Ji-Woong Park, Jae-Suk Lee and colleagues from the Gwangju Institute of Science and Technology, investigated the structures formed when amphiphilic rod-coil block copolymers were dissolved in a solvent specific for the rod block of the copolymer. Until now, most research has focused on the structures formed when the copolymer is exposed to solvent specific to the coil block; where the resulting aggregation of the rod block is driven by solvophobic effects and can lead to a range of aggregate morphologies, with hollow cores, including hockey pucks and needles. 

Park, Lee and colleagues have discovered that placing the rod-coil block copolymer in toluene, a solvent in which the rod component of the amphiphile is soluble, leads to the aggregation of the coil block. The tangential packing of the rod block forms spherical, membrane bound structures that have a shielded core of approximately 20 to 50 nm in diameter, with a thin shell of only 4 nm thickness. This tangential packing, or so-called 'chain wrapping', of the soluble part of the copolymer has not been observed previously; it is the only plausible packing arrangement as the rod block of the copolymer is 38 nm long, so a wrapped (tangential) arrangement is the only way that the rod shell could be confined to a thickness of only 4 nm. The proposed arrangement has been confirmed by Park and Lee and the team via transmission electron microscopy and atomic force microscopy. 

The membrane bound structures are reversibly interchangeable between a vesicular and micellar state, depending on the concentration of the copolymer in the solvent. At low concentrations of the copolymer, the aggregates formed micelles (with a diameter of 40 to 60 nm). At higher concentrations, the aggregates formed vesicles (with diameters of 120-150 nm). The hollow cores of the aggregates were shown to be effectively protected from the surrounding solvent. 

The team postulate that the concentration driven swapping between micellar and vesicular structures is due to competing geometric and thermodynamic constraints, which can also be accredited with the unusual tangential packing of the rod blocks, to produce these unusual, chain-wrapped aggregates. The chain wrapping of the soluble rod block was indirectly evidenced by the successful use of the aggregates as nano-sized reactors for the production of two types of gold nanoparticles. The team also envision that these aggregates may be formed with any type of rod coil block copolymer and could be conjugated with a variety of molecules, making them potentially useful for a broad range of applications.

Katie Dryden-Holt