Post on 09-Jun-2020
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RHEOLOGY AND FLOW-INDUCED STRUCTURES IN POLYSACCHARIDE-BASED MATERIALS
Patrick NavardMines ParisTech
Materials Forming CenterCentre de Mise en Forme des Matériaux – CEMEF
Sophia-AntipolisFrance
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I Introduction
Polymers, including polysaccharides, are chemical species that can form many different organisation in space and time.
When subjected to a flow, these organisations can:• Remain unchanged• Change irreversibly• Change reversibly
Usually, experiments conducted under flow are performed in “blind”manner, i.e. without any indication of the organisational changes that are going on.
And flow is ALWAYS moving molecules from one place to another.
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I Introduction
One way to better understand which organisation is formed under flow is to perform some physical probing while flowing.
This is what is called rheo-physics.
In this talk we will • recall what is rheology• describe several complex fluids• give the basis of rheo-physics• give examples of flow-induced structure studies
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I. Introduction
II. Rheology
III. Complex fluids
IV. Rheo-physics
V. Examples of flow-induced structures
I Introduction
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I Introduction
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Details do not matter
I Introduction
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Polymers: the different statesHermann STAUDINGER (Germany): Nobel prize in chemistry 1953
- polymer
Paul FLORY (USA): Nobel prize in chemistry 1974- physics and chemistry
Pierre Gilles de GENNES (France): Nobel prize in physics 1991- polymer dynamics
HEEGER, McDIARMID (USA), SHIRAKAWA (Japan), Nobel prize in physics 2000- conducting polymers
The different statestype of chain
movementstructure in space
I Introduction
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1- Type of chainLinear
f = 2
Branched
Cross-linked2 < f < 3
Functionality f : number of covalent bonds linking each unit to its neighbours. f drive the structure of the chain and for a large part the physical morphology
I Introduction
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2- Movement
At large scale (notion of glass transition)
At small scale (secondary transitions)
I Introduction
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Viscous (irreversible deformation)-liquid-solid
Gel or elastic network (reversible deformation)
3- Structure in space
- orientation
- order
3-a Not ordered Not oriented
I Introduction
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3-b Oriented Not orderedLiquid crystal (mesomorphic phases)
3-c Ordered Not orientedplastic crystals (CCl4)
3-d Oriented OrderedCrystal
I Introduction
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Simply by looking at a normal polymer, the amount of structures, morphologies, types of organisation is very large.Most of them can influence or change structures during flow.
Why is it important: • processing is always implying a flow.• flow is present in mixing, dissolving and most
preparation• flow is a tool to understand matter properties
and organisation
I Introduction
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II. Rheology
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II Rheology
Rheology, term invented by Bingham in 1929:Rheology is the study of flow and deformation of matter
We are thus considering the interrelation between stress (force over a surface), deformation and rate of deformation.
Uniaxial extension
Shear
Deformation
Rheology is a branch of mechanics. The relations between stress, deformation, rate of deformation are tensorial equations depending on time.
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II Rheology
Polymer are most mainly falling into three categories:
Viscous: stress is not a function of deformation, but of deformation rate (water, oil). Irreversible.
Plastic: stress is not a function of deformation rate, but of deformation (crystal). Irreversible.
Elastic: Reversible deformation (matter comes back to its initial position after stopping deformation)
A lot of materials are complex:• solid metals are visco-elasto-plastic• molten polymers are visco-elastic
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II Rheology
Uniaxial extension• Simpler to understand physically• Simpler to model• Can be easily added into the free energy equation describing
the materialBUT
• Very difficult to perform experiments
Shear• Difficult to understand and model since it couples extension
and rotationBUT
• Easy to perform experiments (capillary rheometers, rotational rheometers
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II Rheology
Rheometry
Conditions• Uniform fluid (same composition and properties in space, in
all the rheometer cell)
• No turbulence
• Laminar flow
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II Rheology
Rate of deformation in shear (shear rate)
Deformation : 0.2
shear rate time
104 s-1 2.10-5 second10 s-1 0.02 second10-1 s-1 2 seconds10-3 s-1 200 seconds
l
hhl
=γ
Shear deformation
timeγγ =&
Shear rate
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Linear regime (low shear rate)
Steady stateConstant viscosity
Newton law
Shear rate
Viscosity η
II Rheology
γση &=
γ&
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Non-linear regime
Steady state flow
Shear thinningeffect
II Rheology
log shear rate
log viscosity
11 −sθ
( )[ ] 2/)1(2
0
1−
∞
∞ +=−− n
γληηηη
&Ex of law: Carreau
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III. Complex fluids
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III Complex fluids
Let’s look at a series of complex polymer fluids
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steady extensional flow
a- Polymers alone
Single chain deformation
III Complex fluids
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Many long chainsEntanglements: topological constrains between chains
III Complex fluids
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Many long chainsEntanglements: topological constrains between chains
Chain orientation and chain stretching (decrease of entropy) will change the rheological response of the fluid
III Complex fluids
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III Complex fluids
b- Incompatible polymer blends
Globular
Layers
Co - continuous
Classical morphologies
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Grace curve (1982) :
III Complex fluids
Incompatible polymer blends: rupture of droplets
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III Complex fluids
Incompatible polymer blends: coalescence
PIB / PDMS
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III Complex fluids
c- Liquid crystalline polymers
Liquid crystal phases lies between the solid and isotropic phases.
