Thermally Induced Distortion of the Hexagonal Close-Packed Lattice of Block Copolymer Micelles

The hexagonal close-packed layers (HCPLs) in the hexagonal close-packed (HCP) lattice of the spherical micelles in block copolymer (BCP)/homopolymer blends were previously shown to contain a significant fraction of stacking faults, which increased in a thermally reversible manner as the temperature decreased. In this study, we further observed that the interlayer distance of the HCPLs expanded more significantly than the lateral dimension during cooling, resulting in a distorted HCP phase, with the unit cell aspect ratio (c/a) exceeding the ideal value of 1.633. The concurrent emergence of unit cell distortion and stacking faults suggested a strong correlation between these two defects. Furthermore, their thermally reversible behavior pointed to a thermodynamic origin. We propose that when the lateral size of the HCPL is small, the entropy gain from mixing HCPLs with different stacking sequences competes with the bulk free energy that favors the ideal HCP packing, leading to the introduction of stacking faults. In BCP/homopolymer blends, a fraction of the homopolymer, initially solubilized in the micelle corona, may be expelled into the interlayer regions to alleviate the bulk free energy penalty associated with stacking faults, thereby increasing the aspect ratio of the unit cell. This study reveals a thermally activated distortion of the close-packed structure in BCP blends, highlighting the complexity and tunability of the spherical phase in BCP systems.