WO2015020874A1

WO2015020874A1 - Ultrastrong bubbles for light -weight materials with high strength and toughness

Info

Publication number: WO2015020874A1
Application number: PCT/US2014/049138
Authority: WO
Inventors: Daeyeon Lee
Original assignee: The Trustees Of The University Of Pennsylvania
Priority date: 2013-08-07
Filing date: 2014-07-31
Publication date: 2015-02-12

Abstract

Stable non-spherical bubbles and a composite comprising the stable non- spherical bubbles and a polymer are provided. The stable non-spherical bubbles have a shell that encapsulates a gas. Also provided are related methods for preparing the stable non-spherical bubbles and the composite.

Description

ULTRASTRONG BUBBLES FOR LIGHT-WEIGHT MATERIALS WITH HIGH

STRENGTH AND TOUGHNESS

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to and claims the benefit of U.S. Provisional

Application No. 61/863,028, entitled "ULTRASTRONG BUBBLES FOR LIGHT-WEIGHT MATERIALS WITH HIGH STRENGTH AND TOUGHNESS" filed 7 August 2013, the contents of which are incorporated herein by reference in their entirety and for all purposes.

FIELD OF THE INVENTION

The invention relates generally to generation of stable non-spherical bubbles and the use thereof.

BACKGROUND OF THE INVENTION

Many scientists and engineers are exploring a number of different approaches to design materials with desirable combinations of mechanical properties; for many, an "ideal" material would exhibit excellent strength and stiffness while remaining light, tough, and durable. A cellular material is an assembly of cells with solid struts or plates, packed together to fill space. Cellular solids made of various materials including polymers, metals and ceramics are increasingly used for advanced structural applications including insulation and energy absorption. By enabling drastic reductions in weight, these cellular materials are likely to play a key role in developing and designing future aerospace and cyber systems for applications in airframes, satellites, and adaptive vehicles. To develop multifunctional low density materials that would be of particular use to a wide range of applications including transportation, aerospace, and construction industries, it has become increasingly important to be able to design and fabricate hierarchical cellular structures that incorporate the benefits of different classes of materials with control over multiple length scales, from nano to macro. For example, a cellular structure that simultaneously exhibits the toughness of polymer foams and the stiffness of ceramic foams would have enormous impact in a variety of applications. Conventional methods of cellular material fabrication do not allow for the control over the structure and composition over a wide range of length scales, making this task very challenging. Also, the formation of hierarchical structure with "designed heterogeneity" is difficult to achieve, limiting the emergent properties that could result from the incorporation of multiple materials.

Nacre is a composite material composed of hexagonal calcium carbonate platelets and elastic biopolymers (e.g., proteins). These two materials are assembled into the so-called brick-and-mortar structure. Despite the fact that majority of nacre is made of a brittle material, calcium carbonate, it exhibits extraordinary toughness in addition to high strength and stiffness. It has been shown that the remarkable combination of stiffness and toughness in nacre is likely due to its brick-and-mortar structure, which prevents the transverse propagation of cracks that leads to the complete failure of the material . Recent developments have shown that artificial structures that mimic the nano/micro-structures of nacre indeed exhibit extraordinary strength and toughness, in some instances, exceeding the properties of natural nacre. These previous studies focused on studying dense (i .e., non-porous) hybrid structures.

3M has successfully commercialized Glass Bubbles. The demand for these Glass Bubbles has been constantly on the rise. To meet the high demand, 3M has recently launched new manufacture facilities in Brazil . The annual sale of 3M Glass Bubbles is projected to be on the order of several hundreds of millions of dollars each year. While 3M™ Glass Bubbles provide high strength and low density, these bubbles tend to be highly polydisperse and thus their properties vary significantly. Even a small number of defective bubbles can lead to catastrophic failure of the final composite structures under mechanical stress. Also, it is extremely difficult to predict the properties of the final composite materials. As the name indicates, these bubbles are made with glass (i.e., silica), which tend to be relatively brittle and weak. It is imperative that disruptive technologies that overcome these drawbacks of currently available glass bu bbles be developed to meet the challenges of the future.

There is a major need to develop techniques that can produce ultrastrong bubbles with high uniformity using a variety of materials such as high strength ceramics. Also the ability to control the shape of these bubbles will be a key challenge that will need to be addressed in order to generate high strength and high toughness composite materials.

SUMMARY OF THE INVENTION

The invention provides a plurality of stable non-spherical bubbles having a shell that encapsulates a gas. The stable platelet bubbles may have an average tensile strength in the range of 0.1- 100 GPa, preferably from 1-100 GPa, more preferably 10- 50 GPa . The stable platelet bubbles may also have a polydispersity of less than 10%.

In some embodiments, the bubbles are platelet bubbles. The platelet bubbles may have an average radius in the range of 1 to 500 pm . The platelet bubbles may also have an average height in the range of 0.1 to 50 pm .

