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15САМОЗАЛЕЧИВАЮЩИЕ МАТЕРИАЛЫ
А. Б. Тарасов
Факультет наук о материалах МГУ им. М. В. Ломоносова Ленинские горы, Москва, 119992 Тел.: 939-47-29. факс: 939-09-98
Introduction
Last 50 years a great development has been made in polymer materials and now they are used very widely. They are used in aerospace, car, military etc. industries, and its consumption grows in all fields.
If you close your eyes and point out to anything around, you will surely point out to an object, which was made out of polymers: cloths, cars, furniture, compact disks and disk boxes, household appliances, plastic coating, interior trim ... even a keyboard, which I used to type this essay, and monitor you are reading it on, are made out of polymers.
Polymer and polymer composites have a lot of advantages over other materials, such as resistance to intrusion of environmental chemicals, increased/beneficial vibration damping, energy absorption, electromagnetic transparency, toughness, control of stiffness, high strength to weight ratios, and being lightweight to decrease dead load as well as transport costs.
Unfortunately, in spite of all these exceptional features, plastic materials show one very painful aspect: many widely used polymers are susceptible to damage in the form of cracks, which are formed deeply inside, where its detection is difficult and its cure is practically impossible. Cracking leads to mechanical degradation of fibre-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Micro-cracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives.
For a long time people dream about self-heal-ing materials — materials inspired by biological systems, where an externally induced damage triggers an internal healing response routines.
It is important to realize that creation of materials, what will be able to self-repair eternally, is impossible of law of conservation of energy and entropy reasons. However, it is urgent to create such materials that will be able to endure critical load and prolong its lifetime until a manual repair will be available, without critical effect. Such materials would be used for medical implants to func-
tion for a certain time until surgery operation, or for aircraft space devices to accomplish landing in extreme situation without an accident.
Self-healing materials is a very young field of research. First investigations have been done in mid 90s by Scott R. White and co-workers. Since that time, there are several approaches and methods of creating these systems has made. Here are some of them.
Encapsulated micro-spheres
Materials, developed on this idea, consist of a self-healing polymer that incorporates a microencapsulated healing agent (dicyclopentadiene, DCPD) and bis-tricyclohexylphosphine benzylidene ruthenium (IV) dichloride, a solid chemical catalyst known as Grubbs' catalyst, in a polymer matrix. When a crack appears in the polymer and grows under cyclic loading in tension, embedded microcapsules are rupturing by this crack growth, and release the dicyclopentadiene into the crack plane through capillary action. Polymerization of the dicyclopentadiene is triggered by contact with the suspended catalyst phase.
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Fig. 1. Schematic of self-healing material with incorporated microspheres during fatigue (<0 2001 Nature Publishing Group)
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poly-DCPD
As the healing reaction progresses, the crack plane fills with polymerized dicy-clopentadiene which glue together a crack periphery. As prof. Brown's works has shown, these materials system recovers up to 90 % of its original fracture toughness.
The healing agent/catalyst couple has been chosen because it satisfies several requirements critical for achieving high healing efficiency. First, the healing agent possesses a low viscosity, which facilitates its flow into the crack plane and allows for complete coverage of the exposed surfaces. Second, this solid-phase catalyst remains reactive during and after curing of the coating. Finally, the catalyst particles quickly dissolve on contact with the monomer in the crack plane and polymerize the DCPD under ambient conditions, producing a tough crosslinked polymer
Technology for embedding microcapsules in polymers already exists and is widely used in the paints industry; the capsules could be produced relatively easily using standard processing techniques; and the insertion of small, spherical particles into a brittle matrix is known not to weaken the material's mechanical properties.
Imperfection of this method lies in the difficulty of using it in low temperature, under high stress intensity and relative complexity of production.
