Control of Self-Assembly by Charge-Transfer Complexation between C60Fullerene and Electron Donating Units of Block Copolymers

Self-assembly allows new opportunities to control the morphology and properties of fullerene-based materials. By using the electron-accepting underivatized C 60 and electron-donating pyridine groups of a polystyrene- block -poly(4-vinylpyridine), PS- block -P4VP, we here show the first example that charge-transfer complexation between fullerenes and block copolymers can essentially modify the self-assembled structures. The morphology of C 60 /PS- block -P4VP mixture is studied in both bulk and thin films by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) upon casting from freshly prepared or differently aged xylene solutions. Selecting PS- block -P4VP that leads to hexagonal self-assembly with P4VP cylindrical cores, closely similar cylindrical structure is observed upon adding a small amount of C 60, when freshly prepared xylene solutions are used to cast solid bulk or thin film samples. In this case the C 60 molecules swell the PS matrix. By contrast, spherical morphology is observed if aged xylene solutions have been used where C 60 has slowly penetrated into the P4VP micellar cores due to charge-transfer complexation, as evidenced by UV−vis and FTIR spectroscopy. This morphological change is unexpected, as according to simple block copolymer theory selective incorporation of a small molecular species or nanoparticles within the minority block of hexagonally self-assembled cylindrical diblock copolymer system should render the morphology toward lamellar structures. This suggests that charge-transfer complexation can be relevant in considering the self-assembled structures of block copolymers and fullerenes.