The shell may comprise a metal oxide or a ceramic. The ceramic may be selected from the group consisting of silicon carbides and nitrides. The average thickness of the shell may be in the range of 10- 10,000 nm . A method of preparing the stable non-spherical bubbles is provided. The method comprises: (a) compressing compound bubbles to generate non-spherical bubbles, wherein the non-spherical bubbles have an inner phase of a gas, a middle phase of water immiscible liquid, and an outer phase of an aqueous liquid, and (b) converting the middle phase of the non-spherical bubbles from step (a) into a shell, wherein the shell encapsulates the gas and is surrounded by the aqueous liquid, whereby stable non-spherical bubbles are formed.

The compound bubbles may be generated in a vertically confined channel. The vertically confined channel may be selected from the group consisting of thin glass capillaries, poly(dimethylsiloxane) channels, etched glass channels, and slit channels between sandwiched glass plates.

Where the stable non-spherical bubbles are platelet bubbles, the preparation method may further comprise modifying at least one of: (a) the ratio of the shell thickness to the bubble height; (b) the stiffness of the shell; and (c) the identity of the shell material ; whereby the stability of the non-spherical bubbles is improved.

The water immiscible liquid may comprise a sol-gel precursor, metal particles, or a preceramic polymer solution. The sol-gel precursor may comprise a metal alkoxide. The metal particles may be selected from the group consisting of Ti0₂ and Al₂0₃ nanoparticles.

The water immiscible liquid may also comprise a solvent, and wherein the converting step comprises removing the solvent from the middle phase. The solvent may be removed by evaporation. The solvent can be directly converted into solid by polymerization or curing.

A composite is provided. The composite comprises a plurality of layers of (a) stable non-spherical bubbles having a shell, wherein the shell encapsulates a gas, and (b) a polymer. The polymer may be selected from the group consisting of

polysaccharide, polyacrylate, polystyrene, polyvinyl alcohol), polyisoprene,

polybutadiene, polyethylene, polypropylene, polyethylene terephthalate, polycarbonate and polyurethane. The polysaccharide may be selected from the group consisting of chitosan, alginate, cellulose and starch. The polyacrylate may be

polymethylmethacrylate.

A method for preparing the composite is also provided. The method comprises applying spin layer-by-layer deposition of non-spherical bubbles and the polymer, spray layer-by-layer deposition of the non-spherical bubbles and the polymer, doctor blade layer-by-layer deposition of the non-spherical bubbles and the polymer, injection molding of the non-spherical bubbles and the polymer, blow molding of the non- spherical bubbles and the polymer, Langmuir-Blodgette technique to the non-spherical bubbles and the polymer, or combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

Figure 1 is a schematic illustration of generation of platelet-like stiff bubbles. A/O/W compound bubbles are compressed between two planar plates. The gap between the two plates is controlled by using a spacer particle (silica particle, 5 ~ 20 pm). The solvent from the oil phase of the A/O/W compound bubbles is removed via evaporation. The shell is further strengthened by sol-gel coating via the Stober process.

Figure 2 is an image of a non-spherical bubble.

Figure 3 is an optical microscopy image of an ellipsoidal bubble (aspect ratio ~ 5) generated using a stretching method.

Figure 4 illustrates conjugation of catechol to chitosan using the EDC chemistry.

Figure 5 shows a bottom-up scheme to generate a bio-inspired hybrid cellular structure with high strength and toughness, (a) Bottom-up hierarchical assembly of nacre-inspired hybrid low-density material. A monolayer of stiff platelet bubbles floating at an air-water interface will be transferred to a polymer-coated substrate. A soft polymer film will be deposited atop bubble monolayer via spin coating. The adhesiveness of the polymer to the bubble surface will be tuned by varying the degree of polymer functionalization. (b) This process will be repeated multiple times to generate a cellular multilayer that is stiff, strong and tough. Layer-by-layer (LbL) assembled cellular structure can be isolated from the substrate using a razor blade or using a sacrificial layer and further modified using layer-by-layer assembly to make a sandwich structure.

Figure 6 shows layer-by-layer assembly of bubbles and polymers, (a) Transfer of a bubble monolayer from an air-water interface to a solid substrate. The glass slide was withdrawn from water at ~ 2 mm/sec. Almost of all of the bubbles on the water surface were transferred to the slide. The width of the substrate is 25 mm. (b)

Scanning electron microscope image of a transferred bubble monolayer (no polymer layer was coated atop the bubble monolayer). Bubbles used for this demonstration are spherical.

Figure 7 shows hypothesized stress-strain response of hybrid cellular multilayers with varying amount of polymers and that of all-inorganic cellular solids (dashed line) derived via calcination. Figure 8 is a schematic illustration showing the self-healing process in the cellular multilayer. During the propagation of a crack, microcapsules containing reactive monomers are compromised and release the precursors into the crack, which undergo polymerization.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to non-spherical bubbles and a composite comprising the non-spherical bubbles and a polymer, and related

preparation methods. These non-spherical bubbles are ultrastrong, and may be used to make light-weight materials with high strength and toughness. These bubbles are also stable, and may be stored without any significant change in size or other properties (e.g., polydispersity, shape, percent fracture, echogenicity and mechanical properties) for an extended period of time.

