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Self-Assembly of Mixtures of Telechelic and Monofunctional Amphiphilic Polymers in Water: From Clusters to Flowerlike Micelles
journal contribution
posted on 2017-09-13, 18:52 authored by Thomas Zinn, Lutz Willner, Kenneth D. Knudsen, Reidar LundWe study the self-assembly of mixtures
of n-alkyl
mono- and difunctionalized poly(ethylene oxide) (PEO) chains in the
dilute concentration regime. The monofunctional PEOs were prepared
by living anionic polymerization with varying n-alkyl
length (n = 14, 16, 22, 28) and constant PEO molecular
weight of 5 kg/mol. The difunctional materials were obtained through
end-to-end coupling of two of the monofunctionalized PEOs via their
terminal hydroxyl groups. The chosen synthetic pathway yields well-defined
model compounds with narrow molecular weight distribution and complete
end-group functionalization. By using both small-angle neutron scattering
(SANS) and dynamic light scattering (DLS) combined with theoretical
data modeling, we have systematically investigated both the global
and inner structure of the self-assembled micellar structures. For
short n-alkyl chain-ends, we find a formation of
clustered micelles with a finite size whereas, intriguingly, at longer n-alkyls, we observe a crossover to flowerlike micelles.
This was confirmed both by DLS, which is very sensitive to formation
of larger clusters, as well as with SANS, which also showed a clear
transition from attractive to repulsive intermicellar interactions
upon increasing n-alkyl length. We attribute this
to the balance between the hydrophobic enthalpic terms that favor
anchoring of both chain-ends to the core and the entropic cost associated
with the bending of the polymer chains. For short n-alkyls, exposure of the chain-ends in the corona structure leads
to net dominance of the attractive interactions while for longer hydrophobic
chains it leads to a stabilization of loops and consequently flowerlike
micellar morphology. Using contrast-variation SANS, the contribution
of mono- and difunctional chains could be separated, confirming the
flowerlike micellar structure.