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“Shake Gels”
Elena Loizou12 May 2006
1. Zebrowski, J.; Prasad, V.; Zhang, W.; Walker, L. M.; Weitz, D. A. “Shake-gels: shear-induced gelation of Laponite/PEO mixtures”, Colloids and Surfaces A: Physicochemical and Engineering Aspects 2003, 213, (2-3), 189-197.
2. Pozzo, D. C.; Walker, L. M. “Reversible shear gelation of polymer–clay dispersions”, Colloids and Surfaces A: Physicochemical and Engineering Aspects 2004, 240, (1-3), 187-198.
What are the shake gels?
• Low viscosity fluids that when shaken form gels.
• Mixtures of a colloid and a polymer at specific range of concentrations
• Characteristics of shear-induced gels: Turbid, stiff, viscoelastic Can support their own weight when the jar is inverted When they left at rest they slowly relax back to a fluid
“Half-cooled gelatin dessert”
Why shake gels are interesting?
• Have potential applications in industry:shock absorbers for cars transporters for materials (e.g. solids)drilling mud for petroleum extraction
Why polymer - colloid dispersions are interesting?
rheological modifiers – paints, cosmetics, foodadditives in coatingsgas or solvent barriers
• Can be used as:
Shake Gels - First observed :
silica spheres (nm) + polyethylene oxide (PEO)
Cabane, B.; Wong, K.; Lindner, P.; Lafuma, F. The society of Rheology 1997, 41, (3), 531-547.
Increasing PEO concentration
“Shake gels” observed near the surface saturation limit
solution gelshake gel
• At Rest:PEO chains weakly adsorbed onto particles
form small aggregates
• Applied Shear:The small aggregates deformexpose additional particle surface to the bulknew polymer segments adsorbed onto the fresh surfaceMore polymer bridges between particles
• Cessation of Shear:thermal motions drive thepolymer to desorb and obtain its original configuration, bridging is reduced gels relax back to a fluid
Mechanism of shear-gelation:Occurs at a regime near the saturation
of the particle surface with polymer
Discoid clay particles
Na0.7+ [(Si8Mg 5.5 Li0.3) O20(OH)4]-0.7
Crystal Structure
Disc particle Stack of particles
•Clay : Laponite charged coin-like particles25-30 nm in diameter1 nm in thickness
Laponite:Dispersion / Exfoliation
Exfoliationplatelets separate fromeach other
Dispersion
Mechanism of gelation - Still a considerable debate
Attractive interactionsRepulsive interactions
OR
Electrostatic Coulomb Repulsion
Van der WaalsAttraction
Tanaka, H.; Meunier, J.; Bonn, D. Physical Review E 2004, 69, 031404
Poly(ethylene oxide) - PEO
Water-soluble, synthetic polymerSimple basic unit : (-CH2CH2O-)n
When dissolves in water, is characterized
Adsorbs onto Laponite platelets
Hydrophilic interactions through O
Hydrophobic interactions through CH2CH2
Phase Diagram of Laponite-PEO
Zebrowski, J.; Prasad, V.; Zhang, W.; Walker, L. M.; Weitz, D. A. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2003, 213, (2-3), 189-197.
Shear Thickening samples
Shake Gel samples
Liquid samples
PEO : Mw = 300 000 g/mol
Phase Diagram of Laponite-PEO
area surfaceclay total
polymer theof masst
Pozzo, D. C.; Walker, L. M.,Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004, 240, (1-3), 187-198.
Characterization TechniquesScattering
-light -neutron-x-ray
Rheology -flow
-oscillatory
Microscopy-SEM-TEM-AFM
Birefringence
Light Scattering
• Dynamic light scattering (DLS)
Relies on time-dependent fluctuations
on the intensity
due to Brownian motions of molecules
Measure the diffusion coefficient of
the molecules
Determine a hydrodynamic radius, Rh
The size range: 1 nm - 500 nm
D
kTR
Dq
tbBtG
h0
2
1
2
6
)exp()(
Second order autocorrelation function
Experimental parameters
Decay rate
Decay time
Diffusion coefficientD
bB
G
,
2
Small Angle Neutron Scattering (SANS)
http://www.ncnr.nist.gov/summerschool/information/SANS_tutorial.pdf
λ: neutron wavelength, θ: scattering angle, Q: scattering vector
Q
2πd
Characteristic dimensionSpacing between particles
Contrast Matched
LaponiteSLD = 4.2x1010 cm-2
(69% D2O)
PEOSLD = 0.6x1010 cm-2
(17% D2O)
Nelson, Andrew, Neutron and Light Scattering Studies of Polymers Adsorbed on Laponite. University of Bristol, 2002
Pynn, Roger, Neutron Scattering - A PRIMER. Los Alamos Neutron Science Center (LANSCE), 1990
SLD = -0.6 x1010 cm-2
SLD = 6.4 x1010 cm-2
Phase Diagram of Laponite-PEO
area surfaceclay total
polymer theof masst
Layer thickness and absorbed amount
Lal, J.; Auvray, L. “Interaction of polymer with clays”, Journal of Applied Crystallography 2000, 33, (1), 673-676.
