Why Nanocomposites?  Multi-functionality “Size does matter” Increased surface area on nanoparticles  Small filler size:  High surface to volume ratio Interaction Zone  Small distance between fillers  bulk interfacial material Particle  Mechanical Properties  Increased ductility with no decrease of strength  Scratching resistance  Optical properties  Light transmission characteristics particle size dependent nanocomposite Stress Traditional a36wt% b c d polymer Strain Scratch Resistant, Transparent, Filtering Coatings Visible Ultraviolet Transmittance rate of 16.7wt.% nanoalumina filled gelatin films coated on 0.1mm thick plastic substrate TEM of the 16.7wt% nano alumina filled gelatin film Application: Barrier properties • Imagine a drop of water trying to get through the film made with nanocomposites. Compared to a film made with conventional composites, the water drop would face more barrier going through the film made with nanocomposites because the distance between fillers is much smaller. • Uses: Packaging in food, medical and pharmaceutical industry. Thermal barrier coatings for Hubble Space Telescope (HST) Current Problem: Hubble Space Telescope Imaging Spectrograph overheats, causing data degradation. Proposed Solution: Carbon Nanotube (CNT) may greatly improve HST’s ability to dissipate excess heat. (2X is the goal) Drug delivery Attributes of nanoparticulate systems: 1.provide a better penetration of the particles inside the body. 2.can be used for intramuscular or subcutaneous applications 3.minimizes the irritant reactions at the injection site. 4.exhibit greater stability, in both longer shelf storage lives and uptake times. 5.and can be designed to elicit the desired kinetics, uptake, and response from the body (i.e. biocompatibility). Nanocomposite as a Multiscale System The ability to consider interactions across the multiple scales is a pre-requisite to the consideration of design decisions that can be made on each scale critical to the performance of the final product.  Macroscale composite structures  Clustering of nanoparticles - micron scale  Interface - affected zones - several to tens of nanometers - gradient of properties  Polymer chain immobilization at particle surface is controlled by electronic and atomic level structure Multiscale Modeling Role  If we can predict: Short Functional Groups R Matrix Property Compatible Enhancing Block Block  Polymer / Particle Interaction (mobility, conformation, crystallinity) and the chemistry to control it  Quantum Mechanic  Molecular Dynamic  Coarse grain Hydrogen Bonding Long Functional oligomers  Particle / Particle interaction  Coarse grain discrete  mesoscale continuum  The effect of these and particle size on local and global mechanical, thermal, electrical, optical behavior  Coarse grain  multiscale continuum  Then we can design and control:  Extent of the interaction zone – polymer mobility  Particle dispersion state – transparency, filtering, defect structure  Glass transition temperature Interaction Zone Particle  Processing  Optimization of multiple functions Consider: Carbon Molecules  Graphite versus Diamond  Graphite: Used as lubricant and pencil lead is composed of sheets of carbon atoms in a large molecule. Only weak van der Waals’ forces hold the sheets together. They slide easily over each other.  Diamond: Carbon atoms stacked in a three-dimensional array (or lattice), giving a very large molecule. This gives diamond its strength. 8 Graphite sheets Diamond structure  Graphite sheet is a molecule of interlocking hexagonal carbon rings. Each carbon bonds covalently with three others, leaving one electron unused. The orbital for these “extra” electrons overlap, allowing electrons to freely move throughout the sheet. This is why graphite conducts electricity. Structure of a sheet of graphite A buckyball  Buckyballs were discovered by Smalley (Rice University), Kroto and Curl in 1985 by vaporizing carbon with a laser and allowing carbon atoms to condense.  A buckyball is short for buckmisterfullerene after Buckminster Fuller, an American architect and engineer, who proposed an arrangement of pentagons and hexagons for geodesic dome structures.  It has 60 carbon atoms in a ball shaped with 20 hexagons and 12 pentagons and has a diameter of about one nanometer. 9  In 1991, carbon nanotubes (CNTs) were discovered by Sumio Iijima of NEC Research Lab. After taking pictures of buckyballs in an electron microscope, he noticed needle shaped structures (i.e., cylindrical carbon molecules).  Single-wall carbon nanotubes (SWNTs) versus multiwalled carbon nanotubes (MWNTs)  The length of CNTs vary, but the smallest diameter seen in SWNTs is about one nm. Strength, stiffness (E modulus), and density of common materials Materials 6061 Aluminum (bulk) 4340 Steel (bulk) Tensile Strength (MPa) Tensile Modulus (GPa) Density (g/cm3) 310 69 2.71 1,030 200 7.83 Nylon 6/6 (polymer) 75 2.8 1.14 Polycarbonate (polymer) 65 2.4 1.20 E-glass fiber 3,448 72 2.54 S-2 glass fiber 4,830 87 2.49 Kevlar 49 aramid fiber 3,792 131 1.44 T-1000G carbon fiber 6,370 294 1.80 10 Carbon nanotubes 30,000 1,000 1.90 A single-walled carbon nanotube CNT A scanning electron microscope (SEM) image of a CNT hanging off the tip of an atomic force microscope (AFM) cantilever.  