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# Mechanical Properties of Biological Nanocomposites

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### Mechanical Properties of Biological Nanocomposites

1. 1. Mechanical Principles of Biological Nanocomposites Greg Orlicz  Introduction / Fracture Mechanics George Keller  Mechanics employed by nature / Role of “nano” Anthony Salvagno  Biological examples / Hierarchy in nature Jingshu Zhu  Characterization / Fabrication of synthetic materials
2. 2. What can we learn from biological materials? • Biological materials (e.g. bone, tooth, shell, and wood) -- excellent mechanical properties: strength, toughness, and resistance to fracture • Much is attributed to nanostructure • Want to understand what factors attribute to strength and toughness by understanding the mechanical forces that result due to the structure • Perhaps we can synthesize materials to meet and surpass the same robustness of biological materials (biomimicking) • Emphasis is material strength and resistance to fracture – fracture mechanics
3. 3. Setting the foundation for fracture mechanics… F A ΔL L0 Stress: σ = F/A related by Hooke’s Law: σ = E ε Strain: ε = ΔL/L0 Young’s Modulus (Stiffness): F L0 E = Tensile Stress/Tensile Strain = A L Analog to spring force: Fs= -kx Strain energy – energy stored in plate like a spring during elastic deformation (can return to same position) (like pushing fist into car door) * When material is unloaded, the strain energy can do work
4. 4. What is plasticity? • Plastic deformation - if stress is high enough, material is strained beyond elastic approximation - internally damaged and deformation is irrecoverable (like punching car door – denting) - linear relationship between stress and strain is lost • Stress-Strain relationship determined by tensile test • Yield Stress = stress under which material no longer deforms elastically (deforms plastically – irrecoverable) • Ultimate Tensile Strength = max stress that material can withstand Extensometer
5. 5. Toughness vs. Strength Strength – how much stress a material can support without failure (usually defined as σUTS = max stress on stress-strain curve) Toughness – amount of energy per unit volume a material can absorb before rupturing energy f toughness d volume 0 σ ε
6. 6. A very close look at elastic fracture Bond energy: Approximate force as half of sine wave: For small displacements: k is analogous to spring constant: Pc Considering all bonds per unit area, Pc E  σc; k  E (Young’s modulus) : Surface energy – energy required to break a plane of atomic bonds to create two free surfaces (energy per surface area): Arrive at critical stress:
7. 7. Cracks and defects in a material magnify the local stress da Vinci – strength of iron wires varied inversely to the wire length -- flaws Stress concentration at crack tip – stress is higher at crack tip than externally applied stress Stress intensity factor k = σA/σ Recall: Location, A Highest local stress Bonds break when σA=σc (describes crack nucleation) Materials do not begin to fail at yield stress (theoretical strength of material) – fail at lower values because flaws create higher local stresses
8. 8. Griffith’s Energy Balance – describes crack propagation Griffith, Irwin, and Orowan Energy balance criteria dE d dW s The total energy must remain the same for a given increase in 0 dA dA dA crack size (strain energy in plate plus work input) 2 2 a B potential energy due to strain energy 0 E Ws 4 aB s work that goes into creating two new surfaces Therefore, 2 dW s R is the energy required to break atomic bonds with further crack d a G = R dA 2 s extension (create two new surfaces). It is a measure of the dA E toughness of a material. G is called the Energy Release Rate, which is thought of as the driving force trying to extend the crack So if the driving force equals the resistance (G=R), then the crack will grow. i.e. * Modification  substitute wf s p for plasticity effects (more energy)
9. 9. R-curves help predict fracture resistance and material toughness • R-curves are studied in literature so we can predict: (1) the conditions under which a crack will extend (2) if crack growth occurs whether it will result in failure of the material (unstable growth) • R-curves can take on different shapes and values, depending on the material, its microstructure, geometry, temperature etc… A flat R-curve indicates a brittle material. A rising R-curve indicates materials that undergo R is ideally an invariant material property. plastic deformation. (e.g. ductile metals) (e.g. glass)
10. 10. Testing resistance to fracture • Test specimens are used to determine resistance to fracture of various materials Both are Mode I loading (load is normal to crack) • A stress intensity factor is associated with each kind of specimen • Usually use displacement control • Arrive at the fracture toughness (how much stress is needed for the crack to grow)
11. 11. Nature has found a way to improve the strength of materials Biological materials can have greater mechanical properties than the individual components that make them up (strength, toughness, fracture resistance) We want to understand nature’s approach to material structuring!