Lal, J.; Auvray, L. “Interaction of polymer with discotic clay particles”, Molecular Crystals and Liquid Crystals 2001, 356, 503-515.
Polymer Layer Thickness: 2-3 nm
On each face: 1-1.5 nm
Absorbed amount :0.6-0.9 mg/m2
Core-Shell Model
Absorbed amount and layer thickness
Nelson, A.; Cosgrove, T. “A Small-Angle Neutron Scattering Study of Adsorbed Poly(ethylene oxide) on Laponite”, Langmuir 2004, 20, (6), 2298-2304.
Core-Shell ModelThe shell is extended to the sides of the clay
Face thickness: 1.5 nm
Edge thickness: 1.5 - 4.5 nm
Absorbed amount : 0.7mg/m2
Phase Diagram of Laponite-PEO
area surfaceclay total
polymer theof masst
Dynamic Light Scattering
Laponite:1.25 wt %
0
~
0τ Decay time of Laponite-PEO mixture
Decay time of Pure Laponite (0.24 ms)
Dim
ensi
onle
ss ti
me
cons
tant
Relaxation after shear-induced gelation
10 C
15 C
30 C
25 C
20 C
1.5% (w/w) Laponite – 0.45% (w/w) PEO/
0
*22* "' teGGGG
G*: complex modulusG’ : elastic modulusG’’ : viscous modulus
Arrhenius plot
T
1
K
ElnA
τ
1ln
B
A
τ : characteristic relaxation time T : absolute temperature (K) A : non thermal constant EA : activation energy (eV) KB : Boltzman’s constant (8.61738 x 10-5 eV/K)
Activation Energy (EA) = 107 kJ/mol
1.5% (w/w) Laponite – 0.45% (w/w) PEO
21 day old
7 day old
1 day old
21 day old
Aging EffectsT=25 C
Scattering Profiles - pure solutions
1.5% (w/w) Laponite Thin particle
Random coils withExcluded VolumeInteractions
Form factor ofNon-interacting thin discsR = 13.3nmH = 0.8 nm
0.45% (w/w) PEO
1.5 % (w/w) Laponite – 0.45% (w/w) PEO - (D2O)
Scattering Profiles
25 C
10 C
Slope: -2Thin Disc
Slope: -1Elongated objects
•Gelled phases are the sameSo T, does not affect the structure
•At T=10 C the relaxation isincomplete and thermal fluctuations notstrong enough to break up the aggregates
Phase Diagram of Laponite-PEO
Contrast Matched the Clay1.5% (w/w) Laponite + PEO (69% D2O)
Shake Gel
Permanent Gel
Foaming solution
25 C
10 C
Highly stretched PEO
High PEO
Low PEOMedium PEO
Medium PEO
Flat adsorbed 2-D structure
PEO coats the clay and adopts its shape
Contrast Matched the PEO
1.5% (w/w) Laponite – 0.45% (w/w) PEO - (17 % D2O)
Shake Gel25 C•The scattering differences are smaller
•The polymer is the one that experience the large deformational changes upon shear
area surfaceclay total
polymer theof masst
Conclusions
ConclusionsYES !!! Shake gels were observed with discoid
Laponite particles when they were mixed with PEOOccur at a regime near saturation
of clay surface with polymerUnder shear formation new polymer-clay bridgesWith cessation of shear slowly relaxation due to
thermal motionsRelaxation depends on:
-temperature
-aging of the sample
Questions?
• Structure Factor: gives information about the correlations of atomic position, and it can be measured only in concentrate systems.
• Form factor: corresponds to the particle shape. In dilute suspensions were the intensity depends only to the form factor, information about the particle size and shape can be obtained. The form factor is a Fourier transformation of the particle pair correlation function.
N
ji,
)rr(qi jieN
1)qS(
2rqi rde Δρ)qP(
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