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The Challenge of Nanomaterials:Routes to reliable materials?
Prof Alun VaughanOctober 2011
The Challenge of Nanomaterials:Routes to reliable materials?
A random walk on the edge
Prof Alun VaughanOctober 2011
Designing materials
44
Motivation
• Increased performance• Increased reliability• Increased power density• Increased functionality• Reduced power losses • Reduced environmental impact
55
The options
• New polymers
• Add something else
– A filler (micro, meso, nano)
– A polymer (immiscible, miscible)
– Small molecules
Which way to go?
66
Overview
• Why nano?
• What is the interphase?
• How much interphase?
• What is required for miscibility?
• How can we modify the interface?
Fillers:Micro, meso, nano?
88
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
Thomas Andritsch, PhD Thesis, 2010
Nano-structuring the epoxy improves performance
99
Publications
• Search ISI Web of Knowledge using terms poly* AND nanocomposite*
• First paper published in 1986
• Period of rapid exponential growth
• Plateau?
Year
2000 2002 2004 2006 2008 2010 2012
Num
ber
of P
aper
s
0
1000
2000
3000
4000
5000
6000
7000
8000
1010
Projects
Improved combinations of properties
1111
Filler chemistry – TiO2
• DC breakdown data for TiO2 in epoxy
• Strength decreases with nanoparticle inclusion
• Strength of the unfilled system ~320 kV mm-1
J Keith Nelson and John C Fothergill, Nanotechnology 15 (2004) 586–595
Nano-structuring the epoxy degrades performance
1212
Filler chemistry – TiO2 and Al2O3
• Addition of micro-sized filler is bad news
• Addition of even 0.1% of nanofiller is bad news
Epoxy / TiO2 Epoxy / Al2O3
S.Singha, M.J.Thomas, IEEE Trans DEI 2008, 15, 12
1313
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
What’s the key feature of nanocomposites?
Silica benign, meso-scopic BN good, even at “high” loadings
1414
The nature of the beast
Local Interactions
Matrix Effects
Aggregation
Local InteractionsLocal Interactions
Matrix EffectsMatrix Effects
AggregationAggregation
• 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/percolationWhich of these factors are important?
What is the interphase?
1616
The multilayer model
T.Tanaka, IEEE Trans DEI 2005, 12,
914
1717
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.
1818
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 T1
H . In the case of protons located within about 1 nm of the MMT surface, T1
H will be reduced directly, while so-called spin-diffusion results in this mechanism propagating into the bulk. Since the measured value of T1
H 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 T1
H 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.
1919
Hard evidence – NMR 2NMR[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
2020
Hard evidence – ESR Theory
2121
Hard evidence – ESR 1ESR
[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
2222
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.”
T.J.Lewis, IEEE Trans DEI 2004, 11, 739
2323
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
T.J.Lewis, IEEE Trans DEI 2004, 11, 739
How much interphase?
2525
More than two phases
pii k
mipiippc 1
How does the fraction of interphase i vary with filler loading p?
2626
2-D – the effect of symmetry
4
224 4 rla Im
r
lrlrlla IIm
12
2/12224 cos2
224
223 2
13 rla Im
3
I
II
Area of matrix phase
r
lrlrlla IIm
122/12223 cos
63
13
2727
Interphase
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.
Three-fold (solid line) and four-fold (dashed line)
20 nm particles
Nanoparticle Area Fraction
0.0 0.1 0.2 0.3 0.4 0.5
Inte
rpha
se A
rea
Fra
ctio
n
0.0
0.2
0.4
0.6
0.8
1.0
4 nm
8 nm
16 nm
32 nm
2828
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 xap
according 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
apai xx 1
ap
n
pai x
r
rx
1
Nanoparticle Area Fraction
0.0 0.1 0.2 0.3 0.4 0.5
Inte
rpha
se A
rea
Fra
ctio
n
0.0
0.2
0.4
0.6
0.8
1.0
4 nm
8 nm
16 nm
32 nm
2929
LatticeConsider 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.
RN
RIRPI
3030
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:
N
RIRNK
N
RMK nn
N
RMK
RN
RIRIRI n
1
3131
Interphase fraction
Three-fold (solid line) and four-fold (dashed line)
20 nm particles
1
2
2pr
rK
Nanoparticle Area Fraction
0.0 0.1 0.2 0.3 0.4 0.5
Inte
rpha
se A
rea
Fra
ctio
n
0.0
0.2
0.4
0.6
0.8
1.0
4 nm
8 nm
16 nm
32 nm
Nanoparticle Area Fraction
0.0 0.1 0.2 0.3 0.4 0.5
Inte
rpha
se A
rea
Fra
ctio
n
0.0
0.2
0.4
0.6
0.8
1.0
1
2
6
17
Interface effects
3333
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.
The nature of the glass transition
3434
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
Confined polyethylene glycol
“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
3535
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
Contributing processes
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.
3636
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.”
Multiple Tgs
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
3737
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)
3838
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?