A plurality of stable non-spherical bubbles are provided. The bubbles have a shell, which encapsulates a gas. As used herein, the term "stable" means that the non- spherical bubbles do not exhibit any substantial change in size, shape, or percent fractured over an extended period of time. By not exhibiting any "substantial change," the bubbles exhibit a change in the relevant property identified by numerical values (e.g., size and polydispersity) of less than about 50%, more preferably less than about 30%, more preferably less than about 10%, even more preferably less than about 5%, still more preferably less than about 2% and most preferably less than about 1%. The period of time during which the bubbles remain stable will depend on application of the bubbles, but preferably is at least 1 hour, one day, or two days, more preferably at least one week, even more preferably at least one month, still more preferably at least six months and most preferably at least one year. Preferably, the non-spherical bubbles of the present invention do not undergo any substantial change over an extended period of time in other properties, such as echogenicity and other mechanical and dynamic properties.

The average thickness of the shell may be in the range of about 10-10,000 nm, preferably about 0.1- 1 pm. The shell may comprise a metal oxide or a ceramic. The ceramic may be selected from the group consisting of silicon carbides and nitrides.

The non-spherical bubbles are strong, preferably ultrastrong. For example, the non-spherical bubbles may have an average tensile strength in the range of about 0.1- 100 GPa, preferably about 1-100 GPa, more preferably about 10-50 GPa.

The non-spherical bubbles are relatively uniform, and may have a polydispersity of less than about 10%, preferably less than about 5%.

The bubbles may be platelet bubbles or needle-like bubbles. The platelet bubbles may have an average radius in the range of about 1-500 pm, preferably about 10-50 μιη, more preferably about 30 pm. The average height of the platelet bubbles may be in the range of about 0.1-50 pm, preferably about 1-20 pm, more preferably about 3-10 pm.

A method for preparing the stable non-spherical bubbles of the present invention is also provided. The method comprises (a) compressing compound bubbles to generate non-spherical bubbles, which have an inner phase of a gas, a middle phase of water immiscible liquid, and an outer phase of an aqueous liquid; and (b) converting the middle phase of the non-spherical bubbles from step (a) into a shell. The shell encapsulates the gas and is surrounded by the aqueous liquid.

The compound bubbles may be spherical and prepared in various ways, for example, as described in U.S. Patent Application Publication No. 2012/0328529. The compound bubbles may be generated in a vertically confined channel. The vertically confined channel may be selected from the group consisting of thin glass capillaries, poly(dimethylsiloxane) channels, etched glass channels, and slit channels between wo- sandwiched glass plates. The compound bubbles may have the same three phases as the non-spherical bubbles generated by the compressing step.

The inner phase may be any gas (e.g., nitrogen, carbon dioxide, or helium) or a combination of two or more gases (e.g., compressed air) and will depend on the particular application of the bubbles as is known in the art. The stability of the non- spherical bubbles may be improved by modifying the identity of the gas and/or shell. A pressure drop across the bubbles may lead to bubble deformation. Therefore, to improve stability, one may select a gas that has a low solubility in the aqueous liquid. Even more preferably, to improve stability, one may select a gas that is similar compositionally to air to minimize diffusion. Some gases that work well are air, nitrogen, or a very hydrophobic gas, while others that do not work very well include carbon dioxide, helium, ammonia, or a very hydrophilic gas.

The middle phase may be amphiphilic and preferably is hydrophobic. Where the middle phase comprises an oil, the compound bubbles or the non-spherical bubbles generated by step (a) may be characterized as air-in-oil-in-water (A/O/W) compound bubbles. The water immiscible liquid comprises a shell forming material. The shell forming material is one that, upon conversion, coalesces and forms a stable shell of the bubbles. Shell forming materials can be dispersed in the water immiscible liquid.

Typical shell forming materials may be nanoparticles, silica particles (preferably silica nanoparticles), polymers, clay particles, monomers, crystallizable liquids, and phase changing liquids. The shell forming materials may also comprise a sol-gel precursor, metal particles, or a preceramic polymer, for example, as described in Laine and Babonneau, Preceramic Polymer Routes To Silicon Carbide, Chem. Mater. 5(3) : 260-279 ~J~

(1993). The sol-gel precursor may comprise a metal alkoxide. The metal particles may comprise Ti0₂ or Al₂0₃ nanoparticles. A phase changing liquid can change phases, for example, from liquid to solid, upon a temperature change or curing, for example, by radiation. For instance, a wax or gel may be used as the shell forming material in making bubbles in the chamber, which is maintained at a temperature at which the wax or gel is liquid. Upon removal of the bubbles from the chamber, the wax or gel may solidify such that the middle phase is converted into a shell encapsulating the gas.

