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Role of cross-links in bundle Role of cross-links in bundle formation, phase separation and formation, phase separation and
gelation of long filamentsgelation of long filaments
Ortal LeviDepartment Of Chemical EngineeringJuly 2013
Branched structures and Branched structures and networksnetworks
• Exists in many physical, chemical and biological systems• Chemical systems- chemical hydrogels• Physical systems- physical hydrogels, wormlike
micelles/micro emulsions, dipolar fluids
• Applications- medical industry (drug delivery and healing) , food industry
Physically cross linked networks
CrosslinkingCrosslinking Physical
crosslinking
• Ionic hydrogel
Chemical and Physical
crosslinking• Cross-l inking without chemical reaction• ionic interaction, hydrogen
bonding, antigen-antibody interaction, supramolecular association
Hydrogel FabricationHydrogel FabricationChemical hydrogels
Physical hydrogels
Hydrogen bonding
hydrophobic interaction
crystall inity
stereocomplex formation
ionic complexation
Covalently crosslinked
Noncovalently crosslinked
Thermoset hydrogels
Thermoplastic hydrogels
Volume phase transit ion
Sol-gel phase transit ion
Reliable shape stabil i ty and memory
Limited shape stabil i ty and memory
• Presenting a phase diagram of a generic system of cross-linked equilibrium chains in terms of two independent transitions in the system
• Prediction of thermodynamic and structural behavior of solutions of long cross-linked filaments.
Research ObjectivesResearch Objectives
The theoretical modelThe theoretical modelThe system-
•A grand-canonical ensemble- network in equilibrium with a reservoir of ends and junctions
•junctions and ends are viewed as “thermal defects” of the system whose “ground state” is an assembly of infinite linear chains
Assumptions-
•No specific interactions between the monomers, except excluded volume-dilute solution
•Mean Field Approximation
•Sparse junctions and ends-
The theoretical modelThe theoretical model
( , )j eφ φ φ=
The theoretical modelThe theoretical modelParameters-
-the monomer density
j
e
j
e
c
φεε
ρφφ
-junction energy
-end energy-cross links density
-end cup molecule density
-number of branching points
-number of free ends points
relative to the bond energy between two monomers in the chain
Per unit volume
Mathematical development-•We start with the grand canonical potential per unit volume-
•The probability of bond breaking- two end formation•The probability of collision of two ends- • a factor of the microscopicall qualities of ends-Chain flexibility and effective collision surface area
The theoretical modelThe theoretical model
21( , , , , ) (1)
2j e j e e jφ µ µ ε ε φ φ φΩ = − −
Excluded volume
2( )/e e Te µ εφ −
2 21e aφ −
1a −
Mathematical development-•In equilibrium-P(break)=P(create)
•For an f component junction formed from f-2 ends and an internal monomer- energy cost for breaking a junction and creating f-2 ends-
The theoretical modelThe theoretical model
( )/ 1/21 (2)e e T
e a e µ εφ φ−=
( 2)( ) lne e j j ff T aε µ ε µ− − − + +
Mathematical development-•Probability of junction break-up:
•Probability of collision of f-2 ends with an internal monomer-
•In equilibrium• coefficient of the microscopical freedom of the junctions, including the configurational entropy of the bonds and monomers in the junction
•
The theoretical modelThe theoretical model
( )/ ( 2)( )/j j e eT f T
j fa e eµ ε ε µφ − − − −
2 21
f fe aφ φ− −
( )/ /2 (3)j j T fj fa e
µ εφ φ−=fa −
Mathematical development-
• Grand canonical potential-
The theoretical modelThe theoretical model
( )/( )/2 1/2 /21