12. 12. • Hard, brittle mineral crystals embedded in soft, elastic protein matrix • The load transfer is accomplished largely by the shearing of the protein matrix between the long sides of mineral platelets • The TSC can be regarded as the primary structure of biological materials
13. 13. • Affects the mechanical properties of the nanostructure such as, load transfer, stiffness, strength and elastic stability • Large ratios make up for softness in the protein matrix • Aspect ratio cannot be infinitely large h
14. 14. • Why is the structure of biological materials always at the nanoscale? • Length Scale: • When mineral exceeds the length scale material is sensitive to crack-like flaws • When mineral drops below the length scale failure is governed by the theoretical strength of material
15. 15. • Protein effectively stabilizes mineral crystals • At a given volume concentration of protein, the critical stress approaches a constant limited value as the aspect ratio becomes sufficiently large • Buckling stress in composite nanostructure is proportional to the geometric means of Young’s moduli of protein and mineral
16. 16. • For nanocomposites, as the mineral bits have nanoscale size, the protein-mineral interfacial area can be enormous • Interface strength depends on both size and geometry • The chain structure of proteins is a crucial factor for the strength of the protein-mineral Figure. Atomistic modeling of protein-mineral interface interface strength showing the mechanical behavior of chain molecules and their interaction with the substrate during interface failure.
17. 17.  Whether or not a bone splits or breaks depends on how efficiently short cracks can be prevented from growing into longer ones  Role of micro cracks  Crack deflection and crack bridging  5X greater toughness in the transverse orientation compared to the longitudinal orientation http://www.lbl.gov/publicinfo/newscenter/ features/assets/img/MSD-bone- tough/Bone-Transverse-Koester.mov
18. 18. • hierarchy is inherent in nature o DNA, proteins, cells, organisms, ecosystems, planets, etc. • differing structures at the meso, micro, and nano scales all play a role hierarchy of crab exoskeleton Example: Crab Exoskeleton -layers of brittle mineral rods organized in a helix -each rod is made of softer protein which are comprised of smaller fibrils
19. 19. • fractal-bone model o self-similar layers repeated N times • bottom-up design process o design lowest level structure first o next level structure determined from current level and characteristics wanted
20. 20. • strength o by combining different compounds, shapes, and structures in a material, strength limitations can be exceeded • toughness o shielding of crack initiation and propagation • flaw-tolerance o lots of small structures handle flaws better than one large structure • stiffness o more hierarchy leads to higher stiffness • effects of more levels of hierarchy: o decrease in strength o increase in fracture energy and flaw- tolerance
21. 21. Bone • platelets in protein matrix bone Wood • complexity from cellular construction wood Seashell (Nacre) • layers of tiles in brick-and- mortar fashion nacre Tendon • tightly packed arrays of collagen
22. 22. Mechanical Properties Hierarchical Organization • multifunctional material • compact bone exterior; spongy • compact bone for strength and interior toughness (structural support for • osteons are concentric rings of body) mineral and collagen • spongy bone for bone marrow • each ring has parallel sheets of fibrils and living cells; also allows for and mineral plates compression in other bone types • tropocollagen forms larger fibrils that • can withstand crack-like flaws at act as a protein matrix many levels of hierarchy • nanocrystals mineralize into plate- like structures
23. 23. Mechanical Properties • high strength due to brick-like arrangement • high resilience due to organic Hierarchy matrix • staggered-tile structure • toughness similar to silicon • mineral "bricks" in an o addition of water enhances organic "mortar" toughness • organic matrix made up of • low crack propagation thin layers of elastic • can undergo microbuckling biopolymers
24. 24. Hierarchy Mechanical Properties • cellulose packed into microfibrils • specific stiffness and strength • bundles of microfibrils packed into comparable to steel larger macrofibrils • microfibril angle plays a large part in o contains regions of crystalline the strength and stiffness structure and amorphous regions o young trees are more flexible and • large fibrils supported by amorphous have larger angles matrix of lignin and hemicellulose o older trees have small MFA and • these fibers organize into a number thus stiffer trunks of cell walls surrounding a given cell • high toughness similar to nacre o cracks don't easily propagate perpendicularly
25. 25. Hierarchy Mechanical Properties • similar to bone's organization • connects muscle to bone • repeat layers of larger and larger • has elastic properties structures • stiffness increases with strain • collagen molecules self assemble • up to 300x stronger than muscle o allows for small sizes into fibrils • fibers provide flexibility • fibrils decorated with proteoglycans and grouped to form fascicles
26. 26. Spider Silk • in nature, needs to absorb high momentum without recoil and endure high stress from impact • high toughness and extensibility(ability to endure strain without failure) • strength comparable to steel Teeth • two layers of protection: enamel and dentin • enamel has high hardness (hardest material in vertibrates) • dentin (similar in design to bone) has high toughness Feathers • require stiffness and flexibility to endure flight • hollow shaft reinforced with "foam" structure • very light but strong
27. 27.  Chemical properties behind biomimicking nanocomposites  Method to fabricate nanocomposites  Characterization Techniques for Nanocomposites  Limitations
28. 28.  The structure-function harmony of nacre and other hard biological tissues has inspired a large class of biomimetic advanced materials and organic/inorganic composites.  The addition of inorganic components, such as clays, to organic polymers noticely improves the mechanical, barrier and thermal properties of polymers and rubbers.  Finding a synthetic pathway to artificial analogs of nacre and bones represents a fundamental milestone in the development of composite materials.