The water immiscible liquid may further comprise a solvent in which a shell forming material is dispersed or dissolved. The non-spherical bubbles may be substantially in a monolayer at the surface of an aqueous liquid (e.g., water). The term "substantially" as used in this manner means that at least about 60%, more preferably 70%, more preferably 80%, still more preferably 90%, even more preferably 95% and most preferably 99% are collected in a reservoir such that the bubbles are in a single layer, not overlapping with each other. This can be achieved by using a collection reservoir that is sized to achieve this effect of having the bubbles in a single layer. The solvent is preferably organic and more volatile than the aqueous liquid, and may be removed from the middle phase of the bubbles by evaporation or washing away such that the middle phase is converted into a shell encapsulating the gas to form stable non-spherical bubbles. Examples of the solvent include toluene, chloroform and dichloromethane, and fluorocarbons generally.

The stability of the non-spherical bubbles may be improved by modifying the stiffness of the shell by, for example, by thermal treatment. The optimal temperature may be selected based on the materials used to make the non-spherical bubbles. For example, the non-spherical bubbles may be heated at a temperature of about 200- 1500°C. In addition, the stiffness of the shell may be modified by selecting a shell forming material that intrinsically increases Young's modulus and/or that does not swell too much in water. Of course, the desired stiffness (and stability) is dependent upon the ultimate use of the bubbles, so one might wish to control the stiffness of the bubbles by, for example, selecting a shell forming material having a desirable Young's modulus.

The water immiscible liquid may comprise, optionally, a functional material. Examples of the functional materials include nanoparticles, polymers, metal, ceramic, magnetic, radiopaque or fluorescent materials, and mixture thereof. Preferably, the functional material is hydrophobic or insoluble in water. The functional material may be pre-mixed in the water immiscible liquid prior to being introduced into the chamber, or added to the immiscible liquid while being introduced into the chamber. In fact, a single material may serve both as the shell-forming material and the functional material. For example, nanoparticles of any known material (e.g., silica, carbon, oxide, metal, or semiconductor) may be a shell forming material and/or a functional material. Different nanoparticles may be mixed (e.g., fluorescent + silica, magnetic + silica, and porous + solid silica) to form desirable stable non-spherical bubbles.

The outer phase is designed to prevent or minimize contact among bubbles having the shell encapsulating the gas, especially before the bubbles cure or harden. It can be water alone but preferably includes an additive that enhances the stability of the bubbles. One such additive is polyvinyl alcohol), which was found to prevent the coalescence of the generated non-spherical bubbles with one another. Other stability- enhancing additives include surfactants, proteins, and ligands. The amount of the stability enhancer is that amount which provides the stable bubbles. Typically, PVA ranges from about 0.1 to 10 wt%, preferably from about 1 to 5 wt %.

The stability of the non-spherical platelet bubbles may be improved by modifying the ratio of the shell thickness to the bubble height. The bubble height may be adjusted by the force used to compress the compound bubbles or by changing the height of the confining channels. The shell thickness may be determined by the shell forming materials.

A composite comprising the stable non-spherical bubbles of the present invention and a polymer is further provided. The polymer may be selected from the group consisting of polysaccharide, polyacrylate, polystyrene, polyvinyl alcohol), polyisoprene, polybutadiene, polyethylene, polypropylene, polyethylene terephthalate, polycarbonate and polyurethane. The polysaccharide may be selected from the group consisting of chitosan, alginate, cellulose and starch. The polyacrylate may be polymethylmethacrylate.

The composite according to the present invention may have ultrastrong hollow structures, and used to make light-weight materials with ultrahigh strength, stiffness and toughness (resistance to brittle fracture). The composite may be used for a wide range of purposes, including bodies and parts of transportation, aerospace, military and construction equipment and vehicles, coatings, underwater/deepwater equipment.

For the composite of the present invention, a method for preparing the composite is provided. The method may involve applying spin layer-by-layer deposition of non-spherical bubbles and the polymer, spray layer-by-layer deposition of the non- spherical bubbles and the polymer, doctor blade layer-by-layer deposition of the non- spherical bubbles and the polymer, injection molding of the non-spherical bubbles and the polymer, blow molding of the non-spherical bubbles and the polymer, Langmuir- Blodgette technique to the non-spherical bubbles and the polymer, or a combination of any of the forgoing techniques. For example, the composite may be prepared by stacking the non-spherical bubbles first using a variety of methods (e.g., spin coating, dip coating, doctor blading), and then infiltrating the interstices with a monomer (e.g., acrylate, styrene, etc.) and a initiator, and finally polymerizing via photo- or thermal- initiation.