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III Complex fluids
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Nematic Phase
• Molecules are rod-like. They can move as a fluid but they keep their main axis along a local common direction.
III Complex fluids
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III Complex fluids
d- Suspensions in polymer fluids
Dispersion of agglomerated particles (carbon black)
Suspension of rods (glass fibres)
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III Complex fluids
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IV. Rheo-physics
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Complex fluid = fluid where the structure changes under the action of flow
DeformationOrientation
CrystallisationFlow heterogeneities
Observation
IV Rheo-physics
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Rheo-physics = rheology + observation
Flow + Physical probing technique
In-situ study of the structure/morphology/orientation underflow
Determine relationships between:
PROCESSING
RHEOLOGY
FLOW MORPHOLOGY
PROPERTIES
IV Rheo-physics
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Objectives:
- To measure the structure induced by a flow in a complexmaterial
- polymers (solution, melts)- blends- phase separation- suspensions, colloids- liquid crystals
- To built adapted tools simulating processingconditions
FLOW
MATERIAL
PROBING TECHNIQUE
IV Rheo-physics
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• Transparent rheometers
Different flow geometries• shear (cone-plan, plate-plate, couette)
• elongation (opposite jets, four roller mill)
• Complex flows (dies, obstacles, …)
Flow geometries
IV Rheo-physics
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• Optical techniques (white light, laser, IR,...)– Optical microscopy (1 - 100 µm)– Light scattering (0,02 - 100 µm)– Birefringence– Dichroism– Polarization– Raman spectroscopy– IR spectroscopy– Diffraction
• X-rays (WAXS : 1 - 20 Å, SAXS : 20 - 1000 Å)– Scattering– Diffraction
• Neutrons (10 - 1000 Å)– Scattering
• NMR (nuclear magnetic resonance)– Imaging
IV Rheo-physics
Examples of useful physical techniques
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Examples of complex flows
Liquid cristalsPolymer flow
Birefringence
stress visualisation
Optics
visualisation of welding lines
IV Rheo-physics
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gap
Optical microscopy observations
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VClassical shear geometry
Counter-rotating shear geometry
The object is convected by flow.
Continuous observations ⇒ motion of the microscope at the same speed
The object is fixed in the laboratoryframework. Continuous observation of the object under shear is possible.
Counter-rotating shear cell
IV Rheo-physics
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• Advantages:– Observations in-situ– Fixed position of the object of interest– Work with low viscosity matrices around room temperature– Or with highly viscoelastic matrices at high temperature
R
ω sup
H
ω inf
HR)ω(ω
γ supinf ×+=&
Counter-rotating shear cell
IV Rheo-physics
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50µm
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25
I / Im
ax
theta (deg)
300
250
200
150
100
50
00 5 10 15 20 25
angle polaire θ (degrés)
beamθ
Imax ≡ I (θ beam)
Polar angle θ (degrees)
I diff
(arb
itrar
yun
its)
Image analysis
White lightLaserSample Shear cell
Software
sizeInformation on shape
mechanismstimescale
10°
θ
Small-angle light scattering
IV Rheo-physics
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V. Examples of flow-induced structures
a- Dispersion of agglomeratesb- Liquid crystalline polymersc- Incompatible polymer blendsd- Flow-induced behaviour of gel particlese- Flow-induced organisation in yoghurt
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- study the effect of hydrodynamic parameters on dispersion of single pellets;
- determine dispersion criteria and laws;
- change the filler/matrix interactions and determine its role on dispersion;
- understand the role of infiltration on dispersion;
Applications:• tyres• filled polymers
Elastomer
Clusters ~ 100µm-a few mm
a- Dispersion of agglomerates
V Examples of flow-induced structures: dispersion of agglomerates
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Erosion
Rupture
Debonding
Collision
0
0,5
1
0 50000 100000 150000τ=ηγ
k erosioncτ α
0
1R
RuptureC ∝τ
( ) tγ ττ αRR τ),(R ErosionC
3t
300 &−=−∀
DWe a
debondingΓ
∝σ
V Examples of flow-induced structures: dispersion of agglomerates
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Nematic polymers are uniaxially oriented. Orientation is not fixed and many orientational defects are present. We have a polydomainstructure, with many orientation separated by orientation defects (disclinations)
b- Liquid crystalline polymers
V Examples of flow-induced structures: liquid crystalline polymers
splay twist
bent
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V Examples of flow-induced structures: liquid crystalline polymers
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Starting point: a polydomainnematic
Under shear, all chains align, giving a uniaxially oriented material
Upon relaxation, a strange structure appears during a certain time
V Examples of flow-induced structures: liquid crystalline polymers
Crossed polarizers
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Different orientations like a composite
