Self-assembly of rigid and coil polymers into hollow spheres in their common solvent

This paper studies self-assembly of a rigid oligomer, polyimide (PI) with carboxyl ends, and a proton-accepting polymer poly(4-vinylpyridine) (PVPy) or poly(styrene-co-4-vinylpyridine) (SVP) in solutions. It was found that these rigid/coil polymer pairs could form submicrometer hollow spheres in their common solvent chloroform. The rigid character of PI and its hydrogen-bonding grafting to PVPy chains are the key factors for such unique self-assembly behavior. Over a broad composition range, decreasing the ratio of PI/PVPy was found unfavorable to the assembly, i.e., leading to larger and broadly distributed hollow spheres. As the weight ratio of PI/PVPy decreased to 0.5 or less, the hollow spheres disappeared and soluble complex formed. PI could form hollow spheres with SVP as well, provided the pyridine unit content in SVP was not less than 11 mol %. With regard to the effect on the aggregate size and its distribution, decreasing the pyridine content in SVP was found equivalent to decreasing the PI/PVPy ratio. The structure of the hollow spheres of PI/SVP could be locked by chemical cross-linking SVP, and TEM studies showed that the cross-linked particles were able to maintain their integrity. Scanning electron micrographs of the solvent-cast films of the hollow-sphere solutions showed a two-dimensional hexagonal close-packing lattice of circular holes in a polymer matrix, similar to that for rod-coil block copolymers reported in the literature. Our work proved that this regular pattern was induced by water droplets and their packing in the process of dewetting of the polymer solutions.