In one embodiment, a cellular structure is fabricated in a bottom-up scheme to generate a bio-inspired hybrid cellular structure with high strength and toughness (Figure 5). The cellular structure comprises anisotropic bubbles that impart mechanical stiffness to the cellular solid, and soft polymers that bind the stiff bubbles together. Briefly, this method relies on the layer-by-layer assembly of a stiff anisotropic bubble and an adhesive polymer. Anisotropic bubbles with high stiffness are generated using a compound bubble template method . Layer-by-layer assembly allows for the precise control over the composition and structure of nanocomposites. Heterostructured multilayers whose composition varies from layer to layer can readily be created by using bubbles with different composition, size and shapes. The cellular structure shown in Figure 5(b) can be further modified by depositing dense layers of polymers and inorganic materials such as clay particles on both sides of the material to create sandwich-type structures. Such sandwich structures are found in natural cellular materials such as plant leaves, bones and shells.

The term "about" as used herein when referring to a measurable value such as an amount, a percentage, and the like, is meant to encompass variations of ±20% or ± 10%, preferably ±5%, more preferably ±1% from the special value, as such

variations are appropriate to perform the disclosed methods.

The following examples are provided to describe exemplary aspects of the invention in greater detail. They are intended to illustrate, not to limit, the invention. Example 1. Generation of platelet-like bubbles with high stiffness

To emulate the mechanical response of nacre under load, it is critical to create a composite that is composed of two materials: an anisotropic material that will impart mechanical strength & stiffness and a soft material that will glue the bubbles together and dissipate the impact energy. Toward this goal, platelet-shaped stiff bubbles will be generated by confining air-in-oil-in-water (A/O/W) compound bubbles between two plates as schematically illustrated in Figure 1. Here, monodisperse A/O/W compound bubbles will be generated using a microfluidic approach. In the oil phase of the A/O/W compound bubbles, hydrophobically modified oxide nanoparticles (Si0₂, Al₂0₃ and Ti0₂), will be dispersed. Different types of nanomaterials including clay platelets and anisotropic particles (e.g., nanorods) can also be used as the structural component to form a stiff bubble shell. Before the complete removal of the solvent, the compound bubbles will be compressed between two stiff plates (e.g., glass slides), to deform the A/O/W compound bubbles. Spacer particles will be used to control the height of the platelet bubbles. Despite being compressed into highly anisotropic shapes, the spreading coefficients of the three fluid phases will be tuned so that air-in-oil-in-water bubble morphology is indeed the thermodynamically stable state. This ensures that the compound bubbles will not undergo destabilization. Subsequently, the solvent will be removed via evaporation. A simple calculation based on the assumption of constant volume (radius of spherical bubble = 30 μητι, height of platelet bubble = 3 ~ 10 pm) confirms that it is possible to generate platelet-like bubbles with the aspect ratio (s) in the range between of 3 to 20 using the proposed method. Previous studies have shown that an artificial nacre-like hybrid composite containing platelets with similar

dimensions and aspect ratios exhibited high toughness and strength. To further stiffen the shell surrounding the gas bubble and to make it more resistant to tensile

deformation, a thin silica shell (thickness 50 ~ 100 nm) will be grown on the surface of the nanoparticle shell using the Stober process. Briefly, this process involves exposing the nanoparticle-shelled bubbles to anhydrous ethanol solution containing

tetraethoxysilane and ammonia. PVA on the surface of the bubbles retains a trace amount of water facilitating the sol-gel process to occur on the surface of the bubbles leading to the formation of a stiff silica shell. Alternatively, titania and alumina coatings also can be generated using titanium isopropoxide and aluminum isopropoxide, respectively, as the precursors for the sol-gel coating. Metal oxide- reinforced bubbles will be thermally treated to complete the sol-gel process to increase the tensile strength of the material. It has been shown that nanoparticle containing sol-gel derived silica fiber treated at 800 °C, for example, has a high tensile strength of close to 2.0 GPa.

The stiffness of the shell comprising nanomaterials and sol-gel reinforcement may . prevent the gas bubbles from relaxing back into the spherical geometry and also prevent the collapse of platelet bubbles during drying. The ratio of the shell thickness to the radius of the highest curvature region (this radius is same as the half of the platelet height) may be the critical parameter that determines whether the shell can sustain non-spherical shapes and also withstand the drying process without collapse. The aspect ratio of the platelet bubbles may be controlled by varying the dimensions (diameter and shell thickness) of A/O/W compound bubbles and using different spacer particles to control the height of the gap during compression. The thickness of the shell may be changed by varying the concentration of the nanoparticle suspension. An imaging algorithm that was developed to analyze the stability of spherical bubbles will be used to analyze the stability of platelet-like bubbles.

The mechanical properties of individual bubbles may be characterized using an indentation method as well as using finite element analysis (FEA). Hysitron Picoindentor can be operated either under an optical microscope or a scanning electron microscope to obtain quantitative load-displacement curves. Anisotropic bubbles may be placed on a stiff substrate using solution casting and may subsequently be indented using a flat punch indenter tip. The flat punch may be designed so that its flat head is slightly larger than the diameter of platelet bubbles but not so much larger to avoid inadvertent contact with the substrate during indentation. Flat punch tips may be purchased from Agilent Technologies. Based on the microfluidic technique and the confinement method described above, bubbles with varying shell thickness, shell material and aspect ratio will be generated.