V Examples of flow-induced structures: liquid crystalline polymers
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Mixed shear and extensionStrain (and strain history) depends on position Slit flow, injection into moulds--transient flowFilling flow (with obstacle):
Weldline
Flow of liquid crystal polymers in complex geometries
V Examples of flow-induced structures: liquid crystalline polymers
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Mold filling: problem of weld lines
V Examples of flow-induced structures: liquid crystalline polymers
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Obstacle divides flow frontDivided fronts meet--what kinds of stresses and strains ? Expect elongation (velocity must be zero at obstacle)
Weld atobstacle
At side wall
Weld line
Obstacle
Air bubble
Weld line structure
V Examples of flow-induced structures: liquid crystalline polymers
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Orientation instabilities
V Examples of flow-induced structures: liquid crystalline polymers
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V Examples of flow-induced structures: liquid crystalline polymers
1 Mold filling2 Steady flow, crossed polars3 Steady flow, parralel polars
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Rupture
Deformation
At rest
Final morphology = result from competitionbetween hydrodynamicforces and interfacial
forces
AB
0m
0AB
m R
RCa
γγη
=γ
γη=
&&
c- Incompatible polymer blends
V Examples of flow-induced structures: incompatible polymer blends
Ca < Cacrit: small stable deformationCa = Cacrit: rupture in two dropsCa > Cacrit: affine long deformation
followed by rupture
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t = 0s
t = 0,6s
t = 2,6s
Shear of a 1% PDMS droplets suspended in PIB – Shear rate 10s-1
V Examples of flow-induced structures: incompatible polymer blends
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t = 14,6s
t = 16,4s
t = 42,7s
Steady state
V Examples of flow-induced structures: incompatible polymer blends
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Butterfly pattern
Pattern withrotational symmetry
Ellipsoidal pattern
Elongated pattern
Size distribution
Shape factor
Wavelength
Filament diameter
Rupture by Rayleigh instabilities
DL
Exact follow-up of morphology
evolution
V Examples of flow-induced structures: incompatible polymer blends
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40000
Diameter (µm) 1% PDMS in PIB
Strainunits0
2
4
6
8
10
12
14
0
3 s-1
1 s-1
0,3 s-1
80000 200000
The lower is the shear rate: This is due the time to eject the fluid layer
the larger is the final size and the slower is the coalescence.
Convection Ejection ofinterparticular
fluid
Rupture of fluid film
Decrease of interfacial stress
coalescence
V Examples of flow-induced structures: incompatible polymer blends
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Gel = biphasic system constituted by a tridimensional network swollen by solvant
stimulusNetworkSolvant
transport
MoleculeActive product release
Storage
Gel sensitive to pH stomach
Skin application : shear, compression….Drug release by mechanical
sollicitation?⇒ Observation: shear
particle deformationliquid release
Polyelectrolyte swollen in an aqueous solution of HPC
V Examples of flow-induced structures: flow of gel particles
d- Flow-induced behaviour of gel particles
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Stress ↑
At rest Deformation/Orientation Apparition of cones Lateral ejection of solvant
Transversal ejection of solvant
• Deformation controlled by the gel elasticity and interfacial stresses
GRCa 20forces elastic forces tensionlinterfaciaforces viscous* +Γ=+= γη &
Particle deformation Solvent release Solvent dispersion
PDMS
Gel particleswollen in solvent Solvent
V Examples of flow-induced structures: flow of gel particles
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Behaviour of a droplet = multiphase system Droplet = starch granules swollen in water suspended in waterMatrix = PDMS
Droplet of suspension of starch granules
100µm
100µm
Droplet of a diluted suspension
100µm
100µm
Volume fract: ~ 10% Volume fract: ~ 100%
relationship between droplet deformation and organolepticproperties
Droplet of a concentrated suspension
V Examples of flow-induced structures: flow of gel particles
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V Examples of flow-induced structures: yoghurt
Objective:
• see if the flow is homogeneous• See if it disrupts caseine aggregates• See how fat globules are behaving
Important information since flow is present during filling of pots.
e- Flow-induced organisation in yoghurt
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Huge local shear rate variations
020406080
100120140160
1 2 3 4 5
Zone of flow
Mea
sure
d sh
ear
rate
-im
pose
dsh
ear
rate
rat
io Zones of flow
1: around an obstacle.
2,3,4: passage through chanels
5: laminar flow, withoutobstacle (shear rate as the one imposed)
V Examples of flow-induced structures: yoghurt
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Creaming
At high shear rates, fat droplets appear in the fluid
Fig 1. Illustration du phénomène d’écrémage dans un yoghourt Bio pendant le cisaillement (lespoches de graisse sont notées G, les agrégats de micelles de caséines A, le grossissement est de 10)
V Examples of flow-induced structures: yoghurt
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