Self-Organization on Multiple Length Scales in "Hairy Rod"-Coil Block Copolymer Supramolecular Complexes

The last two decades have seen an explosion of research activity in the area of self-assembled polymeric and supramolecular materials. Self-assembly schemes rely on an often delicate balance between competing repulsive and attractive forces between structural elements. In traditional coil-coil block copolymers, microphase separated structures are dictated by the balance of immiscibility between blocks and the stretching of chains confined at the microphase interface due to covalent linkages. Mesomorphic behavior is also observed in polymeric comb-like supramolecules created by bonding mesogenic or flexible side chains to a polymer backbone through non-covalent physical interactions such as ionic bonding, hydrogen bonding, metal coordination or various combinations thereof. Increasingly complicated schemes for constructing self-organizing systems now incorporatemultiple building blocks with various structural motifs, taking advantage of competing interactions acting on multiple length scales, and thus giving rise to hierarchical self-assembly. In block copolymers where one block is rod-like, whether owing to helicity or extended p-conjugation, the propensity of the rods to order in liquid crystalline arrangements offers an additional ordering principle. In this case, order can be observed on the scale of a few nm (inter-rod liquid crystalline packing) and on the larger length scale of the block copolymer, typically a few tens of nm. Rod-coil block copolymers where the rod block is an a-helical A peptide-synthetic hybrid block copolymer, poly(ethylene oxide)-block-poly(L-glutamic acid), is shown to form supramolecular complexes with primary alkylamines of varying alkyl chain lengths (8 to 18 methylene units) in organic solvents via acid-base proton transfer and subsequent ionic bonding. The peptidic block being in the a-helical conformation, these materials behave as coil‘‘hairy rod’’ block copolymers, and show hierarchically selforganized nanostructures in the solid state. X-ray scattering measurements show mesomorphic behavior at the length scales of both the overall block copolymer and the polypeptide-alkylammonium complex.