Nanoparticle shell may be stiff enough to stabilize platelet-shaped bubbles. If nanoparticle/sol-gel shell does not provide enough stability, two alternative strategies may be explored. First, ellipsoidal bubbles with a stiff shell may be generated. Stable ellipsoidal bubbles with a stiff shell comprising nanoparticles/polymers may be generated by uniaxially elongating bubbles embedded in a polyvinyl alcohol (PVA) matrix either at an elevated temperature (> 200 °C) or under an organic solvent such as toluene. Ellipsoidal bubbles do not relax back into the spherical shape due to the stiffness of the shell (Figure 3). It is worthwhile to note that natural composites such as bones, which are composed of needle-like (or ellipsoidal) inorganic minerals and polymers, exhibit high strength and toughness. Preceramic polymers such as polymethylsilsesquioxane and polycarbosilane may also be used to generate ceramic- shell stabilized anisotropic bubbles. The advantage of preceramic polymer may lie in the fact that bubbles stabilized by these polymers can be processed into the desired shape and then pyrolyzed to fabricate bubbles with high tensile strength and stiffness. Example 2. Synthesis of catechol-functionalized polymer: controlling the adhesiveness of polymer

Although present in a very small amount, organic materials such as proteins play a crucial role in imparting mechanical toughness to natural composites such as bones and nacre. Poor bonding at the inorganic-organic interface leads to mechanical properties that are inferior to that predicted by theory (Eq. 1).

a_c = aV_pa_p + (\ - V_p)a_m Eq. 1 where a_c is the ultimate tensile strength of the hybrid materials, V_p is the volume fraction of platelets, σ_ρ and a_m are the tensile strength of the platelet and that of the organic material, respectively, a is a constant that depends on the failure mode of the composite and is a function of the aspect ratio of the platelet (s), the yield shear strength of the organic material (τ_γ), and the tensile strength of the platelet (σ_ρ).

The adhesion between the organic material and the mineral platelets need to be strong enough to transfer loads between platelets. More specifically, the shear lag model presented in Eq 1 assumes that the interfacial strength (77) is greater than the yield shear stress of the polymer (r_y). If the inorganic-organic interfacial strength is weak (i.e., r, < r_y), the fracture of the composite occurs via delamination at the organic- inorganic interfaces before the organic matrix undergoes plastic deformation and r, has to be replaced by r_y in Eqs 1 and 2. a = ^- Eq. 2

A composite with weak interfacial strength at the inorganic and organic surface will exhibit low strength. The organic materials also have been shown to dissipate energy by plastically deforming and forming ligaments & filaments during deformation. Thus, it is critical to use an organic material that has tunable adhesion strength and appropriate shear strength to impart mechanical toughness to the composite material. In addition to its importance in controlling the overall strength of the composite, the adhesive force of the polymer should be high enough so that during layer-by-layer assembly, the bubbles remain on surface without floating off due to the buoyancy of the bubbles. Dried bubbles on a surface may be easily detach and float away from the surface when a drop of water is placed above them. To enable the spin coating of a polymer layer atop a monolayer of bubbles on a surface, the bubbles may need to stay adhered avidly to the underlying polymer layer.

To satisfy these criteria, chitosan is chosen as the organic matrix, which has been already shown to lead to high toughness in nacre-like structures. Chitosan can be further functionalized to control the adhesion strength between the polymer and anisotropic bubbles. In addition, by preparing chitosan films from different acid solutions (i .e., acetic, citric, lactic and maleic acids), the tensile strength of chitosan films can be controlled (r_y 4 ~ 100 MPa). The ability to control τ_γ is important because this allows for the control of the critical aspect ratio (s_c) that determines the mode of failure in the composite structures.

To vary the adhesion strength of chitosan to anisotropic bubbles, chitosan will be functionalized using catechol. Catechol has been shown to exhibit excellent adhesion onto all types of surfaces including Teflon and also has high adhesion strength under water as well as in dry conditions. The adhesion strength can be controlled by varying the degree of functionalization of chitosan with catechol. To conjugate catechol to chitosan (CHI-C), a standard carbodiimide chemistry will be used. Deacetylated chitosan, dissolved in water/ethanol mixture, will be reacted with hydrocaffeic acid (HCA) and l-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC) (Figure 4). After ethanol is evaporated, the by-products and unreacted reactants will be removed by dialysis. The final product will be freeze-dried and kept in a moisture- free desiccator before use. The extent of conjugation will be controlled by adding varying amounts of hydrocaffeic acid and EDC. NMR and UV-Vis spectroscopy will be used to measure the catechol content.