1( , , ) / (4)
2j je e
TT fj e fT a e a e µ εµ εφ µ µ φ φ φ−−Ω = − −
Free energyFree energy• The relation between free energy and the grand canonical
potential-
(5) Legendre Transformf µφ=Ω+ →
21 1( , , ) / (ln 1) (ln 1) ( / ln ) / ln ln (6)
2 2 2j e j j e e j j f e e j e
fF T T a Tφ φ φ φ φ φ φ φ φ ε φ ε φ φ φ φ= + − + − + − + − −
Excluded volume
Free energy of “ideal gas” of junctions and ends
Energy cost due to the network constrained-reduction of entropy
free energy free energy TotalTotal
( , , ) ( , , ) ( ) ( )totj j e c j e eF c F F c Fφ φ φ φ φ φ ρ φ= + − + −
Free energy of unbound cross l inkers
Free energy of unbound ends
ψψ
Both molecules as an ideal solut ion-
( ) ( ) (ln 1)e cF F Tψ ψ ψ ψ= = − 1
ln(1 )
ψψ ψ− →−
=
/ /1/2 1/2
/ //2 /2
/ (1 )
/ (1 )
e e
j j
T Te
T Tf fj f f
e e
ca e a e
ε ε
ε ε
φ ρ φ φ
φ φ φ
− −
− −
= +
= +
totF Is minimizes to f ind the end and junction equil ibrium density-
Densit ies vary with
, ,cφ ρ
Density as a function of cross linkers-•Strong cross linkers-
•Therefore-
Total free energyTotal free energy
( 0, 1)jj T
εε < =
j cφ → • Most of the cross linkers are in the junctions
• Junctions tend to form in low temperatures
Total free energyTotal free energyDensity as a function of cross linkers-
•Weak cross linkers-
•Therefore- •The total free energy with :
( 0 , 1)jj T
εε > ?
• Most of the cross linkers are in the solution
• In low temperatures the number of junctions 0 ,j eφ φ
Junctions reduces free energy
Ends reduces free energy
Phase separationPhase separation• Can be caused by high cross linkers density, low temperature
• Requires the matrix of second derivatives of to be positive-definite
• This condition defines the spinodal and the critical point for density-
totF
sc
Dilute phase Dense phase
The predicted phase separation The predicted phase separation is entropic in originis entropic in origin
Phase separationPhase separation
3 ( 0)jf ε= ¬ >
( )sc c φ>
4, 0 , 1
0.05 , 0.005
j ee
c
εε εε
ρ
< =
= =
4 /
4
0.015,
10 / 3 , 0.005
Tf
e
a eε
ε ρε
=
= =
Strong crosslinking
4 ( 0)jf ε= ¬ <
Phase DiagramPhase Diagram
Weak crosslinking
Phase separation and gelationPhase separation and gelationGelation/ Percolation transition• A connected network spanning the entire system-thus
dependent on concentration alone• The transition- Continuous Topological (structural) Un thermodynamicOccurs when-
BundlesBundles • Definition-rigid chains (rods) with connective cross linkers
in a parallel structure- nematically aligned bundles of chains
• Favorable formation in low temperatures-entropy driven• Occurs when - the free energy of the bundle is
lower than that of the isotropic networkb iF Fp
Transitional entropy of the bundles
Reduction in rotational entropy
BundlesBundles
For long chains the bundle formation is characterized by slow kinetics
ConclusionsConclusions This model predicts three transitions-1.Phase separation-dense network and spares network2.Gelation transition-infinite network spanning the entre system3.Bundles- nematic phase-parallel crosslinked chains•Strong crosslinking-most cross linkers are in the junctions, junctions don’t break it low temperatures•Weak crosslinking-low junction density compared to crosslinkiners density, almost non existent in low temperatures
Legendre TransformLegendre Transformהטרנספורם מעביר את הפונקציה לפונקציה חדשה התלויה בנגזרת החלקית •
לפי המשתנה הבלתי תלוי של הפונקציה הישנה.
)נבצע את המעבר:• , , ) ( , , )j e j eFφ µ µ φ φ φΩ →
( , , ) ( , , ) ( , , )j e j e j e j e j j e ej e
F φ φ φ φ µ µ µ µ φ µ µ φ µ φ µµ µ
∂Ω ∂Ω= Ω − − = Ω + +∂ ∂
eφjφ
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