2. Block Copolymers

2. Block Copolymers
2.1 Micelle and gel formation
in amphiphilic block copolymers
2.2 Phase behavior in the bulk
2.3 Structures in thin films
I.W. Hamley, Block Copolymers in Solution. Wiley 2005.
1
Block copolymers
diblock copolymer: AB
triblock copolymer: ABA, BAB
diblock copolymer:
two chemically different
blocks A and B
with repulsive interaction
amphiphilic block copolymers:
A: hydrophilic block
B: hydrophobic block
in aqueous solution:
micelle formation
2
Micelle formation
unimer
micelle
CMC
diblock copolymers
→ star-like micelle
polymer concentration
triblock copolymers
→ folding back into
the core
→ flower-like micelle
CMC = critical micelle concentration
3
Structures in dependence
on architecture and concentration
Förster, Antonietti,
Adv. Mater. 10,
19 (1998)
hydrophobic
4
hydrophilic
4
Poly(alkaneoxide)s
most prominent amphiphilic block copolymer:
Poly(ethylenoxide-b-propylenoxide-b-ethylenoxide)
[CH2-CH2-O]m- [CH-CH2-O]n- [CH2-CH2-O]m
hydrophilic
CH3
hydrophilic
hydrophobic
gel former, dispergant, emulsifier
among others for cosmetics
CGC
critical gel
concentration
micellar
hydrogel:
viscoelastic
5
polymer concentration
Block length dependence of CMC
CMC depends exponentially on
the length of the hydrophobic block
P(EO-b-PO-b-EO): EmPnEm
P(PO-b-EO-b-PO): PnEmPn
P(EO-b-BO-b-EO): EmBnEm
P(BO-b-EO-b-BO): BnEmBn
Nagg (= p) increases with
the length of the hydrophobic block
(•) P(EOm-b-BOn)
(▪) P(BOn/2-b-EOm-b-BOn/2)
(o) P(EOm/2-b-BOn-b-EOm/2)
→ both CMC and aggregation number can be controlled
by the choice of the length of the hydrophobic block
6
Poly(2-oxazoline)s
poly(n-alkyl-2-oxazoline)
CH3 – [N – CH2 – CH2]N –NC4H8NH
I
hydrophilic
CO
main chain
I
CnH2n+1
alkyl side group
methyl:
nonyl:
water soluble
water insoluble
– [N – CH2 – CH2]30 –
|
CO
|
C9H19
[N – CH2 – CH2]6 –
|
CO
|
CH3
7
hydrophobic
hydrophilic
Hydrodynamic radius [nm]
Unimers – micelles
P(Mox30-b-NOx6)
Micelles
Critical
micelle
concentration
10
Unimers
1
10
-8
10
-6
1x10
-4
-2
10
Concentration [M]
Concentration [mol/l]
relative amplitude of the process due to the
diffusion of micelles increases with polymer concentration
→ with increasing polymer concentration
the fraction of micelles increases
and the fraction of unimers stays constant
8
T.B. Bonné, C.M.P. Colloid Polym. Sci. 282, 833 (2004).
Micellar catalysis: The concept
• MMA monomer
* catalyzer metal
MMA
PMMA
polymerization
functionalization of the
hydrophobic blocks
with ligands for metals
serving as catalyzer for
polymerization of monomers
polymerization of nanoparticles
in aqueous solution
then separation of the
catalyzer from the product 9
Micellar catalysis: The realization
challenges during catalyzed polymerization:
• metal catalyst must be removed from polymer product
• aqueous solution: environmentally friendly, efficient heat transfer
amphiphilic diblock copolymer with
• water-soluble poly(2-methyloxazoline) block
• hydrophopic POx block bearing bipyridine
moieties in the side chain
→ atom-transfer radical polymerization (ATRP) of
of methyl methacrylate in presence of the catalyst Cu(I)Br
and the initiator ethyl 2-bromoisobutyrate
→ 96 % monomer conversion after 3 h at 60°C
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T. Kotre, O. Nuyken, R. Weberskirch, Macromol. Rapid Commun. 23, 871 (2002)
Micellar catalysis: Results
rate plot:
MMA monomer conversion as a
function of polymerization time
→ demonstrates that reaction is
of first order
increase of
PMMA molar
mass with time
covalent linkage of bipyridine to polymer
→ separation of Cu/bipyridine ligand from PMMA product
proven by precipitation of polymerization mixture in methanol
→ PMMA contains only very little Cu
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Drug transport using block copolymer micelles
solubilization of highly hydrophobic drugs (here Paclitaxel, PTX)
by di- and triblock copolymers based on poly(2-oxazoline)
→ very large loading capacity for PTX (45 wt.-%)
→ high water solubility of the formulation
→ rather polar micellar core
→ PTX remains fully active in micelles
R. Luxenhofer et al., Biomaterials 31, 4972 (2010)
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Micelle formation
unimers
micelles + unimers
CMC
polymer concentration
below the CMC:
above the CMC:
• exclusively unimers, i.e.
molecularly dissolved polymers
• coexistence of unimers
and micelles
• screening of hydrophobic block
by the hydrophilic block
• the higher the concentration,
the higher the fraction of micelles
• the aggregation number Nagg
(number of unimers per micelle)
is independent of the concentration:
typically Nagg ≅ 10-400
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Thermodynamics of micelle formation
Two models:
1.
open association: continuous distribution of Nagg
is in contradiction to the observed CMC
2.
closed association:
equilibrium between unimers and
micelles with p unimers (p = Nagg)
equilibrium constant:
A⇔
1
Ap
p
[
A ]
K=
1
p
p
[A]
for shift of equilibrium from left to right
by a fraction of α:
where
β = 1−α +
β  α 

⋅ 
K=
1 − α  pβ 
1
p
[A]−1+ 1 p
α
p
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Temperature dependence of the CMC
for large aggregation numbers: 1/p → 0
⇒
K ≈ [A ]
−1
→ free Gibbs energy of association:
∆ mic G 0 = − RT ln K ≈ RT ln[ A]
→ for unimers in ideal dilution
and at 1 mol/l in equilibrium
with micelles, it is obtained:
∆ mic G 0 ≈ RT ln(CMC )
because number of unimers [A]
concentration-independent
→ enthalpy of micelle formation:
(valid for p > ~50)
d ln(CMC )
∆ mic H ≈ R
d (1 / T )
0
assumption: ∆micH0 is temperature independent
∆ mic H 0
→ ln(CMC ) =
+ const.
RT
plot of ln(CMC) vs. 1/T
→ determination of ∆micH0
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equivalent to the plot of
the CMC vs. temperature is
the concentration dependence of
the critical micelle temperature (CMT)
for P(EO21-b-BO8-b-EO21)
unimers
+micelles
unimers
→ ∆micH0 is temperature independent
EO: ethylene oxide
BO: butylene oxide
special für P(EO-BO-EO) system:
the higher the temperature, the lower the CMC
reason: water solubility is temperature dependent
PEO:
hydrophilic
solubility decreases with increasing temperature
PPO, PBO: hydrophobic
solubility increases with temperature
→ strong temperature dependence of the CMC
→
∆micH0
is large and positive → micellization is entropy driven
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Relevance of entropy for micellization
hydrophobic effect (entropic reason):
minimization of the fraction of structured water
shell of structured water
around every monomer
shell of structured water around
each micelle → less water is
affected than for Nagg unimers
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