Example 3. Bottom-up assembly of anisotropic bubbles and catechol- functionalized polymer

Layer-by-layer assembly is a versatile technique that allows for the fabrication of composite structures with precise control over structure and composition, taking advantage of the buoyancy of the bubbles to enable layer-by-layer assembly. A substrate will be first coated with catechol-functionalized chitosan using spin coating. Subsequently, a monolayer of anisotropic bubbles floating at an air-water interface will be transferred as shown in Figure 6. This process is feasible and a dense monolayer of bubbles can be formed on the surface in Figure 6(b). Although the preliminary results shown in Fig . 6 were obtained by withdrawing the substrate manually at a speed of ~ 2 mm/sec, the reliability/repeatability of this process will be improved by using a dipper with controlled withdraw rate. Upon water evaporation, the bubble monolayer on the surface will be coated with catechol-functionalized chitosan again. The sequential deposition of stiff bubbles and chitosan will be repeated to generate a three- dimensional cellular multilayer. Preferably, chitosan has high adhesiveness and will maintain bubbles on the surface during spin coating process. The first and last layers will be chitosan. Assembled structures will be characterized using cross-sectional SEM and optical microscopy. The composition and density of the assembled structure will be characterized using thermogravimetric analysis (TGA) as well as SEM and optical microscopy.

It has been shown that the amount of organic materials significantly influences the toughness of nacre-like composites. It is, thus, important to vary the interstitial void volume as well as the thickness of polymers between platelet bubbles. The structure of LbL-assembled cellular structure will be controlled by varying the concentration of polymers and the angular velocity of spin coating. In addition, the packing of bubbles will be controlled by tuning the aspect ratio and size distribution of platelet bubbles. As shown in Figure 6, monodisperse bubbles are expected to lead to regular array of bubble layer formation at each deposition step, whereas bubbles with broad size distribution will make disordered packing of bubbles. The height of platelet bubbles will be the same regardless of their initial size since the same spacer particles will be used for bubble deformation (Figure 1). By varying the packing density of bubbles, the amount of chitosan that is trapped between the bubbles can be controlled. The regularity (or the lack thereof) of the each bubble layer could play an important role in determining the mechanical response of the LbL assembled cellular structure. The convex water surface may be used to densely pack bubbles at the air-water interface to facilitate the bubble monolayer transfer onto a solid substrate. Although the buoyancy of bubbles is expected to pack them into a dense monolayer as shown in Figure 6(a), they may form loosely-packed structures due to strong directional capillary interactions as was recently reported for anisotropic materials. If this is the case, a Langmuir-Blodgett trough will be used to compress bubbles to form highly packed bubbles prior to the transfer process.

The strength, stiffness and toughness of the cellular multilayers will be increased while the density of the material is decreased.

As long as the aspect ratio of the platelet bubbles are smaller than the critical aspect ratio (s < s_c), the volume fraction of polymer will have a significant influence on the mechanical response (strain vs. strain) of these cellular composites (Figure 7). At a very high polymer concentration, the material will behave very much like the neat polymer (bottom solid curve in Figure 7) . As the volume fraction of stiff platelet bubbles is increased (i.e., polymer vol. frac. decreased), the stiffness of the material will increase. These materials will exhibit yield response and have high toughness, which can be determined by the area under the stress-strain curve (e.g., top solid curve in Figure 7). It is important to note that in this concentration range, the stiffness, strength and toughness of the material will increase while the material becomes lighter. This is the unique benefit that is afforded by introducing the nacre- inspired structure into a cellular material. Once the polymer content is too low, the cellular material is expected to exhibit brittle fracture, similar to those observed in ceramic cellular solids (dashed curve) . To test this hypothesis, the organic matrix will be completely removed by calcinating the hybrid structure at 800 °C. This test will clearly reveal the importance of thin organic layers between the bubbles/cells. It is also important to recognize that organic/inorganic hybrid materials will likely exhibit viscoelasticity, which means that the rate of deformation will play a crucial role in determining their mechanical response.

Example 4. Self-healing process in the cellular multilayers

One major drawback of low-density cellular solids is that, due to their porosity, they are prone to undergo irreversible deformations/damages (i.e., crack/fracture formation) upon impact that may cause serious failure of the material. To enable the use of these low-density solids for transportation, aerospace and structural

applications, it is critical to develop advanced materials with increased safety, extended lifetime and crack avoidance. One emerging approach is the concept of self-healing materials. Self-healing materials are another example of biomimetic materials, which emulate the self-healing capability of many plants and animals. Self-healing materials can be defined as materials that are able to recover (fully or partially) the mechanical strength by crack/pinhole healing. Surprisingly, very little efforts have been devoted to merge the benefits of low density solids with those of self-healing materials so far.

Among different approaches, the release of healing agents from microcapsules upon the formation of cracks and pinholes in solids offers a wide range of

polymerization/crosslinking chemistries that can be explored, for example, the generation of liquid-filled microcapsules that can be used for the fabrication of self- healing materials.

