well in pure polymers and polymer coatings.
A hollow glass fibre approach may be more appropriate for self-healing impact damage in fibre-reinforced polymer composite materials. Impact damage can cause a significant reduction in compressive strength with little damage obvious to the naked eye. Hollow glass fibres containing liquid healing agents (some fibres carrying a liquid epoxy monomer and some the corresponding liquid hardener) are embedded within a composite laminate. Studies have shown significant potential.
Solid-state healing agents
In addition to the sequestered healing agent strategies described above, research into "intrinsically" self-healing materials is also being performed. For example, supramolecular polymers are materials formed by reversibly connected non-covalent bondCovalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s (i.e. hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
), which will disassociate at elevated temperatures. Healing of these supramolecullary based materials is accomplished by heating them and allowing the non-covalent bonds to break. Upon cooling new bonds will be formed and the material will potentially heal any damage. An advantage of this method is that no reactive chemicals or (toxic) catalysts are needed. However, these materials are not "autonomic" as they require the intervention of an outside agent to initiate a healing response.
Biomimetic design approaches
Self-healing materials are widely encountered in natural systems, and inspiration can be drawn from these systems for design. There is evidence in the academic literature of these biomimeticBiomimicry
Biomimicry or biomimetics is the examination of nature, its models, systems, processes, and elements to emulate or take inspiration from in order to solve human problems. The term biomimicry and biomimetics come from the Greek words bios, meaning life, and mimesis, meaning to imitate...
design approaches being used in the development of self-healing systems for polymer composites.