“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(