Structure and thermodynamics of mixed polymeric micelles with crystalline cores: tuning properties via co-assembly

• Published in: Soft matter Vol. 15; no. 39; pp. 7777 - 7786
• Main Authors: König, Nico, Willner, Lutz, Lund, Reidar
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 2019
• Subjects: Alkanes; Aqueous solutions; Assembling; Block copolymers; Calorimetry; Crossovers; Crystal structure; Crystallinity; Crystallization; Dependence; Differential scanning calorimetry; Ethylene oxide; Hydrophobicity; Melting; Melting point; Melting points; Micelles; Polyethylene oxide; Scaling laws; Small angle X ray scattering; Thermodynamics; X-ray scattering
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c9sm01452g

We investigate micelles formed by mixtures of n-alkyl-poly(ethylene oxide) block copolymers, Cn-PEO, with different alkyl block lengths in aqueous solution. This model system has previously been used to shed light on the interplay between exchange kinetics and crystallinity in self-assembling systems [König et al., Phys. Rev. Lett., 2019, 122, 078001]. Now we report on the structure and thermodynamics of these micelles by combining results from small-angle X-ray scattering, differential scanning calorimetry and volumetric measurements. We show that mixed micelles are formed despite the fact that length-mismatched n-alkanes of similar weights in bulk tend to demix below the crystallization temperature. Instead, the system exhibits similar properties as single-component micelles but with a modulated melting region. Interestingly, the melting point depression due to self-confinement within the micellar core can be approximately described by a generalized Gibbs-Thomson equation, similar to single-component micelles [Zinn et al. Phys. Rev. Lett., 2014, 113, 238305]. Furthermore, we find a novel scaling law for these micelles where, at least for larger n, the aggregation number scales with the third power of the length of the hydrophobic block, Nagg ∝ n3. Possibly, there might be a cross-over from the conventional Nagg ∝ n2 behaviour around n ≈ 19. However, the reason for such a transition as well as the strong n dependence remains a challenge and requires more theoretical work.