Homopolymers as structure-driving agents in semicrystalline block copolymer micelles.

A simplified hierarchical self-assembly strategy is presented in which homopolymer additives are used to manipulate the crystallization-driven self-assembly of block copolymer micelles in selective media. By first incorporating the appropriate homopolymer chains within the micelle core, the system then evolves passively to yield crystalline platelets. These lamellae may be considered as self-assembled analogues of the traditional polymeric single crystal, which can be challenging or laborious to obtain otherwise. Used here as the test systems are micelles bearing polycaprolactone as the crystalline subphase in water and a mixed hydrophilic corona of poly(ethylene oxide) and poly(acrylic acid) the composition of which was varied methodically. Comicellization with homo-PCL has no influence at first; instead, the assemblies undergo morphological changes hierarchically, which were probed by electron microscopy and light scattering measurements. For all materials, the final product is consistently lamellar and micrometer-sized; however, lamellar shape variations are encountered as the stabilizing corona is altered. Such lamellae are unexpected based on the composition of most copolymers used here. The phenomenon also depends highly on the nature of the homo-PCL additive. A possible source for the activity of the homo-PCL is suggested, which also provides a strong basis to adapt the strategy for other crystalline materials.