Effectiveness of the healing depends on the amount of time the system has been allowed to heal. DCPD molecules should have time to fill the crack and undergo polymerization before the crack periphery breaks up too much. As the temperature is lowered, the amount of time, required for healing, increases, and this effect, coupled with the relatively high melting point of many reacting monomers, limits the temperature window over which practical self-healing can occur. Using monomers with low melting point can to solve this problem.
This approach also can be used in brittle polymers for prevention incipient fracture. For example, imagine duct coat inside and outside with holeproof cloth which won't allow it to break into pieces after external impact. Then in the course of time, after the pieces will glue up together, material will repair some part of his original fracture toughness. If external coat will be waterproof, it may also be useful in subsea works.
Hollow glass fibres
Usage of composites in airplane production has been growing every year for the past thirty
microcapsule
Fig. 2. SEM of the fracture plane of a self-healing polymer (A. S. Jones et al. J.R.Soc. Interface (2007))
years. Forty percent of that growth is in military aircrafts. A major portion of the growth occurs due to the need for composite-based specialties, such as tails, rudders, doors, and flaps. Advantages are light weight in relation to strength. For example: glass fiber composite is five times stronger than aluminum, carbon reinforcing can give up to 30-40 % structural weight reduction. Boeing's F/A18 Hornet has 50 % of its skin made up of composite material.
Similar approach for self-repairing materials creating is incorporating hollow glass fibers (HGFs) repair components filled, in laminates. Fiber reinforced polymer (FRP) composite materials are leading contenders to improve the efficiency and sus-tainability of many forms of transport. However, despite they show an amazing strength, in many cases materials which able to heal himself are urgent. Self repairing materials can be very suitable for this requirement.
The self-healing fibers can be introduced within a laminate fiber-reinforced polymer as additional plies at each interface, at damage critical interfaces or as individual filaments spaced at predetermined distances within each ply.
A typical hollow fibre self-healing approach used within composite laminates could take the form of fibres containing a one-part resin system, a two-part resin and hardener system, or a resin system with a catalyst or hardener contained within the matrix material.
HGF within composite material filling, using a vacuum-assisted technique, with healing resin (Cytec Cycom 823).
During a damage event, some of these hollow fibres will fracture, thus initiating the recovery of properties by 'healing' whereby a repair agent flowing out from within any broken hollow fibres
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. One-part resin
H — Polymer matrix Hollow fibre
__. Resin system
__Hardener system
--- Hollow fibre
Resin system
_ Micro-encapsulated
hardener
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Fig.3. Schematic of different hollow fibre self-healing approaches (R.S.Trask et al. J.R.Soc. Interface (2007))
to infiltrate the damage zone, and acts to ameliorate the critical effects of matrix cracking and delamination between plies and, most importantly, prevent further damage propagation. This release of repair agent mimics the bleeding mechanism in biological organisms.
Hollow glass fibres are used in preference to embedded microcapsules because they offer the advantage of being able to store functional agents for self-repair as well as integrating easily with and acting as a reinforcement.
In comparison with ordinary FRP, materials with HGFs inclusion have some reduction to an initial strength and thereby have less damage tolerance. However in Williams's and Trask's experiments, has shown, that after small healing time these materials, contain 200 nm HGF, can restore to 87 % of theirs natural strength.
Unfortunately if crack faces will be separated more than 30 mm, capillary forces will be probably insufficient, or there will be inadequate resin volume released to fully infiltrate the damage. It is also necessary to choose fibres materials with low surface wetting energy for reducing of capillary effects.
Much works remains to be done in developing a healing agent suited to this application, providing controllable initiation on demand, robustness in stoichiometry and longevity in the uncured state.
Self-healing materials with microvascular networks
to solve this problem and are capable independently to repair repeated damage events.
