Self-assembly of mixtures of telechelic and monofunctional amphiphilic polymers in water: From clusters to flowerlike micelles

Zinn, T. , Willner, L. , Knudsen, K.D. , Lund, R.
Macromolecules, Vol 50, no. 18 (2017), 7321-7332
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We 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.
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