polymer dynamics and rheology - university of cincinnati
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Polymer Dynamics and Rheology
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Polymer Dynamics and Rheology
Brownian motionHarmonic Oscillator
Damped harmonic oscillatorElastic dumbbell model
Boltzmann superposition principleRubber elasticity and viscous drag
Temporary network model (Green & Tobolsky 1946)Rouse model (1953)
Cox-Merz rule and dynamic viscoelasticityReptation
The gel point
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The Gaussian Chain
Boltzman ProbabilityFor a Thermally Equilibrated System
Gaussian ProbabilityFor a Chain of End to End Distance R
By Comparison The Energy to stretch a Thermally Equilibrated Chain Can be Written
Force Force
Assumptions:-Gaussian Chain
-Thermally Equilibrated-Small Perturbation of Structure (so it is still Gaussian after the deformation)
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Stoke’s Law
F = vςς = 6πηsR
F
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Cox-Merz Rule
Creep Experiment
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Boltzmann Superposition
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Stress Relaxation (liquids)
Creep (solids) J t( ) = ε t( )σ
Dynamic Measurement
Harmonic Oscillator: = 90° for all except = 1/ where = 0°δ δω ω τ
Hookean Elastic
Newtonian Fluid
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Brownian Motion
For short times
For long times 0
E = kT = ½ mV2
F = ma = zV
Integrate
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dE = F dxF = zV
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http://mathworld.wolfram.com/First-OrderOrdinaryDifferentialEquation.html
F = kdxFdx = dU
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The response to any force field
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Both loss and storage are based on the primary response function, so it should be possible to express a relationship between the two.
The response function is not defined at t =∞ or at ω = 0This leads to a singularity where you can’t do the integrals
CauchyIntegral
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W energy = Force * distance
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Parallel Analytic Technique to Dynamic Mechanical(Most of the math was originally worked out for dielectric relaxation)
Simple types of relaxation can be considered, water molecules for instance.
Creep:
InstantaneousResponse
Time-lagResponse
Dynamic:
ε0 Free Space
ε Material
εu Dynamic material
D = ε0E + P = ε0εE Dielectric Displacement
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Rotational Motionat Equilibrium
A single relaxation mode, τ relaxation
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Creep Measurement
Response
K = 1τ
http://mathworld.wolfram.com/First-OrderOrdinaryDifferentialEquation.html
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Apply to a dynamic mechanical measurement
dγ 12 t( )dt
= iωσ120 J * ω( )exp iωt( )
Single modeDebye Relaxation
Multiply by iωτ −1( )iωτ −1( )
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Single modeDebye Relaxation
Symmetric on a log-log plot
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Single modeDebye Relaxation
More complex processes have a broader peak
Shows a broader peak but much narrower than a Debye relaxation
The width of the loss peak indicates the difference between a vibration and a relaxation process
Oscillating system displays a moment of inertiaRelaxing system only dissipates energy
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Equation for a circle in J’-J” space
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dγdt
= −γ t( )τ
+ ΔJσ 0
EquilibriumValue
TimeDependent
Value
Lodge LiquidBoltzmann Superposition
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Boltzmann Superposition
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Rouse DynamicsFlow
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ω 1ω 2
ω12
ω57
NewtonianFlow
EntanglementReptation
RouseBehavior
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ω 1ω 2
ω12
ω57
NewtonianFlow
EntanglementReptation
RouseBehavior
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Lodge Liquid and Transient Network Model
Simple ShearFinger Tensor
Simple ShearStress
First Normal Stress
Second Normal Stress
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For a Hookean Elastic
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For Newtonian Fluid
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Dumbbell Model
x t( ) = dt 'exp −k t − t '( )ξ
⎛⎝⎜
⎞⎠⎟g t( )
−∞
t
∫
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Dilute Solution Chain
Dynamics of the chain
Rouse Motion
Beads 0 and N are special
For Beads 1 to N-1
For Bead 0 use R-1 = R0 and for bead N RN+1 = RN
This is called a closure relationship
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Dilute Solution Chain
Dynamics of the chain
Rouse Motion
The Rouse unit size is arbitrary so we can make it very small and:
With dR/dt = 0 at i = 0 and N
Reflects the curvature of R in i, it describes modes of vibration like on a guitar string
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x, y, z decouple (are equivalent) so you can just deal with z
For a chain of infinite molecular weight there are wave solutions to this series of differential equations
ς Rdzldt
= bR (zl+1 − zl )+ bR (zl−1 − zl )
zl ~ exp − tτ
⎛⎝⎜
⎞⎠⎟ exp ilδ( )
τ −1 = bRζ R
2 − 2cosδ( ) = 4bRζ R
sin2 δ2
Phase shift between adjacent beads
Use the proposed solution in the differential equation results in:
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τ −1 = bRζ R
2 − 2cosδ( ) = 4bRζ R
sin2 δ2
Cyclic Boundary Conditions: zl = zl+NR
NRδ = m2π
NR values of phase shift
δm = 2πNR
m; m = − NR
2−1⎛
⎝⎜⎞⎠⎟ ,..., NR
2
For NR = 10
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τ −1 = bRζ R
2 − 2cosδ( ) = 4bRζ R
sin2 δ2
Free End Boundary Conditions: zl − z0 = zNR−1− zNR−2
= 0
NR −1( )δ = mπ
NR values of phase shift
δm = πNR −1( )m; m = 0,1, 2,..., NR −1( )
dzdl
l = 0( ) = dzdl
l = NR −1( ) = 0
NR Rouse Modes of order “m”
For NR = 10
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τ R =13π 2
ζR
aR2
⎛⎝⎜
⎞⎠⎟
kTR04
Lowest order relaxation time dominates the response
This assumes that ζR
aR2
⎛⎝⎜
⎞⎠⎟
is constant, friction coefficient is proportional to number of monomer units in a Rouse segment
This is the basic assumption of the Rouse model,
ζR ~ aR2 ~ N
NR
= nR
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τ R =13π 2
ζR
aR2
⎛⎝⎜
⎞⎠⎟
kTR04
Lowest order relaxation time dominates the response
Since R02 = a0
2N
τ R ~N 2
kT
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The amplitude of the Rouse modes is given by:
Zm2 = 2
3π 2R02
m2
The amplitude is independent of temperature because the free energy of a mode is proportional to kT and the modes are distributed by Boltzmann statistics
p Zm( ) = exp −FkT
⎛⎝⎜
⎞⎠⎟
90% of the total mean-square end to end distance of the chain originates from the lowest order Rouse-modes so the chain can be often represented as an elastic dumbbell
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Rouse dynamics (like a dumbell response)
dxdt
= −
dUdx
⎛⎝⎜
⎞⎠⎟
ζ+ g(t) = −
ksprxζ
+ g(t)
x t( ) = dt 'exp − t − t 'τ
⎛⎝⎜
⎞⎠⎟
−∞
t
∫ g t( )
τ = ζkspr
Dumbbell Rouse
τ R =ζR
4bR sin2 δ2
δ = πNR −1
m , m=0,1,2,...,NR -1
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Rouse dynamics (like a dumbell response)
g t1( )g t2( ) = 2Dδ t( ) where t = t1 − t2 and δ ( ) is the delta function whose integral is 1
Also, D = kTζ
x t( )x 0( ) =kT exp − t
τ⎛⎝⎜
⎞⎠⎟
ksprτ = ζ
ksprFor t => 0, x2 = kT
kspr
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Predictions of Rouse Model
G t( ) ~ t−12
G ' ω( ) ~ ωη0( )12
η0 = kTρpτ Rπ 2
12~ N
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ω 1ω 2
ω12
ω57
NewtonianFlow
EntanglementReptation
RouseBehavior
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Dilute Solution Chain
Dynamics of the chain
Rouse Motion
Rouse model predicts Relaxation time follows N2 (actually follows N3/df)
Diffusion constant follows 1/N (zeroth order mode is translation of the molecule) (actually follows N-1/df)
Both failings are due to hydrodynamic interactions (incomplete draining of coil)
Predicts that the viscosity will follow N which is true for low molecular weights in the melt and for fully draining polymers in solution
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Dilute Solution Chain
Dynamics of the chain
Rouse Motion
Rouse model predicts Relaxation time follows N2 (actually follows N3/df)
Predicts that the viscosity will follow N which is true for low molecular weights in the melt and for fully draining polymers in
solution
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http://www.eng.uc.edu/~gbeaucag/Classes/MorphologyofComplexMaterials/SukumaranScience.pdf
Chain dynamics in the melt can be described by a small set of “physically motivated, material-specific paramters”
Tube Diameter dTKuhn Length lK
Packing Length p
Hierarchy of Entangled Melts
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Quasi-elastic neutron scattering data demonstrating the existence of the tube
Unconstrained motion => S(q) goes to 0 at very long times
Each curve is for a different q = 1/size
At small size there are less constraints (within the tube)
At large sizes there is substantial constraint (the tube)
By extrapolation to high times a size for the tube can be obtained
dT
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There are two regimes of hierarchy in time dependenceSmall-scale unconstrained Rouse behavior
Large-scale tube behavior
We say that the tube follows a “primitive path”This path can “relax” in time = Tube relaxation or Tube Renewal
Without tube renewal the Reptation model predicts that viscosity follows N3 (observed is N3.4)
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Without tube renewal the Reptation model predicts that viscosity follows N3 (observed is N3.4)
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Reptation predicts that the diffusion coefficient will follow N2 (Experimentally it follows N2)Reptation has some experimental verification
Where it is not verified we understand that tube renewal is the main issue.
(Rouse Model predicts D ~ 1/N)
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Reptation of DNA in a concentrated solution
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Simulation of the tube
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Simulation of the tube
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Plateau Modulus
Not Dependent on N, Depends on T and concentration
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Kuhn Length- conformations of chains <R2> = lKL
Packing Length- length were polymers interpenetrate p = 1/(ρchain <R2>)where ρchain is the number density of monomers
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this implies that dT ~ p
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McLeish/Milner/Read/Larsen Hierarchical Relaxation Model
http://www.engin.umich.edu/dept/che/research/larson/downloads/Hierarchical-3.0-manual.pdf
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