The outer epidermal layer of skin is composed of multiple sublayers that work in concert to continually rebuild the surface of the skin, whereas the underlying dermal layer supplies the epidermis with nutrient-laden blood and regulates temperature. Because skin serves as a protective barrier, any damage must be rapidly and efficiently healed. A cut in the skin triggers blood flow from the capillary network in the dermal layer to the wound site rapidly forming a clot that serves as a matrix through which cells and growth factors migrate as healing ensues. Owing to the vascular nature of this supply system, minor damage to the same area can be healed repeatedly.
Using soft lithographic and directwrite assembly methods materials with complex embedded pervasive microvascular networks can be created. After that, an epoxy coating is deposited on a more ductile substrate that contains a microvascular network. Solid catalyst particles are incorporated within the coating and the network is filled with a liquid healing agent and then sealed.
Healing agent/catalyst couple in this approach has chosen identical as in microencapsulated materials.
The coating-substrate specimen is loaded until crack initiation occurs at the surface of the coating, where the tensile stress is maximum. The resulting cracks are attracted to the more compliant regions of the substrate created by the presence of fluid-filled micro channels and then arrested at the coating-substrate interface. After damage occurs in the coating, healing agent wicks from the micro channels into the crack(s) through capillary action. No external pressure is required. Once in the crack plane, the healing agent inter-
In both previous cases, the major imperfection is impossibility to repeat a healing in same location, because after rupture of the embedded capsules or HGFs localized region is depleted of healing agent and further repair is impossible.
Modern bio-inspired materials, that mimic a human skin, are able
Fig. 4. Crushed-healing fibres located under the impact site viewed under (a) normal and (b) UV illumination. Healing resin bridging cracked interface viewed under (c) normal and (d) UV illumination (R. S. Trash et al.. J. R. Soc. Interface (2007))
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Fig. 5. Self-healing materials with 3D microvascular networks: a — schematic diagram of a capillary network in the dermis layer of skin withz a cut in the epidermis layer: b — schematic diagram of the self healing structure composed of a microvascular substrate and a brittle epoxy coating containing embedded catalyst in a four-point bending configuration monitored with an acoustic-emission sensor; c — high-magnification cross-sectional image of the coating showing that cracks, which Initiate at the surface, propagate towards the microchannel openings at the interface (scale bar = 0.5 mm ); d — optical image of self-healing structure after cracks arc formed in the coating, revealing the presence of excess healing fluid on the coating surface (scale bar — 5 mm) (Jeffrey S.Moore et at., © 2007 Nature Publishing Group)
acts with the catalyst particles in the coating to initiate polymerization, rebonding the crack faces. After a sufficient time period, the cracks are healed and the structural integrity of the coating is restored. As cracks reopen under subsequent loading, the healing cycle is repeated.
As a presence of channels impacts the structural properties of the substrate, networks with maximum channel spacing and minimum channel diameter are desirable. The spacing between channels in a given layer is limited to approximately ten times the channel diameter owing to the vis-coelastic properties of the fugitive ink. In addition, the channel diameter must be large enough for healing agent to flow through the network to the cracks in the coating. The channel diameter of 200 pm is sufficiently small to minimize the total pore volume, yet large enough for ease of fabrication and network operation
The limitations associated with depletion of embedded catalyst and the necessity to re-supply multiple healing agents within these architectures may be overcome by implementing a new microvascular design based on interdigitated dual networks. This improved design will allow new healing chemistries (for example, two-part epoxies) to be exploited, which could ultimately lead to unlimited healing capability. It is possible imagine extending this approach further to integrate pumps, valves and internal reservoirs, as well as to introduce new functionalities, including self-diagnosis or selfcooling, through the circulation of molecular signals, coolants or other species.
Thermosetting epoxy resins with dissolved linear polymer
Described above liquid resin-based systems are capable of autonomic healing, as they do not need any external effect, because the mechanism of healing involves the fracture of the vessels containing the healing resins. However, while autonomic healing is possible in these systems, more efficient recovery of matrix strength is observed at an elevated temperature. Any new resin would need to have equivalent thermo-mechanical properties to at least an epoxy for use as a matrix in fibre composites. Thus their advantage against another approach, the solid-state self-repairing systems is somewhat reduced.
