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The Challenge of Nanomaterials:

The Challenge of Nanomaterials:. Routes to reliable materials?. Prof Alun Vaughan October 2011. The Challenge of Nanomaterials:. Routes to reliable materials? A random walk on the edge. Prof Alun Vaughan October 2011. Designing materials. Motivation.

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The Challenge of Nanomaterials:

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  1. The Challenge of Nanomaterials: Routes to reliable materials? Prof Alun Vaughan October 2011

  2. The Challenge of Nanomaterials: Routes to reliable materials? A random walk on the edge Prof Alun Vaughan October 2011

  3. Designing materials

  4. Motivation • Increased performance• Increased reliability• Increased power density• Increased functionality• Reduced power losses • Reduced environmental impact 4

  5. The options New polymers Add something else A filler (micro, meso, nano) A polymer (immiscible, miscible) Small molecules Which way to go? 5

  6. Overview Why nano? What is the interphase? How much interphase? What is required for miscibility? How can we modify the interface? 6

  7. Fillers:Micro, meso, nano?

  8. Size matters DC breakdown data from 10% BN in epoxy Strength increases as the particle size is reduced Strength of the unfilled system ~165 kV mm-1 Nano-structuring the epoxy improves performance 8 Thomas Andritsch, PhD Thesis, 2010

  9. Publications Search ISI Web of Knowledge using terms poly* AND nanocomposite* First paper published in 1986 Period of rapid exponential growth Plateau? 9

  10. Projects Improved combinations of properties 10

  11. Filler chemistry – TiO2 • DC breakdown data for TiO2 in epoxy • Strength decreases with nanoparticle inclusion • Strength of the unfilled system ~320 kV mm-1 Nano-structuring the epoxy degrades performance J Keith Nelson and John C Fothergill, Nanotechnology 15 (2004) 586–595 11

  12. Filler chemistry – TiO2 and Al2O3 Epoxy / TiO2 Epoxy / Al2O3 • Addition of micro-sized filler is bad news • Addition of even 0.1% of nanofiller is bad news 12 S.Singha, M.J.Thomas, IEEE Trans DEI 2008, 15, 12

  13. Filler chemistry – SiO2 and BN • AC breakdown data for SiO2 and BN in epoxy • Strength of SiO2 largely independent nanoparticle inclusion • Strength increases with BN inclusion Silica benign, meso-scopic BN good, even at “high” loadings What’s the key feature of nanocomposites? 13

  14. The nature of the beast • Nanoparticle size/distribution/aspect ratio • Nanoparticle chemistry/impurities • Nanoparticle structure/crystallography • Nanoparticle surface chemistry • Interactions with matrix material – stoichiometry • Interactions with matrix material – molecular mobility ( ┴) • Charges/ions/polarisation • Matrix morphology • Aggregation/percolation Which of these factors are important? 14

  15. What is the interphase?

  16. The multilayer model T.Tanaka, IEEE Trans DEI 2005, 12, 914 16

  17. Hard evidence – NMR Theory T1 relaxation involves redistributing the populations of the nuclear spin states in order to reach the thermal equilibrium distribution. By definition this is not energy conserving. Moreover, spontaneous emission is negligibly slow at NMR frequencies. Hence truly isolated nuclear spins would show negligible rates of T1 relaxation. However, a variety of relaxation mechanisms allow nuclear spins to exchange energy with their surroundings, the lattice, allowing the spin populations to equilibrate. The fact that T1 relaxation involves an interaction with the surroundings is the origin of the alternative description, spin-lattice relaxation. 17

