Modulating melting points in micellar cores: influence of the corona chain length on the core confinement in binary mixed block copolymer micelles.

Abstract

Self-assembly of polymers with crystallizable blocks may lead to micelles with ordered, gel-like or crystalline cores. Here we investigate binary mixtures of n-alkyl-poly(ethylene oxide), C_n-PEOx (n = 28, x = 3-20 kDa) and study their self-assembly to gain insight into the effect of confinement on the core-crystallization and micellar structure. By employing identical core block length but varying corona block lengths, the size of the core can be tuned by variation of the block ratios. The micelles were characterized by small-angle X-ray scattering (SAXS) to gain insight into the overall and internal structure, including aggregation number, core size, and density distribution of the corona. SAXS curves from examined samples showed a characteristic pattern of spherical core-shell micelles but with broader corona distribution in the binary mixtures as compared to the neat samples. The structural parameters of the micelles were extracted from the SAXS data by employing a spherical core-shell model with dual density profiles in the core. We found that the aggregation number decreases as PEO length increases following a power law predicted in the literature. Furthermore, the melting point and melting enthalpy of crystalline alkyl cores were closely inspected by densitometry and differential scanning calorimetry (DSC). Correlating the core radius obtained from SAXS, we found that the melting point depression caused by the self-confinement in the micellar core can be described by the Gibbs-Thomson equation. These results show that the micellar structure and phase transition of the semicrystalline core can be easily tuned through blending diblock copolymers with different corona block lengths.