Transcript Slide 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 polymer–clay dispersions”,
Colloids and Surfaces A: Physicochemical and Engineering Aspects
2004, 240, (1-3), 187-198.
Elena Loizou
12 May 2006
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 polymer - colloid dispersions
are interesting?
• Can be used as:
rheological modifiers – paints, cosmetics, food
additives in coatings
gas or solvent barriers
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
Shake Gels - First observed :
silica spheres (nm) + polyethylene oxide (PEO)
solution
shake gel
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.
Mechanism of shear-gelation:
• At Rest:
Occurs at a regime near the saturation
of the particle surface
with polymer
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
Discoid clay particles
•Clay : Laponite
charged coin-like particles
25-30 nm in diameter
1 nm in thickness
Crystal Structure
Disc particle
Na0.7+ [(Si8Mg 5.5 Li0.3) O20(OH)4]-0.7
Stack of particles
Laponite:
Dispersion / Exfoliation
Dispersion
Exfoliation
platelets
separate from
each other
Mechanism of gelation Still a considerable debate
Attractive interactions
Repulsive interactions
OR
Electrostatic
Coulomb Repulsion
Tanaka, H.; Meunier, J.; Bonn, D. Physical Review E 2004, 69, 031404
Van der Waals
Attraction
Poly(ethylene oxide) - PEO
 Water-soluble, synthetic polymer
 Simple basic unit : (-CH2CH2O-)n
 When dissolves in water, is characterized
Hydrophilic interactions
through O
Hydrophobic interactions
through CH2CH2
 Adsorbs onto Laponite platelets
Phase Diagram of Laponite-PEO
PEO : Mw = 300 000 g/mol
Shear Thickening samples
Shake Gel samples
Liquid samples
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.
Phase Diagram of Laponite-PEO
t 
mass of the polymer
total clay surface area
Pozzo, D. C.; Walker, L. M.,Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004, 240, (1-3), 187-198.
Characterization Techniques
Scattering
-light
-neutron
-x-ray
Rheology
-flow
-oscillatory
Microscopy
-SEM
-TEM
-AFM
Birefringence
Light Scattering
•
Dynamic light scattering (DLS)
G 2 (t )  B  b exp(t )

Relies on time-dependent fluctuations
   1
on the intensity
  q2D
due to Brownian motions of molecules

Rh 
Measure the diffusion coefficient of
kT
60 D
the molecules


Determine a hydrodynamic radius, Rh
The size range: 1 nm - 500 nm
G 2 Second order autocorrelation function
B, b Experimental parameters
Decay rate


D
Decay time
Diffusion coefficient
Small Angle Neutron Scattering
(SANS)
Characteristic dimension
Spacing between particles
2π
d
Q
λ: neutron wavelength, θ: scattering angle, Q: scattering vector
http://www.ncnr.nist.gov/summerschool/information/SANS_tutorial.pdf
Contrast Matched
SLD = 6.4 x1010 cm-2
Laponite
SLD = 4.2x1010 cm-2
(69% D2O)
PEO
SLD = 0.6x1010 cm-2
(17% D2O)
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
Phase Diagram of Laponite-PEO
t 
mass of the polymer
total clay surface area
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.
Absorbed amount and
layer thickness
Edge thickness:
1.5 - 4.5 nm
Face thickness:
1.5 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.
Phase Diagram of Laponite-PEO
t 
mass of the polymer
total clay surface area
Dynamic Light Scattering

~
 
0
Dimensionless time constant
Laponite:1.25 wt %

Decay time of Laponite-PEO mixture
τ0
Decay time of Pure Laponite (0.24 ms)
Relaxation after
shear-induced gelation
1.5% (w/w) Laponite – 0.45% (w/w) PEO
G *  G ' 2  G" 2  G * e  t / 
0
G*: complex modulus
G’ : elastic modulus
G’’ : viscous modulus
10 C
20 C
25 C
30 C
15 C
Arrhenius plot
EA 1
1
ln   lnA 
KB T
τ
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
Aging Effects
T=25 C
1.5% (w/w) Laponite – 0.45% (w/w) PEO
21 day old
21 day old
7 day old
1 day old
Scattering Profiles - pure solutions
1.5% (w/w) Laponite
0.45% (w/w) PEO
Thin particle
Form factor of
Non-interacting thin discs
R = 13.3nm
H = 0.8 nm
Random coils with
Excluded Volume
Interactions
Scattering Profiles
1.5 % (w/w) Laponite – 0.45% (w/w) PEO - (D2O)
25 C
Slope: -1
Elongated objects
Slope: -2
Thin Disc
10 C
•Gelled phases are the same
So T, does not affect the structure
•At T=10 C the relaxation is
incomplete and thermal fluctuations not
strong enough to break up the aggregates
Phase Diagram of Laponite-PEO
Contrast Matched the Clay
1.5% (w/w) Laponite + PEO (69% D2O)
Shake Gel Flat adsorbed 2-D structure
25 C
Medium PEO
10 C
Medium PEO
Highly stretched PEO
Permanent Gel
Low PEO
Foaming solution
High PEO
PEO coats the clay and adopts its shape
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
Conclusions
t 
mass of the polymer
total clay surface area
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
Questions?
• Structure Factor: gives information about the correlations of
atomic position, and it can be measured only in concentrate systems.

1 N iq(ri  rj )
S( q)   e
N i, j
• 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.
  2

P(q)   Δρ e iq r d r