1
Shake Gels

• 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 polymerclay
  dispersions,
• Colloids and Surfaces A Physicochemical
  and Engineering Aspects
• 2004, 240, (1-3), 187-198.

Elena Loizou 12 May 2006
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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

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Why shake gels are interesting?
Why polymer - colloid dispersions are interesting?

• Can be used as

• rheological modifiers paints, cosmetics, food
• additives in coatings
• gas or solvent barriers

• Have potential applications in industry
• shock absorbers for cars
• transporters for materials (e.g. solids)
• drilling mud for petroleum extraction

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Shake Gels - First observed silica spheres
(nm) polyethylene oxide (PEO)
solution
gel
shake gel
Increasing PEO concentration Shake gels
observed near the surface saturation limit
Cabane, B. Wong, K. Lindner, P. Lafuma, F. The
society of Rheology 1997, 41, (3), 531-547.
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Mechanism of shear-gelationOccurs at a regime
near the saturation of the particle surface
with polymer

• At Rest
• PEO chains weakly adsorbed onto particles
• form small aggregates
• Applied Shear
• The small aggregates deform
• expose additional particle surface to the bulk
• new polymer segments adsorbed onto the fresh
  surface
• More polymer bridges between particles
• Cessation of Shear
• thermal motions drive the
• polymer to desorb and obtain its original
  configuration,
• bridging is reduced ? gels relax back to a
  fluid

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Discoid clay particles

• Clay Laponite
• charged coin-like particles
• 25-30 nm in diameter
• 1 nm in thickness

Crystal Structure
Disc particle
Stack of particles
Na0.7 (Si8Mg 5.5 Li0.3) O20(OH)4-0.7
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LaponiteDispersion / Exfoliation
Dispersion
Exfoliation platelets separate from each other
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Mechanism of gelation - Still a considerable
debate
Attractive interactions
Repulsive interactions
OR
Electrostatic Coulomb Repulsion
Van der Waals Attraction
Tanaka, H. Meunier, J. Bonn, D. Physical Review
E 2004, 69, 031404
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Poly(ethylene oxide) - PEO

• Water-soluble, synthetic polymer
• Simple basic unit (-CH2CH2O-)n
• When dissolves in water, is characterized
• Adsorbs onto Laponite platelets

Hydrophilic interactions through O
Hydrophobic interactions through CH2CH2
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Phase Diagram of Laponite-PEO
PEO Mw 300 000 g/mol
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.
11
Phase Diagram of Laponite-PEO
Pozzo, D. C. Walker, L. M.,Colloids and Surfaces
A Physicochemical and Engineering Aspects, 2004,
240, (1-3), 187-198.
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Characterization Techniques

• Scattering
• -light
• -neutron
• -x-ray
• Rheology
• -flow
• -oscillatory
• Microscopy
• -SEM
• -TEM
• -AFM
• Birefringence

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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

Second order autocorrelation function Experimental
parameters Decay rate Decay time Diffusion
coefficient
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Small Angle Neutron Scattering (SANS)
Characteristic dimension Spacing between particles
? neutron wavelength, ? scattering angle, Q
scattering vector
http//www.ncnr.nist.gov/summerschool/information/
SANS_tutorial.pdf
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Contrast Matched
SLD 6.4 x1010 cm-2
SLD -0.6 x1010 cm-2
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
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Phase Diagram of Laponite-PEO
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Layer thickness and absorbed amount
Polymer Layer Thickness 2-3 nm On each face
1-1.5 nm
Absorbed amount 0.6-0.9 mg/m2
Core-Shell Model
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.
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Absorbed amount and layer thickness
Face thickness 1.5 nm
Edge thickness 1.5 - 4.5 nm
Core-Shell Model The shell is extended to the
sides of the clay
Absorbed amount 0.7mg/m2
Nelson, A. Cosgrove, T. A Small-Angle Neutron
Scattering Study of Adsorbed Poly(ethylene oxide)
on Laponite, Langmuir 2004, 20, (6), 2298-2304.
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Phase Diagram of Laponite-PEO
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Dynamic Light Scattering
Laponite1.25 wt
Dimensionless time constant
Decay time of Laponite-PEO mixture Decay time of
Pure Laponite (0.24 ms)
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Relaxation after shear-induced gelation
1.5 (w/w) Laponite 0.45 (w/w) PEO
G complex modulus G elastic modulus G
viscous modulus
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Arrhenius plot
characteristic relaxation time T
absolute temperature (K) A non thermal
constant EA activation energy (eV) KB
Boltzmans constant (8.61738 x 10-5
eV/K)
Activation Energy (EA) 107 kJ/mol
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Aging Effects
T25 ?C
1.5 (w/w) Laponite 0.45 (w/w) PEO
21 day old
21 day old
7 day old
1 day old
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Scattering Profiles - pure solutions
1.5 (w/w) Laponite
Thin particle
Form factor of Non-interacting thin discs R
13.3nm H 0.8 nm
0.45 (w/w) PEO
Random coils with Excluded Volume Interactions
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Scattering Profiles
1.5 (w/w) Laponite 0.45 (w/w) PEO - (D2O)
Slope -1 Elongated objects
25 ?C
Slope -2 Thin Disc
10 ?C

• Gelled phases are the same
• So T, does not affect the structure
• At T10 ?C the relaxation is
• incomplete and thermal fluctuations not
• strong enough to break up the aggregates

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Phase Diagram of Laponite-PEO
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Contrast Matched the Clay 1.5 (w/w) Laponite
PEO (69 D2O)
Highly stretched PEO
Shake Gel
Flat adsorbed 2-D structure
25 ?C
Permanent Gel
Low PEO
Medium PEO
Foaming solution
10 ?C
High PEO
Medium PEO
PEO coats the clay and adopts its shape
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Contrast Matched the PEO 1.5 (w/w) Laponite
0.45 (w/w) PEO - (17 D2O)
25 ?C
Shake Gel

• The scattering differences are smaller
• The polymer is the one that experience the large
  deformational changes upon shear

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Conclusions
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Conclusions

• YES !!! Shake gels were observed with discoid
  Laponite particles when they were mixed with PEO
• Occur at a regime near saturation
• of clay surface with polymer
• Under shear ? formation new polymer-clay bridges
• With cessation of shear ? slowly relaxation due
  to thermal motions
• Relaxation depends on
• -temperature
• -aging of the sample

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Questions?
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• 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.