Nanofibers and MWNTs: hollow tubular geometries with aspect ratios (L/d) ranging in the thousands. Material Diameter (nm) Length (nm) Young’s Modulus (GPa) Tensile Strength (GPa) Vapor-grown carbon nanofibers 10-200 30,000100,000 400-600 2.7-7.0 ~ 1.3 500-40,000 320-1470 13-52 SWNT 10 m Scanning electron microscope image of vapor-grown carbon nanofibers in a polypropylene matrix 11 300 nm Image of MWNTs in a polystyrene matrix  Challenge: Unlike fibers in conventional laminates, waviness of the nanotubes and nanofiber reinforced materials complicates the material property calculations. Representative volume elements (RVEs) may be modeled as shown below:  Waviness is defined by the waviness factor, 12 w A LNT Predictions of the Young’s modulus of elasticity:  The modulus of the RVE2 (the right diagram in the previous page), Ex= ERVE2, and the effective modulus for randomly oriented nanotubes, E3D-RVE2, have complex formulas, but are both are functions of the waviness factor. E3D-RVE2 as functions of nanotube volume fraction and w, is shown below. 13  Strength prediction: In general, relations for predicting strength are complex. However, for randomly oriented fibers, an approximate equation may be used to estimate the tensile strength, as follows: ~x  S S  2S LT  ST  ln T 2 mf  1    S mf S LT  Where,  x  composite tensile strenght S LT  shear strenght Smf  matrix stress corresponding to the fiber failure strain ST  transverse tensile strength 14 Failure modes of nanofiber-reinforced composites Fracture mechanisms in carbon nanotube-reinforced composites Paving the way to stronger materials By H. Wagner, Nature 2007 The properties of materials reinforced by nanoparticles often fall far short of those predicted by theory, but now a layer-by-layer assembly approach offers a way in which nanocomposite materials could begin to realise their true potential. • There are fundamental differences between manmade composites at the micrometer-scale and those with nanoscale reinforcement. In particular, the paucity of structural defects in high-aspect-ratio nanoparticles means that their strength and stiffness are much closer to the theoretically predicted values. • The comparatively large surface-area-to volume ratio of elongated nano-objects promises materials with much improved mechanical properties. • If there are strong interactions at the interface between the matrix and nanofiller, efficient stress transfer from one to the other leads to increased strength and stiffness of the composite. • In the case of a weaker interface, the friction and energy dissipation associated with extensive pull-out (that is, extraction) of the nanoparticles from the surrounding matrix in an expanding crack translates to substantial toughness. Problem of Nanocomposites: • Aggregation of the nanosized components because of the relatively strong interparticle forces between them • Leads to particle–matrix mixtures with high viscosities which can make the processing of these materials quite challenging. • Most existing nanocomposites have disappointingly low fractions of particle content, and relatively weak mechanical properties when compared with those predicted by theory. • The addition of even tiny amounts of dispersed phase to matrices leads to outstanding properties, at least up to a‘critical mixing threshold’ of particle content above which aggregation effects become significant and the mechanical properties cease to improve. Optimal Nanocomposites requirement: (1) The particle aspect ratio  high-aspect-ratio particles (2) Particle dispersion (3) Particle packing (or alignment) (4) Polymer-to-particle interfacial stress transfer  the adhesion was strengthened in a number of different ways The evolving architecture of nanocomposites (a) Poorly dispersed nanoparticles (blue) form aggregates in a polymer (red) nanocomposite. (b) The bilayer nanocomposites made by Kotov and co-workers comprising clay nanoplatelets (blue) embedded in a poly (vinyl alcohol) matrix (red). (c) Basic ‘unit cells’ of nacre and bone the latter of which has a more regular staggered organization - have densely packed structures and are seen as the archetypes of biological nanocomposites, possessing high modulus, strength and - at larger length scales - toughness. Critical issues in nanocomposites 1. Dispersion Uniform dispersion of nanoparticles, and nanotubes against their agglomeration due to van der Waals bonding is the first step in the processing of nanocomposites.  SWCNTs tend to cluster into ropes and MWCNTs produced by chemical vapor deposition are often tangled together like spaghettis.  The separation of nanotubes in a solvent or a matrix material is a prerequisite for aligning them. 2. Alignment Because of their small sizes, it is exceedingly difficult to align the nanotubes in a polymeric matrix material in a manner accomplished in traditional short fiber composites. The lack of control of their orientation diminishes the effectiveness of nanotube reinforcement in composites, whether for structural or functional performance. 3. Volume and rate High volume and high rate fabrication is fundamental to manufacturing of nanocomposites as a commercially viable product. 4. Cost effectiveness Besides high volume and high rate production, the cost of nanocomposites also hinges on that of the nanoreinforcement material, particularly, nanotubes. It is anticipated that as applications for nanotubes and their composites increase the cost will be dramatically reduced.