This self-healing materials consist of a thermosetting epoxy resin into which a linear polymer is dissolved. The solubility parameters are matched so that dissolved 'healing agent' remains uniformly dissolved in the matrix, without phase separation. The linear polymer should be bonded into the three-dimensional epoxy matrix through hydrogen bonding but become mobile above a particular temperature. Thus, on heating, the 'healing agent* can diffuse throughout the matrix bridging any closed cracks within it.
The requirements for solid-state healing of a thermo-set resin, which used the thermal diffusion of a healing agent, are as follows:
• The healing agent should be reversibly bonded (e. g. through hydrogen bonding) to the cross-linked network of the cured resin below the mini-
Epoxy cootrvj
substrate
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mum healing temperature to limit its effect on thermo-mechanical properties.
• The healing agent should become mobile above this minimum healing temperature so that it can diffuse across a hairline crack, such as a transverse crack, to provide a recovery in strength.
• The healing agent should have sufficient viscosity to avoid entrapment of bubbles in the material.
• The addition of the linear chain molecule should not significantly reduce the thermo-mechanical properties of the resin matrix.
In professor S. A. Hayes investigations, the matrix resin was chosen to be a blend of Epikote 828 (a diglycidylether of bisphenol A) cured with nadic methylene tetrahydrophthalic anhydride (NMA) and the mercaptan accelerator, Capcure 3-800. The healing agent which proved to be effective in his previous papers was a polybisphenol-A-co-epichlorohydrin with A/„ = 44000 gmol"1.
Experiments has shown, that matrix containing 7.5 % (optimum concentration) of healing agent exhibit approximately 50 % repairing of origin impact strength after healing by heating without external restraint for 2h at 130 °C.
Having been included in smart composite system, suggested in S. A. Hayes works, which combines structural health monitoring with a self-healing resin, this materials demonstrate genuine self-repairing properties. For example, self-sensing of micromechanical damage in the form of matrix cracks and delaminations have proved possible for carbon fibre laminates using a triangulation approach. Since the detection method employs the change in resistance in the sensed carbon fibre laminate, the sensor can also be used as a local heating element.
The main advantage of the described solid-state approach is that it is based on conventional matrix resin technology. Therefore, the whole range of current matrix systems has the potential to be modified for healing. In addition, there is no requirement to incorporate special reinforcing fibres or microcapsules; therefore, in principle, conventional. Unfortunately, exactly sensor and heating systems necessity makes this approach application rather difficult.
A hybrid polymer gel with controlled rates of cross-link rupture and self-repair
Damage in the polymer materials can be repaired not only through creation new chemical bonds, as healing agent polymerization, but also by the re-creation of the same chemical bonds that are ruptured in the damage event.
In this case, the loss of stress-bearing after external impact must occur through bond rupture that is reversible, and the bonds must break in a way that does not lead to subsequent, mechanically unproductive reactions.
To preserve a desired structure, reversible interactions might be combined in a composite material with components that preserve a memory of the
Fig. 6. Optical micrographs of a glass fibre composite subjected to two impact and heal cycles showing the closure of damage in a healable matrix composite: a — non-healed and b — after healing (S.A. Hayes et at.. J. R. Soc. Interface (2007))
desired state: encasement within an exoskeleton; tethering to an endoskeleton; or with an additional 'fixed' system in an interpenetrating network.
In prof. F. R. Kersey's works describe a hybrid polymeric consist of permanent, structural component is a polymer network created by cova-lent cross-links between polymer chains and reversible, potentially self-healing component is a metal-ligand coordination complex between a bi-functional Pd(II) or Pt(II) pincer complex, and a polymer side-chain pyridine
Unfortunately I could not find an information about any practical realization of this approach, maybe because it very new, but is seems for me very promising.