  18. Hard evidence – NMR 1 NMR [160] considers the utility of NMR as a potential on-line screening tool for characterizing dispersion in nanocomposites. The rationale behind the approach is that paramagnetic Fe3+ ions present in MMT as impurities will affect the proton longitudinal relaxation time in the polymer, a parameter termed T1H . In the case of protons located within about 1 nm of the MMT surface, T1H will be reduced directly, while so-called spin-diffusion results in this mechanism propagating into the bulk. Since the measured value of T1H will depends upon on the concentration of Fe3+ ions in the system and their proximity to the polymer, the better the MMT dispersion, the greater the reduction in T1H compared with the value determined from the polymer alone. [160] J.W. Gilman, S. Bourbigot, J.R. Shields, M. Nyden, T. Kashiwagi, R.D. Davis, D.L. Vanderhart, W. Demory, C.A. Wilkie, A.B. Morgan, J. Harris, R.E. Lyon, “High Throughput Methods For Polymer Nanocomposites Research: Extrusion, NMR Characterization And Flammability Property Screening”, J. Mater. Sci. 38 (2003) 4451–4460. 18

  19. Hard evidence – NMR 2 NMR [163] used NMR spectroscopy to study nanocomposites based upon styrene-butadiene rubber (SBR) and titania. Although 13C NMR results revealed significant shifts in peak positions, which have been taken to indicate interactions between nanoparticles and polymer chains, spin lattice relaxation experiments suggest that the molecular mobility in both systems is equivalent. [163] T.M. Arantes, K.V. Leao, M.I.B. Tavares, A.G. Ferreira, E. Longo, E.R. Camargo, “NMR study of styrene-butadiene rubber (SBR) and TiO2 nanocomposites”, Polymer Testing 28 (2009) 490–494 19

  20. Hard evidence – ESR Theory 20

  21. Hard evidence – ESR 1 ESR [171] studied nanocomposites of poly(methyl acrylate) (PMA) and synthetic fluoromica, in which the PMA had been modified to include a so-called spin label. That is, a stable free radical, commonly nitroxide, which is introduced into a material that does not have an intrinsic paramagnetic response. This work showed that, in exfoliated systems, the mobility of PMA chains is reduced due to the interactions with the nanofiller. The thickness of the rigid interface region was estimated to be in the range 5-15 nm. In intercalated materials similar results were obtained, in that a fraction of constrained chain segments were detected at the clay interface together with another with a higher mobility. [171] Yohei Miwa, Andrew R. Drews, and Shulamith Schlick, “Detection of the Direct Effect of Clay on Polymer Dynamics: The Case of Spin-Labeled Poly(methyl acrylate)/Clay Nanocomposites Studied by ESR, XRD, and DSC”, Macromolecules 2006, 39, 3304-3311 21

  22. The interphase The “interphase” corresponds to an intermediate region where material properties are representative of neither phase A nor phase B “A frequent situation in nanodielectric systems is one in which the surface or at least a part of the surface of particle A becomes effectively charged and the surrounding phase B responds by establishing a screening countercharge confronting the charge on A.” 22 T.J.Lewis, IEEE Trans DEI 2004, 11, 739

  23. How much interphase? “Interface properties become increasingly prominent if phase A is a particle of finite size and surrounded by B with the AB interface between them …the total interface contribution can become very significant as the particle diameter is reduced.” T J Lewis, Interfaces: nanometric dielectrics, J. Phys. D: Appl. Phys. 38 (2005) 202–212 23 T.J.Lewis, IEEE Trans DEI 2004, 11, 739

  24. How much interphase?

  25. More than two phases How does the fraction of interphase i vary with filler loading p? 25

  26. 2-D – the effect of symmetry Area of matrix phase I II 4 3 26

  27. Interphase Three-fold (solid line) and four-fold (dashed line) 20 nm particles S. Raetzke and J. Kindersberger, Role of Interphase on the Resistance to High-voltage Arcing, on Tracking and Erosion of Silicone/SiO2 Nanocomposites, IEEE Trans. DEI 17, 2010, 607-614. 27

  28. Interphase The form of behaviour is independent of symmetry or dimensionality At low filler loading levels, the area fraction of interphase material increases linearly, according to the relationship: where n indicates the dimensionality of the model (here n = 2) and xai and xap represent the area fractions of interphase and particles respectively. This is independent of symmetry and corresponds to the regime before overlap of neighbouring interphase regions. At high filler loading levels, xai varies with xapaccording to: This is independent of symmetry, dimensionality, or the value chosen for the interphase thickness and corresponds to the regime where all of the area not occupied by the particles themselves corresponds to interphase material. Three-fold (solid line) and four-fold (dashed line) 20 nm particles 28