The concept of imparting self-healing properties to the nacre-like cellular multilayers will be explored. As shown in Figure 8, microcapsules that contain reactive monomers will be incorporated into the cellular multilayers using layer-by-layer assembly. Upon the formation of cracks, these microcapsules will rupture and release monomers that can fill and solidify to heal the damaged low-density solids from undergoing complete mechanical failure. The understanding obtained regarding the failure mechanism of these cellular multilayers under uniaxial tension will be critical since the healing capability will be significantly depend on whether the cellular materials undergo fracture via platelet bubble fracture or pull-out mediated failure. The former failure mechanism will make it difficult to effectively heal the material since the newly created volume that needs to be filled with liquid will be extremely large due to the compromised bubbles. By enabling the formation of cellular composites that "fail" by the pull-out mechanism, monomers will be able to fill the gaps between the intact bubbles to heal the material. The volume fraction of the microcapsules and their mechanical properties may affect the self-healing properties.

Various terms relating to the systems, methods, and other aspects of the present invention are used throughout the specification and claims. Such terms are to be given their ordinary meaning in the art unless otherwise indicated.

The present invention is not limited to the embodiments described and exemplified above, but is capable of variation and modification within the scope and range of equivalents of the appended claims.

Claims

What is Claimed:

1. A plurality of stable non-spherical bubbles having a shell, wherein the shell encapsulates a gas.

2. The plurality of stable non-spherical bubbles of claim 1, wherein the bubbles are platelet bubbles.

3. The plurality of stable non-spherical bubbles of claim 2, having an average radius in the range of 1 to 500 pm.

4. The plurality of stable non-spherical bubbles of claim 2, having an average height in the range of 0.1 to 50 pm.

5. The plurality of stable non-spherical bubbles of claim 1, wherein the shell comprises a metal oxide or a ceramic.

6. The plurality of stable non-spherical bubbles of claim 5, wherein the ceramic is selected from the group consisting of silicon carbides and nitrides.

7. The plurality of stable non-spherical bubbles of claim 1, wherein the average thickness of the shell is in the range of 10-10,000 nm.

8. The plurality of stable non-spherical bubbles of claim 1, having an average tensile strength in the range of 0.1-100 GPa.

9. The plurality of stable non-spherical bubbles of claim 1, having a polydispersity of less than 10%.

10. A method of preparing stable non-spherical bubbles, comprising :

(a) compressing compound bubbles to generate non-spherical bubbles, wherein the non-spherical bubbles have an inner phase of a gas, a middle phase of water immiscible liquid, and an outer phase of an aqueous liquid, and

(b) converting the middle phase of the non-spherical bubbles from step (a) into a shell, wherein the shell encapsulates the gas and is surrounded by the aqueous liquid, whereby stable non-spherical bubbles are formed.

11. The method of claim 10, wherein the compound bubbles are generated in a vertically confined channel.

12. The method of claim 11, wherein the vertically confined channel is selected from the group consisting of thin glass capillaries, poly(dimethylsiloxane) channels, etched glass channels, and slit channels between wo-sandwiched glass plates.

13. The method of claim 10, wherein the stable non-spherical bubbles are platelet bubbles, further comprising modifying at least one of:

(a) the ratio of the shell thickness to the bubble height;

(b) the stiffness of the shell; and

(c) the identity of the shell material ; whereby the stability of the non-spherical bubbles is improved.

14. The method of claim 10, wherein the water immiscible liquid comprises a sol-gel precursor, metal particles, or a preceramic polymer.

15. The method of claim 14, wherein the sol-gel precursor comprises a metal alkoxide.

16. The method of claim 14, wherein the metal particles are selected from the group consisting of Ti0₂ and Al₂0₃ nanoparticles.

17. The method of claim 10, wherein the water immiscible liquid further comprises a solvent, and wherein the converting step comprises removing the solvent from the middle phase.

18. The method of claim 17, wherein the solvent is removed by evaporation.

19. A composite comprising a plurality of layers of

(a) stable non-spherical bubbles having a shell, wherein the shell

encapsulates a gas, and

(b) a polymer.

20. The composite of claim 19, wherein the polymer is selected from the group consisting of polysaccharide, polyacrylate, polystyrene, poly(vinyl alcohol), polyisoprene, polybutadiene, polyethylene, polypropylene, polyethylene terephthalate, polycarbonate and polyurethane.

21. The composite of claim 20, wherein the polysaccharide is selected from the group consisting of chitosan, alginate, cellulose and starch.

22. The composite of claim 20, wherein the polyacrylate is

polymethylmethacrylate.

23. A method for preparing the composite of claim 19, comprising applying spin layer-by-layer deposition of non-spherical bubbles and the polymer, spray layer- by-layer deposition of the non-spherical bubbles and the polymer, doctor blade layer- by-layer deposition of the non-spherical bubbles and the polymer, injection molding of the non-spherical bubbles and the polymer, blow molding of the non-spherical bubbles and the polymer, Langmuir-Blodgette technique to the non-spherical bubbles and the polymer, or combination thereof.