On the Origins of Morphological Complexity in Block Copolymer Surfactants. Amphiphilic compounds such as lipids, soaps, and other surfactants play an important role in numerous arenas, from the biology of living cells to food products to oil recovery. The unique properties of these compounds stem from the existence of hydrophobic and hydrophilic domains, which often prompt the formation of highly organized structures, many which display curved interfaces and well-defined topologies. Block copolymers - macromolecules with dissimilar components linked by covalent junctions - are known to display surfactant behavior, which has led to the discovery and characterization of a cornucopia of complex morphologies.

Figure 1. Three microstructures - vesicles, cylinders with Y-junctions, and spheres - can be observed for aqueous dispersions of PB-PEO diblock copolymers by cryogenic transmission electron microscopy. The existence of these microstructures depends on the weight fraction of PEO. The schematics illustrate that the hydrophobic PB block (red) segregates to the core of the micelle whereas the PEO block (blue) segregates to the corona in contact with the water.

With support from the University of Minnesota MRSEC, Sumeet Jain and Frank Bates have discovered that poly(1,2-butadiene-b-ethylene oxide) (PB-PEO) diblock copolymers in water form three-dimensional networks with Y-junctions when the weight fractions of PEO are adjusted to values between those that prompt the formation of vesicle and wormlike micelle morphologies. Sonication of the mixtures disrupts the reticulated three-dimensional networks to afford an array of discrete micelles that share three essential elements: Y-junctions, spherical end caps, and cylindrical loops. A striking feature of these discrete micelles is their pervasive mirror symmetry, which most likely reflects a balance of free energy within the structure through redistribution of the block copolymer following micelle assembly. Jain and Bates conclude from their data that this type of self-assembly in PB-PEO diblock copolymers appears above a critical molecular weight. The aqueous solutions of these block copolymer surfactants mimic certain low molecular weight three-component (surfactant/water/oil) microemulsions, but they offer substantial practical advantages because a separate hydrophobe (oil) is not required. Furthermore, the ability to manipulate the physical properties of the block copolymer through synthetic design presents an opportunity for new materials with properties that are difficult to achieve with low molecular weight amphiphiles. For more details, see S. Jain and F. S. Bates, Science, 2003, 300, 460.

Figure 2. (left) Cryo-TEM images of a 1 wt% aqueous solutions of PB-PEO diblock copolymers (weight fraction of PEO = 0.34) consists of micelle networks with numerous loops and Y-junctions.(right) Two examples of discrete micelle particles observed after sonication.