  29. Lattice Consider adding the (R + 1)th nanoparticle. The (R + 1)th nanoparticle cannot occupy a cell that is already occupied by a nanoparticle. The (R + 1)th nanoparticle can occupy any one of the unoccupied (N – R) cells that were, previously, either interphase or matrix. The probability of it occupying an interphase cell can therefore be written PI(R), where: and I(R) represent the number of interphase cells present prior to the introduction of the (R + 1)th nanoparticle. 29

  30. Lattice The inclusion of the (R + 1)th nanoparticle will convert neighbouring, previously matrix cells, into interphase cells. The coordination number, Kn , specifies the number of interphase cells per nanoparticle in the limit R 0. At higher fill fractions, there will be a finite probability of each of these Kn interphase cells coinciding with a cell that was not previously of matrix character. The effective number of additional interphase-type cells induced by the addition of the (R + 1)th nanoparticle can then be written: where M(R) represent the number of matrix cells present prior to the introduction of the (R + 1)th nanoparticle. Thus, the effective number of interphase cells after addition of the (R + 1)th nanoparticle, I(R+1) can be written: 30

  31. Interphase fraction Three-fold (solid line) and four-fold (dashed line) 20 nm particles 31

  32. Interface effects

  33. The nature of the glass transition Quench a polymer from a temperature T1 to another temperature T2, where T1 > Tg > T2 The initial glassy state will depend upon both T1 and T2 The Gibbs-DiMarzio theory for a polymer AAAAAAAAAAAAAAAA : At some temperature, the distribution of free volume in the system is such that molecular motion is no longer possible within the time scale of the measurement. Free volume is envisaged as being dynamically created and destroyed locally through the cooperative motion of chain segments. Depends upon local bond conformations and “broken” A-A inter/intra molecular bonds. 33

  34. Confined polyethylene glycol Where a polymer is close to a second medium, we need to consider both polymer – polymer (A-A) and polymer – medium (A-B) interactions This can affect molecular configurations and mobility and, consequently, the measured glass transition Consider polyethylene glycol (low molar mass PEO) confined within porous silica “These results clearly indicate that confined PG exhibits longer relaxation times compared to the bulk dynamics. This finite size effect increases as the temperature is lowered and thus implies a considerable retardation in molecular mobility for confined polyethylene glycol near Tg.” J.Schuller, Y.B.Melnichenko, B.Yu, R.Richert, E.W.Ficher, 1994 Dielectric studies of the glass transition in porous media Phys. Rev. Lett. 73 2224–7 34

  35. Contributing processes Consider toluene in porous silica Two processes can affect the measured Tg A decrease in Tg can occur with decreasing pore size as a result of the material vitrifying under conditions of constant volume (isochoric conditions); modelling indicates that this is an intrinsic size effect related to the influence of a negative hydrostatic pressure on glass formation Interactions with the pore walls tends to reduce inhibit molecular interactions and, hence, increase Tg D.Morineau, Y.D.Xia, C.Alba-Simionesco, 2002 Finite-size and surface effects on the glass transition of liquid toluene confined in cylindrical mesopores J. Chem. Phys. 117 8966–72. 35

  36. Multiple Tgs Consider solutions of polystyrene (PS) in ortho-terphenyl (o-TP) “Interestingly, the DSC thermograms for the o-TP or o-TP/PS solutions confined in the pore show what appear to be two glass transitions. One is at a higher temperature than the bulk state Tg and the other is at a lower temperature.” J.Y.Park, G.B.McKenna, 1999 Size and confinement effects on the glass transition behavior of polystyrene/o-terphenyl polymer solutions Phys. Rev. B 61 6667–76 36

  37. So … Ideas based upon interphases are very reasonable in nanocomposites and include ideas of molecular confinement The interphase is believed to constitute a substantial fraction of the matrix in nanocomposites Evidence from spectroscopy of molecular interactions Tg is intrinsically linked to thermodynamic interactions and molecular confinement Porous systems have been extensively studied Strong Tg effects have been reported and analysed in detail (theory) 37

