Pearl-necklace complexes of flexible polyanions with neutral-cationic diblock copolymers

We study the complexation of very asymmetric diblock copolymers (consisting of a cationic block of 12 lysines connected to a 400 amino acid long hydrophilic polypeptide block with a net charge that is nearly zero) with oppositely charged sodium poly(acrylic acid) (NaPAA) with a range of molar masses between 2 and 1300 kg mol-1. For shorter Na-PAA chains, spherical complex coacervate micelles are formed, but for long Na-PAA chains, with molar masses in excess of 250 kg mol-1, atomic force microscopy indicates the presence of pearl-necklace structures. Complexes most likely consist of only a single NaPAA chain, complexed to multiple diblocks. Hence, the size of the complexes can be fully controlled via the NaPAA molar mass. The occurrence of pearl-necklace complexes at higher NaPAA molar masses is attributed to the packing frustration that arises due to the small size of the cationic block of the diblock copolymers