This strategy — make no resistance for extreme impact, witch may lead to damage, and slow-
Sub-critical current density: Surface detect -> Increased current 2D particle "gas" density: Defect nucleates assembly
Fig. 7. Electrohydrodynamic aggregation of particles (Ilhan Aksay. Princeton University)
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ly restore origin shape after damage event — used already in some nature and safety device. However it can not be realized widely in industry because of high costs of components.
Electro hydrodynamic self-repairing
Another bio-inspired materials are based on classic blood clot scenario, using a principle of electro hydrodynamic (EHD) flow to replicate the same functions in a synthetic material, prof. II-han Aksay from Princeton University suggested.
He describes a test system in which a suspension of colloidal particles is enclosed between the walls of a double-walled cylinder. The cylindrical walls are coated with a thin conducting layer, followed by an insulating film, and are then attached to an electrical power source. This forms a double-wall electrode system that can be activated if the insulating film fails. If the structure is damaged in any way, leading to crack formation in the insulating film, the colloidal particles aggregate (similar to blood coagulation) at the defect site by EHD flow and trigger the first stage of self-healing, he explains.
The system's dependence on a continuous electric field is analogous to biological materials, which rely on a continual sensing mechanism provided by nerve endings.
Conclusions
Because of its unique features, R&D of self-healing polymer materials will grow in many fields of application together with total growth of plastics consumption in a world. Key question of creating industrially used self-healing polymers is economical feasibility of large-scale usage of such materials. They could be undoubtedly used in such fields like medicine, aerospace or nuclear power production, where impossible to overestimate a cost of damage.
that could be induced by a loss of protective or constructive features of used polymer materials.
References
1.VerbergR., Dale A. T., Prashant Kumar, AlexeevA., Balazs A. C. Healing substrates with mobile, particle-filled microcapsules: designing a 'repair and go' system // J. R. Soc. Interface. 2007.
2. Dry C. Self-Repairing Polymer Composites for Airplanes // Natural Process Design. 2005.
3. Kersey F. R., Loveless D. M., Craig S. L. A hybrid polymer gel with controlled rates of crosslink rupture and self-repair // J. R. Soc. Interface. 2007.
4. Toohey K. S., Sottos N. R., Lewis J. A., Moore J. S., WhiteS. R. Self-healing materials with microvascular networks // Nature Materials. 2007.
5. Hayes S. A., Zhang W., Branthwaite M., Jones F. R. Self-healing of damage in fibre-reinforced polymer-matrix composites // J. R. Soc. Interface. 2007.
6. Kessler M. R., Sottos N. R., WhiteS. R. Self-healing structural composite materials // Composites. 2003.
7. Trask R. S., Williams G. J., Bond I. P. Bi-oinspired self-healing of advancedcomposite structures using hollow glass fibres //J. R. Soc. Interface. 2007.
8. White S. R., Sottos N. R., Geubelle P. H., Moore J. S., Kessler M. R., Sriram S. R., Brown E. N.. Viswanathan S. Autonomic healing of polymer composites // Nature. 2001. 409.
9. Jones A. S., Rule J. D., Moore J. S., Sottos N. R., White S. R. Life extension of self-healing polymers with rapidly growing fatigue cracks // J. R. Soc. Interface. 2007.
10. Gould P. Self-help for ailing structures // Materials Today. 2003.
НАНОТЕХНОЛОГИИ
В свет вышла новая научно-популярная книга »Нанотехнологии. Азбука для всех» — и это логичное завершение неофициального Года нанотехнологий в России. Книга составлена студентами и преподавателями Факультета наук о материалах, а также Химического факультета МГУ. В ней разъясняются 140 понятий, которые напрямую связаны с нанотехнологиями. Это том в твердом переплете из 360 страниц. «Азбука» — не энциклопедия или строгий учебник, а увлекательная книга для тех, кто только начинает знакомиться с этой областью науки.
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