  38. So … Ideas based upon interphases are very reasonable in nanocomposites and include ideas of molecular confinement The interphase is believed to constitute a substantial fraction of the the matrix in nanocomposites Evidence from spectroscopy of molecular interactions Tg is intrinsically linked to thermodynamic interactions and molecular confinement Porous systems have been extensively studied Strong Tg effects have been reported and analysed in detail (theory) … how about for nanocomposites? 38

  39. Tg in epoxy/silca systems • Tg is strongly dependent upon resin stoichiometry in both unfilled and filled (5%) systems • Tg is suppressed in nanocomposites of optimum stoichiometry • The value of Dcp varies systematically with stoichiometry/filling • All glass transitions are singular • Width of Tg is constant within experimental error • The complete system is being affected

  40. er´ - varying particle permittivity Plot of the real part of the permittivity against volume fraction of nanoparticles for a random 3-D simulation of an array of nanoparticles (diameter 20 nm). The interphase thickness ti = 20 nm (K3 = 26) and interphase permittivity εi’ = 2.4 throughout; results for nanoparticle permittivity values of εp’  = 6 and εp’  = 10 are shown. The solid and long dashed lines correspond to the upper and lower Wiener bounds respectively and the intermediate Lichtenecker-Rother equation is indicated by the dash/dot/dot line. 40

  41. Varying interphase permittivity Plot of the real part of the permittivity against volume fraction of nanoparticles for a random 3-D simulation of an array of nanoparticles (diameter 20 nm, εp’ = 8) and an interphase thickness of 20 nm (K3 = 26). Results are shown for interphase permittivities εi’ = 2 and εi’  = 2.8. The solid and long dashed lines correspond to the upper and lower Wiener bounds respectively and the intermediate Lichtenecker-Rother equation is indicated by the dash/dot/dot line. 41

  42. Varying interphase thickness Plot of the real part of the permittivity against volume fraction of nanoparticles for a random 3-D simulation of an array of nanoparticles (diameter 20 nm, εp = 8). Results for an interphase permittivity εi’ = 2.4 and interphase thicknesses of 10 nm (K3 = 7) and 40 nm (K3 = 63) are shown. The solid and long dashed lines correspond to the upper and lower Wiener bounds respectively and the intermediate Lichtenecker-Rother equation is indicated by the dash/dot/dot line. 42

  43. MgO The complete system is being affected Effective particle permittivity? Thomas Andritsch, PhD Thesis, 2010 43

  44. Thermodynamics of miscibility

  45. Theory 45

  46. A random model of a three phase system Miscibility The extent to which different systems mix depends on the Gibbs free energy of the system, G In thermodynamic terms, two components will mix intimately provided this results in a reduction in the total free energy of the system: where G12 = Gibbs free energy of mixture G1 = Gibbs free energy of component A G2 = Gibbs free energy of component B If ΔGmisthe Gibbs free energy of mixing and ΔGm < 0, mixing will be favoured thermodynamically:

  47. Entropy and enthalpy In general, the entropy term can be written: where: In general, the enthalpy term can be written:

  48. Theory Entropy Enthalpy 48

  49. Interface chemistry

  50. Sol-gel chemistry The initial reaction is hydrolysis: Si(OR)4 + H2O → HO-Si(OR)3 + R-OH Depending on the amount of water and catalyst present, hydrolysis may proceed to completion, so that all of the OR groups are replaced by OH groups, as follows: Si(OR)4 + 4 H2O → Si(OH)4 + 4 R-OH SILANOL PRODUCTION Hydrolyzed molecules undergo condensation reactions to form siloxane bonds: (OR)3–Si-OH + HO–Si-(OR)3 → [(OR)3Si–O–Si(OR)3] + H-O-H or (OR)3–Si-OR + HO–Si-(OR)3 → [(OR)3Si–O–Si(OR)3] + R-OH POLYMERISATION Polymerisation therefore results in the formation of a 1, 2, or 3- dimensional network of siloxane [Si–O–Si] bonds accompanied by the production of H-O-H and R-O-H species. TEOS tetraethyl orthosilicate tetraethoxysilane 50

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