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

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1 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

2 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

3 Micelle formation unimer micelle diblock copolymers star-like micelle CMC polymer concentration triblock copolymers folding back into the core flower-like micelle CMC = critical micelle concentration 3

4 Structures in dependence on architecture and concentration Förster, Antonietti, Adv. Mater.10, 19 (1998) hydrophobic hydrophilic 4 4

Poly(alkaneoxide)s most prominent amphiphilic block copolymer: Poly(ethylenoxide-b-propylenoxide-b-ethylenoxide) [CH 2 -CH 2 -O] m - [CH-CH 2 -O] n - [CH 2 -CH 2 -O] m

5 Poly(alkaneoxide)s most prominent amphiphilic block copolymer: Poly(ethylenoxide-b-propylenoxide-b-ethylenoxide) [CH 2 -CH 2 -O] m - [CH-CH 2 -O] n - [CH 2 -CH 2 -O] m hydrophilic CH 3 hydrophobic hydrophilic gel former, dispergant, emulsifier among others for cosmetics CGC critical gel concentration micellar hydrogel: viscoelastic polymer concentration 5

6 Block length dependence of CMC CMC depends exponentially on the length of the hydrophobic block P(EO-b-PO-b-EO): E m P n E m P(PO-b-EO-b-PO): P n E m P n P(EO-b-BO-b-EO): E m B n E m P(BO-b-EO-b-BO): B n E m B n N agg (= p) increases with the length of the hydrophobic block ( ) P(EO m -b-bo n ) ( ) P(BO n/2 -b-eo m -b-bo n/2 ) (o) P(EO m/2 -b-bo n -b-eo m/2 ) both CMC and aggregation number can be controlled by the choice of the length of the hydrophobic block 6

7 Poly(2-oxazoline)s hydrophilic main chain alkyl side group methyl: water soluble nonyl: water insoluble poly(n-alkyl-2-oxazoline) CH 3 [N CH 2 CH 2 ] N NC 4 H 8 NH I CO I C n H 2n+1 [N CH 2 CH 2 ] 30 [N CH 2 CH 2 ] 6 CO CO C 9 H 19 CH 3 hydrophobic hydrophilic 7

8 Unimers micelles P(Mox 30 -b-nox 6 ) Hydrodynamic radius [nm] 10 1 Critical micelle concentration Micelles Unimers x 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 T.B. Bonné, C.M.P. Colloid Polym. Sci.282, 833 (2004). 8

9 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

10 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 T. Kotre, O. Nuyken, R. Weberskirch, Macromol. Rapid Commun.23, 871 (2002) 10

11 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 11

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

12 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., Biomaterials31, 4972 (2010) 12

13 Micelle formation unimers micelles + unimers CMC polymer concentration below the CMC: exclusively unimers, i.e. molecularly dissolved polymers screening of hydrophobic block by the hydrophilic block above the CMC: coexistence of unimers and micelles the higher the concentration, the higher the fraction of micelles the aggregation number N agg (number of unimers per micelle) is independent of the concentration: typically N agg

14 Thermodynamics of micelle formation Two models: 1. open association: continuous distribution of N agg is in contradiction to the observed CMC 2. closed association: equilibrium between unimers and micelles with p unimers (p = N agg ) equilibrium constant: 1 A p K = [ A ] p A p [ A] 1 p for shift of equilibrium from left to right β K= 1 α α pβ by a fraction of α: [ ] p where β =1 α + α p 1 p A

15 Temperature dependence of the CMC for large aggregation numbers: 1/p 0 K [ A ] 1 0 free Gibbs energy of association: G = RT ln K RT ln[ A] mic for unimers in ideal dilution and at 1 mol/l in equilibrium with micelles, it is obtained: because number of unimers [A] concentration-independent 0 mic G (valid for p > ~50) RT ln( CMC) enthalpy of micelle formation: 0 mic H R d ln( CMC) d(1/ T ) assumption: mic H 0 is temperature independent 0 mic H ln( CMC) = + const. RT plot of ln(cmc) vs. 1/T determination of mic H 0 15

16 unimers +micelles unimers equivalent to the plot of the CMC vs. temperature is the concentration dependence of the critical micelle temperature (CMT) for P(EO 21 -b-bo 8 -b-eo 21 ) mic H 0 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 mic H 0 is large and positive micellization is entropy driven 16

17 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 N agg unimers 17

Colloid chemistry. Lecture 10: Surfactants

Colloid chemistry. Lecture 10: Surfactants Colloid chemistry Lecture 10: Surfactants Applications of surfactants: cleaning/detergents (40%); textiles; cosmetics; pharmacy; paint; food; etc. Etymology Surfactant micelles surfactant molecule spherical