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 cp varies systematically with stoichiometry/filling
• All glass transitions are singular
• Width of Tg is constant within experimental error
The complete system is being affected
4040
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.
r´ - varying particle permittivity
p
0.00 0.05 0.10 0.15
' c
2.0
2.5
3.0
3.5
4.0
Increasing p'
4141
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.
Varying interphase permittivity
p
0.00 0.05 0.10 0.15
' c
2.0
2.5
3.0
3.5
4.0
Increasing i'
i' = 2
i' = 2.8
4242
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.
Varying interphase thickness
p
0.00 0.05 0.10 0.15
' c
2.0
2.5
3.0
3.5
4.0
Decreasing interphase thickness
1
3
3pr
rK
4343
MgO
p
0.00 0.01 0.02 0.03 0.04
c'
3.0
3.2
3.4
3.6
3.8
4.0
The complete system is being affected
Effective particle permittivity?Thomas Andritsch, PhD Thesis, 2010
Thermodynamics of miscibility
4545
Theory
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46
A random model of a three phase system
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 mixtureG1 = Gibbs free energy of component AG2 = Gibbs free energy of component B
If ΔGm is the Gibbs free energy of mixing and ΔGm < 0, mixing will be favoured thermodynamically:
Miscibility
2112 GGG
mmm STHGGGG 2112
47
In general, the entropy term can be written:
where:
Entropy and enthalpy
lnkSm
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21
21
NN
NN
w12A – B
w22B – B
w11A – A
EnergyInteraction
w12A – B
w22B – B
w11A – A
EnergyInteraction
In general, the enthalpy term can be written:
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4848
Theory
Entropy Enthalpy
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Interface chemistry
5050
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
5151
Surface functionalisation (Dow)• A two percent silane solution can be prepared in the
alcohol of choice and applied to the sample
• Particles, e.g., pigments and fillers, can be silylated by stirring them in a solution for two to three minutes and then decanting the solution. The particles can then be rinsed with alcohol.
• Cure of the silane layer is for 5-10 min at 110 oC or for 24 hr at ambient conditions.
• A 95% ethanol-5% water solution is adjusted to pH 4.5-5.5 with acetic acid; silane is added with stirring to yield a 2-10% final concentration
• Silanetriols are most stable at pH 3-6, but condense rapidly at pH 7-9.3
• For less soluble silanes, 0.1% of a nonionic surfactant could be added and an emulsion rather than a solution is prepared. Stability of aqueous silane solutions varies from hours for the alkyl silanes to weeks for the aminosilanes. Poor solubility parameters limit the use of long chain alkyl and aromatic silanes by this method
5252
Variants
Alkyl substitutedEpoxy compatible
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5353
Another recipe
Angew. Chem. Int. Ed. 2003, 42, 4326 –4331
The solvents were all pre-dried by using standard methods
To dry calcined MCM-41 in THF (15 mL), dichlorodiphenylsilane (0.48 g, 0.19 mmol) was added and stirred at room temperature for 1 hour
(Ph2SiCl2) reacts with external Si-OH moieties and ensures all proceeding silane species reacts at the internal surface of MCM-41).
Elemental analysis found (%): C4.70, H0.87, Br 2.14.
29Si CP MAS NMR indicated significant loss of Si-OH moieties, indicating grafting had occurred
5454
RAFT methods
Colloidal silica particle suspension (48 mL of 30 wt % SiO2 in MIBK, D 20 nm), active silane (1.7 mmol, 0.61 g), and dried THF (6 mL) were added to flask.
The reaction mixture was heated at 85 °C under N2 protection overnight and then cooled to room temperature.
The reaction mixture was then precipitated into a large amount of hexane(500 mL).
The particles were recovered by centrifugation at 3000 rpm for 15 min.
The particles were repeatedly re-dissolved in 20 mL of acetone and reprecipitated in 200 mL of hexane.
5555
Characterisation 1
“Characteristic absorption bands were clearly visible at 1716.3 cm-1 due to the carbonyl group and at 1449.9 and 688 cm-1 due to the phenyl ring.”
Chunzhao Li and Brian C. Benicewicz, Macromolecules 2005, 38, 5929-5936
5656
Characterisation 2
2-stage process – TEOS then trimethyethoxylsilane (TMES)
Clear chemical effects relating to changes in surface chemistry can be seen in both the FTIR and 29Si NMR spectra
Feng-Hsi Huang, Chao-Ching Chang, Tai-Yueh Oyang, Ching-Chung Chen, Liao-Ping Cheng, J
Nanopart Res (2011) 13:3885–3897
5757
But what does the surface need to look like?
Conclusions
5959
MY Conclusions• I think the interfaces are key in both the science and
technology of nanodielectrics
• I think we have ideas but, at present, we don’t have enough understanding of what an interface/interphase is – there are techniques out there that have been used successfully
• I think we don’t have enough understanding of how to characterise interfaces
• I fear that much of our current attempts equate to “fighting the thermodynamics” and, in ameliorating this, are other demons introduced?
• I think that much more systematic study is necessary
• I do not have the tools to do what is necessary – I need to collaborate
60The new hall of the Tony Davies HV Lab.
Thank you