29. 29.  In the case of organic-inorganic nanocomposites, the strength or level of interaction between the organic and inorganic phases is an important issue. 1. hydrogen bonding, van der Waals forces covalent or ionic-covalent bond 2. polarity, molecular weight, hydrophobicity, reactive groups, and so on of the polymer 3. type of solvent and clay mineral type
30. 30. Chemistry Properties behind nanocomposites Extensive coiling of the polyelectrolyte leads to the formation of loops with macromolecular segments linked together by van der Waals and ionic interactions. One surface charge on clay can attract positive headgroups from different parts of the chain resulting in loops. Gradually, the polyelectrolyte molecules become significantly deformed due to sliding of the clay platelets over each other to involve ionic bonds.
31. 31.  More generally, molecular self- assembly seeks to use concepts of supramolecular chemistry and molecular recognition in particular, to cause single-molecule components to automatically arrange themselves into some useful conformation. Photograph of the inner side of a green abalone (Haliotis fulgens) shell, showing the iridescent nacre. Shell diameter is ~20 cm.
32. 32.  Ease of preparation  Versatility  Capability of incorporating high loadings of different types of biomolecules in the films  Fine control over the materials’ structure  Robustness of the products under ambient and physiological conditions
33. 33.  Biomimetics  Biosensors  Drug delivery  Protein and cell adhesion  Mediation of cellular functions  Implantable materials
34. 34. Schematic view of the interface bottom-up synthesis method of crystalline rubeanic acid copper. Layer-by-layer assembly and supramolecular chemistry were used to create an ultrathin-film platform technology for small- molecule delivery using a hydrolytically degradable polyion (see picture, blue waves) and a polymeric cyclodextrin (see picture, red cups).
35. 35. The layered organic-inorganic composites were made from montmorillonite clay platelets (C) and polyelectrolytes (P) by the well-established technique of sequential adsorption of organic and inorganic dispersion, often called layer-by-layer assembly (LBL). The general film structure can be represented by the schematic in Fig. 1c. In nacre, mineral platelets, which are a few hundreds of nanometers in thickness, interlock to form sheets that are stacked on top of each other in a staggered formation.
36. 36.  Atomic force microscope (AFM)  Scanning electron microscopy (SEM)  Transmission electron microscopy (TEM)  Wide-angle X-ray diffraction (WAXD)  Small-angle X-ray scattering (SAXS)
37. 37. a, Phase-contrast AFM image of a (P/C) film on Si substrate.b, Enlarged portion of the film in a showing overlapping clay platelets marked by arrows.
38. 38. e and f,Topographic AFM images of PDDA molecules adsorbed between the clay platelets. Elevated areas of irregular shape represent PDDA coils adsorbed to montmorillonite platelets. Arrows track the partially decoiled macromolecules stretched between the clay platelets. poly(diallyldimethylammonium chloride) (PDDA)
39. 39. Scanning electron microscopy (SEM) examination (a) of the (P/C)100 film cross-section revealed a layered structure which was conceptually similar to that of nacre.The film was dense and uniform in thickness. Scanning process and image formation In a typical SEM, an electron beam is thermionically emitted from an electron gunelectron fitted with a tungsten filament cathode.
40. 40. TEM images showed that the film remained continuous and retained its integrity even when local stress had torn away the epoxy resin serving as an embedding media (b). Perpendicular sectioning slightly expanded the multilayers (c).
41. 41. Challenging problems in the biomimicking synthesis:  Control the size  Geometry  Alignment of nanostructure  Higher levels of hierarchy